Water-table and discharge changes associated with the 2016–2017 seismic sequence in central Italy: hydrogeological data and a conceptual model for fractured carbonate aquifers

Water-table and discharge changes associated with the 2016–2017 seismic sequence in central... A seismic sequence in central Italy from August 2016 to January 2017 affected groundwater dynamics in fractured carbonate aquifers. Changes in spring discharge, water-table position, and streamflow were recorded for several months following nine Mw 5.0–6.5 seismic events. Data from 22 measurement sites, located within 100 km of the epicentral zones, were analyzed. The intensity of the induced changes were correlated with seismic magnitude and distance to epicenters. The additional post-seismic discharge from rivers and springs was found to be higher than 9 m /s, totaling more than 0.1 km of groundwater release over 6 months. This huge and unexpected contribution increased streamflow in narrow mountainous valleys to previously unmeasured peak values. Analogously to the L’Aquila 2009 post- earthquake phenomenon, these hydrogeological changes might reflect an increase of bulk hydraulic conductivity at the aquifer scale, which would increase hydraulic heads in the discharge zones and lower them in some recharge areas. The observed changes may also be partly due to other mechanisms, such as shaking and/or squeezing effects related to intense subsidence in the core of the affected area, where effects had maximum extent, or breaching of hydraulic barriers. . . . . Keywords Earthquake Groundwater monitoring Italy Co-seismic effects Carbonate rocks * Elisabetta Preziosi Laboratori Nazionali del Gran Sasso, Istituto Nazionale di Fisica elisabetta.preziosi@cnr.it Nucleare, Via G. Acitelli, 22, 67100 Assergi (AQ), Italy Dipartimento InGeo, Università BG. d’Annunzio^, via dei Vestini 30, Department of Earth Sciences, Sapienza University of Rome, P.le A. 66013 Chieti, Italy Moro 5, 00185 Rome, Italy Institute of Environmental Geology and Geoengineering, National Water Research Institute, National Research Council, Via Salaria km Research Council, Via Salaria km 29,300, PB 10, 29,300, PB 10, 00015 Monterotondo (Rome), Italy 00015 Monterotondo (Rome), Italy Department of Sciences, University of Roma Tre, L.go San Leonardo Murialdo 1, 00146 Rome, Italy Università Politecnica delle Marche, via Brecce Bianche 12, 60131 Ancona, Italy Institute of Environmental Geology and Geoengineering, National Research Council, at Sapienza University of Rome, Department of Earth Sciences, P.le A. Moro 5, 00185 Rome, Italy Università degli Studi di Cassino e del Lazio Meridionale, DICeM, via Di Biasio 43, 03043 Cassino, Italy Department of Physics and Geology, University of Perugia, Via A. Pascoli snc, 06123 Perugia, Italy Department of Civil, Construction-Architecture and Environmental Centro Nazionale Terremoti, Istituto Nazionale di Geofisica e Engineering, University of Aquila, Via G. Gronchi 18, Vulcanologia, Rome, Italy 67100 L’Aquila, Italy 1010 Hydrogeol J (2018) 26:1009–1026 Introduction Bsustained offset^, which include abrupt rises or falls and sustained gradual rise lasting for several days after the shock The seismic sequence recorded in central Italy in 2016–2017 (Roeloffs 1998; Yan et al. 2014). The most frequent conse- included nine main events (Table 1) withmomentmagnitude quences of earthquakes are spring and river discharge increase (Mw) ≥5.0 (four of which were Mw ≥ 5.5) occurring on four and water-table rise, which are generally attributed to four separate days (August 24th 2016, October 26th and 30th 2016 general classes of possible explanations: (1) co-seismic static and January 18th 2017), as described in detail in Chiaraluce strain increases pore pressure that may contribute to change et al. (2017) and ISIDe Working Group (2016). The main permeability (e.g. Wakita 1975; Jonsson et al. 2003); (2) events caused several observed changes in groundwater dy- earthquake-related dynamic strains may increase permeability, namics, including spring discharge variation, water-table permitting a more rapid flow, which in fractured aquifers can anomalies and river discharge alteration in different basins be enhanced by fracture cleaning, eventually increasing dis- located up to 100 km from the epicentral zone. The fractured charge (e.g. Briggs 1991; Rojstaczer and Wolf 1992; and locally fissured carbonate nature of the aquifers outcrop- Rojstaczer et al. 1995;Satoet al. 2000;Wanget al. 2004a; ping in the earthquake area favors a quick co-seismic response Curry et al. 1994; Amoruso et al. 2011); (3) breaching of in terms of pore pressure propagation; however, the observed hydraulic barriers or seals (e.g. Sibson 1994; Brodsky et al. sustained changes, which developed during several days after 2003;Wangetal. 2004a); (4) the excess of water discharged the main shocks, affected groundwater dynamics for several after the earthquake lies in the shallowest subsurface where months after the seismic events. water is liberated by the consolidation or even liquefaction of Hydrogeological changes caused by earthquakes have been near-surface unconsolidated materials (e.g. Manga 2001; historically reported. Instrumentally measured responses, Manga et al. 2003;Montgomery et al. 2003). however, have become available only in the last few decades. Looking at the relationships between tectonic framework, These responses include changes in water level (Leggette and hydrogeological setting and earthquakes from a wider point of Taylor 1935;Cooperetal. 1965; Roeloffs 1998; Brodsky et al. view, recent research activities have highlighted the role of 2003; Roeloffs et al. 2003; Lachassagne et al. 2011; Shi et al. fluids at crustal scale during the seismic cycle. Doglioni 2015), temperature (Mogi et al. 1989), chemical composition et al. (2014), for instance, suggest that fluid flow rates differ (Claesson et al. 2004; Skelton et al. 2014), stream flow during the different periods of the seismic cycle (inter-seismic, (Manga et al. 2003; Montgomery and Manga 2003;Manga pre-seismic, co-seismic and post-seismic periods), also in con- and Rowland 2009; Muir-Wood and King 1993;Rojstaczer nection with the tectonic style. In particular, they hypothesize et al. 1995), and spring attributes (Wang and Manga 2015). that in extensional tectonic settings like central Italy, the Understanding the origin of these hydrological and hydrogeo- wedge of crust above the brittle ductile transition remains chemical phenomena may have significant impacts on the Bsuspended^ while a dilated area forms during the inter- comprehension of the occurrence of liquefaction (Cox et al. seismic period. This area would trap deep fluids, which when 2012), water supply and quality (Gorokhovich and Fleeger the wedge of crust above the brittle ductile transition starts to 2007), underground storage (Wang et al. 2013) and pore- drop in the pre-seismic period, would be squeezed above due pressure triggered seismicity (Brodsky et al. 2003). to the progressive fracture closing. Consequently, in the co- The effects of earthquakes on groundwater are commonly seismic period, aquifers can host changes in hydrochemistry divided into Btransient oscillations^ (Cooper et al. 1965)and (Barberio et al. 2017) and in water levels, independently from Table 1 Seismic events with Mw ≥ 5.0 recorded in central Italy between 24 August 2016 and 18 January 2017. Data from the four main events having Mw ≥ 5.5 are highlighted in italic. Source: INGV (2017) Date time (UTC) Mw Epicenter location Depth (km) Latitude Longitude August 24th 2016 01:36:32 6.0 Accumoli 8.1 42.6983 13.2335 August 24th 2016 02:33:28 5.3 Norcia 8.0 42.7922 13.1507 October 26th 2016 17:10:36 5.4 Castelsantangelo sul Nera 8.7 42.8802 13.1275 October 26th 2016 19:18:05 5.9 Castelsantangelo sul Nera 7.5 42.9087 13.1288 October 30th 2016 06:40:17 6.5 Norcia 9.2 42.8322 13.1107 January 18th 2017 09:25:40 5.1 Capitignano 10.0 42.5450 13.2768 January 18th 2017 10:14:09 5.5 Capitignano 9.6 42.5310 13.2838 January 18th 2017 10:25:23 5.4 Capitignano 9.4 42.5033 13.2770 January 18th 2017 13:33:36 5.0 Barete 9.5 42.4733 13.2747 Hydrogeol J (2018) 26:1009–1026 1011 changes due to the previously listed local mechanisms, which fault system named Olevano-Antrodoco line (Pierantoni can affect the hydrodynamics of the struck aquifers during and et al. 2005 and references therein). From the upper after the seismic sequence. Such a comprehensive tectonic Miocene to lower Pliocene, thrust migration towards the model allows looking at changes induced in groundwater after east was coupled with the progressive development of earthquakes in a general framework of crustal deformation, fore-deeps in front of the migrating fold-and-thrust belt suggesting the role of deep inputs in triggering the aforemen- (Cipollari and Cosentino 1995). tioned well-known processes (as pore pressure changes, per- Since the upper Miocene–lower Pliocene, extensional meability increase, liquefaction/consolidation, etc.) acting at faulting connected with the opening of the back-arc the aquifer scale. Tyrrhenian Basin has been dissecting the compressive struc- The effects of past earthquakes on groundwater in central tures (Boncio and Lavecchia 2000 and references therein), Italy have been described by previous papers. Esposito et al. leading to the development of intermontane basins filled with (2001) describe the effects of four earthquakes in southern thick continental sequences of Quaternary alluvial, detrital Apennines including the 1980 Irpinia earthquake, which gen- and lacustrine deposits (Cavinato and De Celles 1999). erated important hydrogeological changes as far as 200 km Some normal faults show evidence of Holocene activity, sug- from the epicenter, including a significant increase of gesting that they may be responsible for the seismic activity Caposele spring flow. Amoruso et al. (2011)describethe occurring in this sector of the Apennines (Cello et al. 1998), hydrogeological changes in a fractured aquifer after the mainly confined within the upper part of the crust (<16 km; L’Aquila 2009 earthquake, inferring that those changes were Lavecchia et al. 1994;Boschietal. 1995). probably connected with the increase of bulk hydraulic con- In the study area, the 2016–2017 seismic sequence in- ductivity at the aquifer scale, mainly due to fracture cleaning, cludes some of the largest instrumental earthquakes of the raising hydraulic heads in the discharge zones, and corre- last 40 years (Norcia 1979 Mw = 5.9, Irpinia 1980 Mw = spondingly lowering them in the recharge areas (Adinolfi 6.9, Gubbio 1984 Mw = 5.2, Colfiorito 1997 Mw = 5.9, Falcone et al. 2012; Galassi et al. 2014). L’Aquila 2009 Mw = 6.3; Pantosti and Valensise 1990; The aim of this paper is to present an overview of the Boncio and Lavecchia 2000; Deschamps et al. 2000; effects of the 2016–2017 seismic sequence on the dynam- Chiarabba et al. 2009). The 2016–2017 sequence and its ics of groundwater flow in central Apennines, analyzing main shocks (Table 1) were generated by the Gorzano Mt.- the extent of the impacted area, possible relations between Vettore Mt.-Bove Mt. faults (Galadini and Galli 2003,LMF tectonic environments, geological-hydrogeological set- and MVF in Fig. 2). The seismic crisis started with the ting, and groundwater changes, and providing preliminary August 24th 2016 event (Mw 6.0) and a further significant considerations on the possible causes of the observed event on October 26th. The Vettore Mt. fault experienced a rupture with tectonic segments ~10 km long and a surface- phenomena. displacement of ~30 cm (Smeraglia et al. 2017). The October 30th 2016 event (Mw 6.5) was generated by the Geological and hydrogeological framework rupture of the central zone of the fault by a normal move- ment. The focal mechanism, identical to the previous earth- The central Apennines (Italy) is a Meso-Cenozoic ENE- quakes, was a strike-angle of N155°, a WSW dip slip and a dipping thrust-and-fold belt mainly developed during up- dip angle about of 50° in depth (RCMT 2016). During the per Miocene-Quaternary, composed by a pre-orogenic October 30th 2016 event, the entire Vettore Mt.