TY - JOUR
AU - Ikari, Matt J
AB - SUMMARY Slow slip events (SSEs) and other forms of slow and transient fault slip are becoming increasingly recognized as important, due to their influence on seismicity and potential to provide information on fault zone properties. The majority of our current knowledge of slow fault slip has been obtained from geodetic and seismologic field measurements, but laboratory data of slow slip are comparatively scarce. Here, I present the results of laboratory friction experiments conducted on 11 natural samples from major fault zones around the world, all obtained by scientific drilling. The experiments are conducted water saturated, at room temperature and 10 MPa effective normal stress, representative of in-situ conditions on the shallow portions of fault zones from which the samples were recovered and where slow slip is known to occur. A key component of these experiments is shearing at realistically slow driving rates of cm yr–1, accurately simulating tectonic driving rates. In most samples, these cm yr–1 driving rates produce laboratory SSEs, which are instances of accelerating slip accompanied by a stress drop. The peak slip velocities and stress drops measured in these laboratory SSEs are comparable with those of natural SSEs measured or estimated from geodetic data. A strong correlation is observed between reduced pre-SSE velocity and higher peak slip velocity for the entire laboratory SSE data set. In contrast to the velocity data, significant scatter is observed in the percentage stress drop measurements. The source of this scatter can be attributed to samples with a significant expandable clay component, which tend to exhibit larger stress drops. Results of velocity-stepping tests at cm yr–1 rates show a tendency for velocity-weakening friction not observed at higher sliding velocities, and that the materials with lower values of the rate-dependent friction parameter a–b tend to produce faster SSEs. Critical stiffness analyses within the framework of rate-and-state friction laws show that most of the SSEs observed in this study do not satisfy the condition for slip instability. The SSEs are more consistent with accelerating stable slip, although the stiffness condition allowing such behaviour is not always satisfied. Considering the laboratory SSEs to be accelerating stable slip, I present a conceptual model for their nucleation. Key elements of the model are a healing-dominated departure from steady-state causing partial locking and velocity decrease, followed by a transition to a velocity-dominated phase representing the actual slip event. The model is consistent with observations from geodetic measurements and the experimental observations in this study. In general, characteristics of SSE-producing fault portions such as the ability to strengthen and store elastic strain energy released as stress drops may be expected to enhance coseismic slip from remotely nucleating earthquakes, an effect which may be quite limited but should be investigated further. Creep and deformation, Fault zone rheology, Geomechanics, Ocean drilling, Rheology and friction of fault zones, Dynamics and mechanics of faulting 1 INTRODUCTION Decades since the first observations suggesting the occurrence of slow slip events (SSEs) on plate-boundary faults (e.g. Nason 1969; Goulty & Gilman 1978; Sacks et al.1978, 1981), recently improved geodetic and seismologic coverage has lead to a rapid increase in observations of these phenomena over the past ∼20 yr (e.g. Schwartz & Rokosky 2007; Saffer & Wallace 2015; Bürgmann 2018). One key observation is that discrete slow fault slip occurs over a wide range of timescales: from slow slip events (SSEs) having durations on the order of days to years, to low-frequency earthquakes lasting less than a minute that can still be detected seismologically. Slow slip in its various forms thus represents a continuum between steady fault creep and ordinary, rapidly slipping earthquakes (Ide et al.2007; Peng & Gomberg 2010). Slow slip at plate-boundaries occurs as discrete fault slip events (e.g. Brown et al.2009), and are therefore important because of their influence on ordinary earthquakes. SSEs are in some cases thought to reduce earthquake hazard by relieving stress on otherwise seismogenic fault patches, for example in the Guerrero seismic gap in Mexico (Radiguet et al.2012; Husker et al.2018). However, other studies have suggested that SSEs can increase earthquake hazard on adjacent fault patches through stress transfer (Mazzotti & Adams 2004; Wech & Creager 2011; Koulali et al.2017); for example earthquakes swarms are inferred to have been triggered by SSEs on the same fault in the Hikurangi (Reyners & Bannister 2007; Delahaye et al.2009) and Central Ecuador (Vallée et al.2013) subduction zones. SSEs are reported to have occurred immediately preceding the 2011 Mw 9.0 Tohoku-Oki earthquake in Japan (Ito et al.2013) and the 2014 Mw 8.1 Iquique earthquake in Chile (Ruiz et al.2014) within the eventual main shock rupture patches. These wide-ranging observations are intriguing and raise important questions regarding how and why SSEs occur across multiple regions. A critical component of fault zone studies is laboratory friction experiments simulating fault slip. Many previous studies have focused on the materials and conditions necessary for slip instability, which is responsible for the nucleation of ordinary earthquakes (Dieterich & Kilgore 1996a; Marone 1998a; Scholz 2002). The emergence of slow slip as a continuum between stable (i.e. steady fault creep) and unstable fault slip has led to the expectation that slow slip requires some form of a transitional frictional stability condition (e.g. Dieterich 1986; Scholz 1998; Lowry 2006). Some particular laboratory studies have been able to produce slow stick slip by manipulating parameters controlling frictional stability (Baumberger et al.1999; Leeman et al.2016; Scuderi et al.2016). Other, numerical modelling studies are able to simulate SSEs by having unstable slip nucleate which is then stabilized, for example by spatially-variable stability (Liu & Rice 2005, 2007, 2009) or slip velocity-dependent stability (Shibazaki & Iio 2003; Shibazaki & Shimamoto 2007; Matsuzawa et al.2010; Hawthorne & Rubin 2013). These studies have provided great insight into the role of quasi-frictional slip stability for SSEs, however some particular limitations are that these studies are based on the frictional properties measured for analogue materials (polymer glass, halite, granite or gabbro) rather than materials from natural fault zones, and under conditions that could be more closely matched to in situ conditions. Recently, laboratory studies have demonstrated that laboratory SSEs can be generated during shear deformation at geologically realistic plate tectonic driving rates of cm yr–1 (Ikari et al.2015a; Ikari & Kopf 2017). These experiments were also conducted at low temperature and low effective stress, which are the relevant in situ conditions for slow slip observed on the shallow (a few km up to the surface) portions of major fault zones (e.g. Gladwin et al.1994; Outerbridge et al.2010; Wallace et al.2016; Araki et al.2017). A first study using samples of the plate boundary fault zone recovered from the high coseismic slip region of the 2011 Mw 9 Tohoku-Oki earthquake showed that when driven at the plate convergence rate of ∼8.5 cm yr–1, these samples exhibited discrete slip rate increases concurrent with stress drops interpreted to be laboratory SSEs (Ikari et al.2015a). The peak slip velocity of the laboratory SSEs reaches approximately three times the plate convergence rate, consistent with geodetically-observed SSEs. A following study introduced samples from seven additional major fault zones, revealing that shallow SSEs may be a common phenomenon worldwide (Ikari & Kopf 2017). Here, I expand on the study of Ikari & Kopf (2017) by examining the laboratory SSEs in more detail and introducing additional data and analysis. Additionally, the mechanisms of laboratory SSEs and implications for the overall slip behaviour of major faults, including ordinary earthquake hazard, are discussed. 2 GEOLOGICAL SETTING OF TESTED FAULT ZONES 2.1 Tohoku, Japan Trench The Japan Trench is formed by the subduction of the Pacific Plate beneath the North American/Okhotsk Plate at a rate of ∼8.3 cm yr–1 (DeMets et al.1994). This margin hosted the largely unexpected 2011 Mw 9 Tohoku-Oki earthquake and ensuing tsunami. Based on seismic and tsunami waveform inversions, geodetic data, and repeated bathymetry surveys, this event produced ∼50–80 m of coseismic slip that breached the seafloor (Fig. 1a, Fujiwara et al.2011; Ito et al.2011; Sun et al.2017a). Although the Japan Trench is best known for the Tohoku earthquake, the region has frequently experienced large earthquakes over the last ∼100 yr, mostly of Mw ≥ 7 and occasionally Mw ∼ 8 (e.g. Kawasaki et al.2001). In addition to large and great earthquakes, the Tohoku area exhibits a full range of slow slip, including low frequency earthquakes (Fukao & Kanjo 1980), tectonic tremor (Ito et al.2015), earthquake afterslip (Ohta et al.2012) and SSEs, two of which occurred immediately preceding the Tohoku earthquake (Ito et al.2013). Figure 1. Open in new tabDownload slide Regional maps of study areas: (a) the Japan Trench (modified from Chester et al.2013a and Ito et al.2013), showing the epicentre and region of >50 m coseismic slip of the 2011 Tohoku earthquake, and the 2011 SSE preceding the Tohoku earthquake, (b) the Nankai Trough (modified from Kinoshita et al.2009), showing the rupture areas of the 1944 and 1946 Mw ∼ 8 earthquakes. VLFE locations from Obara & Ito (2005), (c) the Cascadia subduction zone (modified from Westbrook et al.1994). Tremor locations from Wech & Creager (2011), (d) the Costa Rica subduction zone at the Middle America Trench (modified from Morris et al.2003). Tremor/LFE locations from Walter et al. (2011, 2013), SSE locations from Outerbridge et al. (2010). (e) the Lesser Antilles subduction zone near Barbados (modified from Saffer & Tobin 2011), (f) New Zealand, showing the Hikurangi subduction zone offshore the North Island, transitioning to the Alpine Fault on the South Island (modified from Carpenter et al.2014). SSE locations from Wallace et al. (2009), (g) the San Andreas Fault in California, showing the creeping section and location of the 2004 Parkfield earthquake (modified from Zoback et al.2011) and (h) the Woodlark Basin near Papua New Guinea (modified from Wallace et al.2014). Figure 1. Open in new tabDownload slide Regional maps of study areas: (a) the Japan Trench (modified from Chester et al.2013a and Ito et al.2013), showing the epicentre and region of >50 m coseismic slip of the 2011 Tohoku earthquake, and the 2011 SSE preceding the Tohoku earthquake, (b) the Nankai Trough (modified from Kinoshita et al.2009), showing the rupture areas of the 1944 and 1946 Mw ∼ 8 earthquakes. VLFE locations from Obara & Ito (2005), (c) the Cascadia subduction zone (modified from Westbrook et al.1994). Tremor locations from Wech & Creager (2011), (d) the Costa Rica subduction zone at the Middle America Trench (modified from Morris et al.2003). Tremor/LFE locations from Walter et al. (2011, 2013), SSE locations from Outerbridge et al. (2010). (e) the Lesser Antilles subduction zone near Barbados (modified from Saffer & Tobin 2011), (f) New Zealand, showing the Hikurangi subduction zone offshore the North Island, transitioning to the Alpine Fault on the South Island (modified from Carpenter et al.2014). SSE locations from Wallace et al. (2009), (g) the San Andreas Fault in California, showing the creeping section and location of the 2004 Parkfield earthquake (modified from Zoback et al.2011) and (h) the Woodlark Basin near Papua New Guinea (modified from Wallace et al.2014). One year after the Tohoku earthquake, the Japan Trench Fast Drilling Project (JFAST) was conducted as Integrated Ocean Drilling Program (IODP) Expedition 343 (Chester et al.2013a). JFAST drilling was conducted ∼7 km from the deformation front, within the region of highest coseismic slip during the Tohoku earthquake. Core sampling concentrated within ∼650–840 m below seafloor (mbsf) at IODP hole C0019E recovered a relatively structureless siliceous mudstone hanging wall, and a highly localized zone of scaly clay with intense deformation fabric at 821.5–822.5 mbsf. This scaly clay zone is interpreted to be the plate-boundary fault zone, although due to incomplete core recovery it may not necessarily contain the principal slip zone of the Tohoku earthquake (Chester et al.2013b). The sheared clays are inferred to be derived from pelagic clays observed at the base of an older borehole seaward of the Japan Trench, at Site 436 (Kirkpatrick et al.2015). The composition of the clays that make up the fault zone is especially rich in smectite, which comprises up to ∼80 per cent of the bulk sediment (Kameda et al.2015). 2.2 Nankai Trough, Japan Southeast of the Japan Trench, the Nankai Trough is formed by the subduction of the Philippine Sea Plate beneath the Amurian microplate at a rate of ∼4-6.5 cm yr–1 (Miyazaki & Heki 2001; Loveless & Meade 2010) (Fig. 1b). The region is known for a history of destructive earthquakes that may extend over 1300 yr (Ando 1975). Two recent Mw > 8 earthquakes occurred in 1944 and 1946 (Tanioka & Satake 2001; Cummins et al.2002; Ichinose et al.2003; Kikuchi et al.2003) making this region a main focus of several earthquake studies, including scientific drilling expeditions. Deep Sea Drilling Program (DSDP), Ocean Drilling Program (ODP), and IODP expeditions have drilled three northwest-southeast trending borehole transects over 300 km along strike aligned with Cape Ashizuri, Cape Muroto, and through the Kumano Basin. We focus on samples recovered from two particular sites; the first is Site 1174, which penetrated the plate-boundary décollement near the Nankai Trough deformation front during ODP Leg 190 (Moore et al.2001). This site is located updip of the 1944 Tonankai earthquake rupture area (Fig. 1b). The second is Site C0004, which penetrated a major out-of-sequence splay fault known as the “megasplay” during IODP Expedition 316 (Kinoshita et al.2009). Splay faulting has been implicated as a potential host of coseismic slip in the Nankai Trough, based on seismic reflection surveys (Park et al.2002; Moore et al.2007), geodetic and tsunami data (Sagiya & Thatcher 1999; Cummins & Kaneda 2000; Baba et al.2006), numerical models of rupture branching (Kame et al.2003), and indications of frictional heating from vitrinite reflectance measurements on core samples (Sakaguchi et al.2011). Shallow very low frequency earthquakes (VLFE) have been observed within the Nankai accretionary prism, likely on splay faults (Obara & Ito 2005; Ito & Obara 2006a, 2006b) or the plate boundary décollement (Sugioka et al.2012). Recently, Araki et al. (2017) documented fluid pressure transients in borehole observatories indicating a shallow SSE propagating updip which may have reached the trench. 2.3 Cascadia The Cascadia subduction zone is formed by the eastward subduction of the Juan de Fuca Plate beneath the North American Plate, and extends over 1000 km from offshore northern California to Vancouver Island (Fig. 1c). The potential for earthquakes of Mw ≥ 8 in Cascadia has been the subject of debate; early geodetic studies and a lack of recorded large earthquakes over the past ∼180 yr suggested aseismic subduction of the Juan de Fuca Plate (e.g. Ando & Balazs 1979). However, palaeoseismology studies based on shallow sediment stratigraphy provide evidence that large earthquakes have occurred over the past ∼8000 yr with a recurrence interval of ∼500–600 yr (Atwater 1987; Adams 1990; Kelsey et al.2002; Goldfinger et al.2003; Witter et al.2003), with the latest large earthquake having occurred in ∼1700 (Satake et al.1996; Jacoby et al.1997). More recent geodetic measurements indicate plate locking and slip deficit sufficient for future great earthquakes (e.g. Hyndman & Wang 1995; Flück et al.1997; McCaffrey et al.2000; Wang et al.2003; Wang & Tréhu 2016) The Cascadia subduction zone has become well-known for exhibiting diverse types of slow fault slip, particularly SSEs and tectonic tremor (Dragert et al.2001; Miller et al.2002; Rogers & Dragert 2003; Szeliga et al.2008; Wech & Creager 2011). These observations are based on measurements from onland GPS and seismometer networks, and therefore locate these events to the plate interface deeper than 25 km. The Cascadia margin has been drilled during several expeditions; of particular importance to this study is Site 891 drilled during ODP Leg 146, which penetrated the frontal thrust in the prism toe at the southern Cascadia margin offshore central Oregon (Westbrook et al.1994). 2.4 Costa Rica At the Middle America Trench (MAT) offshore Costa Rica, the Cocos Plate subducts beneath the Caribbean Plate at a rate of ∼8.8 cm yr–1 (DeMets et al.2010) (Fig. 1d). We focus here on the Nicoya Peninsula area, which was the location of ODP drilling during Leg 170 (Kimura et al.1997) and Leg 205 (Morris et al.2003). The character of the subducting plate in this region is diverse; offshore northern Nicoya Peninsula the crust originates from the East Pacific Rise, offshore southern Nicoya from the Cocos-Nazca spreading centre. A key difference is that the EPR crust is bathymetrically smooth compared to the CNS crust, on which several 1–2.5 km high seamounts are subducting (Hey 1977; Ranero & von Huene 2000; von Huene et al.2000). Drilling of the input sediment revealed ∼180 m of hemipelagic siliceous ooze and claystone overlying ∼220 m of siliceous ooze, calcareous ooze and pelagic nannofossil chalk. On bathymetric highs such as seamounts, the chalk is exposed at the seafloor rather than the hemipelagic muds that dominate the EPR crust (Spinelli & Underwood 2004). Because the MAT is considered to be undergoing active subduction erosion at a large scale (Meschede et al.1999; Ranero & von Huene 2000; Vannucchi et al.2003) the uppermost sediment layer likely controls megathrust slip behaviour. The Costa Rica subduction zone has recently hosted several large earthquakes of Mw ≥ 7 in the Nicoya region (Protti et al.1995; Husen et al.2002; Yue et al.2013). The shallow megathrust is especially active, exhibiting slow slip events (Outerbridge et al.2010), tectonic tremor (Walter et al.2011) and earthquake afterslip that reaches the trench (Sun et al.2017b). Notably, the Nicoya Peninsula region is where evidence of shallow SSEs was recorded by borehole observatories installed during Leg 205, indicating shallow activity on the megathrust (Solomon et al.2009; Davis et al.2011, 2015). 2.5 Barbados The Barbados accretionary complex is located in the Lesser Antilles subduction zone, where the diffuse boundary between the North American and South American plates is subducting beneath the Caribbean Plate at rate of ∼2 cm yr–1 (Minster & Jordan 1978; DeMets et al.2000, 2010) (Fig. 1e). Seismic reflection surveys revealed a wide accretionary complex with several mud volcanoes, which suggest fluid overpressuring (Westbrook & Smith 1983; Westbrook et al.1988). Exploring the hydrogeology of accretionary prisms provided the motivation for drilling a transect of several boreholes near the deformation front during ODP Legs 110 (Mascle et al.1988) and 156 (Shipley et al.1995). The ODP boreholes are located on a flank of the Tiburon Rise, one of three trench-perpendicular ridges being underthrust at the Lesser Antilles margin. Low seismic activity in the vicinity of these ridges has lead to their classification as ‘aseismic’ (McCann & Sykes 1984; Bouysse & Westercamp 1990). The Lesser Antilles margin overall exhibits low seismicity, with no earthquakes of Mw ≥ 8 since 1900 and only one subduction thrust earthquake of Mw > 6 since 1973 (Dorel 1981; Stein et al.1982; Hayes et al.2014). Although it was suggested that the low seismicity rate is related to the low plate convergence rate and/or aseismic creep, Hayes et al. (2014) suggest that enough elastic strain has accumulated to produce an earthquake of Mw ≥ 8. Slow slip and tectonic tremor in the Lesser Antilles has not yet been reported from the locally installed GPS and seismic networks. 2.6 Hikurangi, New Zealand The Hikurangi subduction zone is located off the east coast of New Zealand's North Island (Fig. 1f). At this margin, the Pacific Plate subducts westward beneath the North Island at a rate of 2–6 cm yr–1, increasing from south to north (Wallace et al.2004). Although this margin has hosted megathrust earthquakes with moment magnitude Mw of up to 7.1, none this large have been recorded over the past century (Doser & Webb 2003). However, geodetic locking estimations indicate that the plate interface has the potential to rupture in earthquakes of Mw 8 or larger (Wallace et al.2009). The Hikurangi subduction zone hosts frequent SSEs, which have been detected geodetically since 2002 by a continuous GPS network (e.g. Douglas et al.2005; Wallace et al.2009, 2012, 2016, 2017; Wallace & Beavan 2010). The SSEs are clearly spatially segmented; north of around 40–41° S the SSEs are shallower (<15 km depth), shorter in duration (1–3 weeks), and recur more frequently (every 1–2 yr), whereas to the south the SSEs are deeper (∼35–60 km depth), longer (>1 yr), and less frequent (recurrence of ∼5 yr). Furthermore, it has been found that the upper depth limit of SSE rupture areas coincides with the depth extent of geodetic plate locking along the margin (Wallace et al.2009, 2012; Wallace & Beavan 2010). Noteworthy facets of the Hikurangi SSEs include mutual triggering relationships with ordinary earthquakes (Koulali et al.2017; Wallace et al.2017, 2018), and the observation of accompanying microseismicity rather than tremor (Delahaye et al.2009). Coring of the subduction inputs, the upper plate, and a frontal thrust was recently completed during IODP Expedition 375 (Saffer et al.2018). In particular, nearly 1200 m of input material was drilled which revealed the major lithologies to be a hemipelagic mud/tubidite trench wedge facies, pelagic chalk, and coarse volcaniclastics. A similar chalk unit was recovered during previous scientific drilling of the incoming sedimentary sequence ∼400 km east of the Hikurangi Trough during ODP Leg 181, at Site 1124 (Carter et al.1999). 2.7 Alpine fault, New Zealand Southwest from the Hikurangi Trough, the Pacific–Australian Plate boundary continues onland to become the Alpine Fault (Fig. 1f). The fault is right-lateral with a displacement rate of 23–27 mm yr–1 over the past ∼50 000 yr, based on radiocarbon dating and offset of geological markers (Cooper & Norris 1994; Norris & Cooper 2000; Sutherland et al.2006). Palaeoseismology studies estimate the recurrence of major earthquakes (Mw > 7.0) to be 260–400 yr over the past 8000 yr (Sutherland & Norris 1995; Bull 1996; Berryman et al.2012), with none occurring in the past ∼300 yr (Cooper & Norris 1990; Sutherland et al.2007). Geodetic measurements indicate that the central segment of the Alpine Fault is fully locked down to depths of 5–8 km and partially locked to ∼18 km, with no creep occurring at the surface (Beavan et al.1999, 2007; Norris & Cooper 2000; Wallace et al.2007). Futhermore, based on earthquake hypocentres the Alpine Fault seismogenic zone is estimated to range from ∼12 km up to near the Earth's surface (Leitner et al.2001). Thus, previous studies collectively indicate that large magnitude earthquake, likely surface-breaking, should be expected in the near future. Evidence that the Alpine Fault is nearing the end of its earthquake cycle has motivated the Deep Fault Drilling Project (DFDP) within the framework of the International Continental Scientific Drilling Project (ICDP). During the initial phase of the project, two shallow boreholes located 80 m apart were drilled through the Alpine Fault: DFDP-1A drilled to 96 m depth, and DFDP-1B drilled to 151 m depth. Both boreholes successfully penetrated principal slip zones (PSZs) consisting of clayey gouge, representing the fault zone proper (Sutherland et al.2012; Townend et al.2013). In DFDP-1B, two PSZs were identified (Toy et al.2015). The main (upper) PSZ, which was tested in this study, is ∼20 cm thick and located within green cataclasites at a depth of ∼128 m. The second PSZ is located at ∼144 m depth; it is not clear which PSZ was most recently active. 2.8 San andreas fault, California The San Andreas Fault runs along western California, and is known for its central creeping section between San Juan Bautista in the north and Cholame in the south (Fig. 1g). The average displacement rate on the San Andreas Fault is ∼25–32 mm yr–1 from GPS measurements (Titus et al.2005, 2006; Ryder & Bürgmann 2008). The fault is locked and accumulating elastic strain outside of the creeping section (Irwin & Barnes 1975), and even within the creeping section some studies suggest slip deficit accumulation that could be released as a Mw ∼6–7 earthquake (Ryder & Bürgmann 2008; Johnson 2013; Maurer & Johnson 2014; Jolivet et al.2015). The San Andreas Fault is one of the first fault zones on which slow slip was identified, from strainmeter measurements near the southern (Goulty & Gilman 1978) and northern (Nason 1969; Gladwin et al.1994; Linde et al.1996) ends of the creeping zone. These are shallow events occurring at <500 m depth. The San Andreas Fault Observatory at Depth (SAFOD) penetrates the San Andreas Fault at ∼2.7 km depth with a northeast-dipping borehole near Parkfield, California (Zoback et al.2011). The drillsite is located at the southern boundary of the SAF central creeping section, approximately 25 km northwest of the fault patch that hosted 7 repeating earthquakes near Parkfield dating back to 1857, the most recent being the Mw 6.0 event in 2004 (Bakun & McEvilly 1984). In the direct vicinity, several clusters of repeating Mw = 2.0 earthquakes have been observed (Nadeau & Johnson 1998; Zoback et al.2011). Borehole geophysical logs, cuttings, and core obtained during SAFOD identified two actively creeping fault strands, termed the Central Deforming Zone (CDZ) and the Southwest Deforming Zone (SDZ). The CDZ is considered the main strand of the active SAF accommodating plate motion, whereas the SDZ marks the southwest edge of the modern SAF damage zone (Zoback et al.2011; Moore 2014; Carpenter et al.2015a). A key finding from the SAFOD core is the large amount of magnesium-smectite (saponite) within the CDZ (e.g. Solum et al.2006; Schleicher et al.2012), implicated in controlling the slip behaviour of the creeping SAF (Carpenter et al.2011, 2012, 2015a; Holdsworth et al.2011; Lockner et al.2011; Hadizadeh et al.2012; Ikari et al.2016a). 