The Crustal Seismicity of the Western Andean Thrust (Central Chile, 33°–34° S): Implications for Regional Tectonics and Seis

Home , Principal Cordillera, Pudahuel, Santiago (commune)

Bulletin of the Seismological Society of America, Vol. 109, No. 5, pp. 1985–1999, October 2019, doi: 10.1785/0120190082 Ⓔ The Crustal Seismicity of the Western Andean Thrust (Central Chile, 33°–34° S): Implications for Regional Tectonics and Seismic Hazard in the Santiago Area by Jean-Baptiste Ammirati, Gabriel Vargas, Sofía Rebolledo, Rachel Abrahami, Bertrand Potin, Felipe Leyton, and Sergio Ruiz

Abstract Most of the recorded seismicity in central Chile can be linked to the subduction of the Nazca plate. To the east, a much smaller fraction is observed at 0–30 km depths beneath the western Andean thrust. Paleoseismic studies evidenced the occurrence of at least two major earthquakes (M > 7) over the past 17 ka, associated with the San Ramón fault (SRF): an important tectonic feature characterizing the west Andean thrust, close the Santiago metropolitan area. To better constrain the crustal seismicity in this area, the Chilean Seismological Center (CSN) extended its permanent seismic network with seven new broadband seismometers deployed around the scarp of the SRF and farther east. The improved azimuthal distribution and reduced station spacing allowed to complete the CSN catalog with more than 900 smaller magnitude earth- 2 5 quakes (ML < : ) detected and located within the study region. The use of a 3D velocity model derived from P-andS-wave travel-time tomography considerably lowered the uncertainties associated with hypocentral locations. Our results show an important seismicity beneath the Principal Cordillera located at a depth of ∼10 km, and a deeper seismicity (~15 km) aligned with the main Andean thrust more to the west, parallel to the scarp of the SRF. Regional stress inversion results suggest that the seismicity of the west Andean thrust is accommodating northeast–southwest compressional stress, consistent with the convergence of the Nazca plate. Based on our improved crustal seismicity, combined with observations from previous studies, we have been able to refine the scenario of an Mw 7.5 earthquake rupturing the SRF. Ground-motion prediction results show peak ground accelerations of ∼0:8g close to the fault scarp.

Supplemental Content: Cross sections of the 3D velocity model used in this study, probabilistic power spectral density (PPSD) plots for the stations of the Chilean Seismological Center (CSN) network located in the study region as well the waveforms corresponding to a seismic event that occurred beneath the west Andean thrust, and tables of hypocentral location and associated errors of the 917 events characterized in this study, and the focal mechanism solutions determined from P-wave first-motion polarity.

Introduction and Geological Setting

Central Chile marks the boundary between the Nazca Valdivia earthquake or more recently, the 2010 Mw 8.9 plate and the South American plate, which makes it one of Maule earthquake (Cifuentes and Silver, 1989; Vigny et al., the most seismically active countries on the planet. In this 2011; Ruiz and Madariaga, 2018) as well as intermediate region (Fig. 1), roughly 95% of the recorded seismicity depth intraplate events, mainly extensional, that have been can be linked to the subduction of the Nazca plate under related to dehydration and brittle failure within the sub- the South American plate, along the Chilean margin. This ducting Nazca plate (Beck et al., 1998; Barrientos et al., major seismogenic zone is characterized by the occurrence 2004; Ruiz et al., 2019). Around 5% of the seismicity between 0 and 50 km depths of some of the largest thrust recorded in central Chile can be observed between 0 and earthquakes ever recorded such as the 1960 Mw 9.5 30 km depths beneath the western flank of the Andean