-Bove Mt. Triassic-Miocene sedimentary succession overlain by fault system gave origin to important surface faulting occur- Miocene and Pliocene synorogenic sediments, resulting rences, reusing the pre-existing fault plane and redisplacing in a highly variable facies and thickness distribution. A the fault segments previously broken. Meso-Cenozoic carbonate platform domain extends in the The acquisition of the interferometric satellite data SE part of the study area (Latium Abruzzi Apennine, Fig. ALOS-2 from JAXA (Japan Aerospace Exploration 1), consisting of a 5,000-m-thick sequence of limestone Agency) and further interferometric analyses (INGV and subordinate dolomite of Upper Triassic to upper Central Italy Earthquake Team 2016)providedanestima- Miocene age (Brandano and Loche 2014 and references tion of the co-seismic subsidence along the NW–SE com- therein). In the western side of the area (Umbro Marchean ponent reaching a maximum of ~80 cm (LOS: Satellite Apennine), a Lower Jurassic carbonate shelf unit is over- Observation Line). The horizontal co-seismic maximum lain by stratified pelagic sediments (middle Lias-lower movements consist of ~40 cm towards NE, ~30 cm towards Miocene), with an overall thickness of 2,500–3,000 m SW as well as a maximum vertical movement of about 20– (Marchegiani et al. 1999). The Apennine orogenesis 40 cm, when considering also the October 30th event overthrusts the Umbria-Marche succession onto the (INGV Working Group GPS 2016;INGV CentralItaly Latium–Abruzzi platform along the main regional thrust Earthquake Team 2016). 1012 Hydrogeol J (2018) 26:1009–1026 Fig. 1 Hydrogeological setting of the study area. 1 Alluvial aquifers and aquifers; 5 Main springs (mean discharge >0.5 m /s); 6 Main streambed aquitards; 2 Synorogenic low-permeability deposits; 3 Latium-Abruzzi springs (mean discharge >0.5 m /s) carbonate fractured aquifers; 4 Umbro-Marchean carbonate fractured In the study area, the fractured carbonate ridges host the Widespread karst development, including endorheic ba- main aquifers, feeding several perennial springs (Nanni and sins, ensures high infiltration rates, from 500 to 700 mm/year, Vivalda 2005; Martarelli et al. 2008; Mastrorillo et al. 2009; in the Umbria Marchean aquifers and up to 900 mm/year in Mastrorillo and Petitta 2014) with steady regimen, located the Latium-Abruzzi aquifers, collectively feeding a total dis- mostly at the external boundaries of the aquifers (Fig. 1). charge of about 300 m /s (Boni et al. 1986, 2010). Fractures Groundwater flows in fissured to locally karstified carbonates. and karst conduits allow for fast vertical flow in the vadose The Miocene-Pliocene synorogenetic silicoclastic sediments zone, while the large thickness of the saturated zone facilitates surrounding the carbonate aquifers, as well as the Plio- a steady flow towards the basal springs that show outstand- Quaternary deposits, filling the intermontane plains and the ingly high and steady discharge (Petitta 2009; Amoruso et al. river valleys, act like aquitards (Petitta et al. 2011). 2014;Fiorilloet al. 2015). Hydrogeol J (2018) 26:1009–1026 1013 Fig. 2 Location of the measurement points, main epicenters and activated Vettore fault (Vettore-Bove faults); NF Norcia fault; CF Cascia fault; faults: R river gauge; W well; S spring. NP Norcia Plain. See Tables 2, 3 LMF Monti della Laga fault (Gorzano Mt.-Campotosto faults); MF and 4 for details. Epicenters are represented by stars: August 24th 2016 Montereale fault; COF Colfiorito fault; CAF Campo Imperatore- event is in purple, October 26th 2016 in red, October 30th 2016 in cyan Assergi fault; MSF Morrone-Sulmona fault. Source: INGV Central and January 18th 2017 in orange. Quaternary fault systems: MVF Monte Italy Earthquake Team (2016) Methodology The monitoring site locations are shown in Fig. 2, distinguishing piezometric heads in monitoring wells Co-seismic changes were examined in several observation (W1–W5), spring discharges (S1–S12) and hydrometric sites located within the area affected by the earthquakes. levels or discharge in river gauging stations (R1–R6). In Within the framework of a continuous monitoring, the collect- addition, in February 2017, water-table levels in the po- ed data refer to piezometric heads in wells and piezometers, rous local aquifer of the Norcia Plain (NP in Fig. 2)were spring discharges and river hydrometric levels or discharges. recorded in 16 wells and compared to a piezometric map realized in 2011. Collected data The monitored springs can be divided into the following: basal springs (S4, S5, S7 and S8), whose steady discharge Altogether, 22 automatic records from continuous moni- shows limited seasonal variation; springs fed by shallow aqui- fers (S1, S2 and S3); and high-elevation springs (S6, S9, S10, toring sites were collected, plus one manually measured. 1014 Hydrogeol J (2018) 26:1009–1026 S11 and S12) having a seasonal slightly variable discharge. where available, have been used in conjunction with stage All the springs, except Torbidone (S5), are tapped for water measurements to determine the river discharges. supply systems. The monitoring wells tap the basal aquifers at depths from Methods of data processing 20 to 250 m. In some cases (W1, W2 and W3), the water level recordings were occasionally affected by the operations for Because of the different nature of data sources, the time series the related water-supply systems. In detail, W1 and W2 were from continuous monitoring may have different measurement affected by operational changes in the tunnel drainage system frequency, with intervals ranging from 0.05 s to 24 h. To of the near San Chiodo spring, whereas the disturbances prior ensure uniformity, data have been aggregated and analyzed to August 2016 in W3 were due to the works for the construc- at daily scale. tion of a new aqueduct. The mean discharge of each data series was calculated The discharge of the monitored rivers can be consid- considering the time intervals before, between and after ered an indicator of hydrogeological changes at the basin the four major seismic events of Mw ≥ 5.5. The first in- scale, because a significant amount of groundwater direct- terval corresponds to the period before the first main seis- ly feeds the rivers’ baseflow by streambed springs. The mic event, and the second, the third and the fourth ones rivers with catchment areas less than 100 km (R1, R2, identify the time intervals between the first–second, sec- R3 and R5) have a steady regimen too and are predomi- ond–third and third–fourth main events; the last one cor- nantly fed by baseflow, whereas the runoff may be con- responds to the period after the fourth main event. The sidered negligible. In the widest river basins (>1,000 km ; discharge/level variation associated with each one of the R4 and R6) the runoff contribution cannot be disregarded. four main events was calculated as the difference between It follows that the river discharge is more variable, despite the daily value prior to and after each event. In cases the clear dominant role of baseflow. where the changes were very abrupt and the difference Data have been recorded by regional hydrographic between the daily values was not appreciable, hourly services, water supply companies or directly by the re- values were considered. Where even the hourly difference search teams monitoring the earthquake zones. All was not evident, changes have been considered as Bnot available data from June 1st 2016 to February 28th significant^ (NS). All calculated values are shown in 2017 were considered. Public service data are available Tables 2, 3 and 4. on-line (ARPA Umbria 2017; Regione Marche 2017; Regione Umbria 2017). Results Methods of data measurement Figures 3, 4, 5 and 6 show the time plots of the available data from monitoring sites (location in Fig. 2), summa- Water-table depths have been recorded in wells and pie- rized in Tables 2, 3 and 4, which refer respectively to zometers by downhole data loggers with atmospheric water levels in piezometers (W1–W5), spring discharge compensation. The horizontal piezometer W4, located in (S1–S12) and river discharge or levels (R1–R6). Red bars the underground National Institute for Nuclear Physics in Figs. 3, 4, 5 and 6 indicate the four main Mw > 5.5 (INFN) laboratory, measures the hydraulic head (pressure events. For the sake of simplicity, the August 24th 2016, in MPa) by a 3-channel 24-bit ADC (Analog to Digital October 26th 2016, October 30th 2016 and January 18th Converter; De Luca et al. 2016). To quantify local head 2017 earthquakes will be named hereinafter 1st, 2nd, 3rd changes during each seismic event, only W4 original pres- and 4th events respectively. See Fig. 2 for the locations of sure data were converted (approximately) to water-table epicenters and the main active fault systems. Both the 1st elevation by multiplying the pressure (MPa) by 100 and and the 2nd events were clearly perceived in the S1, S2 adding the obtained elevation to the elevation of the top and S3 northern sites (Fig. 3a), with abrupt step-like var- of the borehole (987 m a.s.l.). iations. The 3rd event, the strongest of the sequence, only Spring discharges have been measured by automatic water slightly affects the discharge at S1 and S3. level sensors in weirs or in Venturi tubes, and converted into Further south, Forca Canapine spring and Pescara discharge through the related rating curve. Only for the spring (S6 and S7, Fig. 3b) show a clear increase of dis- Torbidone spring, was the discharge manually measured with charge after the 1st event, more step-like for S6 but grad- a portable flow meter, starting on November 11th 2016 at a ual and sustained for S7. These two springs are located at frequency of one measurement about every 5 days. River high elevation (1,350 and 850 m a.s.l.) and they both gauging stations are equipped with water-height data loggers suffered for a sharp decrease after the 3rd event, which or automatic ultrasonic measurement sensors. Rating curves, completely dry up at S6. Aso River nearby (R2, Fig. 3b), Hydrogeol J (2018) 26:1009–1026 1015 Table 2 Water table changes caused by the four main seismic events (Mw ≥ 5.5) Site ID Name Elevation (m a.s.l.) Average level pre Δ Water level Average level Δ Water level Average level Δ Water level Average level Δ Water Average EQ1 (m a.s.l.) EQ1 (m) post EQ1 EQ2 (m) post EQ2 EQ3 (m) post EQ3 level EQ4 level post EQ4 (m a.s.l.) (m a.s.l.) (m a.s.l.) (m) (m a.s.l.) W1 PIEZ. D 779 759.94 + 2.17 761.91 + 1.22 763.58 + 6.96 770.05 0 769.21 W2 PIEZ. G 823 775.54 −1.07 777.02 −1.14 777.39 +6.05 781.23 0 780.57 W3 Renari di Capriglia 329 295.61 + 0.62 296.95 NA NA NA NA NA NA W4 Borehole S13 987 1252 +1.00 1253 0 1196 +1.10 1198 +0.20 1194 W5 Bussi sul Tirino 238 226.27 +0.28 226.07 +0.04 225.96 +0.21 225.81 +0.82 226.14 EQ1: August 24th 2016 seismic event. EQ2: October 26th 2016 seismic event. EQ3: October 30th 2016 seismic event. EQ4: January 18th 2017 seismic event. Negative changes are highlighted in italic NA not available value Table 3 Spring discharge variations caused by the four main seismic events (Mw ≥ 5.5) Site ID Name Elevation Average discharge Δ Discharge Average discharge Δ Discharge Average discharge Δ Discharge Average discharge Δ Discharge Average discharge 3 3 3 3 3 3 3 3 3 (m a.s.l.) pre EQ1 (m /s) EQ1 (m /s) post EQ1 (m /s) EQ2 (m /s) post EQ2 (m /s) EQ3 (m /s) post EQ3 (m /s) EQ4 (m /s) post EQ4 (m /s) S1 Montenero 840 0.057 +0.002 0.044 +0.015 0.050 NS 0.064 NS 0.062 S2 Nicolini 470 0.073 +0.010 0.061 +0.016 0.078 NS 0.060 NS 0.072 S3 Valcimarra 310 0.032 +0.004 0.020 +0.008 0.020 +0.005 0.020 NS NA S4 Alzabove 640 0.298 +0.019 0.278 +0.011 0.269 +0.008 0.259 0 0.256 S5 Torbidone 610 0 0 0 0 0 +1.510 0.990 NS 1.450 S6 Forca Canapine 1350 0.030 +0.045 0.021 +0.003 0.029 −0.029 0NS 0 S7 Pescara 850 0.259 +0.129 0.350 −0.012 0.337 −0.059 0.242 NS 0.240 S8 Lupa 375 0.115 +0.0003 0.091 +0.0005 0.075 +0.004 0.072 NS 0.062 S9 Assergi 967 0.495 +0.058 0.515 NS 0.515 +0.045 0.515 +0.019 0.486 S10 Ruzzo 964 0.979 +0.016 0.995 NS 0.995 +0.085 0.922 NA NA S11 Vacelliera bassa 937 0.193 +0.019 0.200 NS 0.200 +0.021 0.211 NA NA S12 Vacelliera alta 1015 0.134 +0.021 0.140 NS 0.140 +0.019 0.146 NA NA EQ1: August 24th 2016 seismic event. EQ2: October 26th 2016 seismic event. EQ3: October 30th 2016 seismic event. EQ4: January 18th 2017 seismic event. Negative changes are highlighted in italic NA not available value, NS not significant change 1016 Hydrogeol J (2018) 26:1009–1026 monitored at the spring outlet, is also clearly influenced by the 1st event with a step-like increase and limitedly affected by the following events. Different effects were registered in the area closest to the 2nd and 3rd events,inthe SanChiodospringarea (Fig. 4a) and in the Norcia area (Fig. 4b). In the San Chiodo area (Fig. 4a) a water supply system is operating by a tunnel drainage; periodical operational changes, opening and clos- ing of different drainage