2.9 Woodlark Basin, Papua New Guinea The Woodlark Basin is formed by north-south extension between Papua New Guinea to the west and the Solomon Islands to the east (Fig. 1h). The basin lies in a tectonically complex region between the Pacific and Australian plates, where the extension defines the boundary between the northern edge of the Australian Plate and the Woodlark microplate. Extension in the Woodlark basin is transitional from continental rifting at a rate of 3.6 cm yr–1 at the western end, to seafloor spreading at a rate of 6.7 cm yr–1 at the eastern end (Taylor et al.1995; Tregoning et al.1998; Wallace et al.2014). In order to investigate the processes associated with rifting and continental breakup, ODP Leg 180 drilled several boreholes near the Moresby seamount, on the western (continental rifting) side of the Woodlark basin (Taylor et al.2000; Taylor & Huchon 2002). At one site in particular (Site 1117), fault gouge exposed at the seafloor by dip-slip motion was successfully recovered. In this region, earthquake focal mechanisms indicate active normal faulting at depths of ≥10 km, with Mw up to 6.8. Normal faults in the region exhibit a wide range of dips, with several exhibiting low angles including the Moresby seamount itself, which likely represents the footwall of a ∼30° dipping detachment normal fault (Abers 1991; Abers et al.1997). The slip rate on individual normal faults in the region is estimated to be up to ∼1 cm yr–1 (Wallace et al.2014). There are as of yet no reports of slow slip or tectonic tremor in the Woodlark basin. 3 METHODS 3.1 Samples used The samples for this study were selected to be representative of the actively deforming fault zone material at each of the study areas (Table 1). Most of the samples are from subduction zones, but also from two continental strike-slip faults (San Andreas Fault and Alpine Fault) and one low-angle normal fault (Woodlark Basin). Of the subduction zone samples, six were recovered from zones of deformation interpreted to be faults (Japan Trench, Nankai Trough, Cascadia, Costa Rica décollement and Barbados), and two other samples (Costa Rica pelagic chalk, Hikurangi) were recovered from the input sediments on the incoming plate. Incoming sediments at subduction zones are a valuable resource for investigating the mechanical behaviour of the plate boundary because they constitute the eventual shallow megathrust fault (Underwood 2007; Hüpers et al.2017; Ikari et al.2018). Table 1. Experiment and sample details. Location . Sample Type . Expediton . Sample ID . Depth of Recovery (m) . Plate Tectonic Rate (cm yr–1) . Experimental Driving Rate (cm yr–1) . Sample Composition (wt %) (abundances ≥ 5 per cent) . References . JapanTrench Subduction Decollement IODP Exp. 343 Japan Trench Fast Drilling Project (JFAST) C0019E 17R-1 823 8.3 8.4 Smectite 81, Illite 8, Quartz 7 Chester et al. (2013a); DeMets et al. (1994) XRD: Kameda et al. (2015) Nankai Trough Subduction Decollement ODP Leg 190 1174B 73R-1 833 6.3-6.8 6.3 Quartz 39, Smectite 20, Illite 12, Plagioclase 10, Calcite 10 Miyazaki & Heki (2001); Moore et al. (2001) XRD: Steurer & Underwood (2005) Nankai Trough Splay Fault IODP Expedition 316 Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) C0004D 29R-3 277 4.0-6.8 6.3 Quartz 21, Plagioclase 18, Smectite 15, illite 13, Chlorite 6 Miyazaki & Heki (2001); Kinoshita et al. (2009) Cascadia Subduction Frontal Thrust ODP Leg 146 891B 43X 333 4.2 5.3 Plagioclase 25, Quartz 22, Illite 12, Mixed-layer clay 8, K-feldspar 8, Muscovite 5, Chlorite 5 Westbrook et al. (1994) Costa Rica Subduction Decollement ODP Leg 170 1040C 21R-6 359 8.2-8.8 8.7 Smectite 46, Illite 12, Plagioclase 9, Mixed-layer clays 8, Zeolite 8 Kimura et al. (1997) Costa Rica Input Sediment ODP Leg 205 1253A 3R-2 387 8.2-8.8 8.7 Calcite 66, Amorphous Silica 20 Morris et al. (2003) Barbados Subduction Decollement ODP Leg 156 948C 11X-4 513 2.0-3.7 5.3 29 Illite, Kaolinite 12, Smectite 11, Mixed-layer clays 9, Quartz 7, Plagioclase 7, Glauconite 7 DeMets et al. (1994); Shipley et al. (1995) Hikurangi Input Sediment ODP Leg 181 1124C 20X-5, 21X-5, 22X-5 195-205 2.0-6.0 5.3 Calcite 43, Plagioclase 13, Quartz 9, Mixed-layer clays 8 Carter et al. (1999); Wallace et al. (2004) Alpine Fault Principal Slip Zone Deep Fault Drilling Project (DFDP)-1b (1b)59_1.9.DS 127 2.6 5.3 Quartz 20, Plagioclase 18, Illite 10, Smectite 8, Calcite 8, Chlorite 7, K-feldspar 6, Muscovite 5, Clinopyroxene 5 Toy et al. (2015) San Andreas Fault Central Deforming Zone San Andreas Fault Observatory at Depth (SAFOD) G43 ∼2700 2.5 5.3 Smectite 39, Chlorite 17, Illite 10, Chrysotile 6, Mixed-layer clay 5 Titus et al. (2005); Zoback et al. (2011) Woodlark Basin Detatchment Normal Fault ODP Leg 180 1117A 1R-2 9 1.0 (3.7) 5.3 Smectite 12, Pyrite 12, Kaolinite 11, Talc 10, Serpentine 10, Amphibole 8, Chlorite 7, Orthopyroxene 5 Taylor et al. (2000) Location . Sample Type . Expediton . Sample ID . Depth of Recovery (m) . Plate Tectonic Rate (cm yr–1) . Experimental Driving Rate (cm yr–1) . Sample Composition (wt %) (abundances ≥ 5 per cent) . References . JapanTrench Subduction Decollement IODP Exp. 343 Japan Trench Fast Drilling Project (JFAST) C0019E 17R-1 823 8.3 8.4 Smectite 81, Illite 8, Quartz 7 Chester et al. (2013a); DeMets et al. (1994) XRD: Kameda et al. (2015) Nankai Trough Subduction Decollement ODP Leg 190 1174B 73R-1 833 6.3-6.8 6.3 Quartz 39, Smectite 20, Illite 12, Plagioclase 10, Calcite 10 Miyazaki & Heki (2001); Moore et al. (2001) XRD: Steurer & Underwood (2005) Nankai Trough Splay Fault IODP Expedition 316 Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) C0004D 29R-3 277 4.0-6.8 6.3 Quartz 21, Plagioclase 18, Smectite 15, illite 13, Chlorite 6 Miyazaki & Heki (2001); Kinoshita et al. (2009) Cascadia Subduction Frontal Thrust ODP Leg 146 891B 43X 333 4.2 5.3 Plagioclase 25, Quartz 22, Illite 12, Mixed-layer clay 8, K-feldspar 8, Muscovite 5, Chlorite 5 Westbrook et al. (1994) Costa Rica Subduction Decollement ODP Leg 170 1040C 21R-6 359 8.2-8.8 8.7 Smectite 46, Illite 12, Plagioclase 9, Mixed-layer clays 8, Zeolite 8 Kimura et al. (1997) Costa Rica Input Sediment ODP Leg 205 1253A 3R-2 387 8.2-8.8 8.7 Calcite 66, Amorphous Silica 20 Morris et al. (2003) Barbados Subduction Decollement ODP Leg 156 948C 11X-4 513 2.0-3.7 5.3 29 Illite, Kaolinite 12, Smectite 11, Mixed-layer clays 9, Quartz 7, Plagioclase 7, Glauconite 7 DeMets et al. (1994); Shipley et al. (1995) Hikurangi Input Sediment ODP Leg 181 1124C 20X-5, 21X-5, 22X-5 195-205 2.0-6.0 5.3 Calcite 43, Plagioclase 13, Quartz 9, Mixed-layer clays 8 Carter et al. (1999); Wallace et al. (2004) Alpine Fault Principal Slip Zone Deep Fault Drilling Project (DFDP)-1b (1b)59_1.9.DS 127 2.6 5.3 Quartz 20, Plagioclase 18, Illite 10, Smectite 8, Calcite 8, Chlorite 7, K-feldspar 6, Muscovite 5, Clinopyroxene 5 Toy et al. (2015) San Andreas Fault Central Deforming Zone San Andreas Fault Observatory at Depth (SAFOD) G43 ∼2700 2.5 5.3 Smectite 39, Chlorite 17, Illite 10, Chrysotile 6, Mixed-layer clay 5 Titus et al. (2005); Zoback et al. (2011) Woodlark Basin Detatchment Normal Fault ODP Leg 180 1117A 1R-2 9 1.0 (3.7) 5.3 Smectite 12, Pyrite 12, Kaolinite 11, Talc 10, Serpentine 10, Amphibole 8, Chlorite 7, Orthopyroxene 5 Taylor et al. (2000) Open in new tab Table 1. Experiment and sample details. Location . Sample Type . Expediton . Sample ID . Depth of Recovery (m) . Plate Tectonic Rate (cm yr–1) . Experimental Driving Rate (cm yr–1) . Sample Composition (wt %) (abundances ≥ 5 per cent) . References . JapanTrench Subduction Decollement IODP Exp. 343 Japan Trench Fast Drilling Project (JFAST) C0019E 17R-1 823 8.3 8.4 Smectite 81, Illite 8, Quartz 7 Chester et al. (2013a); DeMets et al. (1994) XRD: Kameda et al. (2015) Nankai Trough Subduction Decollement ODP Leg 190 1174B 73R-1 833 6.3-6.8 6.3 Quartz 39, Smectite 20, Illite 12, Plagioclase 10, Calcite 10 Miyazaki & Heki (2001); Moore et al. (2001) XRD: Steurer & Underwood (2005) Nankai Trough Splay Fault IODP Expedition 316 Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) C0004D 29R-3 277 4.0-6.8 6.3 Quartz 21, Plagioclase 18, Smectite 15, illite 13, Chlorite 6 Miyazaki & Heki (2001); Kinoshita et al. (2009) Cascadia Subduction Frontal Thrust ODP Leg 146 891B 43X 333 4.2 5.3 Plagioclase 25, Quartz 22, Illite 12, Mixed-layer clay 8, K-feldspar 8, Muscovite 5, Chlorite 5 Westbrook et al. (1994) Costa Rica Subduction Decollement ODP Leg 170 1040C 21R-6 359 8.2-8.8 8.7 Smectite 46, Illite 12, Plagioclase 9, Mixed-layer clays 8, Zeolite 8 Kimura et al. (1997) Costa Rica Input Sediment ODP Leg 205 1253A 3R-2 387 8.2-8.8 8.7 Calcite 66, Amorphous Silica 20 Morris et al. (2003) Barbados Subduction Decollement ODP Leg 156 948C 11X-4 513 2.0-3.7 5.3 29 Illite, Kaolinite 12, Smectite 11, Mixed-layer clays 9, Quartz 7, Plagioclase 7, Glauconite 7 DeMets et al. (1994); Shipley et al. (1995) Hikurangi Input Sediment ODP Leg 181 1124C 20X-5, 21X-5, 22X-5 195-205 2.0-6.0 5.3 Calcite 43, Plagioclase 13, Quartz 9, Mixed-layer clays 8 Carter et al. (1999); Wallace et al. (2004) Alpine Fault Principal Slip Zone Deep Fault Drilling Project (DFDP)-1b (1b)59_1.9.DS 127 2.6 5.3 Quartz 20, Plagioclase 18, Illite 10, Smectite 8, Calcite 8, Chlorite 7, K-feldspar 6, Muscovite 5, Clinopyroxene 5 Toy et al. (2015) San Andreas Fault Central Deforming Zone San Andreas Fault Observatory at Depth (SAFOD) G43 ∼2700 2.5 5.3 Smectite 39, Chlorite 17, Illite 10, Chrysotile 6, Mixed-layer clay 5 Titus et al. (2005); Zoback et al. (2011) Woodlark Basin Detatchment Normal Fault ODP Leg 180 1117A 1R-2 9 1.0 (3.7) 5.3 Smectite 12, Pyrite 12, Kaolinite 11, Talc 10, Serpentine 10, Amphibole 8, Chlorite 7, Orthopyroxene 5 Taylor et al. (2000) Location . Sample Type . Expediton . Sample ID . Depth of Recovery (m) . Plate Tectonic Rate (cm yr–1) . Experimental Driving Rate (cm yr–1) . Sample Composition (wt %) (abundances ≥ 5 per cent) . References . JapanTrench Subduction Decollement IODP Exp. 343 Japan Trench Fast Drilling Project (JFAST) C0019E 17R-1 823 8.3 8.4 Smectite 81, Illite 8, Quartz 7 Chester et al. (2013a); DeMets et al. (1994) XRD: Kameda et al. (2015) Nankai Trough Subduction Decollement ODP Leg 190 1174B 73R-1 833 6.3-6.8 6.3 Quartz 39, Smectite 20, Illite 12, Plagioclase 10, Calcite 10 Miyazaki & Heki (2001); Moore et al. (2001) XRD: Steurer & Underwood (2005) Nankai Trough Splay Fault IODP Expedition 316 Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) C0004D 29R-3 277 4.0-6.8 6.3 Quartz 21, Plagioclase 18, Smectite 15, illite 13, Chlorite 6 Miyazaki & Heki (2001); Kinoshita et al. (2009) Cascadia Subduction Frontal Thrust ODP Leg 146 891B 43X 333 4.2 5.3 Plagioclase 25, Quartz 22, Illite 12, Mixed-layer clay 8, K-feldspar 8, Muscovite 5, Chlorite 5 Westbrook et al. (1994) Costa Rica Subduction Decollement ODP Leg 170 1040C 21R-6 359 8.2-8.8 8.7 Smectite 46, Illite 12, Plagioclase 9, Mixed-layer clays 8, Zeolite 8 Kimura et al. (1997) Costa Rica Input Sediment ODP Leg 205 1253A 3R-2 387 8.2-8.8 8.7 Calcite 66, Amorphous Silica 20 Morris et al. (2003) Barbados Subduction Decollement ODP Leg 156 948C 11X-4 513 2.0-3.7 5.3 29 Illite, Kaolinite 12, Smectite 11, Mixed-layer clays 9, Quartz 7, Plagioclase 7, Glauconite 7 DeMets et al. (1994); Shipley et al. (1995) Hikurangi Input Sediment ODP Leg 181 1124C 20X-5, 21X-5, 22X-5 195-205 2.0-6.0 5.3 Calcite 43, Plagioclase 13, Quartz 9, Mixed-layer clays 8 Carter et al. (1999); Wallace et al. (2004) Alpine Fault Principal Slip Zone Deep Fault Drilling Project (DFDP)-1b (1b)59_1.9.DS 127 2.6 5.3 Quartz 20, Plagioclase 18, Illite 10, Smectite 8, Calcite 8, Chlorite 7, K-feldspar 6, Muscovite 5, Clinopyroxene 5 Toy et al. (2015) San Andreas Fault Central Deforming Zone San Andreas Fault Observatory at Depth (SAFOD) G43 ∼2700 2.5 5.3 Smectite 39, Chlorite 17, Illite 10, Chrysotile 6, Mixed-layer clay 5 Titus et al. (2005); Zoback et al. (2011) Woodlark Basin Detatchment Normal Fault ODP Leg 180 1117A 1R-2 9 1.0 (3.7) 5.3 Smectite 12, Pyrite 12, Kaolinite 11, Talc 10, Serpentine 10, Amphibole 8, Chlorite 7, Orthopyroxene 5 Taylor et al. (2000) Open in new tab The composition of the samples tested here ranges widely, as measured by X-ray diffraction (XRD) (Table 1). XRD for most samples was performed with a Philips X'Pert Pro multipurpose diffractometer following the double-identification method of Vogt et al. (2002), which employs a preliminary mineral identification with the software MacDiff followed by a full-pattern identification with the QUAX mineral database. XRD data for the Japan Trench fault zone samples was reported in Kameda et al. (2015); for the Nankai Trough décollement sample in Moore et al. (2001) and Steurer & Underwood (2005); and for the Nankai Trough megasplay sample in Kinoshita et al. (2009). 3.2 Experimental procedure For these experiments, the samples were tested as powdered gouge with a maximum grain size of 125 µm. The powders were mixed with 3.5 per cent NaCl brine to form a stiff paste, which was then cold-pressed into a sample cell that holds a cylindrical volume of ∼25 mm height and 25 mm diameter. For the shear experiments, the cell was loaded into a single-direct shear device within which the normal stress is applied to the top face of the sample, aligned parallel to the cylinder axis (Fig. 2). The samples were allowed to consolidate and drain for at least 18 hr (in some cases several days), until the sample height (∼15–20 mm) reaches a steady value. This is done because the apparatus in not equipped with a direct pore pressure measurement system, therefore by allowing full consolidation we may assume that the sample is drained before shearing and that the applied normal stress is equal to the effective normal stress (σn'). All experiments were conducted at room temperature and 10 MPa effective normal stress, which were chosen for two main reasons: (1) to approximate the in-situ conditions of the samples, most of which were recovered from depths of <1 km (Table 1) and (2) due to force limitations of the apparatus. The apparatus stiffness was determined by loading and unloading a steel blank having the same dimensions as the samples, at a slow rate that matches the cm yr–1 shearing velocities in this study. The unloading stiffness under 10 MPa normal load, which provides a direct comparison to stress drops, is 7–11 MPa mm–1 depending on the magnitude of the shear stress. Figure 2. Open in new tabDownload slide Example of experimental data from a sample of Hikurangi input sediment. (a) Coefficient of friction as a function of shear displacement, showing measurement of the steady-state friction coefficient at the 10 µm s–1 run-in, decrease to 1.7 nm s–1 (5.3 cm yr–1) including two SSEs, and velocity step increase to 5.1 nm s–1. (b) Closeup of the velocity step, with experimental data overlain by an inverse model. Inset shows a schematic diagram of the testing apparatus (not to scale). Figure 2. Open in new tabDownload slide Example of experimental data from a sample of Hikurangi input sediment. (a) Coefficient of friction as a function of shear displacement, showing measurement of the steady-state friction coefficient at the 10 µm s–1 run-in, decrease to 1.7 nm s–1 (5.3 cm yr–1) including two SSEs, and velocity step increase to 5.1 nm s–1. (b) Closeup of the velocity step, with experimental data overlain by an inverse model. Inset shows a schematic diagram of the testing apparatus (not to scale). Following consolidation, the apparatus displaces the bottom half of the sample cell relative to the top half inducing planar shear perpendicular to the cylinder axis. As the bottom half of the sample is displaced away, the top half of the sample shears against the lower sample cell. Therefore, although the sample-sample contact reduces with displacement, the full area of the top sample half continuously supports load and experiences deformation throughout the experiment. No systematic changes in friction are observed as a function of displacement, indicating that effects of shearing against the lower sample cell are neglible (see Ikari et al 2015a for further details). During shear, the shear strength (τ) is continuously recorded, from which an (apparent) coefficient of sliding friction µ is calculated as the ratio τ/σn' (Fig. 2). All samples were initially sheared with a constant driving velocity (load point velocity Vlp) of 10 µm s–1 for up to ∼4–5 mm, by which a steady-state shear strength is reached. The slip velocity was then decreased to 5.3–8.7 cm yr–1 (1.7–2.8 nm s–1) for ∼2 mm to simulate naturally slow plate tectonic driving rates (Table 1). At these plate rates, laboratory SSE tend to occur from which important parameters that may be directly compared with natural SSEs are measured: stress drop, which is directly measured from the shear stress record; peak slip velocity during SSE stress drop and initial slip velocity prior to the stress drop; and unloading stiffness, calculated as the maximum slope of the shear stress-sample displacement curve during the stress drop. SSE duration and event slip are also measured, however due to the small size of the samples these values are not directly comparable to natural SSEs without an appropriate scaling factor (e.g. Rabinowitz et al.2018). For most of these experiments, the shear device utilized two horizontal displacement sensors (Fig. 2); one is mounted at the horizontal load cell which monitors the driving velocity, that is the load point velocity Vlp. The second is mounted directly at the shear cell, which measures the offset of the cell plates and therefore the true sample displacement rate V. This dual-measurement system allows us to determine transient changes in slip rate which would occur during frictional slip events. Furthermore, by taking the difference between the sample and driving displacements it can be assessed whether a slip deficit or slip excess is occurring. In some earlier experiments where the load point sensor was not yet installed, slip deficit or excess can still be evaluated by removing the driving rate trend from the sample displacement record. Sample slip velocity velocities are calculated from a running average slope of at least 11 data points from the displacement time series. Due to the importance in sample slip velocity for evaluating SSEs and comparing them to natural geodetic observations, as well as the tendency for velocity perturbations to be small, estimations of error in the slip velocity measurements were made by measuring the standard deviation of the noise level in the displacement sensors when they are stationary. The standard deviation in both sample slip displacement and load point displacement ranges between 0.10 and 0.65 µm; which translates to a maximum uncertainty in slip velocity of ±2.2 cm yr–1 or 37 per cent in the most conservative case. Peak slip velocities for all SSEs in this study are well above this threshold. Following shearing at the plate rate, the slip velocity was then increased by a factor of 3 (but sometimes up to a factor of ∼11) to measure the rate- and state-dependence of friction (RSF) using established inverse modelling techniques (e.g. Reinen & Weeks 1993; Saffer & Marone 2003; Ikari et al.2009, Fig. 2b, Table 3). The frictional response to a velocity step is described as: \begin{eqnarray*} \mu \ = {\mu _o}\ + a\ln \left( {\frac{V}{{{V_o}}}} \right) + {b_1}\ln \left( {\frac{{{V_0}{\theta _1}}}{{{d_{c1}}}}} \right) + {b_2}\ln \left( {\frac{{{V_o}{\theta _2}}}{{{d_{c2}}}}} \right), \end{eqnarray*} (1) \begin{eqnarray*} \frac{{d{\theta _i}}}{{dt}} = \ 1 - \frac{{V{\theta _i}}}{{{d_{ci}}}}, \quad i = 1,2, \end{eqnarray*} (2) where the parameters a, b1 and b2 are dimensionless constants, θ1 and θ2 are state variables having units of time, and dc1 and dc2 are critical slip distances over which frictional state evolves to a new steady value (e.g. Dieterich 1979, 1981; Marone 1998a; Scholz 2002). In most cases the data is adequately described using one state variable, eliminating the last term in eq. (1). In the cases where the data is better described using the two state variables, we report the parameter b as b = b1 + b2. A linear slip-dependent trend in friction is commonly superimposed on the data, which is removed during the modelling process (e.g. Blanpied et al.1998). Eq. (2) describes the time-dependence of friction via the evolution of the state variable θ, known as the ‘Dieterich’, or ‘aging’ law. Other forms of eq. (2) exist, the most prominent being a version in which the state variable evolution depends on slip (i.e. ‘slip law’, Ruina 1983). The choice between the aging and slip laws is a topic of debate; the appropriateness of each law highly depends on its application (e.g. Sleep 2005, 2006, 2012), with the slip law being supported by some recent studies (e.g. Bhattacharya et al.2017). For this study, I employ the Dieterich aging law because it captures the property that friction can change as a function of time and not only slip, which previous studies have verified by analysing ‘slide-hold-slide’ frictional healing experiments (e.g. Beeler et al.1994; Ikari et al.2016b) and by observing time-dependent increase in real area of contact at grain asperities during static loading (e.g. Dieterich & Kilgore 1994, 1996b; Goldsby et al.2004). The aging law has been proven useful for simulating various types of slow fault slip (e.g. Perfettini & Ampuero 2008; Helmstetter & Shaw 2009), with some studies finding that the aging law better describes natural phenomena (Rubin 2008; Kanu & Johnson 2011). I leave open the possibility that the slip law can provide an alternative explanation to the results presented here, but a detailed investigation comparing the two evolution laws is not a specific goal of this work. Regardless of which evolution law is used, at steady state V = dc/θ, so that dθ/dt = 0, and eqs (1) and (2) simplify to: \begin{eqnarray*} a - b\ = \ \frac{{\Delta {\mu _s}}}{{\Delta lnV}}, \end{eqnarray*} (3) where µs is a steady-state value of friction. In addition to a–b values determined by inverse modelling of friction changes due to velocity increases or ‘upsteps’, a second set of a–b values was determined by measuring the steady-state change in friction following the decrease in slip velocity from the run-in value of 10 µm s–1 to the cm yr–1 slip velocities, or ‘downsteps’ (Ikari & Kopf 2017). These were calculated using eq. (3) because modelling of these downsteps proved difficult, likely due to the large velocity difference (∼3.5 orders of magnitude) and low apparatus stiffness (e.g. Bhattacharya et al.2015). The parameter a–b is of great importance for fault slip behaviour because it determines the possibility of unstable frictional slip, which results in stick-slip behaviour in the laboratory and earthquakes in nature (e.g. Dieterich 1986; Tullis 1988; Dieterich & Kilgore 1996a; Marone 1998a, Scholz 2002). A positive value of a–b is known as velocity-strengthening behaviour, a condition that prohibits the nucleation of unstable slip by producing a negative stress drop that damps a potential event. A negative value of a–b indicates velocity-weakening frictional behaviour, which is a requirement for frictional instability. Velocity-weakening does not necessarily guarantee unstable slip, which also requires specific elastic conditions related to the fault surroundings, which is the apparatus in the laboratory and wall rock in nature (Cook 1981; Scholz 1998). Within this framework, various types of slow and transient fault slip events are thought to arise from frictional behaviour near the stability transition or ‘conditional stability’, where ‘self-sustained oscillatory slip’ may occur (e.g. Dieterich 1986; Dieterich & Kilgore 1996a; Scholz 1998). 