1985

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Shin and Teng, 2001; Alvarado et al., 2005; Quigley et al., 2016; Zhang et al., 2016; Instituto Nacional de Prevención Sísmica [INPRES], 2019). To this day, the 1958 Mw 6.3 Las Melosas earthquake is the strongest crustal event to have occurred in our study region that was instrumentally recorded and characterized (Pardo and Acevedo, 1984; Alvarado et al., 2009). It produced several casualties, building collapses, and severely damaged the water distribution infrastructure of the Santiago area (Sepúlveda et al., 2008). The city of Santiago de Chile, located in the central valley on the western Andean piedmont, has expanded over the sediments of the Central Depression, mainly Quaternary detrital material carried from the Mapocho and the Maipo rivers (Fig. 2). An important topographic scarp (∼2700 m) directly to the east marks the beginning of the Principal Cordillera (PC) mostly constituted by Cenozoic volcanic rocks of the Abanico and Farellones formations, intrusive, and highly deformed Jurassic to Cretaceous sequences (Thiele, 1980). Farther east, the Frontal Cordillera (FC) exhibits Permian–Triassic volcanic rocks and intrusive granitoids together with late Paleozoic sequences and Proterozoic metamorphic rocks (Fig. 2). Geological observations in this regions showed that the present structural configuration of the Andes between 33° and 35° S is the result of intense crustal shortening within the upper plate, in the context of subduction (Allmendinger et al., 1990; Cristallini and Ramos, 2000; Giambiagi and Ramos, 2002). It appears that most of this crustal shortening was accommodated since ∼16 Ma by east-vergent structures on the eastern side (Giambiagi, Ramos, et al., 2003). In the western side, the contact between the Abanico formation and the Quaternary sediments of the Central Depression is defined by a west-verging reverse fault that accommodated part of the crustal shortening in the western side: the San Ramón fault (SRF; Armijo et al., 2010; Vargas et al., 2014). Although Figure 1. Seismicity of central Chile recorded by the Chilean the extension in depth of the aforementioned structures and National Network (CSN) between 2000 and 2017. (Inset) Location of our study region at the western margin of South America. Stars their tectonic implications are still debated (Armijo et al., show the epicenter of recent megathrust earthquakes along the 2010; Giambiagi et al., 2015; Riesner et al.,2018), the Chilean margin. Squares mark the location of the CSN permanent SRF certainly raises questions in terms of seismic hazard seismic stations. Note the strike-slip mechanism associated with because of its proximity with the city of Santiago (Fig. 2). three crustal events from the Global Centroid Moment Tensor Recent paleoseismic studies (Vargas et al.,2014)evidenced (CMT) catalog (Ekström et al., 2012) and Alvarado et al. (2005). The solid arrow shows the regional velocity field direction two major seismic events occurred on the SRF, respectively inferred from 20 yr of Global Navigation Satellite Systems (GNSS) ∼8 and ∼17 ka. The slip associated with these events has been measurements (Métois et al., 2016). The color version of this figure estimated to 4.7–4.9 m corresponding to seismic events of is available only in the electronic edition. 7 2 7 4 magnitude : 100 km west of the city). urban centers located in this region. The Santiago metropoli- As an example, the 2010 Mw 8.8 Maule earthquake produced tan area is home to nearly half of the total Chilean population a peak ground acceleration (PGA) of ∼0:3g in the Santiago and concentrates an important fraction of the Chilean eco- area (Boroschek et al.,2012). Previous seismic hazard studies nomic activity. There is no lack of examples to illustrate the simulated ground accelerations generated by rupturing the threat of crustal faults worldwide: Mendoza (Argentina) SRF (Pérez et al., 2014; Estay et al.,2016) and estimated 1862; San Juan (Argentina) 1944; Kobe (Japan) 1995; PGAs of ∼0:7g near the fault scarp. Chi-Chi (Taiwan) 1999; Christchurch (New Zealand) 2010; In this work, we use continuous seismic waveforms and Gorkha (Nepal) 2015 (Kanaori and Kawakami, 1996; recorded by the Chilean seismic network to detect and

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Figure 2. First-order geology of the western margin of South America around 32° S and morphostructural map of the study region. Geological information has been compiled from Armijo et al. (2010), Farías et al. (2010), Giambiagi et al. (2015), and Riesner et al. (2018). The gray polygon delimits the metropolitan area of Santiago. AFTB, Aconcagua fold and thrust belt; SRF, San Ramón fault; WAFTB, west Andean fold and thrust belt. Note the change in structure vergence from east to west, in the Principal Cordillera (PC). The color version of this figure is available only in the electronic edition.

2 5 precisely locate the low-magnitude (ML < : ) seismicity Previous Seismological Studies and Current beneath the west Andean thrust. We also use the first-motion Operations polarity information to determine the focal mechanism asso- The Chilean Seismological Center (Centro Sismológico ciated with some of these events to estimate the regional Nacional or CSN) of the University of Chile started to oper- stress principal directions. This information, along with the ate in March 2013 as a continuation of the Seismological seismic location allows us to discuss about the differences Service of the Department of Geophysics of the same and similarities between our results, tectonic models, and university, which was in charge of the permanent seismic interpretations made in previous studies. In addition, we operations since 1982. use our seismic results to refine the scenario of near-surface To better comprehend the crustal seismicity beneath the rupture in the Santiago area. We then calculate the expected PC, Barrientos et al. (2004) relocated the events recorded by ground accelerations for the metropolitan area of Santiago. the CSN between 1986 and 2001 using an improved 1D

