TY - JOUR
AU - Sammis, Charles, G.
AB - Abstract The central section of the San Andreas Fault (SAF) displays a range of seismic phenomena including normal earthquakes, low-frequency earthquakes (LFE), repeating microearthquakes (REQ) and aseismic creep. Although many lines of evidence suggest that LFEs are tied to the presence of fluids, their geological setting is still poorly understood. Here, we map the seismic velocity structures associated with LFEs beneath the central SAF using surface wave tomography from ambient seismic noise to provide constraints on the physical conditions that control LFE occurrence. Fault perpendicular sections show that the SAF, as revealed by lateral contrasts in relative velocities, is contiguous to depths of 50 km and appears to be relatively localized at depths between about 15 and 30 km. This is consistent with the hypothesis that LFEs are shear-slip events on a deep extension of the SAF. We find that along strike variations in seismic behaviour correspond to changes in the seismic structure, which support proposed connections between fluids and seismicity. LFEs and REQs occur within low-velocity structures, suggesting that the presence of fluids, weaker minerals, or hydrous phase minerals may play an important role in the generation of slow-slip phenomena. Seismicity and tectonics, Continental tectonics: strike-slip and transform 1 INTRODUCTION Low-frequency earthquakes (LFEs) in both subduction zones and beneath the San Andreas Fault (SAF) are sensitive to tidal stress, which has been attributed to conditions of near-lithostatic fluid pressures (Shelly et al. 2006; Ozacar & Zandt 2009; Thomas et al. 2009; Fagereng & Diener 2011; Ide & Tanaka 2014). LFEs beneath the SAF are inferred to represent shear slip (Shelly et al. 2009), although no geodetic signal has been observed (Smith & Gomberg 2009). Petrologic studies suggest that LFEs occur at pressures and temperatures that may coincide with metamorphic dehydration reactions that release fluids (Fagereng & Diener 2011), which is consistent with both observed high Vp/Vs ratios (Ozacar & Zandt 2009) and low electrical resistivity measurements (Becken et al. 2011). Frictional mechanisms have been proposed that invoke stick-slip instabilities where the rupture speeds are either slowed by the interaction with fluids, which causes slip-weakening behaviour (Ikari et al. 2013) or are impeded by dilatant strengthening (Shelly 2015). Correlations between seismic waves speeds and LFE locations can provide insight into (and constraints on) the physical mechanics that produce LFEs and repeating microearthquakes (REQs). Studies have suggested that LFE locations may be controlled by pressure and temperature conditions (Fagereng & Diener 2011), anisotropic fabric (Audet 2015) or contrasts in material strength at the Moho (Chen et al. 2012). For the SAF, LFE and REQ locations along strike also coincide with segments of surface creep which have been tentatively ascribed to the presence of talc, a mineral resulting from hydrothermal fluids interacting with serpentinite (Moore & Rymer 2007). This has been taken as further evidence that LFEs may be closely related to a fluid supply in the central rapidly creeping section (Becken et al. 2011; Zeng et al. 2016). The presence of fluids may also explain REQs’ sensitivity to triggering by neighbouring earthquakes (Chen et al. 2013). Along strike variation in seismic phenomena in the central portion of the SAF defines three subsections (Fig. 1). The creeping section to the north of SAFOD (Titus et al. 2006) hosts standard earthquakes and REQs (Turner et al. 2015). To the south, the Parkfield section is characterized by decreasing surface creep which transitions to the locked section near Cholame (Titus et al. 2006). The Parkfield section also hosts standard earthquakes, REQs and LFEs (Shelly & Hardebeck 2010), whereas the locked segment south of Cholame supports LFEs, but is characterized by a significant reduction in normal seismicity and lack of REQs (Fig. 1). REQs occur within the fault zone core at depths between 2 and 8 km and are observed to be driven by aseismic creep within the seismogenic zone and are not observed in the locked portion of the SAF to the south of Parkfield (Nadeau & McEvilly 2004; Turner et al. 2015). LFEs are observed along the southern 100 km of the creeping section and extend 50 km south of Parkfield beneath the locked portion of the SAF. The shallowest LFEs occur about 15 km beneath SAFOD and progressively