tunnels, produce clear variations in the water levels (black vertical dashed lines in Fig. 4a). The responses of two of the 14 available piezometers (W1 and W2), considered representative of the entire monitoring net- work, and the discharge of the Upper Nera River down- stream at Castelsantangelo (R1), are shown. After the 1st event, the system reacted with a sharp step-like increase in the downgradient part of the aquifer (W1, 779 m a.s.l.) and a clear step-like decrease in the upper part of the aquifer (W2, 823 m a.s.l.). The same happened on the 2nd event, while on the 3rd event the water level in W1 increased gradually by nearly 7 m, while in W2 it firstly decreased, then steadily increasedfor severaldaysupto6min height.After the3rd event, the discharge of the Upper Nera River (R1) doubled; this quick increase reached a steady state in December 2016. In the Norcia area (Fig. 4b) the Torbidone spring (S5), reactivated after the 3rd shock, shows a gradual increase in discharge up to 1.5 m /s in about 3 months after the shock. The Sordo River (R3), receiving the Torbidone dis- charge, also reacted with a clear gradual and sustained in- crease lasting several months, due also to a different direct groundwater inflow. In addition, the water table of the po- rous clastic aquifer of the Norcia Plain has shown a hydrau- lic head increase, reaching +15 m at the contact with the carbonate aquifer, with respect to the water table recorded in 2010–2011. The western-located S4, S8 and W3 (Fig. 4c) registered abrupt positive step-like increases for the 1st, 2nd and 3rd events, with some differences among them. The Nera River at Torre Orsina (Fig. 4d) receives the entire inflow of the aforementioned flow systems, and others not described here due to the absence of recordings; however, its discharge is clearly influenced by the 1st, 2nd and 3rd events, as shown by the abrupt step-like increases coincident with the first three red lines. Although no significant precipitation is recorded, the river discharge did not decrease for the 3 months following the 3rd event. Overall, the Nera River suffered for a total dis- charge increase of about 9 m /s considering the first three main events, corresponding to about 30% surplus of its natural baseflow. The 4th event, which is the southernmost, was not per- ceived at all so far to the north, but locally affected the southern monitoring points (Fig. 5), located very close to its epicenter (Fig. 2). All four springs (Fig. 5a) show sig- nificant abrupt step-like increase of the discharge after the Table 4 River discharge and hydrometric river level variations caused by the four main seismic events (Mw ≥ 5.5) Site Name Elevation Average Δ Average Δ Average Δ Average Δ Average ID ma.s.l. discharge Discharge discharge Discharge discharge Discharge discharge Discharge discharge pre EQ1 EQ1 post EQ1 EQ2 post EQ2 EQ3 (m /s) post EQ3 EQ4 post EQ4 3 3 3 3 3 3 3 3 (m /s) (m /s) (m /s) (m /s) (m /s) (m /s) (m /s) (m /s) R1 Nera at 725 1.760 +0.360 2.060 +0.155 2.180 +2.340 4.260 NS 4.290 Castelsantangelo R2 Aso at Foce 950 0.574 +0.118 0.647 NS 0.682 NS NA NA NA R3 Sordo at Serravalle 510 2.448 +0.179 3.450 NA 3.890 +2.680 5.440 NS 6.901 R4 Nera at Torre Orsina 231 23.214 +1.533 23.459 +2.390 24.888 +5.160 30.738 NS 30.749 a a a a a a R5 Tirino at 313 0.85 NS 0.85 NS 0.85 NS 0.84 +0.14 0.84 Madonnina a a a a a a R6 Aterno at Molina 454 0.23 NS 0.3 NS 0.35 NS 0.38 +0.17 0.40 Hydrometric river level (m) EQ1: August 24th 2016 seismic event. EQ2: October 26th 2016 seismic event. EQ3: October 30th 2016 seismic event. EQ4: January 18th 2017 seismic event NA not available value, NS not significant change Hydrogeol J (2018) 26:1009–1026 1017 Fig. 3 Daily discharge (m /s) of northern monitoring sites. a Central Marche; b Mt. Sibillini eastern slope. See Fig. 2 for location and Tables 3 and 4 for site characteristics. Vertical red bars locate the four main seismic events (Table 1) 1st and the 3rd event, while the 2nd event does not at all Discussion modify the hydrographs. The 4th event, in spite of being so close, only slightly influences the discharge of the sole The hydrogeological changes caused by the 2016–2017 seismic S9 station. sequence are of remarkable intensity specially if compared to Similar evidence has been recorded at a horizontal borehole the relatively limited magnitude of the events; similar or larger (W4, Fig. 6) located in the underground laboratories of the hydrological responses are very rare (Mohr et al. 2015). The INFN in the Gran Sasso massif (Petitta and Tallini 2002; estimated amount of extra discharge drained by springs and Amoruso et al. 2013). The time plot of the pressure head rivers since August 24th 2016 for the following 6 months ex- variation shows sudden increases in the hydraulic pressure ceeds 0.1 km . This amount has been obtained looking at the (MPa) with a sharp rise of about 2 m recorded on the 1st discharge of the entire Nera Basin (R4, Nera at Torre Orsina) and on the 3rd events, while no evidence was recorded after before and after the seismic sequence: the additional discharge 3 6 the 2nd and 4th events (Fig. 6). was about 1.5 m /s between the 1st and 3rd event (about 8 × 10 3 3 Further south, changes in the water table were clearly re- m ), and about of 9 m /s after the 3rd event, which until the end corded in the monitoring well at Bussi sul Tirino (W5, of February 2017 correspond to more than 0.095 km .This Barberio et al. 2016;Fig. 5b). The magnitude of the water estimation does not consider other changes observed in other level variation is about 20 cm for the first three events, with basins, which released a minor amount of discharge. a gradual sustained type variation, while an abrupt step-like Other documented examples of earthquake induced increase up to 90 cm is observed for the closest 4th event. The groundwater release include (ordered by decreasing earth- river monitoring sites R5 and R6 do not show any significant quake magnitude): the Maule Mw 8.8 earthquake in Chile variation of the hydrometric level in the 1st, 2nd and 3rd (1.1 km ,Mohr etal. 2016), the Chi-Chi Mw 7.5 earthquake events (Fig. 5b). However, after the 4th earthquake, both the in Taiwan (0.7 km ,Wanget al. 2004b), and the Hebgen Lake 3 3 Aterno River (R6) and, most clearly, the Tirino River (R5) (Mw 7.3, 0.5 km ), Borah Peak (Mw 6.9, 0.3 km ; Muir- hydrometric level responses show a sharp and sudden increase Wood and King 1993,; USGS 2017), and Loma Prieta earth- of the hydrometric levels, which drop rather quickly to nearly quakes in the USA (Mw 6.9, 0.01 km ;Rojstaczeretal. 1995); the prior level than the earthquake discharge. however, these are earthquakes larger than the central Italy 1018 Hydrogeol J (2018) 26:1009–1026 Fig. 4 Daily discharge (left vertical axis) and daily water- table levels (right vertical axis) of monitoring sites of the Nera River Basin. a San Chiodo spring area. Vertical dashed black lines represent changes caused by the water supply system management; b Norcia Plain area. Blue dots rappresent the manual measurements of Torbidone spring (S5) discharge; c western area; d Nera River closing station. See Fig. 2 for location and Tables 2, 3 and 4 for site characteristics. Vertical red bars locate the four main seismic events (Table 1) Hydrogeol J (2018) 26:1009–1026 1019 Fig. 5 Daily spring discharge, water level and hydrometric level of southern monitoring sites. a Gran Sasso springs; b Aterno– Tirino rivers (hydrometric levels on left vertical axis, water-table elevation on right vertical axis). See Fig. 2 for location and Tables 2, 3 and 4 for site characteristics. Vertical red bars locate the four main seismic events (Table 1) events. At a comparable magnitude, the Mw 6.0 South Napa 2008). Nevertheless, peculiar responses in the study area earthquake (USA) produced extra water of about 0.001 km have been observed in sites very close to the epicenters (Wang and Manga 2015). (<10 km), where abrupt changes have been frequently The extent of the area affected by hydrogeological followed by a sustained increase with time, especially changes approaches 10,000 km . The recorded hydrolog- after the 3rd event (October 30th), having the highest ical variations fall in the known fields of abrupt and magnitude. This behaviour could be due to the fractured sustained water level changes in groundwater due to nature of the aquifer, where pressure changes can easily earthquakes (Fig. 7, modified from Wang and Chia and quickly propagate along the effective porosity Fig. 6 Hydraulic pressure (MPa; Y-axis) vs. day (X-axis) of borehole 2016 to September 16th 2016 and b from October 22nd 2016 to W4. See Fig. 2 for location and Table 2 for site characteristics. The January 31st 2017. The red lines refer to the August 24th, October 30th reported data are 1-min-averaged. Monitoring period a from July 1st and January 18th earthquakes (Table 1) 1020 Hydrogeol J (2018) 26:1009–1026 entity of the response. Differently from the Latium- Abruzzi carbonate aquifers, where a basal aquifer is usu- ally governing groundwater flow, in the Umbria- Marchean aquifers, a network of interconnected faults plays a key role in determining the dynamic groundwater divide location, seepage velocity and extent of the re- charge area. It follows that groundwater flow can be eas- ily influenced by seismic events; furthermore, during the 2016–2017 sequence, the study area suffered from many earthquakes whose epicenters were differently located along the central Apennine chain; therefore, numerous aquifers and springs were hit by repeated events, or by a single group of seismic events only. Generally, a decreasing intensity of the recorded effects was observed moving from the earthquake epicenters towards Fig. 7 Distribution of earthquake-triggered hydrogeological changes as a the distal areas. Figure 8 shows the spring discharge/water function of earthquake magnitude (horizontal axis) and epicentral level variation, as a function of the distance between the mon- distance (vertical axis). Also plotted are the contours (oblique lines) of constant seismic energy density (Wang and Manga 2010)and the itored points and the epicenters of each seismic event. The domains where different types of coseismic water level responses occur result is a cloud of values from about 10—200% in the near- (Wang and Chia 2008). The triangles represent the water level changes in field (less than 10 km from epicenters), clearly decreasing wells or in rivers, the dots the discharge changes in spring or rivers. The with distance. Up to +50% in discharge and +2 m water level four main events are distinguished by different colors: August 24th 2016 increase have been recorded between 20 and 30 km away, (Mw = 6) event is in purple, the October 26th 2016 (Mw = 5.9) event in red, the October 30th 2016 (Mw = 6.5) event in cyan and the January 18th while minor changes have been observed for sites located 2017 (Mw = 5.5) event in orange between 60 and 100 km from the epicenters. Locally, dis- charge decreases have been encountered at less than 10-km network, reaching the boundaries of the aquifers, causing distance from the epicentral area, but only for the October change in hydrodynamic independently from seismic- 30th event, which was the more energetic one. induced stresses (Amoruso et al. 2011). Furthermore, the The mechanism causing both sustained negative and occurrence of several subsequent events repeatedly im- positive hydraulic changes is frequently related to static pacted the aquifers, as happened on October 30th 2016 stress modifications evidenced by comparison between for the third time in 2 months. Similar events have been pre-seismic and post-seismic conditions (Jonsson et al. not reported in the literature so far. 2003; Montgomery and Manga 2003;Parvinetal. 