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2 5 seismic event with a local magnitude ML > : paying particular attention to the Chilean margin, with rapid characterization of large magnitude subduction earthquakes for postseismic response and tsunami alert. During 2017, the CSN extended the national seismic network with seven broadband seismometers (including one 100 m deep bore- hole) permanently deployed at the piedmont of the PC, around the scarp of the SRF and farther east (Fig. 3 and Ⓔ Table S1, available in the supplemental content to this article). The main objective of this permanent instrumenta- tion is to increase the station concentration in the PC, improving the detection and characterization of small- 2 5 magnitude (ML < : ) earthquakes not currently automatically detected by the CSN. Recent studies explored potential rupture scenarios of the Figure 3. Map showing the location of the seismic stations used SRF. Pérez et al. (2014) estimated PGAvalues greater than 0:7g in this work. The solid line corresponds to the scarp of the SRF as described in Armijo et al. (2010). Triangles correspond to the CSN based on five random fractal slip distribution models, account- permanent stations installed prior to 2017. Inverted triangles corre- ing for source directivity effects. Based on transient electro- spond to stations installed during 2017 as part of the SRF project. magnetic imaging, Estay et al. (2016) evidenced surface The diamond corresponds to Global Seismographic Network (GSN) segmentation of the SRF and calculated the PGA associated station Peldehue (PEL). The polygon at the center of the map cor- with the potential rupture of each segment. Their estimated responds to the metropolitan area of Santiago. The color version of this figure is available only in the electronic edition. PGAs are similar to the results of Pérez et al. (2014).The two aforementioned rupture models consistently show stronger velocity model calibrated for their study region. Because of accelerations for the hanging wall (HW), east of the SRF scarp. the sparse station coverage of the Chilean seismic network at that time, this study pointed out the necessity of having Earthquake Characterization Procedure a denser array of seismometers deployed in the Cordilleran sector to improve the detection of smaller events and their The first step of this work consists in analyzing continu- localizations. Results of Barrientos et al. (2004) reveal a more ous records from the stations shown in Figure 3 and Ⓔ Table S1 to detect seismic events related to the seismic clustered seismicity observed at crustal level although not par- activity of the PC. To make good use of the denser CSN array ticularly consistent with the geological feature observed at the around the SRF (Fig. 3), we analyze the continuous wave- surface, in particular the Pocuro fault zone (a Mesozoic nor- forms recorded since the deployment of the seven new sta- mal fault ∼100 km north of Santiago that would have been tions, between 20 January 2017 and 15 March 2019. reactivated as a reverse fault during the Neogene) and the Because the CSN stations record a very large number of SRF more to the south, in our study area (Fig. 2), two impor- earthquakes from different sources, a challenge exists in dif- tant features associated with the uplift of the PC (Charrier ferentiating crustal events occurring beneath the PC et al.,2005; Armijo et al.,2010; Farías et al., 2010). (Ⓔ Fig. S3) from the events occurring along the Chilean Using seismic tomography, Farías et al. (2010) margin or within the subducting slab. For each event, an improved on the relocalization of the crustal events from the automatic detection and a first location are performed using CSN catalog between 1980 and 2004. Their findings show a the methodology described in Poiata et al. (2016). This good correlation between geological observations at the sur- method is based on the statistical characterization of seismic face and structural reconstruction at depth (Giambiagi, signals recorded by a dense seismic network and the stacking Ramos, et al., 2003), which suggests that most of the seis- of time-delay functions. The location process of the method micity is related to mountain building mechanisms respon- allows us to discard all events with hypocenters located out- sible for the uplift of the Andean Cordillera. Interestingly, the side our study area. For each event detected, we visually aforementioned seismological studies evidenced clusters of inspect the corresponding traces to remove eventual false earthquakes beneath the San José (∼33:8° S) and the positives. Because the automatically picked P- and S-wave Tupungatito (∼33:4° S) volcanoes (Fig. 2). arrival is not always accurate, this step is also a good oppor- The main task of the CSN consists of the maintenance tunity to manually pick the P- and S-wave first arrivals to and operation of 105 broadband seismic stations, 128 Global ensure optimized residuals for the final location. When the Navigation Satellite Systems (GNSS) stations, and 297 traces present a high signal-to-noise ratio, polarity informa- accelerometers for strong-motion measurement (Barrientos tion for the first P-wave arrival is reported. and National Seismological Center [CSN] Team, 2018), per- To improve the precision of seismic locations, we need manently deployed along the country. Among other tasks, an accurate wave velocity model with finer crustal details. the CSN automatically detects and characterizes every P-andS-wave arrival times of 11,829 earthquakes occurred