deepen to the north and south, reaching depths of about 30 km (Shelly & Hardebeck 2010). Figure 1. Open in new tabDownload slide Location map: map of the central portion of the SAF showing: local stations (black triangles), NCEDC Earthquakes (blue dots), REQs (green dots, Turner et al. 2015) and LFEs (large magenta dots, Shelly & Hardebeck 2010). Fault traces are shown in red lines. Reference locations are marked with white stars. The SAF's segments are labeled according to its surface creep rates (Titus et al. 2006) as the creeping segment, the Parkfield segment (PkdSeg) and the Locked Segment. The top right inset shows location of map within California in the red box and the stations used. Figure 1. Open in new tabDownload slide Location map: map of the central portion of the SAF showing: local stations (black triangles), NCEDC Earthquakes (blue dots), REQs (green dots, Turner et al. 2015) and LFEs (large magenta dots, Shelly & Hardebeck 2010). Fault traces are shown in red lines. Reference locations are marked with white stars. The SAF's segments are labeled according to its surface creep rates (Titus et al. 2006) as the creeping segment, the Parkfield segment (PkdSeg) and the Locked Segment. The top right inset shows location of map within California in the red box and the stations used. 2 METHODS We map the regional shear wave velocity structure for central California using surface wave ambient noise tomography (ANT, see Supporting Information) and investigate its spatial relation to seismicity on and near the SAF. Although other local velocity models exist (Zeng et al. 2016) and ANT investigations of seismic structure have described the broad trends of the western United States (Shapiro et al. 2005; Lin et al. 2008), the aim of this study is to present a higher resolution model using the ANT for the central SAF region by supplementing the regional data set published in Porritt et al. (2011), with local network coverage. We derive a high-resolution model of shear velocities in the crust and uppermost mantle from data recorded between 2003 and 2015 by the USArray, the Parkfield Hi-Resolution Seismic Network (HRSN) and other regional networks. The ANT method approximates the Green's function between stations by cross-correlating seismic noise between station pairs. Rayleigh wave group and phase velocity maps are constructed from these cross-correlations (Figs S2a and b, Supporting Information) and then inverted to build a 3-D model of shear wave velocity shown in SAF perpendicular (Fig. 2) and SAF parallel (Fig. 3) sections. The key advantage of this method is that it provides resolution that is independent of local seismicity since we measure empirical Green's functions between every available station pair (Figs S1–S6, Supporting Information). We are therefore able to resolve structures along strike of the SAF despite sparse station coverage in some areas. However, the depth resolution of the ANT model is based on Rayleigh waves’ period dependent sensitivity kernels and this causes the resolution to decrease with depth as the sensitivity kernel of each period broadens with increasing period (Fig. S5, Supporting Information). To assess our model's ability to resolve low-velocity structures in the lower and middle crust, we provide tests and results in Fig. S5 in the Supporting Information. Figure 2. Open in new tabDownload slide Shear velocity perpendicular to strike: the top right map shows the locations of the cross-sections shown in Figs 2 and 3. Panels (a)–(d) show cross-sections taken from across strike. Surface trace of the SAF is marked by the red inverted triangles. NCEDC Earthquakes (blue dots), REQs (green dots, Turner et al. 2015) and LFEs (large magenta dots, Shelly & Hardebeck 2010), within 5 km of each section are projected onto the cross-section's plane. Figure 2. Open in new tabDownload slide Shear velocity perpendicular to strike: the top right map shows the locations of the cross-sections shown in Figs 2 and 3. Panels (a)–(d) show cross-sections taken from across strike. Surface trace of the SAF is marked by the red inverted triangles. NCEDC Earthquakes (blue dots), REQs (green dots, Turner et al. 2015) and LFEs (large magenta dots, Shelly & Hardebeck 2010), within 5 km of each section are projected onto the cross-section's plane. Figure 3. Open in new tabDownload slide Shear velocity parallel to strike: shear velocity from cross-sections (e) and (f) along strike of the SAF, as shown in the location map inset of Fig. 2. Reference locations (white stars), REQs (green dots, Turner et al. 2015), LFEs (large magenta dots, Shelly & Hardebeck 2010), within 5 km of each section are projected onto the cross-section's plane. Figure 3. Open in new tabDownload slide Shear velocity parallel to strike: shear velocity from cross-sections (e) and (f) along strike of the SAF, as shown in the location map inset of Fig. 2. Reference locations (white stars), REQs (green dots, Turner et al. 2015), LFEs (large magenta dots, Shelly & Hardebeck 2010), within 5 km of each section are projected onto the cross-section's plane. 