2014; In addition, the characteristics of the fractured aquifers Mohr et al. 2016). The difference between transient dy- impacted by the shocks may also have influenced the namic oscillations (Cooper et al. 1965) and the offset-type Fig. 8 Epicentral distance (horizontal axis) vs the discharge changes, 200%. The four main events are represented by different colors: August expressed as a percentage of the pre-event period mean discharge (left 24th 2016 event is in purple, October 26th 2016 in red, October 30th 2016 vertical axis) and the water level changes (right vertical axis). Torbidone in cyan and January 18th 2017 in orange. The vertical dashed lines show spring (S5), whose ratio to the pre-event discharge would be infinite (as the changes in the near-affected area (<10 km from epicenters) and in a the spring was dry prior to the earthquake) is marked by an asterisk at larger-affected area (up to 60 km from epicenters) Hydrogeol J (2018) 26:1009–1026 1021 Fig. 9 Maps of the entity and directional (positive or negative) changes in discharge (%) and in water level (m) after each main event: a August 24th 2016; b October 26th 2016; c October 30th 2016; d January 18th 2017 1022 Hydrogeol J (2018) 26:1009–1026 water-level changes affecting, in a more permanent way, propagation due to dynamic stresses caused by the seismic groundwater flow, is also well described in Yan et al. waves, which determined the sudden peak, and (2) an increase (2014). Whereas the mechanism for explaining the first of bulk hydraulic conductivity of the fractured aquifer due to type of effects is substantially accepted as being transient fracture cleaning triggered by the pore pressure propagation, oscillations due to crustal dynamic poro-elastic deforma- which induces mobilization by shaking fine particles that tion in an aquifer during the passage of seismic waves block fracture throats (Amoruso et al. 2011). (Rexin et al. 1962; Kitagawa et al. 2006; Yan et al. Recorded hydrological changes for the 2016–2017 earth- 2014), the cause of permanent offsets, corresponding to quakes may be preliminarily attributed to the pore pressure sustained changes, is still debated. propagation in the aquifers, which would be followed by a In this study case, the distribution of changes observed sustained discharge increase attributable to fracture cleaning, after each main event, and their positive or negative ef- mobilizing fine particles from fractures (Brodsky et al. 2003; fects, are synthetized in Fig. 9. The impact on most of the Wang and Manga 2010; Adinolfi Falcone et al. 2012), as springs is the increase of discharge with varying magni- reflected by turbidity increase, clearly recorded in several tude. Most of the recordings in wells and in rivers clearly monitoring points (e.g. R1, S5, S11, S12). In this case, the indicated a sudden and sharp increase simultaneous with superimposition of the post-seismic changes on a recession the earthquakes, generally followed by a steady increase phase makes the post-earthquake evolution clearer. The dy- lasting for a few days after the shock, and a subsequent namic stress due to pore pressure propagation has been smooth decrease. Few points show a decrease of water clearly observed at the high-frequency-monitoring site W4 level or discharge: the disappearance of one tapped spring (Fig. 10), where the short-term (30 s) changes in hydraulic (S6) and a significant discharge decrease of the spring S7, head are the response to the seismic wave of August 24th, both located at high elevation (Fig. 3b). Similar decreases whose effects on groundwater levels ended after a few or drying-up have been observed in other minor not- minutes. monitored springs located in the same recharge area. In this study, the sustained response of several monitored Another monitored site experiencing post-earthquake de- sites to the subsequent seismic events, considering the frac- crease in the water table is in Upper Nera Valley, where tured nature of the struck aquifers, support the hypothesis of only the highest-elevation monitoring well (W2, Fig. 5a) fracture cleaning and the consequent increase of the bulk suffered from a sharp lowering, quickly balanced in the hydraulic conductivity. In many cases, discharge increase following days. has reached its peak a few days after one of the main events, The decrease of discharge or water levels and the disap- followed by a decreasing trend due to the recession phase. pearance of springs have been also been associated in litera- Accordingly, in several cases, mainly located far from epi- ture to proximity to the active faults. With respect to the water- centers, the next earthquakes did not cause permanent level changes observed in the footwall area of the fault acti- changes, but only a temporary discharge increase, favoring vated by the Chi-Chi earthquake, Chia et al. (2001)report that the model of fracture cleaning. The location of negative water-level rise was the predominant effect in most of the area, effects at high-elevation sites confirms this hypothesis, as whereas water-level fall prevailed in a narrow zone adjacent to water-table decrease is expected in recharge zones of struck the fault trace. Amoruso et al. (2011) report that after the aquifers, while increase in discharge is common in low- L’Aquila Mw 6.3 earthquake, the two highest springs, located elevation zones. on the trace of the activated fault, suddenly dried up after the A more complex response has been recorded in the core main shock. The opposite phenomenon, the reactivation of area, i.e. between the epicenters of August 24th and October dry springs or stream, as observed in this case for the 30th events. In the Upper Nera River Valley (R1, W1 and Torbidone spring (S5), is also well known. After the 2014 W2), in the Norcia Plain (S5, R3) and partially in the eastern Mw 6.0 South Napa earthquake, many streams and springs, side of the Sibillini Mts. (S7, R2), the effects of both events is which were dry or nearly dry, started to flow after the earth- quake (Wang and Manga 2015). The possible explanation given by the mentioned authors is the enhanced permeability in the recharge areas. Generally, after the co-seismic peak, discharge and water levels remained on higher values with respect to pre-seismic conditions. A similar mechanism was observed after the 1980 Irpinia earthquake at Caposele spring (Esposito et al. 2001) and, more recently, in the Abruzzi region after the L’Aquila Fig. 10 Water pressure signal (in MPa) at W4 replicating the arrival of 2009 earthquake (Adinolfi Falcone et al. 2012). This last case seismic waves of the August 24th event (modified from De Luca et al. has been explained by a double effect: (1) pore pressure 2016) Hydrogeol J (2018) 26:1009–1026 1023 still evident in the following months. The discharge of springs sometimes dramatic, effects. A generalized decrease and water-table elevations remained very high with respect to of the magnitude of the effects with distance was ob- pre-earthquake conditions, as testified by the Nera main gaug- served. The observed effects may be summarized as ing station (R4), not simply justifiable by seismic stresses. The follows: (1) increase (rarely decrease) of heads mea- new conditions of groundwater flow are testified by the ob- sured in wells/piezometers, (2) positive (rarely nega- served changes in the recession curve of San Chiodo spring tive) variations of spring discharge, (3) positive vari- (R1, W1, Fig. 5a): previous values of α = 0.003 calculated for ation of river baseflow, (4) activation of historically the 2011–2015 period, strongly decrease to α = 0.001 after the intermittent springs, and (5) drying up of high-elevation earthquakes, testifying to a continuous additional contribution springs. A quick oscillation, correlated with dynamic from the aquifer with time. stresses, was observed in a few sites equipped with Consequently, other possible additional factors may have high-frequency recordings. influenced the post-seismic response of the area of Sibillini 2. Within 6 months from August 24th 2016, more than Mts. The impressive increase of spring and river discharge 0.1 km of groundwater has been additionally discharged observed in the Upper Nera River valley and Norcia plain in in the area (about +25% of the natural discharge). the mid-term responses, may be correlated with the subsi- Comparison with similar case studies highlights the rela- dence induced by the toe of Vettore Mt. faulting, which might tively high amount of discharge increase with respect to have created in the core of the Sibillini Mts. aquifer an addi- the limited magnitude of the seismic events; this peculiar- tional Bsqueezing effect^. Other possible mechanisms, not ity could be explained by the succession of four main further investigated in this paper, could be related to shaking events having Mw > 5.5, which continuously struck the of the aquifer, tilting, settlement and uplifting of the fractured aquifers. seismogenetic fault-bounded structures and the consequent 3. The observed response at regional scale is compatible dislocation of permeability thresholds induced by faulting. with the cleaning of fractures and an overall mid-term Further, a possible decrease in aquifer storativity due to increase of the bulk permeability due to the co-seismic fracture-width reduction could be envisaged, which would pore-pressure propagation. have directly triggered the additional volume of groundwater 4. Eventually, the dramatic rise of the water table and of released in the months following the shocks, modifying discharge in the core area could be the result of a groundwater dynamic divides and groundwater flow direc- Bsqueezing effect^ due to the co-seismic subsidence in tions. This hypothesis, still not verified, should be carefully the Sibillini Mts. area, which would act on the storativity evaluated in the future, for prevision purposes. of the aquifer. A change in groundwater flow directions The response of the monitored fractured aquifers to the due to the tilting of the structure and the consequent dis- earthquakes poses questions in terms of future management location of the permeability threshold induced by faulting of groundwater resources of central Apennines. Furthermore, could be an additional factor. the discharge increase recorded along the Nera River basin has raised flood risks for the urban areas struck by the earthquake, Based on the suggested conceptual models, two different as in Castelsantangelo sul Nera and Norcia. As several of the evolutions of the groundwater flow could possibly be faced in monitored springs are tapped for drinking purposes, it is nec- the near future: essary to carefully evaluate the consequences and the mid-to- long-term evolution of the spring discharge. & If and where the fracture cleaning effect is solely respon- sible for discharge increase, gradually the spring and river discharge will return to previous values, without any per- Conclusions manent long-term effects; in this case, the additional vol- umes released by the aquifers will not be recovered, but in The abrupt and sustained variations of spring discharge and some years the groundwater system will probably regain groundwater levels, observed in carbonate fractured aquifers its stability, as happened for the Gran Sasso aquifer after in central Italy by a wide selection of water points during the the 2009 earthquake; 2016–2017 central Apennine seismic sequence, cannot be at- & Alternatively, if and where the effective porosity and tributed to natural hydrological drivers and have therefore to consequently the storativity of the aquifers (at least of be related to the earthquakes. The main findings obtained by the Sibillini Mts. area) have definitely decreased due analyzing data from more than 20 monitoring sites are the to subsidence effects and/or fracture-width reduction, following: a share of the permanent reserves could be lost, and more perceivable effects would affect the groundwater 1. The main shocks affected groundwater as far as 100 km system. In this case, long-term changes are expected in the regime of the springs, which would become from the epicenters, with instrumentally perceivable, 1024 Hydrogeol J (2018) 26:1009–1026 Boncio P, Lavecchia G (2000) A structural model for active extension in more impulsive, leading to a higher seasonal variation. central Italy. J Geodyn 29(3–5):233–244. https://doi.org/10.1016/ Nevertheless, changes in the total amount of recharge, S0264-3707(99)00050-2 and consequently of discharge, are not expected, be- Boni C, Bono P, Capelli G (1986) Schema idrogeologico dell’Italia cause possible storativity reduction does not influence Centrale [Hydrogeological scheme of central Italy]. Mem Soc Geol It 35:991–1012 the infiltration from rainfall. Boni C, Baldoni T, Banzato F, Cascone D, Petitta M (2010) Hydrogeological study for identification, characterization and man- Acknowledgements The authors are thankful to the following water sup- agement of groundwater resources in the Sibillini Mountains ply companies and public bodies for making freely available spring or National Park (central Italy). Ital J Eng Geol Environ 2:21–39. river discharge and water level data: Gran Sasso Acqua and Ruzzo Reti https://doi.org/10.4408/IJEGE.2010-02.O-02 Companies; Nera aqueduct company (S.A.N. spa); CIIP S.P.A. 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Water-table and discharge changes associated with the 2016–2017 seismic sequence in central Italy: hydrogeological data and a conceptual model for fractured carbonate aquifers