4 FRICTIONAL STRENGTH AND VELOCITY DEPENDENCE The steady-state coefficient of sliding friction measured during the 10 µm s–1 portion of these experiments establishes a baseline friction level that can be compared with previous work under similar conditions. These friction values range from 0.12 (San Andreas Fault) to 0.60 (Costa Rica inputs chalk) and exhibit a clear inverse dependence on phyllosilicate mineral content determined by XRD measurements (Fig. 3a, Table 1), a pattern well-established by several previous laboratory friction studies, especially under water-saturated conditions (Byerlee 1978; Lupini et al.1981; Shimamoto & Logan 1981; Logan & Rauenzahn 1987; Morrow et al.1992, 2000; Brown et al.2003; Saffer & Marone 2003; Niemeijer & Spiers 2005; Ikari et al.2007, 2011a,b; Crawford et al.2008; Carpenter et al.2009; Smith & Faulkner 2010; Tembe et al.2010; Giorgetti et al.2015). Some notable observations which match previous results under similar testing conditions include: the extremely low friction of samples from the Tohoku region of the Japan Trench (Remitti et al.2015; Ikari et al.2015a,b), the San Andreas Fault CDZ (Carpenter et al.2011, 2012, 2015a; Lockner et al.2011; Coble et al.2014; Ikari et al.2016a), and the Barbados décollement (Kopf & Brown 2003); the moderately high strength of the Alpine Fault principal slip zone (Boulton et al.2014; Ikari et al.2014, 2015c; Niemeijer et al.2016); the large strength contrast between the weak clay-rich and stronger carbonate-rich samples from Costa Rica (Ikari et al.2013; Kurzawski et al.2016); and the larger strength of the Nankai Trough megasplay at Site C0004 compared to the décollement at Site 1174 (Ikari & Saffer 2011). Figure 3. Open in new tabDownload slide (a) Coefficient of friction and (b) a–b as a function of phyllosilicate mineral content for all samples in this study. Figure 3. Open in new tabDownload slide (a) Coefficient of friction and (b) a–b as a function of phyllosilicate mineral content for all samples in this study. The steady-state sliding friction coefficients measured at plate-rate velocities of 0.0017–0.0028 µm s–1 (5.3–8.7 cm yr–1) exhibit the same dependence on phyllosilicate content as the values obtained at 10 µm s–1, but the absolute values show significant differences (Fig. 3a). This difference allows the calculation of a–b values from the downstep in velocity from 10 µm s–1 to the plate rates using eq. (3), in addition to the a–b values determine by inverse modelling of the 3- to 11-fold velocity upstep from the plate rates. There is significant scatter, however it can be seen that for both sets of a–b values, velocity-weakening occurs for gouges of all strength but that velocity-strengthening is only observed for materials with at least 55 per cent phyllosilicate minerals (Fig. 3b). Furthermore, the a–b values measured from the velocity downsteps exhibit significantly greater velocity weakening, with values as low as –0.010 compared to –0.003 for the modelled velocity upsteps. Since the magnitude of the velocity change is accounted for in the calculation of a–b (eq. 3), the large negative a–b values suggests some fundamental change in frictional behaviour caused by either the large velocity changes or by shearing at extremely slow rates. Upsteps with the same magnitude velocity change as the downsteps (3.6–3.8 orders of magnitude) would be needed to determine if the asymmetry in a–b values depends on the sign of the large velocity change. 5 SITE-SPECIFIC OBSERVATIONS OF LABORATORY SSE 5.1 Japan Trench For this study, the same sample tested previously at 7 MPa effective normal stress by Ikari et al. (2015a) was tested at 10 MPa (Fig. 4). This experiment produced six SSEs having stress drops ranging from 60 to 130 kPa, peak slip velocities of 14–29 cm yr–1 (approximately two to four times the driving rate), and unloading stiffnesses of 3–9 MPa mm–1 (Table 2). The peak slip velocity and a slip excess (or slip deficit recovery) clearly occur during the stress drop. It can also be seen that there is a background friction level, from which loading occurs before the stress drop and returns to after the stress drop. Before the first two SSEs, there is a non-negligible amount of displacement that occurs between the loading and the stress drop, however for the last four events this is not the case. Both the background friction level at the post-stress drop strength and the slip between loadup and stress drop are features that are also observed in several other samples in this study. Figure 4. Open in new tabDownload slide Experimental data for a sample from the Tohoku region of the Japan Trench. (a) Coefficient of friction as a function of displacement. (b) Closeup of SSEs generated during plate-rate shearing. (c) Closeup of shear stress during an individual SSE, showing the measurement of the stress drop Δτ and unloading stiffness Ks. (d) Sample slip velocity V and driving (load point) velocity Vlp, (e) difference between sample displacement and load point displacement and (f) unloading stiffness (measured as –dτ/dx, where x is the sample slip displacement) for the SSE shown in (c). Shading indicates the SSE stress drop. Note that this figure is a full example of the laboratory SSE measurements, for brevity not all measurements may be shown in subsequent Figs 5–14. Figure 4. Open in new tabDownload slide Experimental data for a sample from the Tohoku region of the Japan Trench. (a) Coefficient of friction as a function of displacement. (b) Closeup of SSEs generated during plate-rate shearing. (c) Closeup of shear stress during an individual SSE, showing the measurement of the stress drop Δτ and unloading stiffness Ks. (d) Sample slip velocity V and driving (load point) velocity Vlp, (e) difference between sample displacement and load point displacement and (f) unloading stiffness (measured as –dτ/dx, where x is the sample slip displacement) for the SSE shown in (c). Shading indicates the SSE stress drop. Note that this figure is a full example of the laboratory SSE measurements, for brevity not all measurements may be shown in subsequent Figs 5–14. Table 2. Parameters measured from laboratory SSE. Experiment . Location . Stress drop (kPa) . Percentage stress drop . Slip (μm) . Duration (s) . Pre-SSE velocity (nm s–1) . Pre-SSE velocity (cm yr–1) . Peak velocity (nm s–1) . Peak velocity (cm yr–1) . Vp/Vi . Ks (MPa mm–1) . K (MPa mm–1) . K/Ks . B651 Japan Trench 121 4.6 39.0 11161 2.6 8.2 7.1 22.4 2.7 8.7 8.7 1.0 127 4.8 36.9 8680 2.6 8.0 8.5 26.8 3.3 5.2 8.7 1.7 79 3.1 34.3 7353 1.9 5.9 7.7 24.3 4.2 2.9 8.9 3.1 118 4.6 40.6 8499 2.6 8.2 8.8 27.7 3.4 3.7 8.9 2.4 63 2.5 58.3 19892 2.6 8.2 4.5 14.2 1.7 3.0 8.8 2.9 131 5.1 31.6 6818 2.2 7.1 9.1 28.6 4.0 4.7 8.9 1.9 B518 Nankai Trough Megasplay 33 0.8 8.2 3820 1.1 3.4 3.7 11.7 3.4 5.1 10.7 2.1 36 0.9 11.0 4520 1.8 5.7 4.7 14.9 2.6 4.3 10.8 2.5 15 0.4 5.8 2350 2.0 6.4 3.7 11.6 1.8 3.5 10.7 3.0 26 0.6 19.9 8520 1.7 5.2 3.7 11.6 2.2 2.9 10.5 3.7 33 0.8 13.4 4120 1.4 4.4 5.6 17.5 4.0 3.0 10.7 3.6 51 1.3 18.9 8580 1.1 3.5 4.7 14.7 4.2 4.1 10.7 2.6 26 0.7 11.4 5240 2.5 7.8 5.0 15.9 2.0 3.8 10.5 2.8 42 1.1 12.3 4060 1.4 4.4 4.3 13.7 3.1 4.8 10.5 2.2 61 1.5 21.2 6900 2.0 6.4 7.1 22.4 3.5 5.6 10.5 1.9 B541 Nankai Trough Decollement 0 0 0 0 2.0 2.0 6.3 6.3 1 0 - B565 Cascadia 81 1.7 13.7 6210 0.4 1.3 5.5 17.4 13.4 7.1 10.2 1.4 67 1.4 13.4 4200 0.9 2.8 5.6 17.6 6.2 5.2 10.2 2.0 78 1.6 20.4 4610 1.4 4.4 8.3 26.3 6.0 4.6 10.2 2.2 49 1.0 16.4 6500 1.6 5.0 4.5 14.2 2.9 4.0 10.2 2.5 B517 Costa Rica Decollement 254 11.6 97.1 24990 2.8 8.9 5.8 18.4 2.1 5.6 7.8 1.4 52 2.7 52.3 13410 2.2 7.1 5.0 15.7 2.2 2.0 7.5 3.8 B561 Costa Rica Inputs (chalk) 2466 40.3 12.8 1 0 0 10386.0 32753.3 - 225.9 9.6 0.0 B575 Barbados 725 29.1 180.2 48520 1.5 4.6 11.1 35.1 7.6 6.9 8.9 1.3 93 5.1 48.0 21210 1.0 3.0 3.8 12.0 4.0 3.8 7.4 1.9 880 32.4 188.6 39800 0.9 3.0 13.4 42.2 14.2 8.5 8.8 1.0 B628 Hikurangi Inputs 23 0.5 16.5 4943 0.9 2.8 4.7 14.9 5.3 2.5 8.2 3.2 23 0.5 13.5 5050 1.5 4.8 4.5 14.2 2.9 2.5 8.9 3.6 B675 Hikurangi Inputs 0 0 0 0 1.7 1.7 5.3 5.3 1 0 - B678 Hikurangi Inputs 36 0.8 5.8 3976 1.7 5.3 5.9 18.7 3.5 13.7 8.9 0.7 30 0.7 11.2 2378 1.5 4.7 6.1 19.3 4.1 3.1 8.9 2.9 45 1.0 11.8 4558 1.4 4.3 3.7 11.6 2.7 8.0 8.9 1.1 B560 Alpine Fault (DFDP-1B) 128 2.6 20.1 3130 0.7 2.3 12.7 40.2 17.7 7.6 10.2 1.3 *5 large representative events 138 2.8 18.9 3580 0.8 2.5 12.1 38.2 15.2 10.4 10.2 1.0 155 3.2 20.7 2920 0.5 1.4 17.0 53.7 37.3 11.3 10.2 0.9 155 3.1 23.2 3190 0.5 1.5 14.7 46.4 30.0 8.9 10.2 1.1 155 3.2 23.9 2560 0.6 2.0 21.9 69.2 35.0 8.5 10.2 1.2 B713 Alpine Fault (DFDP-1B) 230 4.6 31.6 3200 0.4 1.4 22.2 70.0 49.9 9.3 10.2 1.1 *5 large representative events 245 4.9 28.8 2700 0.4 1.3 19.8 62.5 48.6 10.7 10.2 1.0 244 4.9 31.3 3100 0.7 2.1 20.9 66.0 31.1 9.8 10.2 1.0 255 5.1 35.7 2200 0.4 1.4 28.1 88.6 64.1 8.3 10.2 1.2 244 4.9 31.9 3000 0.5 1.6 23.2 73.2 46.2 8.8 10.2 1.2 B571 San Andreas Fault CDZ 187 14.0 62.4 19760 1.1 3.3 6.2 19.5 5.9 4.4 7.1 1.6 B594 San Andreas Fault CDZ 217 15.3 877.0 98440 8.1 25.5 15.2 47.9 1.9 1.5 7.2 4.9 364 27.5 953.0 109300 7.2 22.6 20.8 65.5 2.9 2.3 7.0 3.1 401 32.4 788.0 81500 6.7 21.3 14.7 46.5 2.2 2.0 6.8 3.5 B570 Woodlark Basin 16 0.9 7.8 3710 1.5 4.7 2.8 8.9 1.9 2.5 7.4 3.0 47 2.7 27.3 12710 1.6 4.9 4.0 12.5 2.5 3.4 7.4 2.2 32 1.9 17.7 7410 1.5 4.9 2.7 8.7 1.8 3.7 7.4 2.0 30 1.7 17.6 7900 1.6 4.9 4.1 12.8 2.6 4.5 7.4 1.7 Experiment . Location . Stress drop (kPa) . Percentage stress drop . Slip (μm) . Duration (s) . Pre-SSE velocity (nm s–1) . Pre-SSE velocity (cm yr–1) . Peak velocity (nm s–1) . Peak velocity (cm yr–1) . Vp/Vi . Ks (MPa mm–1) . K (MPa mm–1) . K/Ks . B651 Japan Trench 121 4.6 39.0 11161 2.6 8.2 7.1 22.4 2.7 8.7 8.7 1.0 127 4.8 36.9 8680 2.6 8.0 8.5 26.8 3.3 5.2 8.7 1.7 79 3.1 34.3 7353 1.9 5.9 7.7 24.3 4.2 2.9 8.9 3.1 118 4.6 40.6 8499 2.6 8.2 8.8 27.7 3.4 3.7 8.9 2.4 63 2.5 58.3 19892 2.6 8.2 4.5 14.2 1.7 3.0 8.8 2.9 131 5.1 31.6 6818 2.2 7.1 9.1 28.6 4.0 4.7 8.9 1.9 B518 Nankai Trough Megasplay 33 0.8 8.2 3820 1.1 3.4 3.7 11.7 3.4 5.1 10.7 2.1 36 0.9 11.0 4520 1.8 5.7 4.7 14.9 2.6 4.3 10.8 2.5 15 0.4 5.8 2350 2.0 6.4 3.7 11.6 1.8 3.5 10.7 3.0 26 0.6 19.9 8520 1.7 5.2 3.7 11.6 2.2 2.9 10.5 3.7 33 0.8 13.4 4120 1.4 4.4 5.6 17.5 4.0 3.0 10.7 3.6 51 1.3 18.9 8580 1.1 3.5 4.7 14.7 4.2 4.1 10.7 2.6 26 0.7 11.4 5240 2.5 7.8 5.0 15.9 2.0 3.8 10.5 2.8 42 1.1 12.3 4060 1.4 4.4 4.3 13.7 3.1 4.8 10.5 2.2 61 1.5 21.2 6900 2.0 6.4 7.1 22.4 3.5 5.6 10.5 1.9 B541 Nankai Trough Decollement 0 0 0 0 2.0 2.0 6.3 6.3 1 0 - B565 Cascadia 81 1.7 13.7 6210 0.4 1.3 5.5 17.4 13.4 7.1 10.2 1.4 67 1.4 13.4 4200 0.9 2.8 5.6 17.6 6.2 5.2 10.2 2.0 78 1.6 20.4 4610 1.4 4.4 8.3 26.3 6.0 4.6 10.2 2.2 49 1.0 16.4 6500 1.6 5.0 4.5 14.2 2.9 4.0 10.2 2.5 B517 Costa Rica Decollement 254 11.6 97.1 24990 2.8 8.9 5.8 18.4 2.1 5.6 7.8 1.4 52 2.7 52.3 13410 2.2 7.1 5.0 15.7 2.2 2.0 7.5 3.8 B561 Costa Rica Inputs (chalk) 2466 40.3 12.8 1 0 0 10386.0 32753.3 - 225.9 9.6 0.0 B575 Barbados 725 29.1 180.2 48520 1.5 4.6 11.1 35.1 7.6 6.9 8.9 1.3 93 5.1 48.0 21210 1.0 3.0 3.8 12.0 4.0 3.8 7.4 1.9 880 32.4 188.6 39800 0.9 3.0 13.4 42.2 14.2 8.5 8.8 1.0 B628 Hikurangi Inputs 23 0.5 16.5 4943 0.9 2.8 4.7 14.9 5.3 2.5 8.2 3.2 23 0.5 13.5 5050 1.5 4.8 4.5 14.2 2.9 2.5 8.9 3.6 B675 Hikurangi Inputs 0 0 0 0 1.7 1.7 5.3 5.3 1 0 - B678 Hikurangi Inputs 36 0.8 5.8 3976 1.7 5.3 5.9 18.7 3.5 13.7 8.9 0.7 30 0.7 11.2 2378 1.5 4.7 6.1 19.3 4.1 3.1 8.9 2.9 45 1.0 11.8 4558 1.4 4.3 3.7 11.6 2.7 8.0 8.9 1.1 B560 Alpine Fault (DFDP-1B) 128 2.6 20.1 3130 0.7 2.3 12.7 40.2 17.7 7.6 10.2 1.3 *5 large representative events 138 2.8 18.9 3580 0.8 2.5 12.1 38.2 15.2 10.4 10.2 1.0 155 3.2 20.7 2920 0.5 1.4 17.0 53.7 37.3 11.3 10.2 0.9 155 3.1 23.2 3190 0.5 1.5 14.7 46.4 30.0 8.9 10.2 1.1 155 3.2 23.9 2560 0.6 2.0 21.9 69.2 35.0 8.5 10.2 1.2 B713 Alpine Fault (DFDP-1B) 230 4.6 31.6 3200 0.4 1.4 22.2 70.0 49.9 9.3 10.2 1.1 *5 large representative events 245 4.9 28.8 2700 0.4 1.3 19.8 62.5 48.6 10.7 10.2 1.0 244 4.9 31.3 3100 0.7 2.1 20.9 66.0 31.1 9.8 10.2 1.0 255 5.1 35.7 2200 0.4 1.4 28.1 88.6 64.1 8.3 10.2 1.2 244 4.9 31.9 3000 0.5 1.6 23.2 73.2 46.2 8.8 10.2 1.2 B571 San Andreas Fault CDZ 187 14.0 62.4 19760 1.1 3.3 6.2 19.5 5.9 4.4 7.1 1.6 B594 San Andreas Fault CDZ 217 15.3 877.0 98440 8.1 25.5 15.2 47.9 1.9 1.5 7.2 4.9 364 27.5 953.0 109300 7.2 22.6 20.8 65.5 2.9 2.3 7.0 3.1 401 32.4 788.0 81500 6.7 21.3 14.7 46.5 2.2 2.0 6.8 3.5 B570 Woodlark Basin 16 0.9 7.8 3710 1.5 4.7 2.8 8.9 1.9 2.5 7.4 3.0 47 2.7 27.3 12710 1.6 4.9 4.0 12.5 2.5 3.4 7.4 2.2 32 1.9 17.7 7410 1.5 4.9 2.7 8.7 1.8 3.7 7.4 2.0 30 1.7 17.6 7900 1.6 4.9 4.1 12.8 2.6 4.5 7.4 1.7 Open in new tab Table 2. Parameters measured from laboratory SSE. Experiment . Location . Stress drop (kPa) . Percentage stress drop . Slip (μm) . Duration (s) . Pre-SSE velocity (nm s–1) . Pre-SSE velocity (cm yr–1) . Peak velocity (nm s–1) . Peak velocity (cm yr–1) . Vp/Vi . Ks (MPa mm–1) . K (MPa mm–1) . K/Ks . B651 Japan Trench 121 4.6 39.0 11161 2.6 8.2 7.1 22.4 2.7 8.7 8.7 1.0 127 4.8 36.9 8680 2.6 8.0 8.5 26.8 3.3 5.2 8.7 1.7 79 3.1 34.3 7353 1.9 5.9 7.7 24.3 4.2 2.9 8.9 3.1 118 4.6 40.6 8499 2.6 8.2 8.8 27.7 3.4 3.7 8.9 2.4 63 2.5 58.3 19892 2.6 8.2 4.5 14.2 1.7 3.0 8.8 2.9 131 5.1 31.6 6818 2.2 7.1 9.1 28.6 4.0 4.7 8.9 1.9 B518 Nankai Trough Megasplay 33 0.8 8.2 3820 1.1 3.4 3.7 11.7 3.4 5.1 10.7 2.1 36 0.9 11.0 4520 1.8 5.7 4.7 14.9 2.6 4.3 10.8 2.5 15 0.4 5.8 2350 2.0 6.4 3.7 11.6 1.8 3.5 10.7 3.0 26 0.6 19.9 8520 1.7 5.2 3.7 11.6 2.2 2.9 10.5 3.7 33 0.8 13.4 4120 1.4 4.4 5.6 17.5 4.0 3.0 10.7 3.6 51 1.3 18.9 8580 1.1 3.5 4.7 14.7 4.2 4.1 10.7 2.6 26 0.7 11.4 5240 2.5 7.8 5.0 15.9 2.0 3.8 10.5 2.8 42 1.1 12.3 4060 1.4 4.4 4.3 13.7 3.1 4.8 10.5 2.2 61 1.5 21.2 6900 2.0 6.4 7.1 22.4 3.5 5.6 10.5 1.9 B541 Nankai Trough Decollement 0 0 0 0 2.0 2.0 6.3 6.3 1 0 - B565 Cascadia 81 1.7 13.7 6210 0.4 1.3 5.5 17.4 13.4 7.1 10.2 1.4 67 1.4 13.4 4200 0.9 2.8 5.6 17.6 6.2 5.2 10.2 2.0 78 1.6 20.4 4610 1.4 4.4 8.3 26.3 6.0 4.6 10.2 2.2 49 1.0 16.4 6500 1.6 5.0 4.5 14.2 2.9 4.0 10.2 2.5 B517 Costa Rica Decollement 254 11.6 97.1 24990 2.8 8.9 5.8 18.4 2.1 5.6 7.8 1.4 52 2.7 52.3 13410 2.2 7.1 5.0 15.7 2.2 2.0 7.5 3.8 B561 Costa Rica Inputs (chalk) 2466 40.3 12.8 1 0 0 10386.0 32753.3 - 225.9 9.6 0.0 B575 Barbados 725 29.1 180.2 48520 1.5 4.6 11.1 35.1 7.6 6.9 8.9 1.3 93 5.1 48.0 21210 1.0 3.0 3.8 12.0 4.0 3.8 7.4 1.9 880 32.4 188.6 39800 0.9 3.0 13.4 42.2 14.2 8.5 8.8 1.0 B628 Hikurangi Inputs 23 0.5 16.5 4943 0.9 2.8 4.7 14.9 5.3 2.5 8.2 3.2 23 0.5 13.5 5050 1.5 4.8 4.5 14.2 2.9 2.5 8.9 3.6 B675 Hikurangi Inputs 0 0 0 0 1.7 1.7 5.3 5.3 1 0 - B678 Hikurangi Inputs 36 0.8 5.8 3976 1.7 5.3 5.9 18.7 3.5 13.7 8.9 0.7 30 0.7 11.2 2378 1.5 4.7 6.1 19.3 4.1 3.1 8.9 2.9 45 1.0 11.8 4558 1.4 4.3 3.7 11.6 2.7 8.0 8.9 1.1 B560 Alpine Fault (DFDP-1B) 128 2.6 20.1 3130 0.7 2.3 12.7 40.2 17.7 7.6 10.2 1.3 *5 large representative events 138 2.8 18.9 3580 0.8 2.5 12.1 38.2 15.2 10.4 10.2 1.0 155 3.2 20.7 2920 0.5 1.4 17.0 53.7 37.3 11.3 10.2 0.9 155 3.1 23.2 3190 0.5 1.5 14.7 46.4 30.0 8.9 10.2 1.1 155 3.2 23.9 2560 0.6 2.0 21.9 69.2 35.0 8.5 10.2 1.2 B713 Alpine Fault (DFDP-1B) 230 4.6 31.6 3200 0.4 1.4 22.2 70.0 49.9 9.3 10.2 1.1 *5 large representative events 245 4.9 28.8 2700 0.4 1.3 19.8 62.5 48.6 10.7 10.2 1.0 244 4.9 31.3 3100 0.7 2.1 20.9 66.0 31.1 9.8 10.2 1.0 255 5.1 35.7 2200 0.4 1.4 28.1 88.6 64.1 8.3 10.2 1.2 244 4.9 31.9 3000 0.5 1.6 23.2 73.2 46.2 8.8 10.2 1.2 B571 San Andreas Fault CDZ 187 14.0 62.4 19760 1.1 3.3 6.2 19.5 5.9 4.4 7.1 1.6 B594 San Andreas Fault CDZ 217 15.3 877.0 98440 8.1 25.5 15.2 47.9 1.9 1.5 7.2 4.9 364 27.5 953.0 109300 7.2 22.6 20.8 65.5 2.9 2.3 7.0 3.1 401 32.4 788.0 81500 6.7 21.3 14.7 46.5 2.2 2.0 6.8 3.5 B570 Woodlark Basin 16 0.9 7.8 3710 1.5 4.7 2.8 8.9 1.9 2.5 7.4 3.0 47 2.7 27.3 12710 1.6 4.9 4.0 12.5 2.5 3.4 7.4 2.2 32 1.9 17.7 7410 1.5 4.9 2.7 8.7 1.8 3.7 7.4 2.0 30 1.7 17.6 7900 1.6 4.9 4.1 12.8 2.6 4.5 7.4 1.7 Experiment . Location . Stress drop (kPa) . Percentage stress drop . Slip (μm) . Duration (s) . Pre-SSE velocity (nm s–1) . Pre-SSE velocity (cm yr–1) . Peak velocity (nm s–1) . Peak velocity (cm yr–1) . Vp/Vi . Ks (MPa mm–1) . K (MPa mm–1) . K/Ks . B651 Japan Trench 121 4.6 39.0 11161 2.6 8.2 7.1 22.4 2.7 8.7 8.7 1.0 127 4.8 36.9 8680 2.6 8.0 8.5 26.8 3.3 5.2 8.7 1.7 79 3.1 34.3 7353 1.9 5.9 7.7 24.3 4.2 2.9 8.9 3.1 118 4.6 40.6 8499 2.6 8.2 8.8 27.7 3.4 3.7 8.9 2.4 63 2.5 58.3 19892 2.6 8.2 4.5 14.2 1.7 3.0 8.8 2.9 131 5.1 31.6 6818 2.2 7.1 9.1 28.6 4.0 4.7 8.9 1.9 B518 Nankai Trough Megasplay 33 0.8 8.2 3820 1.1 3.4 3.7 11.7 3.4 5.1 10.7 2.1 36 0.9 11.0 4520 1.8 5.7 4.7 14.9 2.6 4.3 10.8 2.5 15 0.4 5.8 2350 2.0 6.4 3.7 11.6 1.8 3.5 10.7 3.0 26 0.6 19.9 8520 1.7 5.2 3.7 11.6 2.2 2.9 10.5 3.7 33 0.8 13.4 4120 1.4 4.4 5.6 17.5 4.0 3.0 10.7 3.6 51 1.3 18.9 8580 1.1 3.5 4.7 14.7 4.2 4.1 10.7 2.6 26 0.7 11.4 5240 2.5 7.8 5.0 15.9 2.0 3.8 10.5 2.8 42 1.1 12.3 4060 1.4 4.4 4.3 13.7 3.1 4.8 10.5 2.2 61 1.5 21.2 6900 2.0 6.4 7.1 22.4 3.5 5.6 10.5 1.9 B541 Nankai Trough Decollement 0 0 0 0 2.0 2.0 6.3 6.3 1 0 - B565 Cascadia 81 1.7 13.7 6210 0.4 1.3 5.5 17.4 13.4 7.1 10.2 1.4 67 1.4 13.4 4200 0.9 2.8 5.6 17.6 6.2 5.2 10.2 2.0 78 1.6 20.4 4610 1.4 4.4 8.3 26.3 6.0 4.6 10.2 2.2 49 1.0 16.4 6500 1.6 5.0 4.5 14.2 2.9 4.0 10.2 2.5 B517 Costa Rica Decollement 254 11.6 97.1 24990 2.8 8.9 5.8 18.4 2.1 5.6 7.8 1.4 52 2.7 52.3 13410 2.2 7.1 5.0 15.7 2.2 2.0 7.5 3.8 B561 Costa Rica Inputs (chalk) 2466 40.3 12.8 1 0 0 10386.0 32753.3 - 225.9 9.6 0.0 B575 Barbados 725 29.1 180.2 48520 1.5 4.6 11.1 35.1 7.6 6.9 8.9 1.3 93 5.1 48.0 21210 1.0 3.0 3.8 12.0 4.0 3.8 7.4 1.9 880 32.4 188.6 39800 0.9 3.0 13.4 42.2 14.2 8.5 8.8 1.0 B628 Hikurangi Inputs 23 0.5 16.5 4943 0.9 2.8 4.7 14.9 5.3 2.5 8.2 3.2 23 0.5 13.5 5050 1.5 4.8 4.5 14.2 2.9 2.5 8.9 3.6 B675 Hikurangi Inputs 0 0 0 0 1.7 1.7 5.3 5.3 1 0 - B678 Hikurangi Inputs 36 0.8 5.8 3976 1.7 5.3 5.9 18.7 3.5 13.7 8.9 0.7 30 0.7 11.2 2378 1.5 4.7 6.1 19.3 4.1 3.1 8.9 2.9 45 1.0 11.8 4558 1.4 4.3 3.7 11.6 2.7 8.0 8.9 1.1 B560 Alpine Fault (DFDP-1B) 128 2.6 20.1 3130 0.7 2.3 12.7 40.2 17.7 7.6 10.2 1.3 *5 large representative events 138 2.8 18.9 3580 0.8 2.5 12.1 38.2 15.2 10.4 10.2 1.0 155 3.2 20.7 2920 0.5 1.4 17.0 53.7 37.3 11.3 10.2 0.9 155 3.1 23.2 3190 0.5 1.5 14.7 46.4 30.0 8.9 10.2 1.1 155 3.2 23.9 2560 0.6 2.0 21.9 69.2 35.0 8.5 10.2 1.2 B713 Alpine Fault (DFDP-1B) 230 4.6 31.6 3200 0.4 1.4 22.2 70.0 49.9 9.3 10.2 1.1 *5 large representative events 245 4.9 28.8 2700 0.4 1.3 19.8 62.5 48.6 10.7 10.2 1.0 244 4.9 31.3 3100 0.7 2.1 20.9 66.0 31.1 9.8 10.2 1.0 255 5.1 35.7 2200 0.4 1.4 28.1 88.6 64.1 8.3 10.2 1.2 244 4.9 31.9 3000 0.5 1.6 23.2 73.2 46.2 8.8 10.2 1.2 B571 San Andreas Fault CDZ 187 14.0 62.4 19760 1.1 3.3 6.2 19.5 5.9 4.4 7.1 1.6 B594 San Andreas Fault CDZ 217 15.3 877.0 98440 8.1 25.5 15.2 47.9 1.9 1.5 7.2 4.9 364 27.5 953.0 109300 7.2 22.6 20.8 65.5 2.9 2.3 7.0 3.1 401 32.4 788.0 81500 6.7 21.3 14.7 46.5 2.2 2.0 6.8 3.5 B570 Woodlark Basin 16 0.9 7.8 3710 1.5 4.7 2.8 8.9 1.9 2.5 7.4 3.0 47 2.7 27.3 12710 1.6 4.9 4.0 12.5 2.5 3.4 7.4 2.2 32 1.9 17.7 7410 1.5 4.9 2.7 8.7 1.8 3.7 7.4 2.0 30 1.7 17.6 7900 1.6 4.9 4.1 12.8 2.6 4.5 7.4 1.7 Open in new tab Table 3. RSF parameters from inverse modelling of velocity step data. Experiment . Location . Vo (µm s–1) . V (µm s–1) . a . b1 . b2 . b . Dc1 . Dc2 . a-b . std a . std b1 . std Dc1 . std b2 . std Dc2 . B651 Japan Trench 0.0027 0.0087 0.0021 0.0036 0.0036 82.4 -0.0014 0.00004 0.00004 2.14 B518 Nankai Trough Megasplay 0.0020 0.0200 0.0031 0.0015 0.0016 0.0031 6.7 58.3 -0.0001 0.00008 0.00008 0.58 0.00004 1.64 B541 Nankai Trough Decollement 0.0020 0.0200 0.0025 0.0010 0.0016 0.0025 14.6 165.8 -0.0001 0.00002 0.00002 0.52 0.00001 1.72 B611 Cascadia 0.0017 0.0051 0.0013 0.0039 0.0039 8.7 -0.0026 0.00030 0.00030 1.08 B517 Costa Rica Decollement 0.0028 0.0280 0.0026 0.0010 0.0010 80.9 0.0017 0.00004 0.00004 4.58 B597 Barbados 0.0017 0.0051 0.0023 0.0030 0.0030 87.8 -0.0007 0.00003 0.00003 1.36 B628 Hikurangi Inputs 0.0017 0.0051 0.0025 0.0014 0.0019 0.0033 3.6 114.1 -0.0008 0.00023 0.00022 0.89 0.00004 3.64 B718 Hikurangi Inputs 0.0017 0.0051 0.0041 0.0035 0.0015 0.0050 2.5 10.2 -0.0009 0.00028 0.00027 0.24 0.00004 4.64 B594 San Andreas Fault 0.0017 0.0051 0.0013 0.0008 0.0008 76.5 0.0006 0.00004 0.00004 10.42 B570 Woodlark Basin 0.0017 0.0051 0.0026 -0.0025 -0.0025 22.5 0.0051 0.00004 0.00004 0.49 Experiment . Location . Vo (µm s–1) . V (µm s–1) . a . b1 . b2 . b . Dc1 . Dc2 . a-b . std a . std b1 . std Dc1 . std b2 . std Dc2 . B651 Japan Trench 0.0027 0.0087 0.0021 0.0036 0.0036 82.4 -0.0014 0.00004 0.00004 2.14 B518 Nankai Trough Megasplay 0.0020 0.0200 0.0031 0.0015 0.0016 0.0031 6.7 58.3 -0.0001 0.00008 0.00008 0.58 0.00004 1.64 B541 Nankai Trough Decollement 0.0020 0.0200 0.0025 0.0010 0.0016 0.0025 14.6 165.8 -0.0001 0.00002 0.00002 0.52 0.00001 1.72 B611 Cascadia 0.0017 0.0051 0.0013 0.0039 0.0039 8.7 -0.0026 0.00030 0.00030 1.08 B517 Costa Rica Decollement 0.0028 0.0280 0.0026 0.0010 0.0010 80.9 0.0017 0.00004 0.00004 4.58 B597 Barbados 0.0017 0.0051 0.0023 0.0030 0.0030 87.8 -0.0007 0.00003 0.00003 1.36 B628 Hikurangi Inputs 0.0017 0.0051 0.0025 0.0014 0.0019 0.0033 3.6 114.1 -0.0008 0.00023 0.00022 0.89 0.00004 3.64 B718 Hikurangi Inputs 0.0017 0.0051 0.0041 0.0035 0.0015 0.0050 2.5 10.2 -0.0009 0.00028 0.00027 0.24 0.00004 4.64 B594 San Andreas Fault 0.0017 0.0051 0.0013 0.0008 0.0008 76.5 0.0006 0.00004 0.00004 10.42 B570 Woodlark Basin 0.0017 0.0051 0.0026 -0.0025 -0.0025 22.5 0.0051 0.00004 0.00004 0.49 Open in new tab Table 3. RSF parameters from inverse modelling of velocity step data. Experiment . Location . Vo (µm s–1) . V (µm s–1) . a . b1 . b2 . b . Dc1 . Dc2 . a-b . std a . std b1 . std Dc1 . std b2 . std Dc2 . B651 Japan Trench 0.0027 0.0087 0.0021 0.0036 0.0036 82.4 -0.0014 0.00004 0.00004 2.14 B518 Nankai Trough Megasplay 0.0020 0.0200 0.0031 0.0015 0.0016 0.0031 6.7 58.3 -0.0001 0.00008 0.00008 0.58 0.00004 1.64 B541 Nankai Trough Decollement 0.0020 0.0200 0.0025 0.0010 0.0016 0.0025 14.6 165.8 -0.0001 0.00002 0.00002 0.52 0.00001 1.72 B611 Cascadia 0.0017 0.0051 0.0013 0.0039 0.0039 8.7 -0.0026 0.00030 0.00030 1.08 B517 Costa Rica Decollement 0.0028 0.0280 0.0026 0.0010 0.0010 80.9 0.0017 0.00004 0.00004 4.58 B597 Barbados 0.0017 0.0051 0.0023 0.0030 0.0030 87.8 -0.0007 0.00003 0.00003 1.36 B628 Hikurangi Inputs 0.0017 0.0051 0.0025 0.0014 0.0019 0.0033 3.6 114.1 -0.0008 0.00023 0.00022 0.89 0.00004 3.64 B718 Hikurangi Inputs 0.0017 0.0051 0.0041 0.0035 0.0015 0.0050 2.5 10.2 -0.0009 0.00028 0.00027 0.24 0.00004 4.64 B594 San Andreas Fault 0.0017 0.0051 0.0013 0.0008 0.0008 76.5 0.0006 0.00004 0.00004 10.42 B570 Woodlark Basin 0.0017 0.0051 0.0026 -0.0025 -0.0025 22.5 0.0051 0.00004 0.00004 0.49 Experiment . Location . Vo (µm s–1) . V (µm s–1) . a . b1 . b2 . b . Dc1 . Dc2 . a-b . std a . std b1 . std Dc1 . std b2 . std Dc2 . B651 Japan Trench 0.0027 0.0087 0.0021 0.0036 0.0036 82.4 -0.0014 0.00004 0.00004 2.14 B518 Nankai Trough Megasplay 0.0020 0.0200 0.0031 0.0015 0.0016 0.0031 6.7 58.3 -0.0001 0.00008 0.00008 0.58 0.00004 1.64 B541 Nankai Trough Decollement 0.0020 0.0200 0.0025 0.0010 0.0016 0.0025 14.6 165.8 -0.0001 0.00002 0.00002 0.52 0.00001 1.72 B611 Cascadia 0.0017 0.0051 0.0013 0.0039 0.0039 8.7 -0.0026 0.00030 0.00030 1.08 B517 Costa Rica Decollement 0.0028 0.0280 0.0026 0.0010 0.0010 80.9 0.0017 0.00004 0.00004 4.58 B597 Barbados 0.0017 0.0051 0.0023 0.0030 0.0030 87.8 -0.0007 0.00003 0.00003 1.36 B628 Hikurangi Inputs 0.0017 0.0051 0.0025 0.0014 0.0019 0.0033 3.6 114.1 -0.0008 0.00023 0.00022 0.89 0.00004 3.64 B718 Hikurangi Inputs 0.0017 0.0051 0.0041 0.0035 0.0015 0.0050 2.5 10.2 -0.0009 0.00028 0.00027 0.24 0.00004 4.64 B594 San Andreas Fault 0.0017 0.0051 0.0013 0.0008 0.0008 76.5 0.0006 0.00004 0.00004 10.42 B570 Woodlark Basin 0.0017 0.0051 0.0026 -0.0025 -0.0025 22.5 0.0051 0.00004 0.00004 0.49 Open in new tab Ito et al. (2013) reported two SSEs prior to the 2011 Tohoku earthquake, one in 2008 and one in 2011 that was ongoing at the time of the Tohoku event. The data presented here clearly confirm that the Tohoku plate boundary material has the ability to generate SSEs. However, the conditions of these experiments represent much shallower depth than the location of the Tohoku SSEs, which occurred at a depth of ∼10–15 km. This difference in depth and therefore stress conditions could explain why the peak slip velocities measured in these experiments only reach 5–10 per cent of the average slip velocity of the 2008 and 2011 Tohoku SSEs, which is estimated to be ∼0.1 µm s–1. 5.2 Nankai trough The sample from the Nankai Trough megasplay fault zone exhibits several SSEs, which are smaller and occur more frequently compared to the Japan Trench sample (Fig. 5). 9 events occur over ∼1.5 mm displacement with stress drops ranging from 15 to 61 kPa, peak slip velocities of 12–22 cm yr–1 (compared to a 6.3 cm yr–1 driving rate), and unloading stiffnesses of ∼3-6 MPa mm–1. In some cases, stress drops are observed that appear to be SSEs, however a peak in sample slip velocity could not be distinguished from the noise level. This can be seen in Figs 5(b)–(e), where two such perturbations are shown in detail. The first stress drop is clearly associated with an increase in slip rate and an episode of slip excess relative to the background driving, the second is not. Therefore, only the first of these two stress drops is considered to be an SSE. In contrast to the megasplay sample, the sample from the Nankai Trough décollement at Site 1174 exhibits no SSEs (Fig. 6). Although some very small stress drops can be seen which are accompanied by some small velocity increases, the velocity perturbations are in phase with the stress perturbations. The peak slip velocity therefore does not coincide with the stress drop, so these perturbations are not considered to be SSEs. Figure 5. Open in new tabDownload slide Experimental data for a sample from the Nankai Trough megasplay. (a) Coefficient of friction as a function of displacement. (b) Closeup of SSEs generated during plate-rate shearing. (c) Closeup of two strength perturbations as candidate SSEs. (d) Sample slip velocity and, (e) sample displacement with the load point displacement trend removed for the two friction perturbations (shading). Note that only the first perturbation shows a slip velocity increase and excess slip above the driving rates, meaning that the first perturbation is considered an SSE and the second is not. Figure 5. Open in new tabDownload slide Experimental data for a sample from the Nankai Trough megasplay. (a) Coefficient of friction as a function of displacement. (b) Closeup of SSEs generated during plate-rate shearing. (c) Closeup of two strength perturbations as candidate SSEs. (d) Sample slip velocity and, (e) sample displacement with the load point displacement trend removed for the two friction perturbations (shading). Note that only the first perturbation shows a slip velocity increase and excess slip above the driving rates, meaning that the first perturbation is considered an SSE and the second is not. Figure 6. Open in new tabDownload slide Experimental data for a sample from the Nankai Trough décollement. (a) Coefficient of friction as a function of displacement. (b) Closeup of plate-rate shearing. The very small friction perturbations do not generate an increase in sample slip velocity at the same time as the stress drop and are therefore not considered SSEs. Figure 6. Open in new tabDownload slide Experimental data for a sample from the Nankai Trough décollement. (a) Coefficient of friction as a function of displacement. (b) Closeup of plate-rate shearing. The very small friction perturbations do not generate an increase in sample slip velocity at the same time as the stress drop and are therefore not considered SSEs. The observation that the megasplay fault sample tested here produces SSEs suggests that megasplay might be expected to host SSE slip, whereas the lack of SSEs observed in the Nankai Trough décollement sample suggests that slow slip may not be able to reach the trench. It should be pointed out, however, that the décollement sample tested here is from a different transect, lying ∼200 km southwest of the observatories which detected the recent SSE (Araki et al.2017). 