in the Chile central region (28°–36° S) and recorded by the square (rms) lower than 1 s, P-andS-wave residuals at each CSN network over a period of ∼20 yr (Fig. 1)werepreviously station are averaged. Final maximum-likelihood hypocenters inverted using a nonlinear approach based on the law of large (Fig. 4 and Ⓔ Table S2) are then estimated taking into account numbers (Potin, 2016). This step yielded 3D variations of the station delays. We note that in general, the dispersion of time velocity structure for both P and S waves. Then, we extracted a residuals is low with a standard deviation σ 0:09 and 0.16 s 3D velocity model centered on our region of interest, which for P and S phases, respectively. The highest residuals can be extends from 33° S to 34.5° S and 69.2° W to 71.6° W, down to observed for stations MT02 (−0:22 sforP phases and 45 km depth and with a grid spacing of 1 km (Ⓔ Fig. S1). −0:31 sforS phases) and MT14 (0.1 and 0.25 s for P and The events previously detected were located using the S phases, respectively). Station MT14 is located in the urban NonLinLoc package (Lomax et al., 2000). This nonlinear, area of Santiago (Fig. 3) and presents a relatively high level of global search method based on the probabilistic formulation anthropogenic noise (Ⓔ Fig. S2), which could be a reason for of inverse problems described in Tarantola and Valette such high residuals. Station MT02 is one of the quietest sta- (1982) is particularly well adapted to the use of 3D velocity tions with a remote location in the Coastal Cordillera and in models because it does not require the calculation of partial general present arrivals with high signal-to-noise ratios (Fig. 3 derivatives (otherwise very difficult to perform using linear Ⓔ Fig. S2). High residuals observed for this station could be approaches). The locations obtained in this work are repre- related to some inadequacy of our 3D velocity model in the sented by a probability density function (PDF) that produces vicinity of this particular station. The rms associated with the more comprehensive uncertainty estimations. The minimum final maximum-likelihood locations is 0.1 s in aver- requirements for an event to be located were set to a mini- age (σ 0:07 s). mum of eight picks (both P and S) and at least two S-wave Location errors are estimated from the 68% confidence picks. Once located, we estimated the local magnitude (ML) ellipsoid as computed from the samples of the location PDF. based on shear-wave maximum amplitude. The average horizontal error is 2.3 km (σ 1:5 km) and the Finally, focal mechanisms and corresponding compres- vertical error is about 3.0 km (σ 2 km). In general, we sion (P) and tension (T) axes were determined from P-wave obtained hypocenters with smaller (below average) vertical first-motion polarities using the grid-search algorithm FPFIT error for events with locations closer to a seismic station. (Reasenberg and Oppenheimer, 1985). We used maximum- Increasing azimuthal gap seems to be associated with likelihood hypocenters and corresponding ray takeoff angles increasing horizontal error (Fig. 5). from the 3D locations as an input. Because the seismicity Twenty-nine events from this work were also detected detected is very low in magnitude (in general lower than and located by the CSN (Fig. 5d). For the central Chile 2 5 ML < : ), the signal-to-noise ratio is often poor, in particu- region, the CSN uses an improved 1D, three layers over a 2–2 5 lar for stations located close to the urbanized area (Fig. 3 and half-space velocity model to locate (ML > : ) earth- Ⓔ Fig. S2), which makes the P-wave polarity hard to read quakes automatically (Barrientos et al., 2004; Barrientos and for these stations. Thus, we determined fault plane solutions National Seismological Center [CSN] Team, 2018). The rms for events with a minimum of six good first-motion polarity for the CSN events is no larger than 0.3 s and the location readings (NR; Ⓔ Fig. S4). errors are in general low (comparable to the ones obtained in Under the main assumptions of a uniform regional stress this work only with a slightly higher standard deviation). tensor in the crust, within our study region and that the slip is However, if we compare the CSN hypocenter solutions with parallel to the direction of the tangential traction (Wallace, the ones obtained in this work, we notice considerable dis- 1951; Bott, 1959), it is possible to find the three principal crepancies, especially in the vertical direction. The hypocen- stress directions that will most closely match the focal ters obtained using the 3D model seem to be shallower and mechanism observations by performing formal stress inver- more gathered around 10 km depth in comparison with the sion (FSI; Hardebeck and Michael, 2006; Martinez-Garzón CSN. The difference tends to increase as we look to the east. et al., 2014). Principal stress axis uncertainties are evaluated Beneath the FC (∼69:6° W), the difference in hypocenter by randomly resampling the focal mechanisms data (boot- depths can reach 15 km (Fig. 5e). The CSN 1D layered straping). In this work, FSI is performed for low quality model has been calibrated for the Andes of central Chile (NR > 6), intermediate quality (NR > 8), and higher quality (Barrientos et al., 2004). We note that the difference in focal (NR > 10) focal mechanisms. Only seismic locations with a depth is higher for events located in the back-arc region of maximum azimuthal gap of 220° have been considered. Argentina, where the real velocity variations might be not well represented by the velocity model. We believe that our 3D velocity model allows more realistic raytracing, hence Results more accurate hypocenter locations. Looking at the distribution of hypocenters (Fig. 4), we Earthquake Locations observe that the seismicity is mainly located along two We obtained 917 probabilistic locations corresponding to north–south stripes. The first stripe extends beneath the PC the crustal seismicity recorded mainly beneath the PC between (70°–70.2° W) between latitudes 33.2° S and 34.1° S. The January 2017 and March 2019. For events with a root mean depth associated with this seismicity appears quite constant

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Figure 4. (Top) Epicentral distribution of the seismic events characterized in this work from 27 January 2017 to 15 March 2019. The polygon shows the limits of the metropolitan area of Santiago. Squares mark the location of the seismic stations used for the location (see Fig. 3 for all station codes). (a–f) Solid lines mark the emplacements of the cross sections. Hypocenters located 0.1° apart from the lines are projected. (Bottom) Corresponding cross sections showing (a–f) the distribution at depth of the seismicity reported in this work. Circled numbers identify clusters of seismicity not directly related to the tectonics of the west Andean thrust: (1) Santiago cluster, (2) Los Bronces open pit mine, and (3) Tupungatito volcano. The color version of this figure is available only in the electronic edition.

at ∼10 km (5–15 km). The second stripe can be observed more 20–30 km (Fig. 4). This seismicity has been evidenced in to thewest consistently aligned with thewestern flank of the PC previous studies although the mechanisms behind it remain (∼70:4° W). Hypocenters in this sector are 10–20 km deep. quite hypothetical (Leyton et al., 2009). During August Interestingly, we observe some clustered seismicity 2017, the CSN network recorded a seismic swarm (Fig. 4) beneath the Central Depression (∼33:6° S) at depths of about located beneath the Tupungatito volcano (Fig. 2)at

Figure 5. (a) Variability of the vertical error depending on earthquake location. Note how lower vertical error decreases when hypocenters are closer to a seismic station. (b) Similar to (a). The map is showing the variation of the horizontal error. This time, higher errors seem to be associated with increasing azimuthal gap (c). (d) Epicenter distribution of some of 29 earthquakes located in this work and by the CSN. (e) Depth distribution for the 29 earthquakes located in this work and by the CSN. The solid line corresponds to the Chile–Argentina international border. The color version of this figure is available only in the electronic edition.