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Abstract

A seismic sequence in central Italy from August 2016 to January 2017 affected groundwater dynamics in fractured carbonate aquifers. Changes in spring discharge, water-table position, and streamflow were recorded for several months following nine Mw 5.0–6.5 seismic events. Data from 22 measurement sites, located within 100 km of the epicentral zones, were analyzed. The intensity of the induced changes were correlated with seismic magnitude and distance to epicenters. The additional post-seismic discharge from rivers and springs was found to be higher than 9 m /s, totaling more than 0.1 km of groundwater release over 6 months. This huge and unexpected contribution increased streamflow in narrow mountainous valleys to previously unmeasured peak values. Analogously to the L’Aquila 2009 post- earthquake phenomenon, these hydrogeological changes might reflect an increase of bulk hydraulic conductivity at the aquifer scale, which would increase hydraulic heads in the discharge zones and lower them in some recharge areas. The observed changes may also be partly due to other mechanisms, such as shaking and/or squeezing effects related to intense subsidence in the core of the affected area, where effects had maximum extent, or breaching of hydraulic barriers. . . . . Keywords Earthquake Groundwater monitoring Italy Co-seismic effects Carbonate rocks * Elisabetta Preziosi Laboratori Nazionali del Gran Sasso, Istituto Nazionale di Fisica elisabetta.preziosi@cnr.it Nucleare, Via G. Acitelli, 22, 67100 Assergi (AQ), Italy Dipartimento InGeo, Università BG. d’Annunzio^, via dei Vestini 30, Department of Earth Sciences, Sapienza University of Rome, P.le A. 66013 Chieti, Italy Moro 5, 00185 Rome, Italy Institute of Environmental Geology and Geoengineering, National Water Research Institute, National Research Council, Via Salaria km Research Council, Via Salaria km 29,300, PB 10, 29,300, PB 10, 00015 Monterotondo (Rome), Italy 00015 Monterotondo (Rome), Italy Department of Sciences, University of Roma Tre, L.go San Leonardo Murialdo 1, 00146 Rome, Italy Università Politecnica delle Marche, via Brecce Bianche 12, 60131 Ancona, Italy Institute of Environmental Geology and Geoengineering, National Research Council, at Sapienza University of Rome, Department of Earth Sciences, P.le A. Moro 5, 00185 Rome, Italy Università degli Studi di Cassino e del Lazio Meridionale, DICeM, via Di Biasio 43, 03043 Cassino, Italy Department of Physics and Geology, University of Perugia, Via A. Pascoli snc, 06123 Perugia, Italy Department of Civil, Construction-Architecture and Environmental Centro Nazionale Terremoti, Istituto Nazionale di Geofisica e Engineering, University of Aquila, Via G. Gronchi 18, Vulcanologia, Rome, Italy 67100 L’Aquila, Italy 1010 Hydrogeol J (2018) 26:1009–1026 Introduction Bsustained offset^, which include abrupt rises or falls and sustained gradual rise lasting for several days after the shock The seismic sequence recorded in central Italy in 2016–2017 (Roeloffs 1998; Yan et al. 2014). The most frequent conse- included nine main events (Table 1) withmomentmagnitude quences of earthquakes are spring and river discharge increase (Mw) ≥5.0 (four of which were Mw ≥ 5.5) occurring on four and water-table rise, which are generally attributed to four separate days (August 24th 2016, October 26th and 30th 2016 general classes of possible explanations: (1) co-seismic static and January 18th 2017), as described in detail in Chiaraluce strain increases pore pressure that may contribute to change et al. (2017) and ISIDe Working Group (2016). The main permeability (e.g. Wakita 1975; Jonsson et al. 2003); (2) events caused several observed changes in groundwater dy- earthquake-related dynamic strains may increase permeability, namics, including spring discharge variation, water-table permitting a more rapid flow, which in fractured aquifers can anomalies and river discharge alteration in different basins be enhanced by fracture cleaning, eventually increasing dis- located up to 100 km from the epicentral zone. The fractured charge (e.g. Briggs 1991; Rojstaczer and Wolf 1992; and locally fissured carbonate nature of the aquifers outcrop- Rojstaczer et al. 1995;Satoet al. 2000;Wanget al. 2004a; ping in the earthquake area favors a quick co-seismic response Curry et al. 1994; Amoruso et al. 2011); (3) breaching of in terms of pore pressure propagation; however, the observed hydraulic barriers or seals (e.g. Sibson 1994; Brodsky et al. sustained changes, which developed during several days after 2003;Wangetal. 2004a); (4) the excess of water discharged the main shocks, affected groundwater dynamics for several after the earthquake lies in the shallowest subsurface where months after the seismic events. water is liberated by the consolidation or even liquefaction of Hydrogeological changes caused by earthquakes have been near-surface unconsolidated materials (e.g. Manga 2001; historically reported. Instrumentally measured responses, Manga et al. 2003;Montgomery et al. 2003). however, have become available only in the last few decades. Looking at the relationships between tectonic framework, These responses include changes in water level (Leggette and hydrogeological setting and earthquakes from a wider point of Taylor 1935;Cooperetal. 1965; Roeloffs 1998; Brodsky et al. view, recent research activities have highlighted the role of 2003; Roeloffs et al. 2003; Lachassagne et al. 2011; Shi et al. fluids at crustal scale during the seismic cycle. Doglioni 2015), temperature (Mogi et al. 1989), chemical composition et al. (2014), for instance, suggest that fluid flow rates differ (Claesson et al. 2004; Skelton et al. 2014), stream flow during the different periods of the seismic cycle (inter-seismic, (Manga et al. 2003; Montgomery and Manga 2003;Manga pre-seismic, co-seismic and post-seismic periods), also in con- and Rowland 2009; Muir-Wood and King 1993;Rojstaczer nection with the tectonic style. In particular, they hypothesize et al. 1995), and spring attributes (Wang and Manga 2015). that in extensional tectonic settings like central Italy, the Understanding the origin of these hydrological and hydrogeo- wedge of crust above the brittle ductile transition remains chemical phenomena may have significant impacts on the Bsuspended^ while a dilated area forms during the inter- comprehension of the occurrence of liquefaction (Cox et al. seismic period. This area would trap deep fluids, which when 2012), water supply and quality (Gorokhovich and Fleeger the wedge of crust above the brittle ductile transition starts to 2007), underground storage (Wang et al. 2013) and pore- drop in the pre-seismic period, would be squeezed above due pressure triggered seismicity (Brodsky et al. 2003). to the progressive fracture closing. Consequently, in the co- The effects of earthquakes on groundwater are commonly seismic period, aquifers can host changes in hydrochemistry divided into Btransient oscillations^ (Cooper et al. 1965)and (Barberio et al. 2017) and in water levels, independently from Table 1 Seismic events with Mw ≥ 5.0 recorded in central Italy between 24 August 2016 and 18 January 2017. Data from the four main events having Mw ≥ 5.5 are highlighted in italic. Source: INGV (2017) Date time (UTC) Mw Epicenter location Depth (km) Latitude Longitude August 24th 2016 01:36:32 6.0 Accumoli 8.1 42.6983 13.2335 August 24th 2016 02:33:28 5.3 Norcia 8.0 42.7922 13.1507 October 26th 2016 17:10:36 5.4 Castelsantangelo sul Nera 8.7 42.8802 13.1275 October 26th 2016 19:18:05 5.9 Castelsantangelo sul Nera 7.5 42.9087 13.1288 October 30th 2016 06:40:17 6.5 Norcia 9.2 42.8322 13.1107 January 18th 2017 09:25:40 5.1 Capitignano 10.0 42.5450 13.2768 January 18th 2017 10:14:09 5.5 Capitignano 9.6 42.5310 13.2838 January 18th 2017 10:25:23 5.4 Capitignano 9.4 42.5033 13.2770 January 18th 2017 13:33:36 5.0 Barete 9.5 42.4733 13.2747 Hydrogeol J (2018) 26:1009–1026 1011 changes due to the previously listed local mechanisms, which fault system named Olevano-Antrodoco line (Pierantoni can affect the hydrodynamics of the struck aquifers during and et al. 2005 and references therein). From the upper after the seismic sequence. Such a comprehensive tectonic Miocene to lower Pliocene, thrust migration towards the model allows looking at changes induced in groundwater after east was coupled with the progressive development of earthquakes in a general framework of crustal deformation, fore-deeps in front of the migrating fold-and-thrust belt suggesting the role of deep inputs in triggering the aforemen- (Cipollari and Cosentino 1995). tioned well-known processes (as pore pressure changes, per- Since the upper Miocene–lower Pliocene, extensional meability increase, liquefaction/consolidation, etc.) acting at faulting connected with the opening of the back-arc the aquifer scale. Tyrrhenian Basin has been dissecting the compressive struc- The effects of past earthquakes on groundwater in central tures (Boncio and Lavecchia 2000 and references therein), Italy have been described by previous papers. Esposito et al. leading to the development of intermontane basins filled with (2001) describe the effects of four earthquakes in southern thick continental sequences of Quaternary alluvial, detrital Apennines including the 1980 Irpinia earthquake, which gen- and lacustrine deposits (Cavinato and De Celles 1999). erated important hydrogeological changes as far as 200 km Some normal faults show evidence of Holocene activity, sug- from the epicenter, including a significant increase of gesting that they may be responsible for the seismic activity Caposele spring flow. Amoruso et al. (2011)describethe occurring in this sector of the Apennines (Cello et al. 1998), hydrogeological changes in a fractured aquifer after the mainly confined within the upper part of the crust (<16 km; L’Aquila 2009 earthquake, inferring that those changes were Lavecchia et al. 1994;Boschietal. 1995). probably connected with the increase of bulk hydraulic con- In the study area, the 2016–2017 seismic sequence in- ductivity at the aquifer scale, mainly due to fracture cleaning, cludes some of the largest instrumental earthquakes of the raising hydraulic heads in the discharge zones, and corre- last 40 years (Norcia 1979 Mw = 5.9, Irpinia 1980 Mw = spondingly lowering them in the recharge areas (Adinolfi 6.9, Gubbio 1984 Mw = 5.2, Colfiorito 1997 Mw = 5.9, Falcone et al. 2012; Galassi et al. 2014). L’Aquila 2009 Mw = 6.3; Pantosti and Valensise 1990; The aim of this paper is to present an overview of the Boncio and Lavecchia 2000; Deschamps et al. 2000; effects of the 2016–2017 seismic sequence on the dynam- Chiarabba et al. 2009). The 2016–2017 sequence and its ics of groundwater flow in central Apennines, analyzing main shocks (Table 1) were generated by the Gorzano Mt.- the extent of the impacted area, possible relations between Vettore Mt.-Bove Mt. faults (Galadini and Galli 2003,LMF tectonic environments, geological-hydrogeological set- and MVF in Fig. 2). The seismic crisis started with the ting, and groundwater changes, and providing preliminary August 24th 2016 event (Mw 6.0) and a further significant considerations on the possible causes of the observed event on October 26th. The Vettore Mt. fault experienced a rupture with tectonic segments ~10 km long and a surface- phenomena. displacement of ~30 cm (Smeraglia et al. 2017). The October 30th 2016 event (Mw 6.5) was generated by the Geological and hydrogeological framework rupture of the central zone of the fault by a normal move- ment. The focal mechanism, identical to the previous earth- The central Apennines (Italy) is a Meso-Cenozoic ENE- quakes, was a strike-angle of N155°, a WSW dip slip and a dipping thrust-and-fold belt mainly developed during up- dip angle about of 50° in depth (RCMT 2016). During the per Miocene-Quaternary, composed by a pre-orogenic October 30th 2016 event, the entire Vettore Mt.