5.3 Cascadia The sample from the southern Cascadia frontal thrust zone offshore Oregon exhibits 4 SSEs with stress drops of ∼50–80 kPa, peak slip velocities of 14–26 cm yr–1, and unloading stiffnesses of ∼4–7 MPa mm–1. Other than the first event, a notable characteristic of the Cascadia laboratory SSEs is the large amount of steady-state slip (up to 0.5 mm) that occurs between the loading phase and the stress drop (Fig. 7). The differential displacement record shows no slip deficit accumulation during the pre-event slipping phase, consistent with steady shearing before the stress drop. Figure 7. Open in new tabDownload slide Experimental data for a sample from the southern Cascadia frontal thrust. (a) Coefficient of friction as a function of displacement. (b) Closeup of SSEs generated during plate-rate shearing. (c) Shear stress, (d) sample slip velocity, and (e) difference between sample displacement and load point displacement for the SSE indicated in (b). Shading indicates the SSE stress drop. Note the significant steady-state slip between the loadup and stress drop in (c). Figure 7. Open in new tabDownload slide Experimental data for a sample from the southern Cascadia frontal thrust. (a) Coefficient of friction as a function of displacement. (b) Closeup of SSEs generated during plate-rate shearing. (c) Shear stress, (d) sample slip velocity, and (e) difference between sample displacement and load point displacement for the SSE indicated in (b). Shading indicates the SSE stress drop. Note the significant steady-state slip between the loadup and stress drop in (c). Little is known about the possibility of slow slip at shallow depths in Cascadia, under conditions similar to those explored in these experiments. A recent study using borehole seismic and geodetic data offshore Vancouver Island show that teleseismic waves do not trigger shallow SSEs or tremor, whereas in other subduction zones such activity does result in triggered slow activity (McGuire et al.2018). On the other hand, Wech & Creager (2011) show some sparse tremor activity seaward, including some events near the Site 891 borehole offshore Oregon. Adopting the assumption of Wech & Creager (2011) that tremor accompanies SSEs in this region, the SSEs observed in the Site 891 sample is consistent with shallow slow slip in the southern Cascadia margin. The lack of triggered slow slip to the north may suggest along-strike variations in mechanical properties at Cascadia, of which there is previously documented evidence (e.g. Brudzinski & Allen 2007; Han et al.2017). 5.4 Costa Rica The clay-rich décollement sample from the Costa Rica subduction zone exhibits two SSEs (along with a third stress perturbation for which an increase in slip velocity could not be resolved, and a fourth that could not be measured due to a machine error, Fig. 8). The first SSE has a much larger stress drop (254 kPa) than the second SSE (52 kPa), but the peak slip velocities are similar ( 18.4 mm -1 for the first SSE , 15.7 mm -1 for the second SSE). The detrended displacement record clearly shows a slip deficit accumulation during the loading phase, and slip excess during the stress drop. The unloading stiffness is 5.6 MPa mm –1 for the first SSE and 2.0 MPa mm–1 for the second SSE; an interesting observation is that the first SSE has an unloading stiffness that is larger than the loading stiffness, however for the second SSE this asymmetry is reversed (Fig. 8e). Figure 8. Open in new tabDownload slide Experimental data for a sample from the Costa Rica décollement. (a) Coefficient of friction as a function of displacement. (b) Closeup of shear stress for both SSEs generated during plate-rate shearing. (c) Sample slip velocity, (d) sample displacement with the load point displacement trend removed and (e) unloading stiffness for the two SSEs. Shading indicates the SSE stress drops. Figure 8. Open in new tabDownload slide Experimental data for a sample from the Costa Rica décollement. (a) Coefficient of friction as a function of displacement. (b) Closeup of shear stress for both SSEs generated during plate-rate shearing. (c) Sample slip velocity, (d) sample displacement with the load point displacement trend removed and (e) unloading stiffness for the two SSEs. Shading indicates the SSE stress drops. The chalk sample recovered from the input sediment seaward of the trench at Costa Rica shows strikingly different behaviour compared to the clay-rich décollement sample. At the plate tectonic driving rate of 8.7 cm yr–1, several closely spaced, repetitive slip events are observed, which strongly resemble ordinary stick-slip events (e.g. Brace & Byerlee 1966; Byerlee & Brace 1968; Savage & Marone 2007, Fig. 9). Unlike the SSEs in the Costa Rica décollement and other samples in this study, the peaks and troughs in frictional strength are relatively uniform and there is no tendency toward steady-state slip at any point. The stress drops are ∼2.5 MPa which represents 40 per cent of the shear strength, the largest value in this study (Table 2). The stress drops clearly correlate with increments of slip. The inter-event slip rate appears to be negative, with indications of a precursory acceleration phase prior to the events. More detailed analysis of an individual event shows that the peak slip velocity reaches 10 µm s–1, orders of magnitude faster than any other slip event in this study. Figure 9. Open in new tabDownload slide Experimental data for a sample of pelagic chalk from the Costa Rica input sediment. (a) Coefficient of friction as a function of displacement. (b) Closeup of shear stress, and (c) sample displacement as a function of time for the slip events generated during plate-rate shearing. (d) Closeup of shear stress and sample displacement as a function of time for a single slip event, showing that the slip velocity is ∼10 µm s–1. Figure 9. Open in new tabDownload slide Experimental data for a sample of pelagic chalk from the Costa Rica input sediment. (a) Coefficient of friction as a function of displacement. (b) Closeup of shear stress, and (c) sample displacement as a function of time for the slip events generated during plate-rate shearing. (d) Closeup of shear stress and sample displacement as a function of time for a single slip event, showing that the slip velocity is ∼10 µm s–1. The shallow subduction zone offshore Costa Rica exhibits both SSEs and VLFEs (e.g. Outerbridge et al.2010; Walter et al.2013; Dixon et al.2014). Due to the erosional nature of this subduction zone, the heterogeneously distributed clay and chalk on the incoming plate are expected to heavily influence the shallow megathrust behaviour. The data from these two lithologies is consistent with the activity at Costa Rica: the clay-rich hemipelagic material produces SSEs in these experiments, whereas the chalk produces much faster stick-slip-like events. The clayey sediments are more widespread on the incoming plate; therefore the observations from these experiments are consistent with the hemipelagic clay producing the broad SSEs, and smaller chalk asperities producing the VLFEs observed at Costa Rica. The average slip velocity during the 2007 Nicoya Peninsula SSE (Outerbridge et al.2010) is estimated to be ∼24 cm yr–1 (∼2 cm slip over 30 d, Jiang et al.2012; Walter et al.2013), a value similar to the peak slip velocities of 16-18 cm yr–1 measured in the sample from the clay-rich décollement. Slip velocity was not estimated for the Costa Rica VLFEs, but the 10 µm s–1 measured for the slip events in the chalk sample approach the lower end of the range 0.05–2 mm s–1 estimated for VLFEs in the Nankai Trough (Ito & Obara 2006b), albeit for a different material. 5.5 Barbados The sample from the Barbados subduction décollement produced three SSEs (excluding two superimposed on velocity step tests, which were not evaluated). The SSEs occurred at a driving rate of 5.3 cm yr–1, two of which had exceptionally large stress drops (725 and 880 kPa), with peak slip rates of 35.1 and 42.2 cm yr–1 and unloading stiffnesses of 6.9 and 8.5 MPa mm–1, respectively (Fig. 10). The stress drops of these two SSEs represent ∼30 per cent of the shear strength, among the higher values in this study (Table 2). A notable feature of these two SSEs is their broad shape in the shear stress-displacement record, exhibiting 180–190 µm of event slip (i.e. slip during the stress drop), which is larger than that of any sample except the San Andreas Fault. A smaller SSE occurred between the two large events which exhibited a stress drop of 93 kPa, and lower peak slip velocity (12.0 cm yr–1) and unloading stiffness (3.8 MPa mm–1) compared to the two larger stress drop events. Figure 10. Open in new tabDownload slide Experimental data for a sample from the Barbados décollement. (a) Coefficient of friction as a function of displacement. Closeup of (b) shear stress, (c) sample slip velocity and (d) difference between sample displacement and load point displacement for the SSE indicated in (a) generated during plate-rate shearing. Shading indicates the SSE stress drop. Note the exceptionally large percentage stress drop. Figure 10. Open in new tabDownload slide Experimental data for a sample from the Barbados décollement. (a) Coefficient of friction as a function of displacement. Closeup of (b) shear stress, (c) sample slip velocity and (d) difference between sample displacement and load point displacement for the SSE indicated in (a) generated during plate-rate shearing. Shading indicates the SSE stress drop. Note the exceptionally large percentage stress drop. The lack of reported slow slip activity near Barbados hampers comparison between laboratory and natural observations. However, the observation that SSEs occur in the Barbados décollement sample does suggest that this material has the ability to store some amount of elastic strain energy, which supports the assertion of Hayes et al. (2014) that enough geodetically-measured strain deficit has accumulated with the potential for a Mw ∼8 earthquake. 5.6 Hikurangi, New Zealand Three experiments were conducted on the Hikurangi sediment input sample at 10 MPa effective normal stress; these experiments were previously included in studies by Rabinowitz et al. (2018) and Ikari et al. (2019). Two of these experiments exhibited a total of five SSEs, with one experiment exhibiting no SSEs. The Hikurangi SSE stress drops are generally small and range from 23 to 46 kPa, peak slip velocities range from 11.6 to 19.3 cm yr–1, and unloading stiffnesses are 2.5–13.7 MPa mm–1 (Fig. 11). Little slip deficit is observed during the loading phase, whereas slip excess can clearly be seen during the stress drop. The form of the stress drop is variable, in some cases there is little to no slip between the loading and the stress drop (Fig. 11B), whereas in other cases significant pre-SSE slip (up to 190 µm) occurs before the stress drop (cf. Fig. 6 in Rabinowitz et al.2018). Figure 11. Open in new tabDownload slide Experimental data for a sample from the Hikurangi input sediment. (a) Coefficient of friction as a function of displacement. Closeup of (b) shear stress, (c) sample slip velocity and (d) sample displacement with the load point displacement trend removed for the SSE indicated in (a) generated during plate-rate shearing. Shading indicates the SSE stress drop. Figure 11. Open in new tabDownload slide Experimental data for a sample from the Hikurangi input sediment. (a) Coefficient of friction as a function of displacement. Closeup of (b) shear stress, (c) sample slip velocity and (d) sample displacement with the load point displacement trend removed for the SSE indicated in (a) generated during plate-rate shearing. Shading indicates the SSE stress drop. The peak slip velocities of the laboratory SSEs are comparable to the lower range of southern Hikurangi SSE slip rates of 9–110 cm yr–1, but are significantly slower than the northern Hikurangi SSEs, which exhibit peak slip velocities of at least 73 cm yr–1 (Wallace & Beavan 2010; Wallace et al.2012). As reported by Ikari et al. (2019), the form of the slip history curve of the laboratory SSEs closely resembles the displacement history during SSEs from cGPS measurements, and the general comparison between laboratory and natural Hikurangi SSEs for the stress drop and peak slip velocity data at 10 MPa effective stress also applies to tests conducted at 1, 5 and 15 MPa. 5.7 Alpine Fault, New Zealand Two experiments were conducted on a sample of the principal slip zone of the Alpine Fault recovered in borehole DFDP-1B. Both experiments showed repetitive, uniform stick-slip-like events (Fig. 12), which are similar to but clearly slower and less energetic than the events seen in the Costa Rica chalk. Representative sequences of 5 consecutive events were analysed from each experiment, which showed differences in stress drop (128–155 kPa and 230–255 kPa) but similarities in peak slip velocity (∼40–70 cm yr–1 and ∼60–90 cm yr–1) and unloading stiffness (∼8–11 MPa mm–1 for both experiments). Unlike SSEs in the other samples in this study (except the Costa Rica chalk), the interevent slip rate approaches zero. There is an observable phase of increasing slip rate prior to the stress drop, as also observed for the Costa Rica chalk sample. The unloading stiffness in this case can be seen to be much lower than the stiffness during the loading phase prior to the stress drop. Figure 12. Open in new tabDownload slide Experimental data for a sample from the Alpine Fault DFDP-1B principal slip zone. (a) Coefficient of friction as a function of displacement. (b) Closeup of the slip events generated generated during plate-rate shearing. Closeup of (c) shear stress, (d) sample slip velocity, (e) difference between sample displacement and load point displacement and (f) unloading stiffness for a sequence of five slip events indicated in (b). Shading indicates the SSE stress drop. Note the large peak slip velocities and low interevent velocity. Figure 12. Open in new tabDownload slide Experimental data for a sample from the Alpine Fault DFDP-1B principal slip zone. (a) Coefficient of friction as a function of displacement. (b) Closeup of the slip events generated generated during plate-rate shearing. Closeup of (c) shear stress, (d) sample slip velocity, (e) difference between sample displacement and load point displacement and (f) unloading stiffness for a sequence of five slip events indicated in (b). Shading indicates the SSE stress drop. Note the large peak slip velocities and low interevent velocity. On the Alpine Fault, the shallow portion is locked and slow slip and tremor are only observed deeper than ∼20-25 km (Wech et al.2012; Chamberlain et al.2014); thus far there has been no indication of shallow slow slip on the Alpine Fault. The repetitive slip events observed in the laboratory tests on the DFDP-1B sample indicate the potential for shallow slip instability which is consistent with the near-surface locking, therefore slip events would be expected to occur as small magnitude earthquakes (e.g. Leitner et al.2001), not SSEs or VLFEs. 5.8 San Andreas Fault Two experiments on a sample from the San Andreas Fault Central Deforming Zone produced four total SSEs. One of these SSEs occurred at a driving rate of 5.3 cm yr–1, but the other three were generated at a driving rate of 25 cm yr–1 (10 times the long-term slip rate, e.g. Titus et al.2005). The San Andreas Fault SSEs exhibit stress drops of ∼190–400 kPa, peak slip velocities of 20–66 cm yr–1, and unloading stiffnesses of 1.5–4.4 MPa mm–1. The one SSE generated at 5.3 cm yr–1 has the smallest stress drop, lowest peak slip velocity and the highest unloading stiffness (Fig. 13). The San Andreas Fault SSEs bear several similarities to SSEs produced in the Barbados sample, including the broad form of the stress curve, large event slip, large percentage stress drop, and large slip deficit accumulation during loading. An interesting feature of the SSE generated at 5.3 cm yr–1 is a distinct two-phase stress drop, with an initially small, steep drop before a more gradual drop. The initial drop is associated with the peak slip velocity, with the velocity during the gradual drop being smaller but still elevated above the driving rate. This feature also occurs in the SSEs at the faster driving rate, but is not as prominent. Figure 13. Open in new tabDownload slide Experimental data for a sample from the San Andreas Fault CDZ. (a) Coefficient of friction as a function of displacement. Closeup of (b) shear stress, (c) sample slip velocity and (d) difference between sample displacement and load point displacement for the SSE indicated in (a). Figure 13. Open in new tabDownload slide Experimental data for a sample from the San Andreas Fault CDZ. (a) Coefficient of friction as a function of displacement. Closeup of (b) shear stress, (c) sample slip velocity and (d) difference between sample displacement and load point displacement for the SSE indicated in (a). Based on data presented by Gladwin et al. (1994), the slip rate of northern San Andreas Fault creep events near Hollister can be roughly estimated to range from ∼3 cm yr–1 (slightly higher than the long-term slip rate) to 73 cm yr–1 (nearly 30 times the long-term slip rate). Similar estimates from the creepmeter data of Goulty & Gilman (1978) for the southern creep events near Parkfield are approximately 10–38 cm yr–1, or 4–15 times the long-term slip rate. A curiosity of the SSEs observed in the SAFOD gouge in this study is that the SSEs occurred at driving rates that are 2–10 times faster than the plate rate of ∼2.5 cm yr–1 on the San Andreas Fault. However, all four laboratory SSEs exhibit peak slip velocities that are ∼2–6 times the driving rate, consistent with the lower range of the creepmeter observations. 5.9 Woodlark Basin, Papua New Guinea The sample from the Woodlark Basin produced four regularly-spaced SSEs. The first SSE is notably smaller than the other three in terms of stress drop (16 kPa) and event slip and duration. The other three SSEs are remarkably similar (Fig. 14). These three SSEs exhibit stress drops of 30–47 kPa, peak slip velocities of ∼9-13 cm yr–1 and unloading stiffnesses of 3.4–4.5 MPa mm–1. Unlike most of the SSEs in this study, large slip deviations (i.e. slip deficit and/or excess) are not observed. In addition to their periodicity (roughly every ∼0.5 mm) a noticeable feature of these three Woodlark SSEs is the ∼110–150 µm of pre-event slip between the loading phase and the stress drop, which is similar to the behaviour of the Cascadia and some of the Hikurangi SSEs. Shallow slow slip activity on the Woodlark Basin detachment normal fault has not been documented, but the laboratory observations presented here suggest that SSEs are possible in this region. Figure 14. Open in new tabDownload slide Experimental data for a sample from the Woodlark Basin detachment normal fault. (a) Coefficient of friction as a function of displacement. Closeup of (b) shear stress, (c) sample slip velocity, (d) unloading stiffness and (e) difference between sample displacement and load point displacement for the three SSEs shown in (b). Figure 14. Open in new tabDownload slide Experimental data for a sample from the Woodlark Basin detachment normal fault. (a) Coefficient of friction as a function of displacement. Closeup of (b) shear stress, (c) sample slip velocity, (d) unloading stiffness and (e) difference between sample displacement and load point displacement for the three SSEs shown in (b). 6 MECHANISMS OF SLOW SLIP BEHAVIOUR 6.1 Slow slip in the context of critical stiffness theory Since observational studies of plate-boundary fault motion have generally shown that slow slip tends to occur near the edges of geodetically locked zones, a common assumption is that slow slip arises at the transition from stable to unstable frictional behaviour (Schwartz & Rokosky 2007; Outerbridge et al.2010; Wallace et al.2012; Dixon et al.2014; Saffer & Wallace 2015). This is consistent with expectations from RSF, which predict that near the stability transition ‘self-sustained oscillatory motion’ is expected (e.g. Dieterich 1981, 1986; Scholz 1998) which may be analogous to slow slip. How exactly the stability transition can cause slow slip to arise has been explored in several numerical modelling studies, which have shown that SSEs can be simulated using: a velocity-weakening fault stabilized by dilatancy hardening (Rubin 2008; Liu & Rubin 2010; Segall et al.2010); fault patches located near the downdip transition from velocity-strengthening to velocity-weakening frictional behaviour (Liu & Rice 2005, 2007, 2009); heterogeneity in stress (Helmstetter & Shaw 2009) or in the distribution of velocity-weakening and velocity-strengthening patches (Skarbek et al.2012); velocity-neutral friction (a–b ∼ 0) (Kanu & Johnson 2011); and a transition from velocity weakening to velocity strengthening as the fault accelerates, known as the cutoff velocity (Shibazaki & Iio 2003, Shibazaki & Shimamoto 2007; Matsuzawa et al.2010; Hawthorne & Rubin 2013). Despite the successes of these models each has its specific limitations, and they are still informed from older friction data on analogue fault materials such as granite (Kilgore et al.1993; Blanpied et al.1998) or halite (Shimamoto 1986), obtained under conditions that may not be completely relevant to slow fault slip. Such studies would benefit from improved constraints on frictional behaviour from targeted laboratory experiments on natural samples from slow slip faults. The RSF parameters from velocity steps (Table 3) can be used to determine the frictional properties of natural fault materials that exhibit slow slip, and examine any potential relationship between the RSF and SSE parameters. The RSF formulation (eqs 1–3) provides a quantitative means for determining whether stable or unstable slip is expected, assuming the system can be approximated by a simple spring-slider model (e.g. Rice & Ruina 1983; Gu et al.1984). The data presented here show that changes in shear stress are approximately proportional to the deviation between load point velocity and sample slip velocity (Figs 4, 5, 7, 8, 10–14, Table 2). In some cases the proportionality is not always constant, however in most cases they are sufficient for the system to be considered an approximate, if not ideal, spring-slider system. The criterion for slip instability, defined as self-acceleration to slip speed only limited by inertia, is: \begin{eqnarray*} K \lt {K_c} = \frac{{\left( {b - a} \right)\sigma _n'}}{{{d_c}}}, \end{eqnarray*} (4) where K is the stiffness of the fault surroundings, which must be lower than the critical stiffness value Kc (Dieterich 1981, 1986; Rice & Ruina 1983; Gu et al.1984; Dieterich & Kilgore 1996a; Scholz 1998; Marone 1998a). For simplicity, I assume that dc is dc1 from eqs (1) and (2) in the case that two state variables are employed. It can be seen from eq. (4) that if b–a < 0, indicating a velocity-strengthening material, it is impossible to satisfy the condition K < Kc and therefore instability should not occur. If this condition is satisfied, audible stick-slip events occur in laboratory experiments which are analogous to ordinary earthquakes (Brace & Byerlee 1966; Cook 1981). Thus, the concept of a critical stiffness for unstable slip was used in many early studies to describe the seismic cycle (e.g. Cao & Aki 1986; Dieterich 1986, 1992, 1994; Okubo 1989; Rice 1993; Dieterich & Kilgore 1996a). The critical stiffness criterion described by eq. (4) is useful for predicting slip instabilities that are analogous to ordinary earthquakes, but it is not certain whether various types of slow slip should also be considered instabilities. For example, SSEs do not emit seismic signals and have limited velocity (usually a few times the plate rate) and therefore may not be true slip instabilities, even though they must also nucleate and accelerate. Therefore, I do not make an a priori assumption as to whether the experimentally observed slip events should be classified as instabilities or not. I consider a general model of slip event nucleation that sequentially includes: a perturbation in the initial slip velocity, accelerating slip on a growing fault patch, and finally runaway slip acceleration (instability) after a critical patch size has been attained (e.g. Dieterich 1986, 1992; Roy & Marone 1996; Perfettini & Ampuero 2008). The initial acceleration phase for an event that later becomes fully unstable is the slowly slipping earthquake nucleation phase (Iio 1992, 1995; Ellsworth & Beroza 1995). If it does not become a full instability it can be considered a slow slip event, assuming that the nucleation process is independent of the final slip velocity that is attained. The criterion for the accelerating slip phase before instability is: \begin{eqnarray*} K \lt {K_b} = \frac{{b\sigma _n'}}{{{d_c}}} + \frac{T}{{{V_i}}}, \end{eqnarray*} (5) where T is an external shear stressing rate dτ/dt and Vi is the initial slip velocity; this second term on the right-hand side is commonly neglected (e.g. Dieterich 1992). As noted by Dieterich (1992), since the parameter b rather than b-a appears in eq. (5), it is possible for velocity-strengthening materials to exhibit accelerating slip, a property that has been successfully used in numerical simulations of slow slip and earthquake afterslip (Perfettini & Ampuero 2008; Kanu & Johnson 2011). 