3 SECTIONS ACROSS THE SAF In Fig. 2, the SAF is localized to depths of 50 km. No major velocity structure crosses the SAF in any of the sections south of the San Andreas Observatory station (Fig. 2a). In the shallow crust, the strongest lateral velocity contrast is delineated by repeating events within the fault zone (Fig. 2b). This section also displays the strongest lateral velocity contrast in the lower crust. These clear lateral discontinuities support the hypothesis that a relatively narrow shear zone is maintained at depth across the SAF. Although the resolution of ANT decreases with depth due to the properties of the Rayleigh wave sensitivity kernels (see Fig. S5, Supporting Information), this velocity contrast between the east and west side of the fault observed in the upper and lower crust, persists along strike south of Cholame. These observations support models based on mantle xenolith fabrics from the deep SAF (Titus et al. 2007) as well as lithosphere–asthenosphere steps (Ford et al. 2014) suggesting the SAF extends through the lithosphere. The lateral discontinuity we observe in the lower crust (Figs 2b and d) coincides with the locations of the LFEs that occur on the interface, supporting the idea that the LFEs represent shear slip on the relatively localized fault root of the deep extent of the SAF (Shelly et al. 2009). LFEs plot on a downward projection of the crustal fault and they occur at the transition between a low-velocity structure (about 3.25 km s−1) belonging to the Pacific plate and a higher velocity structure of the North American plate (about 3.75 km s−1). In the cross-section near Cholame (Fig. 2d), the LFEs occur predominantly on the SAF fault interface on the edge of the lower relative velocity structure of the Pacific plate. Near SAFOD (Fig. 2c), although the LFEs are still in the downward projection of the fault, they are located in a broader region of low-velocity material. In contrast, no LFEs are observed to the north (Fig. 2a), as well as a more homogenous lower crust and uppermost mantle across the plate boundary beneath the SAF. This suggests that the LFEs occur within the fault zone in a relatively narrow interface of contrasting material types and are associated with a low-velocity structure. 4 SECTIONS PARALLEL TO THE SAF The low-velocity structure observed near SAFOD in the lower crust and uppermost mantle, highlighted by the dip in the 4.25 km s−1 contour line 35 km deep, beneath the SAF trace (Fig. 2c), is consistent with the presence of fluids, weaker mineral phases and a deeper fluid source (Becken et al. 2011). This interpretation is consistent with geological evidence of hydrothermal fluids interacting with serpentinite at depth (Moore & Rymer 2007). The low-velocity structure in the lower crust and uppermost mantle, extends ∼50 km north of SAFOD along strike of the SAF (Fig. 3f), coinciding with the SAF's section's highest creep rate (Titus et al.2006) and suggests a connection between lower crustal structure and surface creep. Becken et al. (2011) also support this interpretation showing a large anomalously low resistivity area with similar extent to the low-velocity structure in the lower crust and uppermost mantle. This is also corroborated by Zeng et al. (2016) who a show a lower velocity zone at mid-crustal depths to the west of the SAF in the same region. Above this deeper low-velocity structure, another low-velocity structure extends horizontally along strike at depths from 15 to 25 km (Figs 2b–d). The low-velocity structure, located west of the SAF, is strongest to the north of SAFOD (Fig. 2b) and extends to the south (Fig. 2d) and beyond Cholame. Near SAFOD, a low-velocity structure at 15–25 km depth is present on both sides of the fault but is stronger on the east side of the fault, shown by a slight deepening of the 3.75 and 4 km s−1 contour lines across the fault (Fig. 2c). Along strike, this low-velocity structure gently dips to the north from SAFOD and to the south toward Cholame, following the 3.75 km s−1 contour on the west side of the fault, shown in Fig. 3(f). Near SAFOD, it is highlighted by the 3.75 km s−1 contour on the east side of the fault shown in Fig. 3(e), extending for about 50 km north of SAFOD between 20 and 25 km depth. The LFEs are shallowest beneath SAFOD, which corresponds to the shallowest low-velocity structure (Fig. 3e). The LFEs deepen to the north and to the south of that point, which follows the contours of the low-velocity structures belonging to the west side of the fault (Fig. 3f). 