10–15 km depth. Furthermore, we can observe a small clus- can occur on preexisting zones of weakness in directions not ter of seismicity at shallow depths (0–5 km) beneath the Los always geometrically consistent with regional stress directions Bronces open pit mine. These clusters are not related to the (McKenzie, 1969). Because the seismicity found in this work tectonics of the western Andean thrust; hence, we do not con- is low in magnitude, we expect the corresponding focal mech- sider them in our further interpretations. anisms to be associated with small fractures with a broad Sections across the study area (Fig. 4) show that the range of orientations rather than larger regional structures seismicity depth presents a general westward increase. globally aligned with observed geological features. Seismicity in the Argentine back-arc is limited to the north- Considering this, the focal mechanism parameters (strike, east of our study area and can be observed at shallow depth dip, and rake) are not limited by regional structural observa- (although we acknowledge that the location errors are higher tions as seen in previous works (e.g., Pérez et al.,2014). in this sector due to a higher azimuthal gap). We obtained a total 104 focal mechanism solutions from The local magnitudes estimated in this work are in gen- P-wave first-motion polarities (Fig. 6 and Ⓔ Table S3). Fifty- 2 5 Ⓔ eral lower than ML < : ( Table S2). However, In the two of them were determined using a minimum of NR > 8 Argentine back-arc, the magnitude can be higher with events polarity readings and 27 has a minimum of NR > 10 polarity 3 0 with magnitude ML > : . readings. The limited number of polarity data and their weak coverage of the focal sphere did not permit to accurately constrain the fault plane orientations for each individual event. Focal Mechanism Solutions Thus, we do not relate them with geological structures but A single focal mechanism only constrains the stress direc- rather look at the set of solutions as a whole. tion within the dilatational quadrant, which implies that vari- In general, our solutions (Fig. 6a and Ⓔ Table S3) cover ous focal mechanisms are necessary to further constrain the a large variety of focal mechanisms. However, we observe a regional stress directions. In a rock volume under stress, slip major concentration of T axis closer to the center of the focal

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right-lateral strike-slip component compatible with the focal mechanism of intermediate magnitude earthquake located at shallow depths, in the PC sector (Fig. 1).

Implications for Regional Tectonics Both improved hypocentral locations and regional stress tensor obtained in this work can be discussed in the frame- work of the regional tectonics in our study area. The hypocenter distribution (Fig. 4) seems to coincide with tectonic and geological observations at the surface (Fig. 2). We observe that an important amount of the shallow seismicity lies beneath the highly deformed Mesozoic sequences of the PC also known as the Aconcagua fold and thrust belt (AFTB). Around 33.5° S, this ∼20 km wide feature has been described as a thin-skinned thrust belt with east-vergent structures, overthrusting Neogene intramoun- tain basins. The timing of the deformation suggests that this feature contributed to the accommodation of the Andean orogeny between ∼20 and ∼10 Ma (Giambiagi, Alvarez, et al., 2003). The basal décollement of the AFTB has been estimated by Giambiagi, Alvarez, et al. (2003) to be 5–10 km deep, which is quite consistent with the distribution of our hypocenters for this area (Fig. 7). However, recent re-evalu- ation of the structural configuration by Riesner et al. (2018) suggests that the AFTB roots onto a very shallow, 2–3km deep décollement level, well above the seismicity level high- lighted in the present work. Figure 6. (a) Inferior hemisphere projection of the P (compression) and T (tensional) axes for the focal mechanisms found in this Two controversial models have been proposed to study with a minimum of 6, 8, and 10 polarity readings, respec- explain the tectonic evolution of the PC and the FC more to tively. (b) Formal stress inversion (FSI) results. The crosses corre- the east. Giambiagi et al. (2015) proposed that the AFTB spond to the best regional stress orientations. The data have been accommodated crustal shortening in the PC during the randomly resampled (2000 times) for uncertainty estimations. The Miocene and then migrated to the east, affecting the dots show the distribution of the bootstrap samples. The color – version of this figure is available only in the electronic edition. Permian Triassic sequences of the FC. This has been con- strued as two successive east-vergent thrusts with two sep- – – sphere and P axis more distributed closer to the edges of the arate décollement levels at 5 10 and 15 20 km depths, respectively. The total estimated shortening associated with focal sphere suggesting compressive regional stress. these structures is ∼47 km (Giambiagi and Ramos, 2002). FSI results (Fig. 6b) show a general northeast–southwest Alternatively, Armijo et al. (2010) consider the AFTB as orientation of the principal stress axis (σ1). The minimum a secondary feature that accommodated a maximum of stress axis (σ3) appears well constrained at the center of 10 km of crustal shortening and have then been carried over the focal sphere, which suggests a northeast–southwest-ori- the basement of the FC by a deep west-vergent ramp ented compressional crustal stress state for our study region. throughout a continuous crustal shortening accommodation These results are consistent with the regional velocity field during the Neogene. The important seismicity observed in inferred from GNSS data (Métois et al., 2016). the present work (Figs. 4 and 7) beneath the AFTB is com- In detail, the uncertainty associated with principal stress patible with the uplift of the FC basement as mentioned by orientations does not appear to be sensitive to data quality. Riesner et al. (2018) although they imply that this structure The bootstrap samples are tightly surrounding the best sol- was only active during the late Middle Miocene (∼11 Ma). ution especially when the number of polarity readings is low Our results thus suggest that quite an important fraction 6 8 (