6.2 Critical stiffness evaluation Values of b–a and b from velocity upsteps from 0.0017 to 0.0028 µm s–1 are compared with the size of SSEs generated in the same material. Here, the SSE size is quantified as the ratio of the peak slip velocity Vp to the initial slip velocity Vi measured prior to the stress drop (Table 2). The velocity increase was chosen to represent SSE size rather than stress drop for two reasons: (1) stress drops without a distinguishable velocity increase were sometimes observed and (2) the velocity increase in the laboratory SSEs can be directly related to natural faults by comparison with geodetic measurements. Fig. 15 shows that the largest SSEs tend to occur in materials which exhibit large values of b–a, which is generally consistent with critical stiffness theory. However, there are several instances of SSEs occurring in samples that exhibit velocity-strengthening behaviour (i.e. negative b–a) and b–a values that are essentially zero, which suggest the inability to produce a stress drop and therefore are inconsistent with the appearance of SSEs in these samples. Figure 15. Open in new tabDownload slide Ratio of SSE slip velocity increase Vp/Vi as a function of (a) b–a and (b) b, as measured from velocity step tests. (c) Schematic of a velocity step test, showing the measurement of Kc and Kb. Figure 15. Open in new tabDownload slide Ratio of SSE slip velocity increase Vp/Vi as a function of (a) b–a and (b) b, as measured from velocity step tests. (c) Schematic of a velocity step test, showing the measurement of Kc and Kb. Comparing the SSE velocity increase with the parameter b shows a similar relationship as observed with b–a despite significant scatter, showing generally higher Vp/Vi for samples with larger b (Fig. 15). A key aspect of the critical stiffness criterion for accelerating stable slip using Kb is that nearly all samples have the capability for stress drop, and therefore stably accelerating slip can occur even in velocity-strengthening materials. The exception is the sample from the Woodlark Basin, which exhibits a negative value of b and still produced SSEs. Using eqs (4) and (5), the critical stiffness values for instability Kc and for accelerating slip Kb can be calculated. Incorporating the measured apparatus stiffness K is a useful indicator for the type of slip expected, i.e. unstable for K/Kc < 1 and stable accelerating for K/Kc > 1 and K/Kb < 1. Stable creep, that is lack of accelerating slip, is expected if K/Kb > 1. Note that the essence of these stiffness criteria is that the stiffness of the surroundings must be lower than the ability of the sample to unload stress. Therefore, the directly measured unloading stiffness during the SSE stress drop is also a critical stiffness value, designated here Ks. Thus, there are three permutations of critical stiffness from these experiments which may be compared. Comparison of critical stiffness values shows that in most cases (Japan Trench, Costa Rica décollement, Barbados, San Andreas Fault, Woodlark Basin) Ks values are much larger than Kc and Kb, some of which are vanishingly small (Fig. 16). For the Nankai Trough megasplay, Cascadia and Hikurangi, Kb more closely matches the SSE unloading stiffnesses. K/Ks values range from 0.7 to 4.9, with the exception of an exceptionally small value of 0.04 observed for the large events in the Costa Rica chalk. In contrast, many of the K/Kc values have exceedingly large values ranging up to ∼160. These large values are clearly due to the small values of Kc calculated from the velocity step data. Similar to the Kc data, K/Kc is only consistent with K/Ks for the Nankai Trough megasplay and Hikurangi samples. In the cases of the Costa Rica décollement, San Andreas Fault, and Woodlark Basin, K/Kc could not be calculated due to velocity-strengthening behaviour, also the case for K/Kbfor the Woodlark Basin due to negative b values. Figure 16. Open in new tabDownload slide (a) Comparison of the stiffnesses Kc, Kb and Ks. Ks = 225 MPa mm–1 for CRc. (b) Comparison of the stiffness ratios K/Kc, K/Kb and K/Ks, where the apparatus stiffness K = 7–11 MPa mm–1. Note that negative stiffness ratios due to a–b > 0 or b < 0 are not shown. JT, Japan Trench; NTm, Nankai Trough megasplay; Ca, Cascadia; CRd, Costa Rica décollement; CRc, Costa Rica inputs chalk; Ba, Barbados; Hk, Hikurangi; AF, Alpine Fault; SA, San Andreas fault; WB, Woodlark Basin. Figure 16. Open in new tabDownload slide (a) Comparison of the stiffnesses Kc, Kb and Ks. Ks = 225 MPa mm–1 for CRc. (b) Comparison of the stiffness ratios K/Kc, K/Kb and K/Ks, where the apparatus stiffness K = 7–11 MPa mm–1. Note that negative stiffness ratios due to a–b > 0 or b < 0 are not shown. JT, Japan Trench; NTm, Nankai Trough megasplay; Ca, Cascadia; CRd, Costa Rica décollement; CRc, Costa Rica inputs chalk; Ba, Barbados; Hk, Hikurangi; AF, Alpine Fault; SA, San Andreas fault; WB, Woodlark Basin. A small number of K/Ks values are < 1, observed for the Japan Trench, Costa Rica chalk, Hikurangi and Alpine Fault samples (Fig. 16). The Hikurangi sample also exhibits a K/Kb value <1. The Costa Rica décollement, Barbados, San Andreas Fault, and Woodlark Basin did not exhibit any stiffness ratios <1, regardless of the method used in the calculation. Repetitive stick-slip-like events generated in the Costa Rica chalk sample exhibited a K/Ks value of 0.04, and those for the Alpine Fault sample which exhibited K/Ks of 0.9–1.3. Note that the stick-slip-like events in the Costa Rica chalk and the Alpine Fault samples did not allow the extraction of RSF parameters to facilitate comparison. This analysis implies that the SSEs observed in these laboratory experiments, with the possible exception of the Costa Rica chalk and Alpine Fault events, are not frictional instabilities but instances of accelerating stable slip. Using Kc from eq. (4) is therefore not an appropriate predictor of SSE behaviour, whereas Kb and Ks appear to be more consistent with the observed SSE behaviour. However, using Kb and Ks only predict accelerating slip in a few cases, which is not consistent with the majority of the laboratory SSE data set. One possibility is that the measured ratio bσn'/dc may underestimate the stiffness of the sample. However, direct measurement of the peak unloading stiffness during velocity step tests showed values similar to Ks, therefore this possibility can be ruled out. Another possibility is that SSEs can be generated within a range of K/Kb or K/Ks values from positive to negative, in a continuum that does not exhibit a sharp transition or divergent behaviour at the critical value. For the data in this study, this suggests that accelerating stable slip can occur for K/Ks values up to ∼5. K/Kb values are significantly more scattered, but separate into two populations: K/Kb < 3 (Nankai Trough, Cascadia, Hikurangi), and K/Kb > 19 (Japan Trench, Costa Rica decollement, Barbados and San Andreas Fault). Slow stick slip occurring in combination with stiffness ratios slightly greater than 1 were also observed by Scuderi et al. (2016) and Leeman et al. (2016), albeit for a different material (quartz silt) and driving velocity (10 µm s–1). For the purposes of discussing the current data, I will consider the slip events observed in these laboratory experiments to be SSEs originating from accelerating stable slip after a perturbation but without reaching true instability. Exactly why these events occur for stiffness ratios >1 should be examined in future research. An exception is the Costa Rica chalk sample, which may represent LFE or VLFE and emit inaudible low-frequency signals although this must be verified with further measurements. Although the critical stiffness formulation presents difficulties in predicting whether SSEs will occur, laboratory SSE behaviour collectively does follow a pattern consistent with expectations from critical stiffness theory. Ikari & Kopf (2017) showed that larger peak slip velocities correlated with higher percentage stress drops for laboratory SSEs, a characteristic also observed for stick-slip instabilities in granite (Lockner & Okubo 1983; Ohnaka et al.1987; Beeler et al.2012). Fig. 17 shows the important observation that the size of an SSE correlates with increasing stiffness, and therefore decreasing K/Ks. Therefore, the systematics of SSEs, as accelerating stable slip events, are similar to those expected for full slip instabilities. Figure 17. Open in new tabDownload slide Ratio of SSE slip velocity increase Vp/Vi as a function of (a) measured SSE unloading stiffness Ks and (b) unloading stiffness ratio K/Ks. Figure 17. Open in new tabDownload slide Ratio of SSE slip velocity increase Vp/Vi as a function of (a) measured SSE unloading stiffness Ks and (b) unloading stiffness ratio K/Ks. 6.3 Conceptual model of SSE generation Dieterich (1992) demonstrated that the critical stiffness for stably accelerating slip following a perturbation is bσn'/dc (eq. 5). A specific requirement here is that the instantaneous slip velocity V >> dc/θ, so that the system is far away from steady-state. This makes the term Vθ/dc large so that the 1 in eq. (2) can be neglected. Since the time-dependence of friction arises from the 1 in the aging law, the approximation neglecting the 1 has been referred to as the ‘no healing’ condition (Rubin & Ampuero 2005). However, in both natural SSEs and the laboratory SSEs observed here, the peak slip velocities are often small and close to the plate tectonic driving rates. In eq. (2), it can be seen that there is another way to deviate from steady state, and that is with a very large θ (and/or small dc, which I do not consider here for simplicity). Although in this case Vθ/dc >> 1, it cannot be referred to as a no-healing condition because large θ indicates that the healing process is dominant. A key question is how the nucleation of slip acceleration to a higher velocity can be initiated in the first place, including the nature of the initial perturbation. In nature, such a perturbation could be driven externally, for example from movement on a neighboring fault or fault patch. However, the experiments here show that SSEs occur spontaneously with no apparent external influence. Previous numerical simulations of the nucleation process have shown that the condition Vθ/dc >> 1 is necessary for initially stably accelerating slip (Dieterich 1992; Rubin & Ampuero 2005; Ampuero & Rubin 2008; Perfettini & Ampuero 2008; Rubin 2008), however these studies also noted that it is not always straightforward when the condition Vθ/dc >> 1 is valid because V and θ are inversely related. Therefore, V and θ could drive the system away from steady state in combination. I suggest that the initiation of stable accelerating slip that results in SSEs could arise from advanced healing, that is rapid time-dependent frictional strengthening due to advanced evolution of θ. This process can be summarized as follows: (1) at initially steady-state sliding at the plate rate, state evolution during sliding begins to outpace the slip rate, increasing θ so that additional healing during sliding occurs, (2) the additional healing forces V to decrease to a small but non-zero value, which allows θ to become large and drives the system away from steady state, (3) failure of the strengthened material is reached, resulting in an increase in V, which functions as the initial slip rate perturbation, and (4) the perturbation in V causes θ to decrease, driving V higher so that V >> θ/dc and slip may accelerate according to eq. (5). This is illustrated semi-schematically in Fig. 18; data from the first SSE in the Costa Rica décollement sample (Fig. 8) is used as an example for the shear stress and slip velocity. In this figure, the quantity θ is shown schematically, since away from steady state θ ≠ dc/V and hence is not well constrained. Figure 18. Open in new tabDownload slide Illustration of the conceptual model for SSE generation using Dieterich's aging law. Shear stress and slip velocity data from the large SSE observed for the Costa Rica décollement sample shown as an example for reference. Note that the slip velocity curve follows the experimental data, while the curve for the state variable θ is shown as a schematic and is not meant to represent actual values. Figure 18. Open in new tabDownload slide Illustration of the conceptual model for SSE generation using Dieterich's aging law. Shear stress and slip velocity data from the large SSE observed for the Costa Rica décollement sample shown as an example for reference. Note that the slip velocity curve follows the experimental data, while the curve for the state variable θ is shown as a schematic and is not meant to represent actual values. One key feature of this model is a transition from a healing-dominated departure from steady-state to a velocity-dominated departure from steady-state. This transition necessitates a point at which V and θ are balanced and steady-state is reached. This near-steady-state condition is likely short-lived, however a temporary steady-state condition explains the observation of a measureable amount of steady-state slip after the loading phase and before the stress drop of some of the SSEs, one example being the SSEs in the Cascadia sample. Another important aspect of this model is that the initial perturbation is in the slip velocity, not necessarily in the shear stress as is usually employed in numerical models (e.g. Roy & Marone 1996; Perfettini & Ampuero 2008), although the velocity and shear stress are of course closely related. For most of the SSEs in these experiments the pre-event slip velocity Vi is below the driving rate, ranging from 1.3 to 8.9 cm yr–1 for driving rates of 5.3–8.7 cm yr–1. Exceptions are the San Andreas Fault, which exhibited some SSEs at a higher driving rate (25 cm yr–1) and the Costa Rica chalk events which exhibited Vi ∼ 0. All but four of the SSEs discussed in this study have initial slip velocities that drop below the uncertainty in slip velocity defined by noise in the displacement sensors. Normalizing the initial slip velocity and peak slip velocity by the driving (load point) velocity for all slip events in this study shows a clear inverse relationship between Vi/Vlp and Vp/Vlp (Fig. 19). This demonstrates that the farther the velocity decreases below the driving rate, the larger the eventual peak slip velocity becomes, but that the velocity does not need to approach 0. For natural faults this means that full geodetic locking is not a requirement for SSE generation, but larger events result from stronger locking. This is consistent with the observation that ordinary earthquakes result from true geodetic locking, and also with geodetic measurements during the interval between repeating SSEs, which document a decrease in velocity that does not necessarily reach full locking before the slip reversal signaling the event (McCaffrey 2009; Vergnolle et al.2010; Wallace & Beavan 2010; Jiang et al.2012). Figure 19. Open in new tabDownload slide Peak slip velocity normalized by the driving velocity Vp/Vlp as a function of the initial pre-SSE velocity normalized by the driving velocity Vi/Vlp. The point 1:1 indicates no velocity perturbation and therefore no SSEs, which is the case for the Nankai Trough décollement and one experiment using the Hikurangi input sediment. Figure 19. Open in new tabDownload slide Peak slip velocity normalized by the driving velocity Vp/Vlp as a function of the initial pre-SSE velocity normalized by the driving velocity Vi/Vlp. The point 1:1 indicates no velocity perturbation and therefore no SSEs, which is the case for the Nankai Trough décollement and one experiment using the Hikurangi input sediment. 6.4 Role of frictional healing The origin of the decrease in initial velocity or partial locking could be advanced healing which leads to large values of θ. I suggest that such healing is facilitated by the extremely low slip velocities in these experiments, which allow the state to advance further than in standard friction experiments conducted in the µm s–1 to mm s–1 range. An advanced healing mechanism at cm yr–1 driving rates is consistent with the RSF data in this study, including the large strength increase upon the velocity downsteps and the tendency for velocity-weakening friction (Fig. 3, Ikari & Kopf 2017). This mechanism is also consistent with a study by Kanu & Johnson (2011), who conducted numerical simulations of a creep event on the Hayward fault and concluded that evolution in θ from the aging law is necessary to simulate the event. The time- (state-) dependent evolution of friction known as frictional healing can be isolated and measured in slide-hold-slide friction experiments. In these tests, sliding at a constant rate is interrupted by periods of 0 driving rate (the ‘hold’) of various durations; the resulting excess peak in strength above the steady-state strength depends on the duration of the hold (Dieterich 1972). Most laboratory studies conduct SHS tests for hold times up to ∼3000 s, which show that frictional healing depends linearly on the logarithm of the hold time, and that at room temperature the rate of healing varies depending on the gouge composition, background driving rate, and normal stress (e.g. Beeler et al.1994; Marone 1998b; Niemeijer & Spiers 2006; Ikari et al.2012; Tesei et al.2012; Carpenter et al.2016). However, healing tests up to 3000 s are insufficient to explain the observations at low driving velocities near the plate rate. For example, assuming a grain length of 100 µm, an asperity contact being displaced at 1 µm s–1 would exist for 100 s; whereas at 1.7 nm s–1 (5.3 cm yr–1) this contact would last for ∼60  000 s. Previous studies using the same San Andreas Fault sample tested here indicated very small healing rates at typical laboratory hold times of up to 3000 s (Carpenter et al.2011, 2012, 2015a), which would suggest that a healing mechanism leading to partial locking and a decrease in slip rate would not be effective. This would therefore present difficulty explaining the observation of SSEs in this sample and also the repeating creep events on the San Andreas Fault. Using this same San Andreas Fault sample, Ikari et al. (2016a) demonstrated that for hold times up to 106 s, there is a distinct increase in healing at very large hold times (>3000 s). In this case, the healing rate is better fit with a power law rather than a log-linear function. This suggests that extremely slow driving rates of cm yr–1 are necessary to allow enough time for advanced healing to occur, which facilitates the partial locking leading to accelerating stable slip. Whether the other samples in this study also exhibit increased healing at very large hold times will require further testing. Power-law healing is also observed in calcite samples (Chen et al.2015; Carpenter et al.2015b), which could explain why the Costa Rica chalk sample exhibited full locking and very fast slip events. On natural faults, mechanisms other than healing may also contribute to an initial decrease in slip velocity prior to SSEs. One possibility is geometric irregularities, such as subducting topographic highs or specific fault surface roughness. Another is heterogeneous or patchy strength, which could arise from spatially varying lithology or compartmentalized pore fluid pressure. These mechanisms do not contradict the effect of advanced healing proposed here; rather their effects likely enhance each other. For example, faults which have the ability to heal rapidly would be more sensitive to small perturbations that may be introduced by sliding against such geometric irregularities. 6.5 Effect of mineralogic composition An intriguing aspect of the San Andreas Fault sample which exhibits advanced healing that could allow the generation of SSEs, is its notably high smectite content. This has led to the conclusion that smectite controls the mechanical behaviour of the central San Andreas Fault to a large degree (e.g. Carpenter et al.2011, 2012, 2015a; Holdsworth et al.2011; Lockner et al.2011; Hadizadeh et al.2012; Schleicher et al.2012; Moore 2014; Ikari et al.2016a). Three other notable fault zones which exhibit SSEs in this study have been previously implied to be controlled or heavily influenced by smectite: the Japan Trench (Kameda et al.2015; Ikari et al.2015a,b), Barbados (Deng & Underwood 2001; Kopf & Brown 2003), and Costa Rica (Spinelli & Underwood 2004) subduction zones. Smectite, and other expandable clays, are notable for having the unique property of strongly attracting water molecules to their surfaces and interlayers, heavily affecting their mechanical behaviour (e.g. Byerlee 1978; Lupini et al.1981; Bird 1984; Ikari et al.2007; Behnsen & Faulkner 2013). For the samples used in this study, the Japan Trench sample is composed of 81 per cent smectite, the Costa Rica décollement 46 per cent smectite and 8 per cent mixed-layer clays, the San Andreas Fault sample 39 per cent smectite and 5 per cent mixed-layer clays, and the Barbados sample 11 per cent smectite and 9 per cent mixed-layer clays. Mixed-layer clays are typically alternating layers of smectite with illite (or another clay) and therefore included as expandable clays (Środón 1999). All other samples in this study have an expandable clay component of 3–25 per cent. Note that it is these four samples which produced SSEs despite exhibiting very large K/Kb values, which should lead to an expectation of stable non-accelerating slip rather than SSEs. Strong evidence for an effect of expandable clays can be seen in the SSE stress drop data. Stress drop is one measure of slip event size and can be expected to correlate positively with peak slip velocity (e.g. Lockner & Okubo 1983, Ohnaka et al.1987; Beeler et al.2012). Therefore, larger percentage stress drops should also correlate with lower initial slip velocity, Vi. However, these relationships are obscured by significant scatter (Fig. 20). Closer inspection of the data shows that the scatter is caused by the samples with a significant expandable clay component: the Japan Trench, Costa Rica décollement, Barbados and the San Andreas Fault. After separating these samples out, the remaining percentage stress drop data show the expected relationships with Vi/Vlp and Vp/Vlp (Fig. 20). The scatter can thus be attributed to the extraordinarily large stress drops produced by the four expandable clay-bearing samples. It is not clear why the samples bearing expandable clay produce larger stress drops, but may be related to an advanced healing mechanism observed in the San Andreas Fault sample (Ikari et al.2016a). Considering this compositional effect, a general correlation can be observed between both low overall clay and low expandable clay with Vp/Vlp (Fig. 21). However, the correlation between expandable clay and measured SSE parameters is not a strictly direct one and should be considered a general tendency; for example the Nankai Trough megasplay sample has a smectite content of 25 per cent but does not exhibit especially large stress drops. Figure 20. Open in new tabDownload slide Percentage stress drop as a function of peak slip velocity normalized by the driving velocity Vp/Vlp (A, B, C) and as a function of the initial pre-SSE velocity normalized by the driving velocity Vi/Vlp (D, E, F). Panels (a) and (d) show the complete population of laboratory SSEs, (b) and (e) show samples from the Japan Trench, Costa Rica décollement, Barbados and San Andreas Fault which have an expandable clay content of 20–81 per cent and (c) and (f) show all other samples which have an expandable clay content of 3–25 per cent. Note that the Costa Rica chalk is only shown in panels (a) and (d) due to a very large value of Vp/Vlp = 3765. Figure 20. Open in new tabDownload slide Percentage stress drop as a function of peak slip velocity normalized by the driving velocity Vp/Vlp (A, B, C) and as a function of the initial pre-SSE velocity normalized by the driving velocity Vi/Vlp (D, E, F). Panels (a) and (d) show the complete population of laboratory SSEs, (b) and (e) show samples from the Japan Trench, Costa Rica décollement, Barbados and San Andreas Fault which have an expandable clay content of 20–81 per cent and (c) and (f) show all other samples which have an expandable clay content of 3–25 per cent. Note that the Costa Rica chalk is only shown in panels (a) and (d) due to a very large value of Vp/Vlp = 3765. Figure 21. Open in new tabDownload slide Ratio of SSE slip velocity increase Vp/Vi as a function of (a) total phyllosilicate content and (b) expandable clay content. Figure 21. Open in new tabDownload slide Ratio of SSE slip velocity increase Vp/Vi as a function of (a) total phyllosilicate content and (b) expandable clay content. 