5 DISCUSSION AND CONCLUSIONS Our observations of a low-velocity structure 15–25 km deep are consistent with local receiver function observations of a low-velocity layer in the lower Salinian crust (Cox et al. 2016) near SAFOD, as well as high Vp/Vs ratios near Parkfield (Ozacar & Zandt 2009; Audet 2015). The uppermost mantle low-velocity feature may be the deeper fluid source responsible for these lower velocity layers within the lower crust. In combination with a higher permeability in the lower Salinian crust relative to the lower crust across the fault (Becken et al. 2011), this could explain both the along fault extent of the low-velocity layer as well as the strong velocity contrast across the fault in the lower crust (Fig. 2b). There is evidence that geological structure controls the depth of LFEs along strike. Using receiver functions, Audet (2015) found that the LFEs are located within an anisotropic layer of weak minerals potentially related to serpentinization of the lower crust. Many authors have pointed out that LFE are located near Moho depths (Beroza & Ide 2011, and references therein), and Chen et al. (2012) argue that LFE locations may be related to the increase in strength at the Moho where the compositional contrast leads to a change from the more ductile lower crustal material to the stronger rheology of olivine rich uppermost mantle rocks. This interpretation is supported by observations that the crust thickens from 25 to 35 km along the Cholame segment (Fuis & Mooney 1990; Levander & Miller 2012) in concert with the increase in depth of the LFEs. Our findings support the interpretation that the LFEs are located within a low-velocity layer distinctly above the Moho. Although the Moho deepens in the Cholame segment with a similar trend to the LFE locations, our along-strike cross-sections find that the low-velocity layer shown by the dip in the 3.75 km s−1 contour in Fig. 3(f) also deepens to the north and to the south away from SAFOD, along strike. Our observation that LFEs occur in relatively low-velocity materials suggests that weaker minerals and/or high fluid pressures, and not an increase in strength at the Moho, are mechanistically important. LFEs and REQs are similar in that both correspond to families of events that repeatedly rupture the same location. Both are families of small low stress-drop events and are detected using similar methods (Nadeau & McEvilly 2004; Shelly et al. 2009). REQs have been interpreted as small asperities that rupture repeatedly in response to aseismic creep within the seismogenic crust. They are thought to be either annular cracks loaded by creep on the surrounding fault plane (Nadeau & McEvilly 2004) or weak patches at the edges of stronger fault asperities that are similarly loaded by creep (Sammis & Rice 2001). We observe that LFEs occur within low-velocity structures suggesting that, like REQs, they may be linked to aseismic creep. The observation that LFEs are located in low-velocity structures and are probably occurring in a material that is more prone to creeping behaviour is consistent with several independent lines of evidence. By modeling spatial and temporal fluctuations of off-fault seismicity, Sammis et al. (2016) derived a fault creep model similar to that derived here from ANT in Fig. 3. Additionally, we note similarities between our lower relative velocity structures and those found from InSAR and GPS inversions (Jolivet et al. 2015). Jolivet et al. (2015) show a sustained higher creep rate that generally matches the LFE locations that deepen to the south of Parkfield, as well as a locked portion above them. Our higher relative velocity mid-crustal structures to the north and to the south of SAFOD correspond to areas that are geodetically inferred to be locked asperities or have lower creep rates. This spatial correlation of the locked portion above the low-velocity structure