-Bove Mt. Triassic-Miocene sedimentary succession overlain by fault system gave origin to important surface faulting occur- Miocene and Pliocene synorogenic sediments, resulting rences, reusing the pre-existing fault plane and redisplacing in a highly variable facies and thickness distribution. A the fault segments previously broken. Meso-Cenozoic carbonate platform domain extends in the The acquisition of the interferometric satellite data SE part of the study area (Latium Abruzzi Apennine, Fig. ALOS-2 from JAXA (Japan Aerospace Exploration 1), consisting of a 5,000-m-thick sequence of limestone Agency) and further interferometric analyses (INGV and subordinate dolomite of Upper Triassic to upper Central Italy Earthquake Team 2016)providedanestima- Miocene age (Brandano and Loche 2014 and references tion of the co-seismic subsidence along the NW–SE com- therein). In the western side of the area (Umbro Marchean ponent reaching a maximum of ~80 cm (LOS: Satellite Apennine), a Lower Jurassic carbonate shelf unit is over- Observation Line). The horizontal co-seismic maximum lain by stratified pelagic sediments (middle Lias-lower movements consist of ~40 cm towards NE, ~30 cm towards Miocene), with an overall thickness of 2,500–3,000 m SW as well as a maximum vertical movement of about 20– (Marchegiani et al. 1999). The Apennine orogenesis 40 cm, when considering also the October 30th event overthrusts the Umbria-Marche succession onto the (INGV Working Group GPS 2016;INGV CentralItaly Latium–Abruzzi platform along the main regional thrust Earthquake Team 2016). 1012 Hydrogeol J (2018) 26:1009–1026 Fig. 1 Hydrogeological setting of the study area. 1 Alluvial aquifers and aquifers; 5 Main springs (mean discharge >0.5 m /s); 6 Main streambed aquitards; 2 Synorogenic low-permeability deposits; 3 Latium-Abruzzi springs (mean discharge >0.5 m /s) carbonate fractured aquifers; 4 Umbro-Marchean carbonate fractured In the study area, the fractured carbonate ridges host the Widespread karst development, including endorheic ba- main aquifers, feeding several perennial springs (Nanni and sins, ensures high infiltration rates, from 500 to 700 mm/year, Vivalda 2005; Martarelli et al. 2008; Mastrorillo et al. 2009; in the Umbria Marchean aquifers and up to 900 mm/year in Mastrorillo and Petitta 2014) with steady regimen, located the Latium-Abruzzi aquifers, collectively feeding a total dis- mostly at the external boundaries of the aquifers (Fig. 1). charge of about 300 m /s (Boni et al. 1986, 2010). Fractures Groundwater flows in fissured to locally karstified carbonates. and karst conduits allow for fast vertical flow in the vadose The Miocene-Pliocene synorogenetic silicoclastic sediments zone, while the large thickness of the saturated zone facilitates surrounding the carbonate aquifers, as well as the Plio- a steady flow towards the basal springs that show outstand- Quaternary deposits, filling the intermontane plains and the ingly high and steady discharge (Petitta 2009; Amoruso et al. river valleys, act like aquitards (Petitta et al. 2011). 2014;Fiorilloet al. 2015). Hydrogeol J (2018) 26:1009–1026 1013 Fig. 2 Location of the measurement points, main epicenters and activated Vettore fault (Vettore-Bove faults); NF Norcia fault; CF Cascia fault; faults: R river gauge; W well; S spring. NP Norcia Plain. See Tables 2, 3 LMF Monti della Laga fault (Gorzano Mt.-Campotosto faults); MF and 4 for details. Epicenters are represented by stars: August 24th 2016 Montereale fault; COF Colfiorito fault; CAF Campo Imperatore- event is in purple, October 26th 2016 in red, October 30th 2016 in cyan Assergi fault; MSF Morrone-Sulmona fault. Source: INGV Central and January 18th 2017 in orange. Quaternary fault systems: MVF Monte Italy Earthquake Team (2016) Methodology The monitoring site locations are shown in Fig. 2, distinguishing piezometric heads in monitoring wells Co-seismic changes were examined in several observation (W1–W5), spring discharges (S1–S12) and hydrometric sites located within the area affected by the earthquakes. levels or discharge in river gauging stations (R1–R6). In Within the framework of a continuous monitoring, the collect- addition, in February 2017, water-table levels in the po- ed data refer to piezometric heads in wells and piezometers, rous local aquifer of the Norcia Plain (NP in Fig. 2)were spring discharges and river hydrometric levels or discharges. recorded in 16 wells and compared to a piezometric map realized in 2011. Collected data The monitored springs can be divided into the following: basal springs (S4, S5, S7 and S8), whose steady discharge Altogether, 22 automatic records from continuous moni- shows limited seasonal variation; springs fed by shallow aqui- fers (S1, S2 and S3); and high-elevation springs (S6, S9, S10, toring sites were collected, plus one manually measured. 1014 Hydrogeol J (2018) 26:1009–1026 S11 and S12) having a seasonal slightly variable discharge. where available, have been used in conjunction with stage All the springs, except Torbidone (S5), are tapped for water measurements to determine the river discharges. supply systems. The monitoring wells tap the basal aquifers at depths from Methods of data processing 20 to 250 m. In some cases (W1, W2 and W3), the water level recordings were occasionally affected by the operations for Because of the different nature of data sources, the time series the related water-supply systems. In detail, W1 and W2 were from continuous monitoring may have different measurement affected by operational changes in the tunnel drainage system frequency, with intervals ranging from 0.05 s to 24 h. To of the near San Chiodo spring, whereas the disturbances prior ensure uniformity, data have been aggregated and analyzed to August 2016 in W3 were due to the works for the construc- at daily scale. tion of a new aqueduct. The mean discharge of each data series was calculated The discharge of the monitored rivers can be consid- considering the time intervals before, between and after ered an indicator of hydrogeological changes at the basin the four major seismic events of Mw ≥ 5.5. The first in- scale, because a significant amount of groundwater direct- terval corresponds to the period before the first main seis- ly feeds the rivers’ baseflow by streambed springs. The mic event, and the second, the third and the fourth ones rivers with catchment areas less than 100 km (R1, R2, identify the time intervals between the first–second, sec- R3 and R5) have a steady regimen too and are predomi- ond–third and third–fourth main events; the last one cor- nantly fed by baseflow, whereas the runoff may be con- responds to the period after the fourth main event. The sidered negligible. In the widest river basins (>1,000 km ; discharge/level variation associated with each one of the R4 and R6) the runoff contribution cannot be disregarded. four main events was calculated as the difference between It follows that the river discharge is more variable, despite the daily value prior to and after each event. In cases the clear dominant role of baseflow. where the changes were very abrupt and the difference Data have been recorded by regional hydrographic between the daily values was not appreciable, hourly services, water supply companies or directly by the re- values were considered. Where even the hourly difference search teams monitoring the earthquake zones. All was not evident, changes have been considered as Bnot available data from June 1st 2016 to February 28th significant^ (NS). All calculated values are shown in 2017 were considered. Public service data are available Tables 2, 3 and 4. on-line (ARPA Umbria 2017; Regione Marche 2017; Regione Umbria 2017). Results Methods of data measurement Figures 3, 4, 5 and 6 show the time plots of the available data from monitoring sites (location in Fig. 2), summa- Water-table depths have been recorded in wells and pie- rized in Tables 2, 3 and 4, which refer respectively to zometers by downhole data loggers with atmospheric water levels in piezometers (W1–W5), spring discharge compensation. The horizontal piezometer W4, located in (S1–S12) and river discharge or levels (R1–R6). Red bars the underground National Institute for Nuclear Physics in Figs. 3, 4, 5 and 6 indicate the four main Mw > 5.5 (INFN) laboratory, measures the hydraulic head (pressure events. For the sake of simplicity, the August 24th 2016, in MPa) by a 3-channel 24-bit ADC (Analog to Digital October 26th 2016, October 30th 2016 and January 18th Converter; De Luca et al. 2016). To quantify local head 2017 earthquakes will be named hereinafter 1st, 2nd, 3rd changes during each seismic event, only W4 original pres- and 4th events respectively. See Fig. 2 for the locations of sure data were converted (approximately) to water-table epicenters and the main active fault systems. Both the 1st elevation by multiplying the pressure (MPa) by 100 and and the 2nd events were clearly perceived in the S1, S2 adding the obtained elevation to the elevation of the top and S3 northern sites (Fig. 3a), with abrupt step-like var- of the borehole (987 m a.s.l.). iations. The 3rd event, the strongest of the sequence, only Spring discharges have been measured by automatic water slightly affects the discharge at S1 and S3. level sensors in weirs or in Venturi tubes, and converted into Further south, Forca Canapine spring and Pescara discharge through the related rating curve. Only for the spring (S6 and S7, Fig. 3b) show a clear increase of dis- Torbidone spring, was the discharge manually measured with charge after the 1st event, more step-like for S6 but grad- a portable flow meter, starting on November 11th 2016 at a ual and sustained for S7. These two springs are located at frequency of one measurement about every 5 days. River high elevation (1,350 and 850 m a.s.l.) and they both gauging stations are equipped with water-height data loggers suffered for a sharp decrease after the 3rd event, which or automatic ultrasonic measurement sensors. Rating curves, completely dry up at S6. Aso River nearby (R2, Fig. 3b), Hydrogeol J (2018) 26:1009–1026 1015 Table 2 Water table changes caused by the four main seismic events (Mw ≥ 5.5) Site ID Name Elevation (m a.s.l.) Average level pre Δ Water level Average level Δ Water level Average level Δ Water level Average level Δ Water Average EQ1 (m a.s.l.) EQ1 (m) post EQ1 EQ2 (m) post EQ2 EQ3 (m) post EQ3 level EQ4 level post EQ4 (m a.s.l.) (m a.s.l.) (m a.s.l.) (m) (m a.s.l.) W1 PIEZ. D 779 759.94 + 2.17 761.91 + 1.22 763.58 + 6.96 770.05 0 769.21 W2 PIEZ. G 823 775.54 −1.07 777.02 −1.14 777.39 +6.05 781.23 0 780.57 W3 Renari di Capriglia 329 295.61 + 0.62 296.95 NA NA NA NA NA NA W4 Borehole S13 987 1252 +1.00 1253 0 1196 +1.10 1198 +0.20 1194 W5 Bussi sul Tirino 238 226.27 +0.28 226.07 +0.04 225.96 +0.21 225.81 +0.82 226.14 EQ1: August 24th 2016 seismic event. EQ2: October 26th 2016 seismic event. EQ3: October 30th 2016 seismic event. EQ4: January 18th 2017 seismic event. Negative changes are highlighted in italic NA not available value Table 3 Spring discharge variations caused by the four main seismic events (Mw ≥ 5.5) Site ID Name Elevation Average discharge Δ Discharge Average discharge Δ Discharge Average discharge Δ Discharge Average discharge Δ Discharge Average discharge 3 3 3 3 3 3 3 3 3 (m a.s.l.) pre EQ1 (m /s) EQ1 (m /s) post EQ1 (m /s) EQ2 (m /s) post EQ2 (m /s) EQ3 (m /s) post EQ3 (m /s) EQ4 (m /s) post EQ4 (m /s) S1 Montenero 840 0.057 +0.002 0.044 +0.015 0.050 NS 0.064 NS 0.062 S2 Nicolini 470 0.073 +0.010 0.061 +0.016 0.078 NS 0.060 NS 0.072 S3 Valcimarra 310 0.032 +0.004 0.020 +0.008 0.020 +0.005 0.020 NS NA S4 Alzabove 640 0.298 +0.019 0.278 +0.011 0.269 +0.008 0.259 0 0.256 S5 Torbidone 610 0 0 0 0 0 +1.510 0.990 NS 1.450 S6 Forca Canapine 1350 0.030 +0.045 0.021 +0.003 0.029 −0.029 0NS 0 S7 Pescara 850 0.259 +0.129 0.350 −0.012 0.337 −0.059 0.242 NS 0.240 S8 Lupa 375 0.115 +0.0003 0.091 +0.0005 0.075 +0.004 0.072 NS 0.062 S9 Assergi 967 0.495 +0.058 0.515 NS 0.515 +0.045 0.515 +0.019 0.486 S10 Ruzzo 964 0.979 +0.016 0.995 NS 0.995 +0.085 0.922 NA NA S11 Vacelliera bassa 937 0.193 +0.019 0.200 NS 0.200 +0.021 0.211 NA NA S12 Vacelliera alta 1015 0.134 +0.021 0.140 NS 0.140 +0.019 0.146 NA NA EQ1: August 24th 2016 seismic event. EQ2: October 26th 2016 seismic event. EQ3: October 30th 2016 seismic event. EQ4: January 18th 2017 seismic event. Negative changes are highlighted in italic NA not available value, NS not significant change 1016 Hydrogeol J (2018) 26:1009–1026 monitored at the spring outlet, is also clearly influenced by the 1st event with a step-like increase and limitedly affected by the following events. Different effects were registered in the area closest to the 2nd and 3rd events,inthe SanChiodospringarea (Fig. 4a) and in the Norcia area (Fig. 4b). In the San Chiodo area (Fig. 4a) a water supply system is operating by a tunnel drainage; periodical operational changes, opening and clos- ing of different drainage tunnels, produce clear