6.6 Implications for general fault slip behaviour Due to their potential effect on coseismic fault slip, it is of interest to discuss the implications of these laboratory SSEs in the context of the complete spectrum of fault slip behaviour. A caveat to these analyses is that these experiments were performed at 10 MPa effective normal stress and at room temperature, which represents the shallowest portions of major faults (∼1–2 km). Although slow slip can propagate to such shallow levels (e.g. Wallace et al.2016; Araki et al.2017), care should be taken in comparing results from these testing conditions with natural SSEs that originate near the updip limit of geodetic locking and/or seismogenesis. A key finding of this work and of Ikari & Kopf (2017) is that naturally slow, plate-rate shearing can result in velocity-weakening friction not observed under more commonly used shearing rates. Velocity-weakening friction suggests the potential for unstable slip, however the critical stiffness analysis performed here indicates that the majority of the SSEs observed in these experiments are accelerating stable slip events and not frictional slip instabilities. This nominally suggests that velocity-weakening friction on shallow, SSE-producing fault portions may not significantly enhance the risk of seismic slip. On the other hand, the occurrence of SSEs does indicate the ability to store and release elastic strain energy, which could be a factor contributing to enhanced coseismic slip. There are two observations from the laboratory SSEs that may be especially relevant to coseismic slip propagation. The first is that the SSEs demonstrate the ability for velocity-strengthening faults to exhibit a stress drop, even the Woodlark Basin sample which is velocity-strengthening and exhibits a negative b. This implies that RSF does not always completely describe a materials’ ability to release stored energy. Secondly, during SSEs the frictional strength increases from a baseline level, before a stress drop of equal magnitude back to the baseline level. This implies that while extra strengthening may allow additional energy to be stored on SSE faults, the occurrence of the SSEs does not necessarily relieve stress below the background level. Therefore, based on the observations in this study, SSE-producing fault portions may be expected to enhance coseismic slip from a propagating earthquake rather than resisting it. However, the overall small stress drops observed in this study, the stably-sliding nature of the SSEs, and the small overall strength increase at plate-rate shearing suggests that this effect is likely to be small. As noted by Faulkner et al. (2011), weak clay-rich faults, which describes most of the samples in this study, are expected to provide little resistance against propagating earthquake slip despite velocity-strengthening frictional behaviour at intermediate slip rates. The effect of elastic strain energy accumulation in weak, SSE-generating materials on coseismic slip propagation merits further study. As a final note, many of the experiments presented here were performed on samples from well-studied fault zones with records of recent large earthquakes (e.g. Nankai Trough, Japan Trench and Cosa Rica). However, the complete data set suggests that SSEs can occur in regions without recently recorded SSEs or large earthquakes, such as Barbados and the Woodlark Basin. We may therefore expect to find that new instrumentation on previously unstudied fault zones would reveal SSEs, which may interact with future large earthquakes that have not been recorded in the past. 7 CONCLUSIONS This paper presents the results of laboratory friction experiments conducted on 11 natural samples from major fault zones around the world, conducted at cm yr–1 shearing rates and simulating in situ conditions on the shallow (upper few km) region of major fault zones. The cm yr–1 driving rates produce spontaneously occurring laboratory SSEs, which exhibit peak slip velocities and stress drops comparable to those of natural SSEs estimated from geodetic data. Generally, the stress drops range from 10s to several hundreds of kPa, or >1–32 per cent stress drop. Slip velocity tends to decrease below the driving rate prior to the SSE stress drops, then accelerates to peak slip velocities that generally range from ∼9 to ∼90 cm yr–1, for driving rates of 5.3–8.7 cm yr–1. An exception is a sample of pelagic chalk from the incoming plate of the Costa Rica subduction zone, which produced stick-slip like events with relatively rapid (10 µm s–1) slip velocities and large (40 per cent) stress drops which are likely analogous to VLFEs rather than SSEs. Despite the overall similarity of the laboratory SSEs, some characteristics are observed which are specific to certain samples that, in many cases, are consistent with geodetic observations in the regions from which they were sampled. The laboratory SSE data set shows a strong correlation between pre-SSE velocity and peak slip velocity, where a larger decrease in slip velocity before the event results in a faster event. The size of the SSEs, quantified by the slip velocity increase, correlates with higher values of unloading stiffness during the SSE stress drop, suggesting that SSEs behave similarly to true frictional instabilities. Results of velocity-stepping tests at cm yr–1 rates show a tendency for velocity-weakening friction not observed at higher sliding velocities, and that the materials with lower values of the rate-dependent friction parameter a–b and larger values of b tend to produce faster SSEs. Using RSF measurements on samples that produce SSEs, critical stiffness analyses show that the condition for slip instability is not satisfied. Rather, the SSEs should be considered accelerating stable slip events, based on the observation that the stiffness condition for this type of slip behaviour is satisfied more often. Based on Dieterich's aging law, I develop a conceptual model for the nucleation of SSEs. The model utilizes a competition between the state variable θ and the instantaneous slip velocity V, which drives the system away from steady state in two phases: an initial ‘healing-dominated’ phase where advanced healing mechanism causes the onset of partial locking and velocity decrease, followed by failure of the strengthened material and the ensuing transition to a ‘velocity-dominated’ phase of accelerating (stable) slip. This model explains several aspects of the laboratory SSEs and is also consistent with geodetic measurements of natural SSEs, most notably the relationship between low pre-SSE velocity and high peak slip velocity. The role of sample composition is explored, with the data showing that the frictional strength and velocity-dependence of friction depend on the phyllosilicate mineral content, consistent with previous studies. Faster SSEs tend to occur in low-phyllosilicate materials, but the presence of expandable clays (typically smectite) can result in exceptionally larger stress drops. Applying the behaviour observed in SSE-producing samples to natural fault zones, the appearance of SSEs indicates the ability to strengthen above the steady-state plate-rate level, thereby storing elastic strain energy later released as the SSE stress drop. The data show that this can occur even in gouges that are velocity-strengthening and exhibit stably accelerating slip rather than slip instability. This can be interpreted to enhance coseismic slip in the specific case of an earthquake propagating onto the SSE-producing fault portion. However, several lines of evidence such as the fact that the criteria for slip instability is rarely satisfied and the small overall stress drops indicate that the enhancement of coseismic slip may be relatively small, although this effect warrants further study. ACKNOWLEDGEMENTS This research uses samples and/or data provided by the Ocean Drilling Program (ODP) and Integrated Ocean Drilling Program (IODP). I thank Achim Kopf for enlightening discussions, and three anonymous reviewers for helpful comments. This work was supported by the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme Grant #714430 to M. Ikari. REFERENCES Abers G.A. , 1991 . Possible seismogenic shallow-dipping normal faults in the Woodlark-D'Entrecasteaux extensional province, Papua New Guinea , Geology , 19 , 1205 – 1208 .. 10.1130/0091-7613(1991)019%3c1205:PSSDNF%3e2.3.CO;2 Google Scholar Crossref Search ADS WorldCat Abers G.A. , Mutter C.Z., Fang J., 1997 . Shallow dips of normal faults during rapid extension: earthquakes in the Woodlark-D'Entrecasteaux rift system, Papua New Guinea , J. geophys. Res. , 102 , 15 301 – 15 317 .. 10.1029/97JB00787 Google Scholar Crossref Search ADS WorldCat Adams J. , 1990 . Paleoseismicity of the Cascadia subduction zone: evidence from turbidites off the Oregon-Washington margin , Tectonics , 9 , 569 – 583 .. 10.1029/TC009i004p00569 Google Scholar Crossref Search ADS WorldCat Ampuero J.-P. , Rubin A.M., 2008 . Earthquake nucleation on rate and state faults—aging and slip laws , J. geophys. Res. , 113 , B01302 , doi:10.1029/2007JB005082 . Google Scholar Crossref Search ADS WorldCat Ando M. , 1975 . Source mechanisms and tectonic significance of historical earthquakes along the Nankai Trough, Japan , Tectonophysics , 27 , 119 – 140 .. 10.1016/0040-1951(75)90102-X Google Scholar Crossref Search ADS WorldCat Ando M. , Balazs E.I., 1979 . Geodetic evidence for aseismic subduction of the Juan de Fuca plate , J. geophys. Res. , 84 , 3023 – 3028 .. 10.1029/JB084iB06p03023 Google Scholar Crossref Search ADS WorldCat Araki E. , Saffer D.M., Kopf A., Wallace L.M., Kimura T., Machida Y., Ide S., 2017 . Recurring and triggered slow slip events near the trench at the Nankai Trough subduction megathrust , Science , 356 , 1157 – 1160 .. 10.1126/science.aan3120 Google Scholar Crossref Search ADS PubMed WorldCat Atwater B.F. , 1987 . Evidence for great Holocene earthquakes along the outer coast of Washington state , Science , 236 , 942 – 944 .. 10.1126/science.236.4804.942 Google Scholar Crossref Search ADS PubMed WorldCat Baba T. , Cummins P.R., Hori T., Kaneda Y., 2006 . High precision slip distribution of the 1944 Tonankai earthquake inferred from tsunami waveforms: possible slip on a splay fault , Tectonophysics , 426 , 119 – 134 .. 10.1016/j.tecto.2006.02.015 Google Scholar Crossref Search ADS WorldCat Bakun W.H. , McEvilly T.V., 1984 . Recurrence models and Parkfield, California, earthquakes , J. geophys. Res. , 89 , 3051 – 3058 .. 10.1029/JB089iB05p03051 Google Scholar Crossref Search ADS WorldCat Baumberger T. , Berthoud P., Caroli C., 1999 . Physical analysis of the state- and rate-dependent friction law. II. Dynamic friction , Phys. Rev. B , 60 , 3928 – 3939 .. 10.1103/PhysRevB.60.3928 Google Scholar Crossref Search ADS WorldCat Beavan J. , Ellis S., Wallace L., Denys P., 2007 . Kinematic constraints from GPS on oblique converence of the Pacific and Australian plates, central South Island, New Zealand , eds. Okaya D., Stern T., Davey , F., A Continental Plate Boundary:teconics At South Island, New Zealand, Geophys. Mon. Ser. , 175 , American Geophysical Union , p p. 75 – 94 . Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC Beavan J. et al. ., 1999 . Crustal deformation during 1994–1998 due to oblique continental collision in the central Southern Alps, New Zealand, and implications for seismic potential of the Alpine fault , J. geophys. Res. , 104 , 25 233 – 25 255 .. 10.1029/1999JB900198 Google Scholar Crossref Search ADS WorldCat Beeler N. , Kilgore B., McGarr A., Fletcher J., Evans J., Baker S.R., 2012 . Observed source parameters for dynamic rupture with non-uniform initial stress and relatively high fracture energy , J. Struct. Geol. , 38 , 77 – 89 .. 10.1016/j.jsg.2011.11.013 Google Scholar Crossref Search ADS WorldCat Beeler N.M. , Tullis T.E., Weeks J.D., 1994 . The roles of time and displacement in the evolution effect in rock friction , Geophys. Res. Lett. , 21 , 1987 – 1990 .. 10.1029/94GL01599 Google Scholar Crossref Search ADS WorldCat Behnsen J. , Faulkner D.R., 2013 . Permeability and frictional strength of cation-exchanged montmorillonite , J. geophys. Res.: Solid Earth , 118 , doi:10.1002/jgrb.50226 . Google Scholar OpenURL Placeholder Text WorldCat Berryman K.R. , Cochran U.A., Clark K.J., Biasi G.P., Langridge R.M., Villamor P., 2012 . Major earthquakes occur regularly on an isolated plate boundary fault , Science , 336 , 1690 – 1693 .. 10.1126/science.1218959 Google Scholar Crossref Search ADS PubMed WorldCat Bhattacharya P. , Rubin A.M., Bayart E., Savage H.M., Marone C., 2015 . Critical evaluation of state evolution laws in rate and state friction: fitting large velocity steps in simulated fault gouge with time-, slip- and stress-dependent constitutive laws , J. geophys.: Res. Solid Earth , 120 , doi:10.1002/2015JB012437 . Google Scholar OpenURL Placeholder Text WorldCat Bhattacharya P. , Rubin A.M., Beeler N.M., 2017 . Does fault strengthening in laboratory rock friction experiments really depend primarily upon time and not slip? . J. geophys. Res.: Solid Earth , 122 , 6389 – 6430 .. 10.1002/2017JB013936 Google Scholar Crossref Search ADS WorldCat Bird P. , 1984 . Hydration-phase diagrams and friction of montmorillonite under laboratory and geologic conditions with implications for shale compaction, slope stability, and strength of fault gouge , Tectonophysics , 107 , 235 – 260 .. 10.1016/0040-1951(84)90253-1 Google Scholar Crossref Search ADS WorldCat Blanpied M.L. , Marone C.J., Lockner D.A., Byerlee J.D., King D.P., 1998 . Quantitative measure of the variation in fault rheology due to fluid-rock interactions , J. geophys. Res. , 103 , 9691 – 9712 .. 10.1029/98JB00162 Google Scholar Crossref Search ADS WorldCat Boulton C. , Moore D.E., Lockner D.A., Toy V.G., Townend J., Sutherland R., 2014 . Frictional properties of exhumed fault gouges in DFDP-1 cores, Alpine Fault, New Zealand , Geophys. Res. Lett. , 41 , 356 – 362 .. 10.1002/2013GL058236 Google Scholar Crossref Search ADS WorldCat Bouysse P. , Westercamp D., 1990 . Subduction of Atlantic aseismic ridges and late Cenozoic evolution of the Lesser Antilles island arc , Tectonophysics , 175 , 349 – 380 .. 10.1016/0040-1951(90)90180-G Google Scholar Crossref Search ADS WorldCat Brace W.F. , Byerlee J.D., 1966 . Stick-slip as a mechanism for earthquakes , Science , 153 , 990 – 992 .. 10.1126/science.153.3739.990 Google Scholar Crossref Search ADS PubMed WorldCat Brown J.R. et al. ., 2009 . Deep low-frequency earthquakes in tremor localize to the plate interface in multiple subduction zones , Geophys. Res. Lett. , 36 , L19306 , doi:10.1029/2009GL040027 . Google Scholar Crossref Search ADS WorldCat Brown K.M. , Kopf A., Underwood M.B., Weinberger J.L., 2003 . Compositional and fluid pressure controls on the state of stress on the Nankai subduction thrust: a weak plate boundary , Earth planet. Sci. Lett. , 214 , 589 – 603 .. 10.1016/S0012-821X(03)00388-1 Google Scholar Crossref Search ADS WorldCat Brudzinski M.R. , Allen R.M., 2007 . Segmentation in episodic tremor and slip all along Cascadia , Geology , 35 , 907 – 910 .. 10.1130/G23740A.1 Google Scholar Crossref Search ADS WorldCat Bull W.B. , 1996 . Prehistorical earthquakes on the Alpine Fault, New Zealand , J. geophys. Res. , 101 , 6037 – 6050 .. 10.1029/95JB03062 Google Scholar Crossref Search ADS WorldCat Byerlee J. , 1978 . Friction of rocks . Pure appl. Geophys. , 116 , 615 – 626 .. 10.1007/BF00876528 Google Scholar Crossref Search ADS WorldCat Byerlee J.D. , Brace W.F., 1968 . Stick slip, stable sliding, and earthquakes—effect of rock type, pressure, strain rate, and stiffness , J. geophys. Res. , 73 , 6031 – 6037 .. 10.1029/JB073i018p06031 Google Scholar Crossref Search ADS WorldCat Bürgmann R. , 2018 . The geophysics, geology and mechanics of slow fault slip , Earth planet. Sci. Lett. , 495 , 112 – 134 .. 10.1016/j.epsl.2018.04.062 Google Scholar Crossref Search ADS WorldCat Cao T. , Aki K., 1986 . Seismicity simulation with a rate- and state-dependent friction law , Pure appl. Geophys. , 124 , 487 – 513 .. 10.1007/BF00877213 Google Scholar Crossref Search ADS WorldCat Carpenter B.M. , Ikari M.J., Marone C., 2016 . Laboratory observations of time-dependent frictional strengthening and stress relaxation in natural and synthetic fault gouges , J. geophys. Res.: Solid Earth , 121 , 1183 – 1201 .. 10.1002/2015JB012136 Google Scholar Crossref Search ADS WorldCat Carpenter B.M. , Kitajima H., Sutherland R., Townend J., Toy V.G., Saffer D.M., 2014 . Hydraulic and acoustic properties of the active Alpine Fault, New Zealand: laboratory measurements on DFDP-1 drill core , Earth planet. Sci. Lett. , 390 , 45 – 51 .. 10.1016/j.epsl.2013.12.023 Google Scholar Crossref Search ADS WorldCat Carpenter B.M. , Marone C., Saffer D.M., 2009 . Frictional behavior of materials in the 3D SAFOD volume , Geophys. Res. Lett. , 36 , L05302 , doi:10.1029/2008GL036660 . Google Scholar Crossref Search ADS WorldCat Carpenter B.M. , Marone C., Saffer D.M., 2011 . Weakness of the San Andreas Fault revealed by samples from the active fault zone , Nat. Geosci. , 4 , 251 – 254 .. 10.1038/ngeo1089 Google Scholar Crossref Search ADS WorldCat Carpenter B.M. , Mollo S., Viti C., Collettini C., 2015b . Influence of calcite decarbonation on the frictional behavior of carbonate-bearing gouge: implications for the instability of volcanic flanks and fault slip , Tectonophysics , 658 , 128 – 136 .. 10.1016/j.tecto.2015.07.015 Google Scholar Crossref Search ADS WorldCat Carpenter B.M. , Saffer D.M., Marone C., 2012 . Frictional properties and sliding stability of the San Andreas fault from deep drill core , Geology , 40 , 759 – 762 .. 10.1130/G33007.1 Google Scholar Crossref Search ADS WorldCat Carpenter B.M. , Saffer D.M., Marone C., 2015a . Frictional properties of the active San Andreas fault at SAFOD: Implications for fault strength and slip behavior , J. geophys. Res.: Solid Earth , 120 , 5273 – 5289 .. 10.1002/2015JB011963 Google Scholar Crossref Search ADS WorldCat Carter R.M. et al. ., 1999 . Proc. Ocean Drill. Program, Initial Rep. , Vol. 181 , College Station, Texas , Ocean Drill, Program , doi:10.2973/odp.proc.ir.181.2000 . Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC Chamberlain C.J. , Shelly D.R., Townend J., Stern T., 2014 . Low-frequency earthquakes reveal punctuated slow slip on the deep extent of the Alpine Fault, New Zealand , Geochem. Geophys. Geosyst. , 15 , 2984 – 2999 .. 10.1002/2014GC005436 Google Scholar Crossref Search ADS WorldCat Chen J. , Verberne B.A., Spiers C.J., 2015 . Effects of healing on the seismogenic potential of carbonate fault rocks: experiments on samples from the Longmenshan Fault, Sichuan, China , J. geophys. Res. Solid Earth , 120 , doi:10.1002/2015JB012051 . Google Scholar OpenURL Placeholder Text WorldCat Chester F.M. , Mori J.J., Eguchi N., Toczko S., the Expedition 343/343T Scientists , 2013a . Proceedings of the Integrated Ocean Drilling Program, 343/343T , Integrated Ocean Drilling Program Management International, Inc , Tokyo Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC Chester F.M. et al. ., 2013b . Structure and composition of the plate-boundary slip zone for the 2011 Tohoku-Oki earthquake , Science , 342 , 1208 – 1211 .. 10.1126/science.1243719 Google Scholar Crossref Search ADS WorldCat Coble C.G. , French M.E., Chester F.M., Chester J.S., Kitajima H., 2014 . In situ frictional properties of San Andreas Fault gouge at SAFOD , Geophys. J. Int. , 199 , 956 – 967 .. 10.1093/gji/ggu306 Google Scholar Crossref Search ADS WorldCat Cook N. , 1981 . Stiff testing machines, stick slip sliding, and the stability of rock deformation , in Mechanical Behavior of Crustal Rocks: The Handin Volume, Geophys. Monogr. Ser. , 24 , 93 – 102 ., eds Carter N.L., Friedman M., Logan J.M., Stearns D.W., American Geophysical Union . Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC Cooper A.F. , Norris R.J., 1990 . Estimates for the timing of the last coseismic displacements on the Alpine Fault, northern Fiordland, New Zealand , N. Zeal. J. Geol. Geophys. , 33 , 303 – 307 .. 10.1080/00288306.1990.10425688 Google Scholar Crossref Search ADS WorldCat Cooper A.F. , Norris R.J., 1994 . Anatomy, structural evolution, and slip rate of a plate-boundary thrust: the Alpine fault at Gaunt Creek, Westland, New Zealand , Bull. geol. Soc. Am. , 106 , 627 – 633 .. 10.1130/0016-7606(1994)106%3c0627:ASEASR%3e2.3.CO;2 Google Scholar Crossref Search ADS WorldCat Crawford B.R. , Faulkner D.R., Rutter E.H., 2008 . Strength, porosity, and permeability development during hydrostatic and shear loading of synthetic quartz-clay fault gouge , J. geophys. Res. , 113 , B03207 , doi:10.1029/2006JB004634 . Google Scholar Crossref Search ADS WorldCat Cummins P.R. , Baba T., Kodaira S., Kaneda Y., 2002 . The 1946 Nankai earthquake and segmentation of the Nankai Trough , Phys. Earth planet. Inter. , 132 , 75 – 87 .. 10.1016/S0031-9201(02)00045-6 Google Scholar Crossref Search ADS WorldCat Cummins P.R. , Kaneda Y., 2000 . Possible splay fault slip during the 1946 Nankai earthquake , Geophys. Res. Lett. , 27 , 2725 – 2728 .. 10.1029/1999GL011139 Google Scholar Crossref Search ADS WorldCat Davis E. , Heesemann, Wang K., 2011 . Evidence for episodic aseismic slip across the subduction seismogenic zone off Costa Rica: CORK borehose pressure observations at the subduction prism toe , Earth planet. Sci. Lett. , 309 , 299 – 305 .. 10.1016/j.epsl.2011.04.017 Google Scholar Crossref Search ADS WorldCat Davis E.E. , Villinger H., Sun T., 2015 . Slow and delayed deformation and uplift of the outermost subduction prism following ETS and seismogenic slip events beneath Nicoya Peninsula, Costa Rica , Earth planet. Sci. Lett. , 410 , 117 – 127 .. 10.1016/j.epsl.2014.11.015 Google Scholar Crossref Search ADS WorldCat Delahaye E.J. , Townend J., Reyners M.E., Rogers G., 2009 . Microseismicity but no tremor accompanying slow slip in the Hikurangi subduction zone, New Zealand , Earth planet. Sci. Lett. , 277 , 21 – 28 .. 10.1016/j.epsl.2008.09.038 Google Scholar Crossref Search ADS WorldCat DeMets C. , Gordon R.G., Argus D.F., 2010 . Geologically current plate motions , Geophys. J. Int. , 181 , 1 – 80 .. 10.1111/j.1365-246X.2009.04491.x Google Scholar Crossref Search ADS WorldCat DeMets C. , Gordon R.G., Argus D.F., Stein S., 1994 . Effect of recent revisions to the geomagnetic reversal time scale on estimates of current plate motions , Geophys. Res. Lett. , 21 , 2191 – 2194 .. 10.1029/94GL02118 Google Scholar Crossref Search ADS WorldCat DeMets C. , Jansma P.E., Mattioli G.S., Dixon T.H., Farina F., Bilham R., Calais E., Mann P., 2000 . GPS geodetic constraints on Caribbean-North America plate motion , Geophys. Res. Lett. , 27 , 437 – 440 .. 10.1029/1999GL005436 Google Scholar Crossref Search ADS WorldCat Deng X. , Underwood M.B., 2001 . Abundance of smectite and the location of a plate-boundary fault, Barbados accretionary prism , Bull. geol. Soc. Am. , 113 , 495 – 507 .. 10.1130/0016-7606(2001)113%3c0495:AOSATL%3e2.0.CO;2 Google Scholar Crossref Search ADS WorldCat Dieterich J. , 1994 . A constitutive law for rate of earthquake production and its application to earthquake clustering , J. geophys. Res. , 99 , 2601 – 2618 .. 10.1029/93JB02581 Google Scholar Crossref Search ADS WorldCat Dieterich J.H. , 1972 . Time-dependent friction in rocks , J. geophys. Res. , 77 , 3690 – 3697 .. 10.1029/JB077i020p03690 Google Scholar Crossref Search ADS WorldCat Dieterich J.H. , 1979 . Modeling of rock friction 1. Experimental results and constitutive equations , J. geophys. Res. , 84 , 2161 – 2168 .. 10.1029/JB084iB05p02161 Google Scholar Crossref Search ADS WorldCat Dieterich J.H. , 1981 . Constitutive properties of faults with simulated gouge , in Mechanical Behavior of Crustal Rocks: The Handin Volume, Geophys. Monogr. Ser. , Vol. 24 , pp. 103 – 120 ., eds Carter N.L., Friedman M., Logan J.M., Stearns D.W., American Geophysical Union . Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC Dieterich J.H. , 1986 . A model for the nucleation of earthquake slip , in Earthquake Source Mechanics, Geophys. Monogr. Ser. , eds Das S., Boatwright J., Scholz C.H., Vol. 37 , pp. 37 – 47 ., American Geophysical Union . Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC Dieterich J.H. , 1992 . Earthquake nucleation on faults with rate- and state-dependent strength , Tectonophysics , 211 , 115 – 134 .. 10.1016/0040-1951(92)90055-B Google Scholar Crossref Search ADS WorldCat Dieterich J.H. , Kilgore B., 1994 . Direct observation of frictional contacts: New insights for state-dependent properties , Pure appl. Geophys. , 143 , 283 – 302 .. 10.1007/BF00874332 Google Scholar Crossref Search ADS WorldCat Dieterich J.H. , Kilgore B.D., 1996a . Implications of fault constitutive properties for earthquake prediction , Proc. Nat. Acad. Sci. USA , 93 , 3787 – 3794 .. 10.1073/pnas.93.9.3787 Google Scholar Crossref Search ADS WorldCat Dieterich J.H. , Kilgore B.D., 1996b . Imaging surface contacts: power law contact distributions and contact stresses in quartz, calcite, glass and acrylic plastic , Tectonophysics , 256 , 219 – 239 .. 10.1016/0040-1951(95)00165-4 Google Scholar Crossref Search ADS WorldCat Dixon T.H. , Jiang Y., Malservisi R., McCaffrey R., Voss N., Protti M., Gonzalez V., 2014 . Earthquake and tsunami forecasts: Relation of slow slip events to subsequent earthquake rupture , Proc. Nat. Acad. Sci. U.S.A. , 111 , 17 039 – 17 044 .. 10.1073/pnas.1412299111 Google Scholar Crossref Search ADS WorldCat Dorel J. , 1981 . Seismicity and seismic gap in the Lesser Antilles arc and earthquake hazard in Guadaloupe , Geophys. J. R. astr. Soc. , 67 , 679 – 695 .. 10.1111/j.1365-246X.1981.tb06947.x Google Scholar Crossref Search ADS WorldCat Doser D.I. , Webb T.H., 2003 . Source parameters of large historical (1917-1961) earthquakes, North Island, New Zealand , Geophys. J. Int. , 152 , 795 – 832 .. 10.1046/j.1365-246X.2003.01895.x Google Scholar Crossref Search ADS WorldCat Douglas A. , Beavan J., Wallace L., Townend J., 2005 . Slow slip on the northern Hikurangi subduction interface, New Zealand , Geophys. Res. Lett. , 32 , L16305 , doi:10.1029/2005GL023607 . Google Scholar Crossref Search ADS WorldCat Dragert G. , Wang K., James T.S., 2001 . A silent slip event on the deeper Cascadia subduction interface , Science , 292 , 1525 – 1528 .. 10.1126/science.1060152 Google Scholar Crossref Search ADS PubMed WorldCat Ellsworth W.L. , Beroza G.C., 1995 . Seismic evidence for an earthquake nucleation phase , Science , 286 , 851 – 855 .. 10.1126/science.268.5212.851 Google Scholar Crossref Search ADS WorldCat Faulkner D.R. , Mitchell T.M., Behnsen J., Hirose T., Shimamoto T., 2011 . Stuck in the mud? Earthquake nucleation and propagation through accretionary forearcs , Geophys. Res. Lett. , 38 , L18303 , doi:10.1029/2011GL048552 . Google Scholar Crossref Search ADS WorldCat Flück P. , Hyndman R.D., Wand K., 1997 . Three-dimensional dislocation model for great earthquakes of the Cascadia subduction zone , J. geophys. Res. , 102 , 20539 – 20550 .. 10.1029/97JB01642 Google Scholar Crossref Search ADS WorldCat Fujiwara T. , Kodaira K., No T., Kaiho Y., Takahashi N., Kaneda Y., 2011 . The 2011 Tohoku-Oki earthquake: Displacement reaching the trench axis , Science , 334 , 1240 , doi:10.1126/science.1211554 . Google Scholar Crossref Search ADS PubMed WorldCat Fukao Y. , Kanjo K., 1980 . A zone of low-frequency earthquakes beneath the inner wall of the Japan Trench , Tectonophysics , 67 , 153 – 162 .. 