in the transition zone and a higher velocity body was also observed by Thurber et al. (2006). The southernmost REQs occur in a region of lower relative velocity structure, which in their study is also ascribed to creep. The similarity of the locations of our observed lower relative velocity structure near SAFOD, that deepens to the south in the Cholame segment and to the north of SAFOD, matches the locations of the LFEs, as well as inferred higher creep rates. This suggests a compositional or rheological explanation due to the fluid source at depth for the LFE as well as the higher creeping rates in these sections. Acknowledgments Data were obtained through Incorporated Research Institutions for Seismology and the Parkfield High-Resolution Seismic Network borehole seismic network. Earthquake data were obtained from NCEDC. We thank Robert Nadeau for sharing the repeating earthquake catalogue (16). Cross-sections and map were generated using the Generic Mapping Tools version GMT-5 (http://gmt.soest.hawaii.edu). This work was supported by the Southern California Earthquake Center (SCEC). SCEC is funded by the National Science Foundation Cooperative Agreement EAR-1033462 and United States Geological Survey Cooperative Agreement G12AC20038. REFERENCES Audet P. , 2015 . Layered crustal anisotropy around the San Andreas fault near Parkfield, California , J. geophys. Res. 120 ( 5 ), 3527 – 3543 . Google Scholar Crossref Search ADS WorldCat Barmin M.P. , Ritzwoller M.H., Levshin A.L., 2001 . A fast and reliable method for surface wave tomography , Monitoring the Comprehensive Nuclear-Test-Ban Treaty: Surface Waves . 1351 – 1375 , Birkhäuser Basel . OpenURL Placeholder Text WorldCat Becken M. , Ritter O., Bedrosian P.A., Weckmann U., 2011 . Correlation between deep fluids, tremor and creep along the central San Andreas fault , Nature 480 ( 7375 ), 87 – 90 . 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(a) Group and (b) phase velocity maps: Rayleigh wave group and phase velocity maps relative to a local mean shown in the bottom left in km s−1 for 8, 10, 15, 20, 25, 30 and 35 s periods. Stations are plotted in black triangles, reference locations are marked with white stars and fault traces are shown in red lines. Figure S3. Resolution tests for phase velocity maps: yellow lines in panels (a)–(c) represent ray paths between two stations used for 8, 15 and 30 s periods. The colours of each cell in panels (d)–(f) show ray path density for the same periods. Panels (g)–(i) show the checkerboard test results for 8, 15 and 30 s periods. The input checkers are 0.5° × 0.5° and correspond to a perturbation of ±10 per cent relative to a mean of 5 km s−1. Stations are plotted in black triangles, reference locations are marked with white stars and fault traces are shown in red lines. Figure S4. Resolution maps: Rayleigh wave resolution maps estimated using the resolution tests described in Barmin et al. (2001) for 8, 10, 15, 20, 25, 30 and 35 s periods. Smaller numbers indicate better resolution. Stations are plotted in black triangles, reference locations are marked with white stars and fault traces are shown in red lines. Figure S5. Sensitivity kernels: phase and group velocity sensitivity kernels for Rayleigh waves to shear velocity with depth calculated for 8, 10, 15, 20, 25, 30 and 35 s periods. Figure S6. Inversion to depth tests: inversion to depth test for alternating layers of faster and lower velocity structure. The input models tested vary in the mid-crustal low-velocity structure depth and thickness shown in red and the recovered models are in blue. Tests use the same set of periods (8–35 s) and both group and phase velocity as in the 3-D model inversion. Table S1. Number of correlations used for group and phase velocity map calculations listed per period. Please note: Oxford University Press is not responsible for the content or functionality of any supporting materials supplied by the authors. Any queries (other than missing material) should be directed to the corresponding author for the paper. © The Authors 2017. Published by Oxford University Press on behalf of The Royal Astronomical Society.
TI - Relating seismicity to the velocity structure of the San Andreas Fault near Parkfield, CA
JO - Geophysical Journal International
DO - 10.1093/gji/ggx131
DA - 2017-06-01
UR - https://www.deepdyve.com/lp/oxford-university-press/relating-seismicity-to-the-velocity-structure-of-the-san-andreas-fault-nlIueWawYP
SP - 1740
VL - 209
IS - 3
DP - DeepDyve
ER -