variations in the water levels (black vertical dashed lines in Fig. 4a). The responses of two of the 14 available piezometers (W1 and W2), considered representative of the entire monitoring net- work, and the discharge of the Upper Nera River down- stream at Castelsantangelo (R1), are shown. After the 1st event, the system reacted with a sharp step-like increase in the downgradient part of the aquifer (W1, 779 m a.s.l.) and a clear step-like decrease in the upper part of the aquifer (W2, 823 m a.s.l.). The same happened on the 2nd event, while on the 3rd event the water level in W1 increased gradually by nearly 7 m, while in W2 it firstly decreased, then steadily increasedfor severaldaysupto6min height.After the3rd event, the discharge of the Upper Nera River (R1) doubled; this quick increase reached a steady state in December 2016. In the Norcia area (Fig. 4b) the Torbidone spring (S5), reactivated after the 3rd shock, shows a gradual increase in discharge up to 1.5 m /s in about 3 months after the shock. The Sordo River (R3), receiving the Torbidone dis- charge, also reacted with a clear gradual and sustained in- crease lasting several months, due also to a different direct groundwater inflow. In addition, the water table of the po- rous clastic aquifer of the Norcia Plain has shown a hydrau- lic head increase, reaching +15 m at the contact with the carbonate aquifer, with respect to the water table recorded in 2010–2011. The western-located S4, S8 and W3 (Fig. 4c) registered abrupt positive step-like increases for the 1st, 2nd and 3rd events, with some differences among them. The Nera River at Torre Orsina (Fig. 4d) receives the entire inflow of the aforementioned flow systems, and others not described here due to the absence of recordings; however, its discharge is clearly influenced by the 1st, 2nd and 3rd events, as shown by the abrupt step-like increases coincident with the first three red lines. Although no significant precipitation is recorded, the river discharge did not decrease for the 3 months following the 3rd event. Overall, the Nera River suffered for a total dis- charge increase of about 9 m /s considering the first three main events, corresponding to about 30% surplus of its natural baseflow. The 4th event, which is the southernmost, was not per- ceived at all so far to the north, but locally affected the southern monitoring points (Fig. 5), located very close to its epicenter (Fig. 2). All four springs (Fig. 5a) show sig- nificant abrupt step-like increase of the discharge after the Table 4 River discharge and hydrometric river level variations caused by the four main seismic events (Mw ≥ 5.5) Site Name Elevation Average Δ Average Δ Average Δ Average Δ Average ID ma.s.l. discharge Discharge discharge Discharge discharge Discharge discharge Discharge discharge pre EQ1 EQ1 post EQ1 EQ2 post EQ2 EQ3 (m /s) post EQ3 EQ4 post EQ4 3 3 3 3 3 3 3 3 (m /s) (m /s) (m /s) (m /s) (m /s) (m /s) (m /s) (m /s) R1 Nera at 725 1.760 +0.360 2.060 +0.155 2.180 +2.340 4.260 NS 4.290 Castelsantangelo R2 Aso at Foce 950 0.574 +0.118 0.647 NS 0.682 NS NA NA NA R3 Sordo at Serravalle 510 2.448 +0.179 3.450 NA 3.890 +2.680 5.440 NS 6.901 R4 Nera at Torre Orsina 231 23.214 +1.533 23.459 +2.390 24.888 +5.160 30.738 NS 30.749 a a a a a a R5 Tirino at 313 0.85 NS 0.85 NS 0.85 NS 0.84 +0.14 0.84 Madonnina a a a a a a R6 Aterno at Molina 454 0.23 NS 0.3 NS 0.35 NS 0.38 +0.17 0.40 Hydrometric river level (m) EQ1: August 24th 2016 seismic event. EQ2: October 26th 2016 seismic event. EQ3: October 30th 2016 seismic event. EQ4: January 18th 2017 seismic event NA not available value, NS not significant change Hydrogeol J (2018) 26:1009–1026 1017 Fig. 3 Daily discharge (m /s) of northern monitoring sites. a Central Marche; b Mt. Sibillini eastern slope. See Fig. 2 for location and Tables 3 and 4 for site characteristics. Vertical red bars locate the four main seismic events (Table 1) 1st and the 3rd event, while the 2nd event does not at all Discussion modify the hydrographs. The 4th event, in spite of being so close, only slightly influences the discharge of the sole The hydrogeological changes caused by the 2016–2017 seismic S9 station. sequence are of remarkable intensity specially if compared to Similar evidence has been recorded at a horizontal borehole the relatively limited magnitude of the events; similar or larger (W4, Fig. 6) located in the underground laboratories of the hydrological responses are very rare (Mohr et al. 2015). The INFN in the Gran Sasso massif (Petitta and Tallini 2002; estimated amount of extra discharge drained by springs and Amoruso et al. 2013). The time plot of the pressure head rivers since August 24th 2016 for the following 6 months ex- variation shows sudden increases in the hydraulic pressure ceeds 0.1 km . This amount has been obtained looking at the (MPa) with a sharp rise of about 2 m recorded on the 1st discharge of the entire Nera Basin (R4, Nera at Torre Orsina) and on the 3rd events, while no evidence was recorded after before and after the seismic sequence: the additional discharge 3 6 the 2nd and 4th events (Fig. 6). was about 1.5 m /s between the 1st and 3rd event (about 8 × 10 3 3 Further south, changes in the water table were clearly re- m ), and about of 9 m /s after the 3rd event, which until the end corded in the monitoring well at Bussi sul Tirino (W5, of February 2017 correspond to more than 0.095 km .This Barberio et al. 2016;Fig. 5b). The magnitude of the water estimation does not consider other changes observed in other level variation is about 20 cm for the first three events, with basins, which released a minor amount of discharge. a gradual sustained type variation, while an abrupt step-like Other documented examples of earthquake induced increase up to 90 cm is observed for the closest 4th event. The groundwater release include (ordered by decreasing earth- river monitoring sites R5 and R6 do not show any significant quake magnitude): the Maule Mw 8.8 earthquake in Chile variation of the hydrometric level in the 1st, 2nd and 3rd (1.1 km ,Mohr etal. 2016), the Chi-Chi Mw 7.5 earthquake events (Fig. 5b). However, after the 4th earthquake, both the in Taiwan (0.7 km ,Wanget al. 2004b), and the Hebgen Lake 3 3 Aterno River (R6) and, most clearly, the Tirino River (R5) (Mw 7.3, 0.5 km ), Borah Peak (Mw 6.9, 0.3 km ; Muir- hydrometric level responses show a sharp and sudden increase Wood and King 1993,; USGS 2017), and Loma Prieta earth- of the hydrometric levels, which drop rather quickly to nearly quakes in the USA (Mw 6.9, 0.01 km ;Rojstaczeretal. 1995); the prior level than the earthquake discharge. however, these are earthquakes larger than the central Italy 1018 Hydrogeol J (2018) 26:1009–1026 Fig. 4 Daily discharge (left vertical axis) and daily water- table levels (right vertical axis) of monitoring sites of the Nera River Basin. a San Chiodo spring area. Vertical dashed black lines represent changes caused by the water supply system management; b Norcia Plain area. Blue dots rappresent the manual measurements of Torbidone spring (S5) discharge; c western area; d Nera River closing station. See Fig. 2 for location and Tables 2, 3 and 4 for site characteristics. Vertical red bars locate the four main seismic events (Table 1) Hydrogeol J (2018) 26:1009–1026 1019 Fig. 5 Daily spring discharge, water level and hydrometric level of southern monitoring sites. a Gran Sasso springs; b Aterno– Tirino rivers (hydrometric levels on left vertical axis, water-table elevation on right vertical axis). See Fig. 2 for location and Tables 2, 3 and 4 for site characteristics. Vertical red bars locate the four main seismic events (Table 1) events. At a comparable magnitude, the Mw 6.0 South Napa 2008). Nevertheless, peculiar responses in the study area earthquake (USA) produced extra water of about 0.001 km have been observed in sites very close to the epicenters (Wang and Manga 2015). (<10 km), where abrupt changes have been frequently The extent of the area affected by hydrogeological followed by a sustained increase with time, especially changes approaches 10,000 km . The recorded hydrolog- after the 3rd event (October 30th), having the highest ical variations fall in the known fields of abrupt and magnitude. This behaviour could be due to the fractured sustained water level changes in groundwater due to nature of the aquifer, where pressure changes can easily earthquakes (Fig. 7, modified from Wang and Chia and quickly propagate along the effective porosity Fig. 6 Hydraulic pressure (MPa; Y-axis) vs. day (X-axis) of borehole 2016 to September 16th 2016 and b from October 22nd 2016 to W4. See Fig. 2 for location and Table 2 for site characteristics. The January 31st 2017. The red lines refer to the August 24th, October 30th reported data are 1-min-averaged. Monitoring period a from July 1st and January 18th earthquakes (Table 1) 1020 Hydrogeol J (2018) 26:1009–1026 entity of the response. Differently from the Latium- Abruzzi carbonate aquifers, where a basal aquifer is usu- ally governing groundwater flow, in the Umbria- Marchean aquifers, a network of interconnected faults plays a key role in determining the dynamic groundwater divide location, seepage velocity and extent of the re- charge area. It follows that groundwater flow can be eas- ily influenced by seismic events; furthermore, during the 2016–2017 sequence, the study area suffered from many earthquakes whose epicenters were differently located along the central Apennine chain; therefore, numerous aquifers and springs were hit by repeated events, or by a single group of seismic events only. Generally, a decreasing intensity of the recorded effects was observed moving from the earthquake epicenters towards Fig. 7 Distribution of earthquake-triggered hydrogeological changes as a the distal areas. Figure 8 shows the spring discharge/water function of earthquake magnitude (horizontal axis) and epicentral level variation, as a function of the distance between the mon- distance (vertical axis). Also plotted are the contours (oblique lines) of constant seismic energy density (Wang and Manga 2010)and the itored points and the epicenters of each seismic event. The domains where different types of coseismic water level responses occur result is a cloud of values from about 10—200% in the near- (Wang and Chia 2008). The triangles represent the water level changes in field (less than 10 km from epicenters), clearly decreasing wells or in rivers, the dots the discharge changes in spring or rivers. The with distance. Up to +50% in discharge and +2 m water level four main events are distinguished by different colors: August 24th 2016 increase have been recorded between 20 and 30 km away, (Mw = 6) event is in purple, the October 26th 2016 (Mw = 5.9) event in red, the October 30th 2016 (Mw = 6.5) event in cyan and the January 18th while minor changes have been observed for sites located 2017 (Mw = 5.5) event in orange between 60 and 100 km from the epicenters. Locally, dis- charge decreases have been encountered at less than 10-km network, reaching the boundaries of the aquifers, causing distance from the epicentral area, but only for the October change in hydrodynamic independently from seismic- 30th event, which was the more energetic one. induced stresses (Amoruso et al. 2011). Furthermore, the The mechanism causing both sustained negative and occurrence of several subsequent events repeatedly im- positive hydraulic changes is frequently related to static pacted the aquifers, as happened on October 30th 2016 stress modifications evidenced by comparison between for the third time in 2 months. Similar events have been pre-seismic and post-seismic conditions (Jonsson et al. not reported in the literature so far. 2003; Montgomery and Manga 2003;Parvinetal. 