10.1016/0040-1951(80)90170-5 Google Scholar Crossref Search ADS WorldCat Giorgetti C. , Carpeter B.M., Collettini C., 2015 . Frictional behavior of talc-calcite mixtures , J. geophys. Res.: Solid Earth , 120 , 6614 – 6633 .. 10.1002/2015JB011970 Google Scholar Crossref Search ADS WorldCat Gladwin M.T. , Gwyther R.L., Hart R.H.G., 1994 . Measurements of the strain field associated with episodic creep events on the San Andreas fault at San Juan Bautista, California , J. geophys. Res. , 99 , 4559 – 4565 .. 10.1029/93JB02877 Google Scholar Crossref Search ADS WorldCat Goldfinger C. , Nelson C.H., Johnson J.E., the Shipboard Scientific Party , 2003 . Holocene earthquake records from the Cascadia subduction zone and northern San Andreas Fault based on precise dating of offshore turbidites , Annu. Rev. Earth planet. Sci. , 31 , 555 – 577 .. 10.1146/annurev.earth.31.100901.141246 Google Scholar Crossref Search ADS WorldCat Goldsby D.L. , Rar A., Pharr G.M., Tullis T.E., 2004 . Nanoindentation creep of quartz, with implications for rate- and state-variable friction laws relevant to earthquake mechanics , J. Mater. Res. , 19 , 357 – 365 .. 10.1557/jmr.2004.19.1.357 Google Scholar Crossref Search ADS WorldCat Goulty N.R. , Gilman R., 1978 . Repeated creep events on the San Andreas Fault near Parkfield, California, recorded by a strainmeter array , J. geophys. Res. , 83 , 5415 – 5419 .. 10.1029/JB083iB11p05415 Google Scholar Crossref Search ADS WorldCat Gu J.-C. , Rice J.R., Ruina A.L., Tse S.T., 1984 . Slip motion and stability of a single degree of freedom elastic system with rate and state dependent friction , J. Mech. Phys. Solids , 32 , 167 – 196 .. 10.1016/0022-5096(84)90007-3 Google Scholar Crossref Search ADS WorldCat Hadizadeh J. , Mittempergher S., Gratier J.-P., Renard F., Di Toro G., Richard J., Babaie H.A., 2012 . A microstructural study of fault rocks from the SAFOD: Implications for the deformation mechanisms and strength of the creeping segment of the San Andreas Fault , J. Struct. Geol. , 42 , 246 – 260 .. 10.1016/j.jsg.2012.04.011 Google Scholar Crossref Search ADS WorldCat Han S. , Bangs N.L., Carbotte S.M., Saffer D.M., Gibson J.C., 2017 . Links between sediment consolidation and Cascadia megathrust slip behaviour , Nat. Geosci. , 10 , 954 – 959 .. 10.1038/s41561-017-0007-2 Google Scholar Crossref Search ADS WorldCat Hawthorne J.C. , Rubin A.M., 2013 . Laterally propagating slow slip events in a rate and state friction model with a velocity-weakening to velocity-strengthening transition , J. geophys. Res.: Solid Earth , 118 , doi:10.1002/jgrb.50261 . Google Scholar OpenURL Placeholder Text WorldCat Hayes G.P. , McNamara D.E., Seidman L., Roger J., 2014 . Quantifying potential earthquake and tsunami hazard in the Lesser Antilles subduction zone of the Caribbean region , Geophys. J. Int. , 196 , 510 – 521 .. 10.1093/gji/ggt385 Google Scholar Crossref Search ADS WorldCat Helmstetter A. , Shaw B.E., 2009 . Afterslip and aftershocks in the rate-and-state friction law , J. geophys. Res. , 114 , B01308 , doi:10.1029/2007JB005077 . Google Scholar Crossref Search ADS WorldCat Hey R. , 1977 . Tectonic evolution of the Cocos-Nazca spreading center , Bull. geol. Soc. Am. , 88 , 1404 – 1420 .. 10.1130/0016-7606(1977)88%3c1404:TEOTCS%3e2.0.CO;2 Google Scholar Crossref Search ADS WorldCat Holdsworth R.E. , van Diggelen E.W.E., Spiers C.J., de Bresser J.H.P., Walker R.J., Bowen L., 2011 . Fault rocks from the SAFOD core samples: implications for weakening at shallow depths along the San Andreas Fault, California , J. Struct. Geol. , 33 , 132 – 144 .. 10.1016/j.jsg.2010.11.010 Google Scholar Crossref Search ADS WorldCat Husen S. , Kissling E., Quintero R., 2002 . Tomographic evidence for a subducted seamount beneath the Gulf of Nicoya, Costa Rica: the cause of the 1990 Mw = 7.0 Gulf of Nicoya earthquake , Geophys. Res. Lett. , 29 , 1238 , doi:10.1029/2001GL014045 . Google Scholar Crossref Search ADS WorldCat Husker A. , Ferrari L., Arango-Galván C., Corbo-Camargo F., Arzate-Flores J.A., 2018 . A geologic recipe for transient slip within the seismogenic zone: insight from the Guerrero seismic gap, Mexico , Geology , 46 , 35 – 38 .. 10.1130/G39202.1 Google Scholar Crossref Search ADS WorldCat Hyndman R.D. , Wang K., 1995 . The rupture zone of Cascadia great earthquakes from current deformation and the thermal regime , J. geophys. Res. , 100 , 22 133 – 22 154 .. 10.1029/95JB01970 Google Scholar Crossref Search ADS WorldCat Hüpers A. et al. ., 2017 . Release of mineral-bound water prior to subduction tied to shallow seismogenic slip off Sumatra , Science , 356 , 841 – 844 .. 10.1126/science.aal3429 Google Scholar Crossref Search ADS PubMed WorldCat Ichinose G.A. , Thio H.K., Somerville P.G., 2003 . Rupture process of the 1944 Tonankai earthquake (M s 8.1) from the inversion of teleseismic and regional seismograms , J. geophys. Res. , 108 , 2497 , doi:10.1029/2003JB002393 . Google Scholar Crossref Search ADS WorldCat Ide S. , Beroza G.C., Shelly D.R., Uchide T., 2007 . A scaling law for slow earthquakes , Nature , 447 , 76 – 79 .. 10.1038/nature05780 Google Scholar Crossref Search ADS PubMed WorldCat Iio Y. , 1992 . Slow initial phase of the P-wave velocity pulse generated by microearhquakes , Geophys. Res. Lett. , 19 , 477 – 480 . Google Scholar Crossref Search ADS WorldCat Iio Y. , 1995 . Observations of the slow initial phase generated by microearthquakes: implications for earthquake nucleation and propagation , J. geophys. Res. , 100 , 15 333 – 15 349 .. 10.1029/95JB01150 Google Scholar Crossref Search ADS WorldCat Ikari M.J. , Carpenter B.M., Kopf A.J., Marone C., 2014 . Frictional strength, rate-dependence, and healing in DFDP-1 borehole samples from the Alpine Fault, New Zealand , Tectonophysics , 630 , 1 – 8 .. 10.1016/j.tecto.2014.05.005 Google Scholar Crossref Search ADS WorldCat Ikari M.J. , Carpenter B.M., Marone C., 2016b . A microphysical interpretation of rate- and state-dependent friction for fault gouge , Geochem. Geophys. Geosyst. , 17 , 1660 – 1677 .. 10.1002/2016GC006286 Google Scholar Crossref Search ADS WorldCat Ikari M.J. , Carpenter B.M., Vogt C., Kopf A.J., 2016a . Elevated time-dependent strengthening rates observed in San Andreas Fault drilling samples , Earth planet. Sci. Lett. , 450 , 164 – 172 .. 10.1016/j.epsl.2016.06.036 Google Scholar Crossref Search ADS WorldCat Ikari M.J. , Ito Y., Ujiie K., &Kopf A.J., 2015a . Spectrum of slip behaviour in Tohoku fault zone samples at plate tectonic slip speeds , Nat. Geosci. , 8 , 870 – 874 .. 10.1038/ngeo2547 Google Scholar Crossref Search ADS WorldCat Ikari M.J. , Kameda J., Saffer D.M., Kopf A.J., 2015b . Strength characteristics of Japan Trench borehole samples in the high-slip region of the 2011 Tohoku-Oki earthquake , Earth planet. Sci. Lett. , 412 , 35 – 41 .. 10.1016/j.epsl.2014.12.014 Google Scholar Crossref Search ADS WorldCat Ikari M.J. , Knuth M.W., Marone C., Saffer D.M., 2012 . Data Report: Frictional healing and compressibility of sheared sediment from fault zones, Sites C0004 and C0007 , in Proc. IODP, 314/315/316 , eds Kinoshita M., Tobin H., Ashi J., Kimura G., Lallemant S., Screaton E.J., Curewitz D., Masago H., Moe K.T., the Expedition 314/315/316 Scientists, Washington, DC , Integrated Ocean Drilling Program Management International Inc , doi:10.2204/iodp.proc.314315316.219.2012 . Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC Ikari M.J. , Kopf A.J., 2017 . Seismic potential of weak, near-surface faults revealed at plate tectonic slip rates , Sci. Adv. , 3 , e1701269 , doi:10.1126/sciadv.1701269 . Google Scholar Crossref Search ADS PubMed WorldCat Ikari M.J. , Kopf A.J., Hüpers A., Vogt C., 2018 . Lithologic control of frictional strength variation in subduction zone sediment inputs , in Subduction Top to Bottom 2, Geosphere , Vol. 14 , eds Bebout G.E., Scholl D.W., Stern R.J., Wallace L.M., Agard P., doi:10.1130/GES01546.1 . Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC Ikari M.J. , Marone C., Saffer D.M., 2011a . On the relation between fault strength and frictional stability , Geology , 39 , 83 – 86 .. 10.1130/G31416.1 Google Scholar Crossref Search ADS WorldCat Ikari M.J. , Niemeijer A.R., Marone C., 2011b . The role of fault zone fabric and lithification state on frictional strength, constitutive behavior, and deformation microstructure , J. geophys. Res. , 116 , B08404 , doi:10.1029/2011JB008264 . Google Scholar Crossref Search ADS WorldCat Ikari M.J. , Niemeijer A.R., Spiers C.J., Kopf A.J., Saffer D.M., 2013 . Experimental evidence linking slip instability with seafloor lithology and topography at the Costa Rica convergent margin , Geology , 41 , 891 – 894 .. 10.1130/G33956.1 Google Scholar Crossref Search ADS WorldCat Ikari M.J. , Saffer D.M., 2011 . Comparison of frictional strength and velocity dependence between fault zones in the Nankai accretionary complex , Geochem. Geophys. Geosyst. , 12 , Q0AD11 , doi:10.1029/2010GC003442 . Google Scholar Crossref Search ADS WorldCat Ikari M.J. , Saffer D.M., Marone C., 2007 . Effect of hydration state on the frictional properties of montmorillonite-based fault gouge , J. geophys. Res. , 112 , B06423 , doi:10.1029/2006JB004748 . Google Scholar Crossref Search ADS WorldCat Ikari M.J. , Saffer D.M., Marone C., 2009 . Frictional and hydrologic properties of clay-rich fault gouge , J. geophys. Res. , 114 , B05409 , doi:10.1029/2008JB006089 . Google Scholar Crossref Search ADS WorldCat Ikari M.J. , Trütner S., Carpenter B.M., Kopf A.J., 2015c . Shear behavior of DFDP-1 borehole samples from the Alpine Fault, New Zealand, under a wide range of experimental conditions , Int. J. Earth Sci. , 104 , 1523 – 1535 .. 10.1007/s00531-014-1115-5 Google Scholar Crossref Search ADS WorldCat Ikari M.J. , Wallace L.M., Rabinowitz H.S., Savage H.M., Hamling I.J., Kopf A.J., 2019 . Observations of Laboratory and Natural Slow Slip Events: Hikurangi Subduction Zone, New Zealand , Geochem. Geophys. Geosyst . Google Scholar OpenURL Placeholder Text WorldCat Irwin W.P. , Barnes I., 1975 . Effect of geologic structure and metamorphic fluids on seismic behavior and of the San Andreas fault system in central and northern California . Geology , 3 , pp. 713 – 706 . Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC Ito Y. , Hino R., Suzuki S., Kaneda Y., 2015 . Episodic tremor and slip near the Japan Trench prior to the 2011 Tohoku-Oki earthquake , Geophys. Res. Lett. , 42 , 1725 – 1731 .. 10.1002/2014GL062986 Google Scholar Crossref Search ADS WorldCat Ito Y. , Obara K., 2006a . Dynamic deformation of the accretionary prism excites very low frequency earthquakes , Geophys. Res. Lett. , 33 , L02311 , doi:10.1029/2005GL025570 . Google Scholar OpenURL Placeholder Text WorldCat Ito Y. , Obara K., 2006b . Very low frequency earthquakes within accretionary prisms are very low stress-drop earthquakes , Geophys. Res. Lett. , 33 , L09302 , doi:10.1029/2006GL025883 . Google Scholar OpenURL Placeholder Text WorldCat Ito Y. et al. ., 2011 . Frontal wedge deformation near the source region of the 2011 Tohoku-Oki earthquake , Geophys. Res. Lett. , 38 , L00G05 , doi:10.1029/2011GL048355 . Google Scholar Crossref Search ADS WorldCat Ito Y. et al. ., 2013 . Episodic slow slip events in the Japan subduction zone before the 2011 Tohoku-Oki earthquake , Tectonophysics , 600 , 14 – 26 .. 10.1016/j.tecto.2012.08.022 Google Scholar Crossref Search ADS WorldCat Jacoby G.C. , Bunker D.E., Benson B.E., 1997 . Tree-ring evidence for an A.D. 1700 Cascadia earthquake in Washington and northern Oregon , Geology , 25 , 999 – 1002 .. 10.1130/0091-7613(1997)025%3c0999:TREFAA%3e2.3.CO;2 Google Scholar Crossref Search ADS WorldCat Jiang Y. , Wdowinski S., Dixon T.H., Hackl M., Protti M., Gonzalez V., 2012 . Slow slip events in Costa Rica detected by continuous GPS observations, 2002–2011 , Geochem. Geophys. Geosyst. , 13 , Q04006 , doi:10.1029/2012GC004058 . Google Scholar Crossref Search ADS WorldCat Johnson K.M. , 2013 . Is stress accumulating on the creeping section of the San Andreas fault? . Geophys. Res. Lett. , 40 , 6101 – 6105 .. 10.1002/2013GL058184 Google Scholar Crossref Search ADS WorldCat Jolivet R. , Simons M., Agram P.S., Duputel Z., Shen Z.-K., 2015 . Aseismic slip and seismogenic coupling along the central San Andreas Fault , Geophys. Res. Lett. , 42 , 297 – 306 .. 10.1002/2014GL062222 Google Scholar Crossref Search ADS WorldCat Kameda J. , Shimizu M., Ujiie K., Hirose T., Ikari M., Mori J., Oohashi K., Kimura G., 2015 . Pelagic smectite as an important factor in tsunamigenic slip along the Japan Trench , Geology , 43 , 155 – 158 .. 10.1130/G35948.1 Google Scholar Crossref Search ADS WorldCat Kame N. , Rice J.R., Dmowska R., 2003 . Effects of prestress state and rupture velocity on dynamic fault branching , J. geophys. Res. , 108 , 2265 , doi:10.1029/2002JB002189 . Google Scholar Crossref Search ADS WorldCat Kanu C. , Johnson K., 2011 . Arrest and recovery of frictional creep on the southern Hayward fault triggered by the 1989 Loma Prieta, California, earthquake and implications for future earthquakes , J. geophys. Res. , 116 , B04403 , doi:10.1029/1010JB007927 . Google Scholar Crossref Search ADS WorldCat Kawasaki I. , Asai Y., Tamura Y., 2001 . Space-time distribution of interplate moment release including slow earthquakes and the seismo-geodetic coupling in the Sanriku-oki region along the Japan trench , Tectonophysics , 330 , 267 – 283 .. 10.1016/S0040-1951(00)00245-6 Google Scholar Crossref Search ADS WorldCat Kelsey H.M. , Witter R.C., Hemphill-Haley E., 2002 . Plate-boundary earthquakes and tsunamis of the past 5500 yr, Sixes River estuary, southern Oregon , Bull. geol. Soc. Am. , 114 , 298 – 314 .. 10.1130/0016-7606(2002)114%3c0298:PBEATO%3e2.0.CO;2 Google Scholar Crossref Search ADS WorldCat Kikuchi M. , Nakamura M., Yoshikawa K., 2003 . Source rupture processes of the 1944 Tonankai earthquake and the 1945 Mikawa earthquake derived from low-gain seismograms , Earth Planets Space , 55 , 159 – 172 .. 10.1186/BF03351745 Google Scholar Crossref Search ADS WorldCat Kilgore B.D. , Blanpied M.L., Dieterich J.H., 1993 . Velocity dependent friction of granite over a wide range of conditions , Geophys. Res. Lett. , 20 , 903 – 906 .. 10.1029/93GL00368 Google Scholar Crossref Search ADS WorldCat Kimura G. , Silver E., Blum P. et al. ., 1997 . Proc. Ocean Drill. Program, Initial Rep. , Vol. 170 , Ocean Drilling Program . Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC Kinoshita M. et al. ., 2009 . Proc. IODP, 314/315/316 , Integrated Ocean Drilling Program Management International , doi:10.2204/iodp.proc.314315316.2009 . Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC Kirkpatrick J.D. et al. ., 2015 . Structure and lithology of the Japan Trench subduction plate boundary fault , Tectonics , 34 ( 1 ), 53 – 69 .. 10.1002/2014TC003695 Google Scholar Crossref Search ADS WorldCat Kopf A. , Brown K.M., 2003 . Friction experiments on saturated sediments and their implications for the stress state of the Nankai and Barbados subduction thrusts , Mar. Geol. , 202 , 193 – 210 .. 10.1016/S0025-3227(03)00286-X Google Scholar Crossref Search ADS WorldCat Koulali A. , McClusky S., Wallace L., Allgeyer S., Tregoning P., D'Anastasio E., Benavente R., 2017 . Slow slip events and the 2016 TeAraroa M w 7.1 earthquake interaction: Northern Hikurangi subduction, New Zealand , Geophys. Res. Lett. , 44 ( 16 ), 8336 – 8344 .. 10.1002/2017GL074776 Google Scholar Crossref Search ADS WorldCat Kurzawski R.M. , Stipp M., Niemeijer A.R., Spiers C.J., Behrmann J.H., 2016 . Earthquake nucleation in weak subducted carbonates , Nat. Geosci. , 9 , 717 – 722 .. 10.1038/ngeo2774 Google Scholar Crossref Search ADS WorldCat Leeman J.R. , Saffer D.M., Scuderi M.M., Marone C., 2016 . Laboratory observations of slow earthquakes and the spectrum of tectonic fault slip modes , Nat. Commun. , 7 , 11104 , doi:10.1038/ncomms11104 . Google Scholar Crossref Search ADS PubMed WorldCat Leitner B. , Eberhart-Phillips D., Anderson H., Nabelek J.H., 2001 . A focused look at the Alpine fault, New Zealand: seismicity, focal mechanisms, and stress observations , J. geophys. Res. , 106 , 2193 – 2220 .. 10.1029/2000JB900303 Google Scholar Crossref Search ADS WorldCat Linde A.T. , Gladwin M.T., Johnston M.J.S., Gwyther R.L., Bilham R.G., 1996 . A slow earthquake sequence on the San Andreas fault , Nature , 383 , 65 – 68 .. 10.1038/383065a0 Google Scholar Crossref Search ADS WorldCat Liu Y. , Rice J.R., 2005 . Aseismic slip transients emerge spontaneously in three-dimensional rate and state modeling of subduction earthquake sequences , J. geophys. Res. , 110 , B08397 , doi:10.1029/2004JB003424 . Google Scholar OpenURL Placeholder Text WorldCat Liu Y. , Rice J.R., 2007 . Spontaneous and triggered aseismic deformation transients in a subduction fault model , J. geophys. Res.: Solid Earth , 112 , B09404 , doi: 10.1029/2007JB004930 . Google Scholar OpenURL Placeholder Text WorldCat Liu Y. , Rice J.R., 2009 . Slow slip predictions based on granite and gabbro friction data compared to GPS measurements in northern Cascadia . J. geophys. Res. , 114 , B09407 , doi:10.1029/2008JB006142 . Google Scholar Crossref Search ADS WorldCat Liu Y. , Rubin A.M., 2010 . Role of fault gouge dilatancy on aseismic deformation transients , J. geophys. Res. , 115 , B10414 , doi:10.1029/2010JB007522 . Google Scholar Crossref Search ADS WorldCat Lockner D.A. , Morrow C., Moore D., Hickman S., 2011 . Low strength of deep San Andreas fault gouge from SAFOD core , Nature , 472 , 82 – 85 .. 10.1038/nature09927 Google Scholar Crossref Search ADS PubMed WorldCat Lockner D.A. , Okubo P.G., 1983 . Measurements of frictional heating in granite , J. geophys. Res. , 88 , 4313 – 4320 .. 10.1029/JB088iB05p04313 Google Scholar Crossref Search ADS WorldCat Logan J.M. , Rauenzahn K.A., 1987 . Frictional dependence of gouge mixtures of quartz and montmorillonite on velocity, composition, and fabric , Tectonophysics , 144 , 87 – 108 .. 10.1016/0040-1951(87)90010-2 Google Scholar Crossref Search ADS WorldCat Loveless J.P. , Meade B.J., 2010 . Geodetic imaging of plate motions, slip rate, and partitioning of deformation in Japan , J. geophys. Res. , 115 , B02410 , doi:10.1029/2008JB006248 . Google Scholar Crossref Search ADS WorldCat Lowry A.R. , 2006 . Resonant slow fault slip in subduction zones forced by climatic load stress , Nature , 442 , 802 – 805 .. 10.1038/nature05055 Google Scholar Crossref Search ADS PubMed WorldCat Lupini J.F. , Skinner A.E., Vaughan P.R., 1981 . The drained residual strength of cohesive soils , Geotechnique , 31 , 181 – 213 .. 10.1680/geot.1981.31.2.181 Google Scholar Crossref Search ADS WorldCat Marone C. , 1998a . Laboratory-derived friction laws and their application to seismic faulting , Annu. Rev. Earth planet. Sci. , 26 , 643 – 696 .. 10.1146/annurev.earth.26.1.643 Google Scholar Crossref Search ADS WorldCat Marone C. , 1998b . The effect of loading rate on static friction and the rate of fault healing during the earthquake cycle , Nature , 391 , 69 – 72 .. 10.1038/34157 Google Scholar Crossref Search ADS WorldCat Mascle A. et al. ., 1988 . Proc. Ocean Drill. Program, Initial Rep. , Vol. 110 , Ocean Drilling Program , doi:10.2973/odp.proc.ir.110.1988 . Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC Matsuzawa T. , Hirose H., Shibazaki B., Obara K., 2010 . Modeling short- and long-term slow slip events in the seismic cycle of large subduction earthquakes , J. geophys. Res. , 115 , B12301 , doi:10.1029/2010B007566 . Google Scholar Crossref Search ADS WorldCat Maurer J. , Johnson K., 2014 . Fault coupling and the potential for earthquakes on the creeping section of the San Andreas Fault , J. geophys. Res.: Solid Earth , 119 , 4414 – 4428 .. 10.1002/2013JB010741 Google Scholar Crossref Search ADS WorldCat Mazzotti S. , Adams J., 2004 . Variability of near-term probability for the next great earthquake on the Cascadia subduction zone , Bull. seism Soc. Am. , 94 , 1954 – 1959 .. 10.1785/012004032 Google Scholar Crossref Search ADS WorldCat McCaffrey R. , 2009 . Time-dependent inversion of three-component continuous GPS for steady and transient sources in northern Cascadia , Geophys. Res. Lett. , 36 , L07304 , doi:10.1029/2008GL036784 . Google Scholar Crossref Search ADS WorldCat McCaffrey R. , Long M.D., Goldfinger C., Zwick P.C., Nabelek J.L., Johnson C.K., Smith C., 2000 . Rotation and plate locking at the southern Cascadia subduction zone , Geophys. Res. Lett. , 27 , 3117 – 3120 .. 10.1029/2000GL011768 Google Scholar Crossref Search ADS WorldCat McCann W.R. , Sykes L.R., 1984 . Subduction of aseismic ridges beneath the Caribbean plate: Implications for the tectonics and seismic potential of the northeastern Caribbean , J. geophys. Res. , 89 , 4493 – 4519 .. 10.1029/JB089iB06p04493 Google Scholar Crossref Search ADS WorldCat McGuire J.J. , Collins J.A., Davis E., Becker K., Heesemann M., 2018 . A lack of dynamic triggering of slow slip and tremor indicates that the shallow Cascadia megathrust offshore Vancouver Island is likely locked , Geophys. Res. Lett. , 45 ( 20 ), 11095 – 11103 .. 10.1029/2018GL079519 Google Scholar Crossref Search ADS WorldCat Meschede M. , Zweigel P., Kiefer E., 1999 . Subsidence and extension at a convergent plate margin: evidence for subduction erosion off Costa Rica , Terra Nova , 11 , 112 – 117 .. 10.1046/j.1365-3121.1999.00234.x Google Scholar Crossref Search ADS WorldCat Miller M.M. , Melbourne T., Johnson D.J., Sumner W.Q., 2002 . Periodic slow earthquakes from the Cascadia subduction zone , Science , 295 , 2423 , doi:10.1126/science.1071193 . Google Scholar Crossref Search ADS PubMed WorldCat Minster J.B. , Jordan T.H., 1978 . Present-day plate motions , J. geophys. Res. , 83 , 5331 – 5354 .. 10.1029/JB083iB11p05331 Google Scholar Crossref Search ADS WorldCat Miyazaki S. , Heki K., 2001 . Crustal velocity field of southwest Japan: subduction and arc-arc collision , J. geophys. Res. , 106 , 4305 – 4326 .. 10.1029/2000JB900312 Google Scholar Crossref Search ADS WorldCat Moore D.E. , 2014 . Comparative mineral chemistry and textures of SAFOD fault gouge and damage-zone rocks , J. Struct. Geol. , 68 , 82 – 96 .. 10.1016/j.jsg.2014.09.002 Google Scholar Crossref Search ADS WorldCat Moore G.F. , Bangs N.L., Taira A., Kuramoto S., Pangborn E., Tobin H.J., 2007 . Three-dimensional splay fault geometry and implications for tsunami generation , Science , 318 ( 5853 ), 1128 – 1131 .. 10.1126/science.1147195 Google Scholar Crossref Search ADS PubMed WorldCat Moore G.F. et al. ., 2001 . Proc. Ocean Drill. Program, Initial Rep. , Vol. 190 , Ocean Drilling Program , doi:10.2973/odp.proc.ir.190.2001 . Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC Morris J.D. et al. ., 2003 . Proc. Ocean Drill. Program, Initial Rep. , Vol. 205 , Ocean Drilling Program , doi:10.2973/odp.proc.ir.205.2003 . Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC Morrow C. , Radney B., Byerlee J., 1992 . Frictional strength and the effective pressure law of montmorillonite and illite clays , in Fault Mechanics and Transport Properties of Rocks , pp. 69 – 88 ., eds Evans B., Wong T.-F., Academic Press . Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC Morrow C.A. , Moore D.E., Lockner D.A., 2000 . The effect of mineral bond strength and adsorbed water on fault gouge frictional strength , Geophys. Res. Lett. , 27 , 815 – 818 .. 10.1029/1999GL008401 Google Scholar Crossref Search ADS WorldCat Nadeau R.M. , Johnson L.R., 1998 . Seismological studies at Parkfield VI: moment release rates and estimates of source parameters for small repeating earthquakes , Bull. seism. Soc. Am. , 88 , 790 – 814 . Google Scholar OpenURL Placeholder Text WorldCat Nason R.D. , 1969 . Preliminary instrumental measurements of fault creep slippage on the San Andreas fault, California , Earthq. Notes , 40 , 7 – 10 . Google Scholar OpenURL Placeholder Text WorldCat Niemeijer A.R. , Boulton C., Toy V.G., Townend J., Sutherland R., 2016 . Large-displacement, hydrothermal frictional properties of DFDP-1 fault rocks, Alpine Fault, New Zealand: Implications for deep rupture propagation , J. geophys. Res.: Solid Earth , 121 , 624 – 647 .. 10.1002/2015JB012593 Google Scholar Crossref Search ADS PubMed WorldCat Niemeijer A.R. , Spiers C.J., 2005 . Influence of phyllosilicates on fault strength in the brittle-ductile transition: insights from rock analogue experiments , Geol. Soc. Lond. Spec. Publ. , 245 , 303 – 327 .. 10.1144/GSL.SP.2005.245.01.15 Google Scholar Crossref Search ADS WorldCat Niemeijer A.R. , Spiers C.J., 2006 . Velocity dependence of strength and healing behavior in simulated phyllosilicate-bearing fault gouge , Tectonophysics , 427 , 231 – 253 .. 10.1016/j.tecto.2006.03.048 Google Scholar Crossref Search ADS WorldCat Norris R.J. , Cooper A.F., 2000 . Late Quaternary slip rates and slip partitioning on the Alpine Fault, New Zealand , J. Struct. Geol. , 23 , 507 – 520 .. 10.1016/S0191-8141(00)00122-X Google Scholar Crossref Search ADS WorldCat Obara K. , Ito Y., 2005 . Very low frequency earthquakes excited by the 2004 off the Kii Peninsula earthquakes: a dynamic deformation process in the large accretionary prism , Earth Planets Space , 57 , 321 – 326 .. 10.1186/BF03352570 Google Scholar Crossref Search ADS WorldCat Ohnaka M. , Kuwahara Y., Yamamoto K., 1987 . Constitutive relations between dynamic physical parameters near a tip of the propagating slip zone during stick-slip shear failure , Tectonophysics , 144 , 109 – 125 .. 10.1016/0040-1951(87)90011-4 Google Scholar Crossref Search ADS WorldCat Ohta Y. et al. ., 2012 . Geodetic constraints on afterslip characteristics following the March 9, 2011, Sanriku-oki earthquake, Japan , Geophys. Res. Lett. , 39 , L16304 , doi:10.1029/2012Gl052430 . Google Scholar Crossref Search ADS WorldCat Okubo P.G. , 1989 . Dynamic rupture modeling with laboratory-derived constitutive relations , J. geophys. Res. , 94 , 12 321 – 12 335 .. 