Figure 7. Integrated cross section showing the depth distribution and the normalized density of the seismicity characterized in this study between 33° S and 34.5° S. Solid lines are structures inferred from geological observations (Riesner et al., 2018). Dashed lines correspond to major structures inferred from our seismic results. Dotted lines delimit the seismogenic zone associated to the tectonics of the study region. The circled numbers refer to clusters of seismicity not directly related to the tectonics west Andean thrust (see Fig. 4). The color version of this figure is available only in the electronic edition.

Abanico and Farellones formations as well as Quaternary of at least 50 km. This length value is more consistent with intrusive bodies seem to be rather deformed and uplifted by empirical scaling laws between 4.7 and 4.9 slip estimated by deep-cored structures sometimes referred as west Andean fold Vargas et al. (2014) and the rupture length at the surface and thrust belt (WAFTB); Riesner et al.,2017). The PC is (Wells and Coppersmith, 1994). However, we acknowledge bounded to the west by east-dipping faults that mounted the that the amount slip measured at the surface is not always rep- aforementioned Cenozoic formations above the Quaternary resentative of the magnitude, because the rupture is not nec- sediments of the Central Depression resulting in a very abrupt essarily uniform along the fault plane. As an example, the mountain front (Fig. 2). Farther east, a series of anticlines and 1999 Mw 7.6 Chi-Chi earthquake (Shin and Teng, 2001)was synclines was developed as the underlying structures (former characterized by a slip and surface rupture length way above WAFTB) accommodated the Andean compression for the past the values expected from magnitude scaling relationships. 20–25 Ma. According to the west-verging model (Armijo In the Andes of central Chile, most of the tectonic fea- et al.,2010; Riesner et al.,2017), the WAFTB would be the tures observed at the surface present a north–south general main structure responsible for the building of the Andes orientation. This is particularly true for the three main afore- between 33° and 35° S. Recent quantification of the WAFTB mentioned structures: the AFTB in the eastern PC, the kinematics from structural and geochronological observations WAFTB in the western PC, and the SRF (Fig. 2). These (Riesner et al.,2017) allowed to estimate the depth of the structures accommodated the Andean shortening in a very décollement level associated with the WAFTB at 12–15 km. clear east–west direction. However, the regional stress tensor The hypocenter distribution found along the western front of displays a northeast–southwest orientation, parallel to the the PC, lying at ∼15 km depth is compatible with this struc- convergence orientation of the Nazca plate. This observation ture (Fig. 4). Nearby Santiago de Chile, the west Andean front raises the question of how the upper plate accommodates is characterized by the presence of the SRF. The seismicity northeast–southwest motion induced by the subduction associated with the WAFTB appears quite parallel to this dynamics with north–south structure? Geological observa- structure (Fig. 4) although we do not observe any hypocenters tions mainly support the idea of reverse faulting. As an exam- on the SRF fault plane, at shallow levels, between the surface ple, the SRF on the PC piedmont previously interpreted as a and the base of the ramp. Paleoseismic studies suggest that at normal fault (Brüggen, 1950) has been recently studied more least two ruptures occurred along the SRF during the past in details driving to the conclusion that it rather corresponds 17,000 yr (Vargas et al.,2014) and were associated to 7:2 > to the expression of a west-vergent reverse structure thrusting 7 5 Mw > : earthquakes. We believe that such high-magnitude the Miociene volcanic rocks of the Abanico Formation over events would occur when the entire ramp is ruptured. the Quaternary sediments of the Central Depression (Armijo Although the SRF scarp at the surface was mapped as a et al., 2010; Vargas et al., 2014). More to the east, the west- ∼30 km long structure (Armijo et al.,2010), the seismicity verging folds that characterize the WAFTB present clear (found in this work) at its base clearly extends farther south. north–south axial planes and suggest east–west deformation Considering this observation, there is a possibility that the (Riesner et al., 2017). The same observation would apply to SRF plane is extending southward as well, totaling a length the AFTB (Armijo et al., 2010; Riesner et al., 2018).