2014; In addition, the characteristics of the fractured aquifers Mohr et al. 2016). The difference between transient dy- impacted by the shocks may also have influenced the namic oscillations (Cooper et al. 1965) and the offset-type Fig. 8 Epicentral distance (horizontal axis) vs the discharge changes, 200%. The four main events are represented by different colors: August expressed as a percentage of the pre-event period mean discharge (left 24th 2016 event is in purple, October 26th 2016 in red, October 30th 2016 vertical axis) and the water level changes (right vertical axis). Torbidone in cyan and January 18th 2017 in orange. The vertical dashed lines show spring (S5), whose ratio to the pre-event discharge would be infinite (as the changes in the near-affected area (<10 km from epicenters) and in a the spring was dry prior to the earthquake) is marked by an asterisk at larger-affected area (up to 60 km from epicenters) Hydrogeol J (2018) 26:1009–1026 1021 Fig. 9 Maps of the entity and directional (positive or negative) changes in discharge (%) and in water level (m) after each main event: a August 24th 2016; b October 26th 2016; c October 30th 2016; d January 18th 2017 1022 Hydrogeol J (2018) 26:1009–1026 water-level changes affecting, in a more permanent way, propagation due to dynamic stresses caused by the seismic groundwater flow, is also well described in Yan et al. waves, which determined the sudden peak, and (2) an increase (2014). Whereas the mechanism for explaining the first of bulk hydraulic conductivity of the fractured aquifer due to type of effects is substantially accepted as being transient fracture cleaning triggered by the pore pressure propagation, oscillations due to crustal dynamic poro-elastic deforma- which induces mobilization by shaking fine particles that tion in an aquifer during the passage of seismic waves block fracture throats (Amoruso et al. 2011). (Rexin et al. 1962; Kitagawa et al. 2006; Yan et al. Recorded hydrological changes for the 2016–2017 earth- 2014), the cause of permanent offsets, corresponding to quakes may be preliminarily attributed to the pore pressure sustained changes, is still debated. propagation in the aquifers, which would be followed by a In this study case, the distribution of changes observed sustained discharge increase attributable to fracture cleaning, after each main event, and their positive or negative ef- mobilizing fine particles from fractures (Brodsky et al. 2003; fects, are synthetized in Fig. 9. The impact on most of the Wang and Manga 2010; Adinolfi Falcone et al. 2012), as springs is the increase of discharge with varying magni- reflected by turbidity increase, clearly recorded in several tude. Most of the recordings in wells and in rivers clearly monitoring points (e.g. R1, S5, S11, S12). In this case, the indicated a sudden and sharp increase simultaneous with superimposition of the post-seismic changes on a recession the earthquakes, generally followed by a steady increase phase makes the post-earthquake evolution clearer. The dy- lasting for a few days after the shock, and a subsequent namic stress due to pore pressure propagation has been smooth decrease. Few points show a decrease of water clearly observed at the high-frequency-monitoring site W4 level or discharge: the disappearance of one tapped spring (Fig. 10), where the short-term (30 s) changes in hydraulic (S6) and a significant discharge decrease of the spring S7, head are the response to the seismic wave of August 24th, both located at high elevation (Fig. 3b). Similar decreases whose effects on groundwater levels ended after a few or drying-up have been observed in other minor not- minutes. monitored springs located in the same recharge area. In this study, the sustained response of several monitored Another monitored site experiencing post-earthquake de- sites to the subsequent seismic events, considering the frac- crease in the water table is in Upper Nera Valley, where tured nature of the struck aquifers, support the hypothesis of only the highest-elevation monitoring well (W2, Fig. 5a) fracture cleaning and the consequent increase of the bulk suffered from a sharp lowering, quickly balanced in the hydraulic conductivity. In many cases, discharge increase following days. has reached its peak a few days after one of the main events, The decrease of discharge or water levels and the disap- followed by a decreasing trend due to the recession phase. pearance of springs have been also been associated in litera- Accordingly, in several cases, mainly located far from epi- ture to proximity to the active faults. With respect to the water- centers, the next earthquakes did not cause permanent level changes observed in the footwall area of the fault acti- changes, but only a temporary discharge increase, favoring vated by the Chi-Chi earthquake, Chia et al. (2001)report that the model of fracture cleaning. The location of negative water-level rise was the predominant effect in most of the area, effects at high-elevation sites confirms this hypothesis, as whereas water-level fall prevailed in a narrow zone adjacent to water-table decrease is expected in recharge zones of struck the fault trace. Amoruso et al. (2011) report that after the aquifers, while increase in discharge is common in low- L’Aquila Mw 6.3 earthquake, the two highest springs, located elevation zones. on the trace of the activated fault, suddenly dried up after the A more complex response has been recorded in the core main shock. The opposite phenomenon, the reactivation of area, i.e. between the epicenters of August 24th and October dry springs or stream, as observed in this case for the 30th events. In the Upper Nera River Valley (R1, W1 and Torbidone spring (S5), is also well known. After the 2014 W2), in the Norcia Plain (S5, R3) and partially in the eastern Mw 6.0 South Napa earthquake, many streams and springs, side of the Sibillini Mts. (S7, R2), the effects of both events is which were dry or nearly dry, started to flow after the earth- quake (Wang and Manga 2015). The possible explanation given by the mentioned authors is the enhanced permeability in the recharge areas. Generally, after the co-seismic peak, discharge and water levels remained on higher values with respect to pre-seismic conditions. A similar mechanism was observed after the 1980 Irpinia earthquake at Caposele spring (Esposito et al. 2001) and, more recently, in the Abruzzi region after the L’Aquila Fig. 10 Water pressure signal (in MPa) at W4 replicating the arrival of 2009 earthquake (Adinolfi Falcone et al. 2012). This last case seismic waves of the August 24th event (modified from De Luca et al. has been explained by a double effect: (1) pore pressure 2016) Hydrogeol J (2018) 26:1009–1026 1023 still evident in the following months. The discharge of springs sometimes dramatic, effects. A generalized decrease and water-table elevations remained very high with respect to of the magnitude of the effects with distance was ob- pre-earthquake conditions, as testified by the Nera main gaug- served. The observed effects may be summarized as ing station (R4), not simply justifiable by seismic stresses. The follows: (1) increase (rarely decrease) of heads mea- new conditions of groundwater flow are testified by the ob- sured in wells/piezometers, (2) positive (rarely nega- served changes in the recession curve of San Chiodo spring tive) variations of spring discharge, (3) positive vari- (R1, W1, Fig. 5a): previous values of α = 0.003 calculated for ation of river baseflow, (4) activation of historically the 2011–2015 period, strongly decrease to α = 0.001 after the intermittent springs, and (5) drying up of high-elevation earthquakes, testifying to a continuous additional contribution springs. A quick oscillation, correlated with dynamic from the aquifer with time. stresses, was observed in a few sites equipped with Consequently, other possible additional factors may have high-frequency recordings. influenced the post-seismic response of the area of Sibillini 2. Within 6 months from August 24th 2016, more than Mts. The impressive increase of spring and river discharge 0.1 km of groundwater has been additionally discharged observed in the Upper Nera River valley and Norcia plain in in the area (about +25% of the natural discharge). the mid-term responses, may be correlated with the subsi- Comparison with similar case studies highlights the rela- dence induced by the toe of Vettore Mt. faulting, which might tively high amount of discharge increase with respect to have created in the core of the Sibillini Mts. aquifer an addi- the limited magnitude of the seismic events; this peculiar- tional Bsqueezing effect^. Other possible mechanisms, not ity could be explained by the succession of four main further investigated in this paper, could be related to shaking events having Mw > 5.5, which continuously struck the of the aquifer, tilting, settlement and uplifting of the fractured aquifers. seismogenetic fault-bounded structures and the consequent 3. The observed response at regional scale is compatible dislocation of permeability thresholds induced by faulting. with the cleaning of fractures and an overall mid-term Further, a possible decrease in aquifer storativity due to increase of the bulk permeability due to the co-seismic fracture-width reduction could be envisaged, which would pore-pressure propagation. have directly triggered the additional volume of groundwater 4. Eventually, the dramatic rise of the water table and of released in the months following the shocks, modifying discharge in the core area could be the result of a groundwater dynamic divides and groundwater flow direc- Bsqueezing effect^ due to the co-seismic subsidence in tions. This hypothesis, still not verified, should be carefully the Sibillini Mts. area, which would act on the storativity evaluated in the future, for prevision purposes. of the aquifer. A change in groundwater flow directions The response of the monitored fractured aquifers to the due to the tilting of the structure and the consequent dis- earthquakes poses questions in terms of future management location of the permeability threshold induced by faulting of groundwater resources of central Apennines. Furthermore, could be an additional factor. the discharge increase recorded along the Nera River basin has raised flood risks for the urban areas struck by the earthquake, Based on the suggested conceptual models, two different as in Castelsantangelo sul Nera and Norcia. As several of the evolutions of the groundwater flow could possibly be faced in monitored springs are tapped for drinking purposes, it is nec- the near future: essary to carefully evaluate the consequences and the mid-to- long-term evolution of the spring discharge. & If and where the fracture cleaning effect is solely respon- sible for discharge increase, gradually the spring and river discharge will return to previous values, without any per- Conclusions manent long-term effects; in this case, the additional vol- umes released by the aquifers will not be recovered, but in The abrupt and sustained variations of spring discharge and some years the groundwater system will probably regain groundwater levels, observed in carbonate fractured aquifers its stability, as happened for the Gran Sasso aquifer after in central Italy by a wide selection of water points during the the 2009 earthquake; 2016–2017 central Apennine seismic sequence, cannot be at- & Alternatively, if and where the effective porosity and tributed to natural hydrological drivers and have therefore to consequently the storativity of the aquifers (at least of be related to the earthquakes. The main findings obtained by the Sibillini Mts. area) have definitely decreased due analyzing data from more than 20 monitoring sites are the to subsidence effects and/or fracture-width reduction, following: a share of the permanent reserves could be lost, and more perceivable effects would affect the groundwater 1. The main shocks affected groundwater as far as 100 km system. In this case, long-term changes are expected in the regime of the springs, which would become from the epicenters, with instrumentally perceivable, 1024 Hydrogeol J (2018) 26:1009–1026 Boncio P, Lavecchia G (2000) A structural model for active extension in more impulsive, leading to a higher seasonal variation. central Italy. J Geodyn 29(3–5):233–244. https://doi.org/10.1016/ Nevertheless, changes in the total amount of recharge, S0264-3707(99)00050-2 and consequently of discharge, are not expected, be- Boni C, Bono P, Capelli G (1986) Schema idrogeologico dell’Italia cause possible storativity reduction does not influence Centrale [Hydrogeological scheme of central Italy]. Mem Soc Geol It 35:991–1012 the infiltration from rainfall. Boni C, Baldoni T, Banzato F, Cascone D, Petitta M (2010) Hydrogeological study for identification, characterization and man- Acknowledgements The authors are thankful to the following water sup- agement of groundwater resources in the Sibillini Mountains ply companies and public bodies for making freely available spring or National Park (central Italy). Ital J Eng Geol Environ 2:21–39. river discharge and water level data: Gran Sasso Acqua and Ruzzo Reti https://doi.org/10.4408/IJEGE.2010-02.O-02 Companies; Nera aqueduct company (S.A.N. spa); CIIP S.P.A. 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