10.1029/JB094iB09p12321 Google Scholar Crossref Search ADS WorldCat Outerbridge K.C. et al. ., 2010 . A tremor and slip event on the Cocos-Caribbean subduction zone as measured by a global positioning system (GPS) and seismic network on the Nicoya Peninsula, Costa Rica , J. geophys. Res. , 115 , B10408 , doi:10.1029/2009JB006845 . Google Scholar Crossref Search ADS WorldCat Park J.-O. , Tsuru T., Kodaira S., Cummins P.R., Kaneda Y., 2002 . Splay fault branching along the Nankai subduction zone , Science , 297 , 1157 – 1160 .. 10.1126/science.1074111 Google Scholar Crossref Search ADS PubMed WorldCat Peng Z. , Gomberg J., 2010 . An integrated perspective of the continuum between earthquakes and slow-slip phenomena , Nat. Geosci. , 3 , 599 – 607 .. 10.1038/ngeo940 Google Scholar Crossref Search ADS WorldCat Perfettini H. , Ampuero J.-P., 2008 . Dynamics of a velocity-strengthening fault region: Implications for slow earthquakes and postseismic slip , J. geophys. Res. , 113 , B09411 , doi:10.1029/2007JB005398 . Google Scholar Crossref Search ADS WorldCat Protti M. et al. ., 1995 . The March 25, 1990 (Mw = 7.0, ML = 6.8), earthquake at the entrance of the Nicoya Gulf, Costa Rica: Its prior activity, foreshocks, aftershocks, and triggered seismicity , J. geophys. Res. , 100 , 20 345 – 20 358 .. 10.1029/94JB03099 Google Scholar Crossref Search ADS WorldCat Rabinowitz H.S. , Savage H.M., Skarbek R.M., Ikari M.J., Carpenter B.M., Collettini C., 2018 . Frictional behavior of input sediments to the Hikurangi Trench, New Zealand , Geochem., Geophys., Geosyst. , 19 ( 9 ), 2973 – 2990 .. 10.1029/2018GC007633 Google Scholar Crossref Search ADS WorldCat Radiguet M. , Cotton F., Vergnolle M., Campillo M., Walpersdorf A., Cotte N., Kostoglodov V., 2012 , Slow slip events and strain accumulation in the Gurerrero gap, Mexico , J. geophys. Res. , 117 , B04305 , doi:10.1029/2011JB008801 . Google Scholar Crossref Search ADS WorldCat Ranero C.R. , von Huene R., 2000 . Subduction erosion along the Middle America convergent margin , Nature , 404 , 748 – 752 .. 10.1038/35008046 Google Scholar Crossref Search ADS PubMed WorldCat Reinen L.A. , Weeks J.D., 1993 . Determination of rock friction constitutive parameters using an iterative least squares method , J. geophys. Res. , 98 , 15937 – 15950 .. 10.1029/93JB00780 Google Scholar Crossref Search ADS WorldCat Remitti F. , Smith S.A.F., Mittempergher S., Gualtieri A.F., Di Toro G., 2015 . Frictional properties of fault zone gouges from the J-FAST drilling project (M w 9.0 Tohoku-Oki earthquake) , Geophys. Res. Lett. , 42 ( 8 ), 2691 – 2699 .. 10.1002/2015GL063507 Google Scholar Crossref Search ADS WorldCat Reyners M. , Bannister S., 2007 . Earthquakes triggered by slow slip at the plate interface in the Hikurangi subduction zone, New Zealand , Geophys. Res. Lett. , 34 , L14305 , doi:10.1029/2007GL030511 . Google Scholar Crossref Search ADS WorldCat Rice J.R. , 1993 . Spatio-temporal complexity of slip on a fault , J. geophys. Res. , 98 , 9885 – 9907 .. 10.1029/93JB00191 Google Scholar Crossref Search ADS WorldCat Rice J.R. , Ruina A.L., 1983 . Stability of steady frictional slipping , J. Appl. Mech. , 50 , 343 – 349 .. 10.1115/1.3167042 Google Scholar Crossref Search ADS WorldCat Rogers G. , Dragert H., 2003 . Episodic tremor and slip on the Cascadia subduction zone: the chatter of silent slip , Science , 300 , 1942 – 1943 .. 10.1126/science.1084783 Google Scholar Crossref Search ADS PubMed WorldCat Roy M. , Marone C., 1996 . Earthquake nucleation on model faults with rate- and state-dependent friction: effects of inertia , J. geophys. Res. , 101 , 13919 – 13932 .. 10.1029/96JB00529 Google Scholar Crossref Search ADS WorldCat Rubin A.M. , 2008 . Episodic slow slip events and rate-and-state friction , J. geophys. Res. , 113 , B11414 , doi:10.1029/2008JB005642 . Google Scholar Crossref Search ADS WorldCat Rubin A.M. , Ampuero J.-P., 2005 . Earthquake nucleation on (aging) rate and state faults , J. geophys. Res. , 110 , B11312 , doi:10.1029/2005JB003686 . Google Scholar Crossref Search ADS WorldCat Ruina A. , 1983 . Slip instability and state variable friction laws , J. geophys. Res. , 88 , 10 359 – 10 370 .. 10.1029/JB088iB12p10359 Google Scholar Crossref Search ADS WorldCat Ruiz S. et al. ., 2014 . Intense foreshocks and a slow slip event preceded the 2014 Iquique M w 8.1 earthquake , Science , 345 , 1165 – 1169 .. 10.1126/science.1256074 Google Scholar Crossref Search ADS PubMed WorldCat Ryder I. , Bürgmann R., 2008 . Spatial variations in slip deficit on the central San Andreas Fault , Geophys. J. Int. , 175 , 837 – 852 .. 10.1111/j.1365-246X.2008.03938.x Google Scholar Crossref Search ADS WorldCat Sacks I.S. , Linde A.T., Snoke J.A., Suyehiro S., 1981 . A slow earthquake sequence following the Izu-Oshima earthquake of 1978 , in Earthquake Prediction: An International Review , Vol. 4 , pp. 617 – 628 ., eds Simpson D.W., Richards P.G., American Geophysical Union . Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC Sacks I.S. , Suyehiro S., Linde A.T., Snoke J.A., 1978 . Slow earthquakes and stress redistribution , Nature , 275 , 599 – 602 .. 10.1038/275599a0 Google Scholar Crossref Search ADS WorldCat Saffer D.M. , Marone C., 2003 . Comparison of smectite- and illite-rich gouge frictional properties: Application to the updip limit of the seismogenic zone along subduction megathrusts , Earth planet. Sci. Lett. , 215 , 219 – 235 .. 10.1016/S0012-821X(03)00424-2 Google Scholar Crossref Search ADS WorldCat Saffer D.M. , Tobin H.J., 2011 . Hydrogeology and mechanics of subduction zone forearcs: Fluid flow and pore pressure , Annu. Rev. Earth. planet. Sci. , 39 , 157 – 186 .. 10.1146/annurev-earth-040610-133408 Google Scholar Crossref Search ADS WorldCat Saffer D.M. , Wallace L.M., 2015 . The frictional, hydrologic, metamorphic and thermal habitat of shallow slow earthquakes , Nat. Geosci. , 8 , 594 – 600 .. 10.1038/ngeo2490 Google Scholar Crossref Search ADS WorldCat Saffer D.M. , Wallace L.M., Petronotis K., the Expedition 375 Scientists , 2018 . Expedition 375 Preliminary Report: Hikurangi Subduction Margin Coring and Observatories , International Ocean Discovery Program , doi:1014379.iodp.pr.375.2018 . Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC Sagiya T. , Thatcher W., 1999 . Coseismic slip resolution along a plate boundary megathrust: the Nankai Trough, southwest Japan , J. geophys. Res. , 104 , 1111 – 1129 .. 10.1029/98JB02644 Google Scholar Crossref Search ADS WorldCat Sakaguchi A. et al. ., 2011 . Seismic slip propagation to the updip end of plate boundary subduction interface faults: Vitrinite reflectance geothermometry on Integrated Ocean Drilling Program NanTroSEIZE cores , Geology , 39 , 395 – 398 .. 10.1130/G31642.1 Google Scholar Crossref Search ADS WorldCat Satake K. , Shimazaki K., Tsuji Y., Ueda K., 1996 . Time and size of a giant earthquake in Cascadia inferred from Japanese tsunami records of January 1700 , Nature , 379 , 246249 . 10.1038/379246a0 Google Scholar Crossref Search ADS WorldCat Savage H.M. , Marone C., 2007 . Effects of shear velocity oscillations on stick-slip behavior in laboratory experiments , J. geophys. Res. , 112 , B02301 , doi:10.1029/2005JB004238 . Google Scholar OpenURL Placeholder Text WorldCat Schleicher A.M. , van der Pluijm B.A., Warr L.N., 2012 . Chlorite-smectite clay minerals and fault behavior: New evidence from the San Andreas Fault Observatory at Depth (SAFOD) core , Lithosphere , 4 , 209 – 220 .. 10.1130/L158.1 Google Scholar Crossref Search ADS WorldCat Scholz C.H. , 1998 . Earthquakes and friction laws , Nature , 391 , 37 – 42 .. 10.1038/34097 Google Scholar Crossref Search ADS WorldCat Scholz C.H. , 2002 . The Mechanics of Earthquakes and Faulting , 2nd edn, Cambridge Press . Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC Schwartz S.Y. , Rokosky J.M., 2007 . Slow slip events and seismic tremor at circum-Pacific subduction zones , Rev. Geophys. , 45 , RG3004 , doi:10.1029/2006RG000208 . Google Scholar Crossref Search ADS WorldCat Scuderi M.M. , Marone C., Tinti E., Di Stefano G., Collettini C., 2016 . Precursory changes in seismic velocity for the spectrum of earthquake failure modes , Nat. Geosci. , 9 , 695 – 700 .. 10.1038/ngeo2775 Google Scholar Crossref Search ADS PubMed WorldCat Segall P. , Rubin A.M., Bradley A.M., Rice J.R., 2010 . Dilatant strengthening as a mechanism for slow slip events . J. geophys. Res. , 115 , B12305 , doi:10.1029/2010JB007449 . Google Scholar Crossref Search ADS WorldCat Shibazaki B. , Iio Y., 2003 . On the physical mechanism of silent slip events along the deeper part of the seismogenic zone , Geophys. Res. Lett. , 30 , 1489 , doi:10.1029/2003GL017047 . Google Scholar Crossref Search ADS WorldCat Shibazaki B. , Shimamoto T., 2007 . Modelling of short-interval silent slip events in deeper subduction interfaces considering the frictional properties at the unstable-stable transition regime , Geophys. J. Int. , 171 , 191 – 205 .. 10.1111/j.1365-246X.2007.03434.x Google Scholar Crossref Search ADS WorldCat Shimamoto T. , 1986 . Transition between frictional slip and ductile flow for halite shear zones at room temperature , Science , 231 , 711 – 714 .. 10.1126/science.231.4739.711 Google Scholar Crossref Search ADS PubMed WorldCat Shimamoto T. , Logan J.M., 1981 . Effects of simulated clay gouges on the sliding behavior of Tennessee sandstone , Tectonophys , 75 , 243 – 255 .. 10.1016/0040-1951(81)90276-6 Google Scholar Crossref Search ADS WorldCat Shipley T.H. et al. ., 1995 . Proc. Ocean Drill. Program, Initial Rep. , Vol. 156 , Ocean Drilling Program , doi:10.2973/odp.proc.ir.156.1995 . Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC Skarbek R.M. , Rempel A.W., Schmidt D.A., 2012 . Geologic heterogeneity can produce aseismic slip transients , Geophys. Res. Let. , 39 , L21306 , doi:10.1029/2012GL053762 . Google Scholar Crossref Search ADS WorldCat Sleep N.H. , 2005 . Physical basis of evolution laws for rate and state friction , Geochem. Geophys. Geosyst. , 6 , Q11008 , doi:10.1029/2005GC000991 . Google Scholar Crossref Search ADS WorldCat Sleep N.H. , 2006 . Real contacts and evolution laws for rate and state friction , Geochem. Geophys. Geosyst. , 7 , Q08012 , doi:10.1029/2005GC001187 . Google Scholar OpenURL Placeholder Text WorldCat Sleep N.H. , 2012 . Microscopic elasticity and rate and state friction evolution laws , Geochem. Geophys. Geosyst. , 13 , Q12002 , doi:10.1029/2012GC004393 . Google Scholar Crossref Search ADS WorldCat Smith S.A.F. , Faulkner D.R., 2010 . Laboratory measurements of the frictional properties of the Zuccale low-angle normal fault, Elba Island, Italy , J. geophys. Res. , 115 , B02407 , doi:10.1029/2008JB006274 . Google Scholar Crossref Search ADS WorldCat Solomon E.A. , Kastner M., Wheat C.G., Jannasch H., Robertson G., Davis E.E., Morris J.D., 2009 . Long-term hydrogeochemical records in the oceanic basement and forearc prism at the Costa Rica subduction zone , Earth planet. Sci. Lett. , 282 , 240 – 251 .. 10.1016/j.epsl.2009.03.022 Google Scholar Crossref Search ADS WorldCat Solum J.G. , Hickman S.H., Lockner D.A., Moore D.E., van der Pluijm B.A., Schleicher A.M., Evans J.P., 2006 . Mineralogical characterization of protolith and fault rocks from the SAFOD main hole , Geophys. Res. Let. , 33 , L21314 , doi:10.1029/2006GL027285 . Google Scholar Crossref Search ADS WorldCat Spinelli G.A. , Underwood M.B., 2004 . Character of sediments entering the Costa Rica subduction zone: Implications for partitioning of water along the plate interface , Island Arc , 13 , 432 – 451 .. 10.1111/j.1440-1738.2004.00436.x Google Scholar Crossref Search ADS WorldCat Stein S. , Engeln J.F., Wiens D.A., Fujita K., Speed R.C., 1982 . Subduction seismicity and tectonics in the Lesser Antilles arc , J. geophys. Res. , 87 , 8642 – 8664 .. 10.1029/JB087iB10p08642 Google Scholar Crossref Search ADS WorldCat Steurer J.F. , Underwood M.B., 2005 . Clay mineralogy of mudstones from the Nankai Trough reference Sites 1173 and 1177 and frontal accretionary prism Site 1174 , in Proceedings of the Ocean Drilling Program Scientific Results, 190/196 , pp. 1 – 25 ., eds Mikada H., Moore G.F., Taira A., Becker K., Moore J.C., Klaus A., College Station, TX , Ocean Drilling Program . Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC Sugioka H. et al. ., 2012 . Tsunamigenic potential of the shallow subduction plate boundary inferred from slow seismic slip , Nat. Geosci. , 5 , 414 – 418 .. 10.1038/ngeo1466 Google Scholar Crossref Search ADS WorldCat Sun T. , Davis E.E., Wang K., Jiang Y., 2017b . Trench-breaking afterslip following deeper coseismic slip of the 2012 M w 7.6 Costa Rica earthquake constrained by near-trench pressure and land-based geodetic observations , Earth planet. Sci. Lett. , 479 , 263 – 272 .. 10.1016/j.epsl.2017.09.021 Google Scholar Crossref Search ADS WorldCat Sun T. , Wang K., Fujiwara T., Kodaira S., He J., 2017a . Large fault slip peaking at the trench in the 2011 Tohoku-oki earthquake , Nat. Commn. , 8 , doi:10.1038/ncomms14044 . Google Scholar OpenURL Placeholder Text WorldCat Sutherland R. , Berryman K., Norris R., 2006 . Quaternary slip rate and geomorphology of the Alpine fault: Implications for kinematics and seismic hazard in southwest New Zealand , Bull. geol. Soc. Am. , 118 , 464 – 474 .. 10.1130/B25627.1 Google Scholar Crossref Search ADS WorldCat Sutherland R. , Norris R.J., 1995 . Late Quaternary displacement rate, paleoseismicity, and geomorphic evolution of the Alpine Fault: Evidence from the Hokuri Creek, South Westland, New Zealand , N. Zeal. J. Geol. Geophys. , 38 , 419 – 430 .. 10.1080/00288306.1995.9514669 Google Scholar Crossref Search ADS WorldCat Sutherland R. et al. ., 2007 . Do great earthquakes occur on the Alpine Fault in central South Island, New Zealand? in A Continental Plate Boundary: Tectonics at South Island, New Zealand , Geophys. Mon. Ser. , Vol. 175 , pp. 235 – 251 ., eds Okaya D., Stern T., Davey F., AGU , doi:10.1029/175GM12 . Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC Sutherland R. et al. ., 2012 . Drilling reveals fluid control on architecture and rupture of the Alpine Fault, New Zealand , Geology , 40 , 1143 – 1146 .. 10.1130/G33614.1 Google Scholar Crossref Search ADS WorldCat Szeliga W. , Melbourne T., Santillan M., Miller M., 2008 . GPS constraints on 34 slow slip events within the Cascadia subduction zone , J. geophys. Res. , 113 , B04404 , doi:10.1029/2007JB004948 . Google Scholar Crossref Search ADS WorldCat Środón J. , 1999 . Nature of mixed-layer clays and mechanisms of their formation and alteration , Annu. Rev. Earth. planet. Sci. , 27 , 19 – 53 .. 10.1146/annurev.earth.27.1.19 Google Scholar Crossref Search ADS WorldCat Tanioka Y. , Satake K., 2001 . Detailed coseismic slip distribution of the 1944 Tonankai earthquake estimated from tsunami waveforms , Geophys. Res. Lett. , 28 , 1075 – 1078 .. 10.1029/2000GL012284 Google Scholar Crossref Search ADS WorldCat Taylor B. , Goodliffe A., Martinez F., Hey R., 1995 . Continental rifting and initial sea-floor spreading in the Woodlark basin , Nature , 374 , 534 – 537 .. 10.1038/374534a0 Google Scholar Crossref Search ADS WorldCat Taylor B. , Huchon P., 2002 . Active continental extension in the western Woodlark Basin: a synthesis of Leg 180 results , in Proc. ODP, Sci. Results , vol. 180 , 1 – 36 ., eds Huchon P., Taylor B., Klaus A., College Station, TX , Ocean Drilling Program , doi:10.2973/odp.proc.sr.180.150.2002 . Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC Taylor B. et al. ., 2000 . Proc. Ocean Drill. Program, Initial Rep. , Vol. 190 , Ocean Drilling Program , doi:10.2973/odp.proc.ir.180.2000 . Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC Tembe S. , Lockner D.A., Wong T.-F., 2010 . Effect of clay content and mineralogy on frictional sliding behavior of simulated gouges: Binary and ternary mixtures of quartz, illite, and montmorillonite , J. geophys. Res.: Solid Earth , 115 , doi:10.1029/2009JB006383 . Google Scholar OpenURL Placeholder Text WorldCat Tesei T. , Collettini C., Carpenter B.M., Viti C., Marone C., 2012 . Frictional strength and healing behavior of phyllosilicate-rich faults , J. geophys. Res. , 117 , B09402 , doi:10.1029/2012JB009204 . Google Scholar Crossref Search ADS WorldCat Titus S.J. , DeMets C., Tikoff B., 2005 . New slip rate estimates for the creeping segment of the San Andreas fault, California , Geology , 33 , 205 – 208 .. 10.1130/G21107.1 Google Scholar Crossref Search ADS WorldCat Titus S.J. , DeMets C., Tikoff B., 2006 . Thirty-five-year creep rates for the creeping section of the San Andreas Fault and the effects of the 2004 Parkfield earthquake: constraints from alignment arrays, continuous global positioning system, and creepmeters , Bull. seism. Soc. Am. , 96 , S250 – S268 ., doi:10.1785/0120050811 . Google Scholar Crossref Search ADS WorldCat Townend J. , Sutherland R., Toy V.G., Eccles J.D., Boulton C., Cox S.C., McNamara D., 2013 . Late-interseismic state of a continental plate-bounding fault: petrophysical results from DFDP-1 wireline logging and core analysis, Alpine Fault, New Zealand , Geochem. Geophys. Geosyst. , 14 ( 9 ), 3801 – 3820 .. 10.1002/ggge.20236 Google Scholar Crossref Search ADS WorldCat Toy V.G. et al. ., 2015 . Fault rock lithologies and architecture of the central Alpine fault, New Zealand, revealed by DFDP-1 drilling , Lithosphere , L395-1, doi:10.1130/L395.1 . Google Scholar OpenURL Placeholder Text WorldCat Tregoning P. et al. ., 1998 . Estimation of current plate motions in Papua New Guinea from Global Positioning System observations , J. geophys. Res. , 103 , 12181 – 12203 .. 10.1029/97JB03676 Google Scholar Crossref Search ADS WorldCat Tullis T.E. , 1988 . Rock friction constitutive behavior from laboratory experiments and its implications for an earthquake prediction field monitoring program , Pure Appl. Geophys. , 126 , 555 – 588 . Google Scholar Crossref Search ADS WorldCat Underwood M.B. , 2007 . Sediment inputs to subduction zone: Why lithostratigraphy and clay mineralogy matter , in The Seismogenic Zone of Subduction Thrust Faults , pp. 42 – 85 ., eds Dixon T.H., Moore J.C., Columbia University Press . Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC Vallée M. et al. ., 2013 . Intense interface seismicity triggered by a shallow slow slip event in the Central Ecuador subduction zone , J. geophys. Res.: Solid Earth , 118 , 2965 – 2981 .. 10.1002/jgrb.50216 Google Scholar Crossref Search ADS WorldCat Vannucchi P. , Ranero C.R., Galeotti S., Straub S.M., Scholl D.W., McDougall-Ried K., 2003 . Fast rates of subduction erosion along the Costa Rica Pacific margin: implications for nonsteady rates of crustal recycling at subduction zones , J. geophys. Res. , 108 , 2511 , doi:10.1029/2002Jb002207 . Google Scholar Crossref Search ADS WorldCat Vergnolle M. , Walpersdorf A., Kostoglodov V., Tregoning P., Santiago J.A., Cotte N., Franco S.I., 2010 . Slow slip events in Mexico revised from the processing of 11 year GPS observations , J. geophys. Res. , 115 , B08403 , doi:10.1029/2009JB006852 . Google Scholar Crossref Search ADS WorldCat Vogt C. , Lauterjung J., Fischer R., 2002 . Investigation of the clay fraction (< 2 µm) of the Clay Minerals Society reference clays , Clays Clay Miner. , 50 ( 3 ), 388 – 400 . Google Scholar Crossref Search ADS WorldCat von Huene R. , Ranero C.R., Weinrebe W., Hinz K., 2000 . Quaternary convergent margin tectonics of Costa Rica, segmentation of the Cocos Plate, and Central American volcanism , Tectonics , 19 , 314 – 334 .. 10.1029/1999TC001143 Google Scholar Crossref Search ADS WorldCat Wallace L.M. , Beavan J., 2010 . Diverse slow slip behavior at the Hikurangi subduction margin, New Zealand , J. geophys. Res. , 115 , B12402 , doi:10.1029/2010JB007717 . Google Scholar Crossref Search ADS WorldCat Wallace L.M. , Beavan J., Bannister S., Williams C., 2012 . Simulataneous long-term and short-term slow slip events at the Hikurangi subduction margin, New Zealand: implications for processes that control slow slip event occurrence, duration, and migration , J. geophys. Res. , 117 , B11402 , doi:10.1029/2012JB009489 . Google Scholar OpenURL Placeholder Text WorldCat Wallace L.M. , Beavan J., McCaffrey R., Berryman K., Denys P., 2007 . Balancing the plate motion budget in the South Island, New Zealand using GPS, geological and seismological data , Geophys. J. Int. , 168 , 332 – 352 .. 10.1111/j.1365-246X.2006.03183.x Google Scholar Crossref Search ADS WorldCat Wallace L.M. , Beavan J., McCaffrey R., Darby D., 2004 . Subduction zone coupling and tectonic block rotations in the North Island, New Zealand , J. geophys. Res. , 109 , B12406 , doi:10.1029/2004JB003241 . Google Scholar Crossref Search ADS WorldCat Wallace L.M. , Hreinsdóttir S., Ellis S., Hamling I., D'Anastasio E., Denys P., 2018 . Triggered slow slip and afterslip on the southern Hikurangi subduction zone following the Kaikoura earthquake , Geophys. Res. Lett. , 45 ( 10 ), 4710 – 4718 .. 10.1002/2018GL077385 Google Scholar Crossref Search ADS WorldCat Wallace L.M. , Kaneko Y., Hreinsdóttir S., Hamling I., Peng Z., Bartlow N., D'Anastasio E., Fry B., 2017 . Large-scale dynamic triggering of shallow slow slip enhanced by overlying sedimentary wedge , Nat. Geosci. , 10 , 765 – 770 .. 10.1038/ngeo3021 Google Scholar Crossref Search ADS WorldCat Wallace L.M. , Reyners M., Cochran U., Bannister S., 2009 . Characterizing the seismogenic zone of a major plate boundary subduction thrust: Hikurangi Margin, New Zealand , Geochem. Geophys. Geosyst. , 10 , Q10006 , doi:10.1029/2009GC002610 . Google Scholar OpenURL Placeholder Text WorldCat Wallace L.M. , Webb S.C., Ito Y., Mochizuki K., Hino R., Henrys S., Schwartz S.Y., Sheehan A.F., 2016 . Slow slip near the trench at the Hikurangi subduction zone, New Zealand , Science , 352 , 701 – 704 .. 10.1126/science.aaf2349 Google Scholar Crossref Search ADS PubMed WorldCat Wallace L.M. et al. ., 2014 . Continental breakup and UHP rock exhumation in action: GPS results from the Woodlark Rift, Papua New Guinea , Geochem., Geophys. Geosyst. , 15 ( 11 ), 4267 – 4290 .. 10.1002/2014GC005458 Google Scholar Crossref Search ADS WorldCat Walter J.I. , Schwartz S.Y., Protti M., Gonzalez V., 2011 . Persistent tremor within the northern Costa Rica seismogenic zone , Geophys. Res. Lett. , 38 , L01307 , doi:10.1029/2010GL045586 . Google Scholar Crossref Search ADS WorldCat Walter J.I. , Schwartz S.Y., Protti M., Gonzalez V., 2013 . The synchronous occurrence of shallow tremor and very low frequency earthquakes offshore of the Nicoya Peninsula, Costa Rica , Geophys. Res. Lett. , 40 , 1517 – 1522 .. 10.1002/grl.50213 Google Scholar Crossref Search ADS WorldCat Wang K. , Tréhu A.M., 2016 . Invited review paper: some outstanding issues in the study of great megathrust earthquakes – the Cascadia example , J. Geodyn. , 98 , 1 – 18 .. 10.1016/j.jog.2016.03.010 Google Scholar Crossref Search ADS WorldCat Wang K. , Wells R., Mazzotti S., Hyndman R.D., 2003 . A revised dislocation model of interseismic deformation of the Cascadia subduction zone , J. geophys. Res. , 108 , 2026 , doi:10.1029/2001JB001227 . Google Scholar OpenURL Placeholder Text WorldCat Wech A.G. , Boese C.M., Stern T.A., Townend J., 2012 . Tectonic tremor and deep slow slip on the Alpine Fault , Geophys. Res. Lett. , 39 , L10303 , doi:10.1029/2012GL051751 . Google Scholar Crossref Search ADS WorldCat Wech A.G. , Creager K.C., 2011 . A continuum of stress, strength and slip in the Cascadia subduction zone , Nat. Geosci. , 4 , 624 – 628 .. 10.1038/ngeo1215 Google Scholar Crossref Search ADS WorldCat Westbrook G.K. , Ladd J.W., Buhl P., Bangs N., Tiley G.J., 1988 . Cross section of an accretionary wedge: Barbados Ridge complex , Geology , 16 , 631 – 635 .. 10.1130/0091-7613(1988)016%3c0631:CSOAAW%3e2.3.CO;2 Google Scholar Crossref Search ADS WorldCat Westbrook G.K. , Smith M.J., 1983 . Long decollements and mud volcanoes: Evidence from the Barbados Ridge Complex for the role of high pore-fluid pressure in the development of an accretionary complex , Geology , 11 , 279 – 283 .. 10.1130/0091-7613(1983)11%3c279:LDAMVE%3e2.0.CO;2 Google Scholar Crossref Search ADS WorldCat Westbrook G.K. et al. ., 1994 . Proc. Ocean Drill. Program, Initial Rep. , Vol. 146 , Ocean Drilling Program , doi:10.2973/odp.proc.ir.146-1.1994 . Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC Witter R.C. , Kelsey H.M., Hemphill-Haley E., 2003 . Great Cascadia earthquakes and tsunamis of the past 6700 years, Coquille River estuary, southern coastal Oregon , Bull. geol. Soc. Am. , 115 , 1289 – 1306 .. 10.1130/B25189.1 Google Scholar Crossref Search ADS WorldCat Yue H. , Lay T., Schwartz S.Y., Rivera L., Protti M., Dixon T.H., Owen S., Newman A.V., 2013 . The 5 September 2012 Nicoya, Costa Rica M w 7.6 earthquake rupture process from joint inversion of high-rate GPS, strong-motion, and teleseismic P wave data and its relationship to adjacent plate boundary interface properties , J. geophys. Res.: Solid Earth , 118 , 5453 – 5466 .. 10.1002/jgrb.50379 Google Scholar Crossref Search ADS WorldCat Zoback M. , Hickman S., Ellsworth W., SAFOD Science Team , 2011 . Scientific drilling into the San Andreas fault zone – an overview of SAFOD's first five years , Sci. Drill. , 11 , 14 – 28 .. 10.5194/sd-11-14-2011 Google Scholar Crossref Search ADS WorldCat © The Author(s) 2019. Published by Oxford University Press on behalf of The Royal Astronomical Society. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited. © The Author(s) 2019. Published by Oxford University Press on behalf of The Royal Astronomical Society.
TI - Laboratory slow slip events in natural geological materials
JF - Geophysical Journal International
DO - 10.1093/gji/ggz143
DA - 2019-07-01
UR - https://www.deepdyve.com/lp/oxford-university-press/laboratory-slow-slip-events-in-natural-geological-materials-oS7bUqhpci
SP - 354
EP - 387
VL - 218
IS - 1
DP - DeepDyve
ER -