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The Las Melosas earthquake is the largest crustal event They obtained PGA values between 0.7 and 0:8g around the instrumentally recorded in the central Andes. It occurred on 4 scarp of the SRF. September 1958 in the PC, ∼60 km southeast from Santiago. Estay et al. (2016) focused their work on improving the The hypocenter was located at around 10 km depth, beneath near-surface fault geometry. They found evidence of at least the locality of Las Melosas where the earthquake was felt four different subsegments of ∼10 km length, along the SRF. with the strongest intensity (IX on the Mercalli scale) accord- They concluded that those segments would be activated inde- ing to Sepúlveda et al. (2008). Using teleseismic waveform pendently resulting in four different rupture scenarios that 6 2 6 7 modeling and moment tensor inversion, Alvarado et al. involve earthquakes of magnitudes : 5) events in this We use ground-motion prediction equations derived from sector would be necessary to validate this particular point. the Next Generation Attenuation (NGA) database (Pacific Earthquake Engineering Research Center [PEER], 2019)to compute PGAs in our study area (Abrahamson et al.,2014; Implications for Seismic Hazard Graizer and Kalkan, 2016; hereafter, ASK14 and GK15, respectively). The NGA database includes ground-motion SRF Rupture Scenario observations for a large number of earthquakes, which allows The scarp of the SRF has been identified bordering the the calculation of PGA median value and standard deviation. eastern side of the metropolitan area of Santiago. It extends For this reason, we assume that PGA variations due to source from the Cerro Alvarado, on the north bank of the Mapocho effects are taken into account through the dispersion of the river (−33:35° S) to the south of the Maipo river at ∼33:6°S empirical data. The HW effect has been defined as a system- (Armijo et al., 2010). Previous SRF rupture scenarios (Pérez atic amplification of ground-motion amplitudes measured on et al., 2014; Estay et al., 2016) are based on approximation the HW (Abrahamson and Somerville, 1996). Unlike GK15, of Armijo et al. (2010) of fault plane geometry. the ASK14 model allows to specifically quantify this effect. Pérez et al. (2014) estimated the PGA from five random For this reason, we compute PGA variations using both mod- composite fractal slip distribution scenarios and calculated els to explore their similarities and differences. the corresponding PGA within a 122-node grid. This Our SRF scenario is based on the most recently available approach accounts for the directivity effect associated with geological and seismological knowledge. It considers the the rupture propagation. This effect strongly influences the rupture of every point on the fault plane (Fig. 8), which does wave amplitudes depending on the angle between the not mean that the rupture would be uniform along the entire receiver and the rupture propagation main direction. On the fault plane but that we consider every possible source– other hand, they neglected site parameters that could locally receiver distances. We compute PGAs for a grid of ∼900 m influence the ground acceleration such as soil qualifications (30 arcsec) spacing for which each node is characterized with (VS30) and/or basin thickness variations. Their rupture sce- elevation (h), VS30 and the closest distance to the rupture narios were computed for a conservative magnitude Mw 6.9 (rmin). The distance to the rupture is calculated considering earthquake rupturing an S 30 × 16 km2 SRF fault plane. the topography and the depth of the fault plane. Thus, for

−1 (footwall) is filled with relatively low VS30 (450–500 ms ) detrital alluvium in comparison to the HW, mostly composed −1 by higher VS30 (>1200 ms ) rocks from the Abanico formation (Fig. 2 and Ⓔ Fig. S5). This difference in VS30 seems to compensate the HW effect. However, our highest PGA values are observed on the HW north of our study area (∼33:4°S) and more to the south (∼33:6°S) where the Mapocho and Maipo rivers (Figs. 2 and 9) open to the Santiago basin. These sectors are characterized by low −1 VS30 (∼500 ms ) and are located close to the SRF scarp, Figure 8. SRF plane geometry taken into account for a major hence the strong PGAs observed locally. To the northwest rupture scenario. The solid line corresponds to the scarp of the SRF of the Santiago metropolitan area (Fig. 9), the presence of as described in Armijo et al. (2010). The dashed line is the southern volcanic ashes (Pudahuel ignimbrites) is characterized by extension of the SRF inferred from the crustal seismicity reported in −1 a very low VS30 (∼150 ms ) and contributes to a locally this work (see the Results section). The rupture of the entire SRF higher PGA (0:3 0:15g) in this sector despite being fault plane would generate an earthquake of magnitude Mw 7.5. The metropolitan area of Santiago (white polygon) is home to more than located further from the fault scarp. This observation is par- ∼6:5 million inhabitants. The color version of this figure is available ticularly visible on the GK15 model (Fig. 9a and Ⓔ Fig. S5). only in the electronic edition. Overall, our models show that ground-motion attenuation with distance is lower than observed in Pérez et al. the foot wall (west of the SRF scarp) rmin corresponds to the (2014) and Estay et al. (2016). On the footwall, 10 km from closest distance to the scarp whereas some points on the HW the SRF scarp, our models show PGA average values of can be closer to the underlying fault plane. The shear-wave 0:4–0:5g, in which the work mentioned earlier obtained val- velocity to a depth of 30 m (VS30) has been estimated in the ues of ∼0:25 and ∼0:3g, respectively. This difference in study area by Leyton et al. (2011) based on soil and rock ground-motion attenuation can be related to the higher mag- characteristics from local geology. We considered basin nitude earthquake modeled in this study. thicknesses (b) from Yáñez et al. (2015), which varies Values and distribution of PGAs estimated in this study between 0.2 km to the south and 0.5 km to the northeast of are consistent with ground-motion measurements correspond- – the Santiago basin. The Joyner Boore distance (closest dis- ing to the 1999 Mw 7.6 Chi-Chi earthquake, a case similar to tance to the surface projection of the rupture plane: RJB)is our SRF scenario in terms of tectonic setting although with a also calculated, as it is required by ASK14. The anelastic slightly higher magnitude and a rupture length of nearly 90 km attenuation factor (Q0) is required to constrain far source (Shin and Teng, 2001). Chi-Chi generated PGA values greater attenuation in the GK15 model (which in our case will only than 0:9g close to the rupture scarp, on the HW and PGA val- have little effect). We set Q0 450 based on Ji et al. (2010). ues of 0:3g at ∼25 km to the west, on the footwall. Another comparable example would be the 2011 Mw 6.2 Christchurch earthquake with recorded PGA greater than 0:8g on the HW Results and Discussion although with a rapid decrease at short distances PGAs computed in this work are shown in Figure 9.In (PGA 0:25g recorded at 10 km from the epicenter), prob- general, both models show quite similar results close to the ably due to the lower magnitude compared to our study. Also, SRF scarp with maximum PGA of 0:8 0:4g and the Christchurch area is characterized by poorer quality soils 0:7 0:3g for the ASK14 and GK15 models, respectively. than observed in the Santiago basin. Unlike, the SRF or the We observe that footwall PGAs obtained from ASK14 1999 Chi-Chi case, the fault responsible for the 2011 decrease more rapidly with distance compared to the GK15 Christchurch earthquake was blind, which means that no sur- model. Values obtained in this work are consistent with Pérez face rupture was observed. et al. (2014) and Estay et al. (2016) although our rupture Shallow crustal earthquakes this size occurring on reverse scenario is considering a much bigger earthquake (Mw 7.5). faults are expected to generate surface rupture, which was the However, our results show quite a different repartition of case for the Chi-Chi earthquake and locally resulted in a ∼9 m PGAs compared to the aforementioned works. Close to the scarp. This amount of slip close to the surface reminds us that SRF scarp, the first-order parameter that controls the amount the slip distribution can be quite heterogeneous along the rup- of expected acceleration appears to be the VS30, a parameter ture and that patches of high slip can locally increase PGA closely related to the observed geology in the study area values. Considering the 4.9 m surface displacement observed (Leyton et al., 2011). Pérez et al. (2014) and Estay et al. by Vargas et al. (2014), there is no doubt that the rupture of the (2016) estimated stronger PGAs systematically located on SRF will reach the surface. In this case, differential ground the HW. Although we specifically considered the amplifica- movements would be particularly detrimental for any build- tion caused by the HW effect, our results (Fig. 9) show ings or infrastructure constructed above and right next to the relatively higher PGAs mostly characterizing the footwall. rupture area. We do not discard the occurrence of smaller The reason for this observation is that the Santiago basin events, large enough to cause damage but with a slip

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Figure 9. Peak ground acceleration (PGA) estimations corresponding to our rupture scenario for the SRF calculated from the (a) GK15 and (b) ASK14 models, following Graizer and Kalkan (2016) and Abrahamson et al. (2014), respectively. The solid white line corresponds to the SRF scarp considered in this case. Both models clearly show high-PGA values on the footwall, east from the scarp of the SRF. The strongest accelerations are observed on the hanging wall (HW) in areas filled with sedimentary material carried by the Mapocho river to the north and then by the Maipo river to the south (Fig. 2). This observation is particularly clear on the (b) ASK14 model that specifically accounts for the HW effect (see the Implications for Seismic Hazard section). Variations of PGA mean values for the GK15 and ASK14 models along cross section AB are shown within 1 standard deviation in (c) and (d), respectively. The dashed line shows topography variations along cross section AB. The color version of this figure is available only in the electronic edition.

distribution that would not rupture up to the surface. Finally, N° 41, 20 June 2016 (Monitoreo sismico y potencial sismogénico de la Falla strong accelerations related to the SRF rupture could trigger San Ramón). The authors thank Sergio Barrientos and the Chilean Seismological Center (CSN) team (in particular Sebastián Arriola) for the landslides and rock falls in areas characterized by high easy access to the continuous waveforms and station metadata used in the topography. present work. J.-B. A. is thankful to Chelsea Mackaman-Lofland for fruitful discussions about the tectonics of the central Andes. J.-B. A., G. V., and S. R. Conclusions are thankful to Ruben Boroschek for his feedback on peak ground acceleration (PGA) calculations. The authors are grateful to Associate Editor Mark In this study, we analyzed 26 months (27 January 2017 to W. Stirling, as well as Gregory De Pascale and another anonymous reviewer 15 March 2019) of continuous waveforms recorded by 17 for their very constructive comments on the original article. broadband seismometers and obtained 917 probabilistic locations of seismic events associated to the west Andean thrust References between 33° and 34° S. Our results evidence seismic activity mainly distributed beneath the eastern PC between 5 and Abrahamson, N. A., and P. G. Somerville (1996). Effects of the hanging wall 15 km depths and beneath the western PC between 10 and and footwall on ground motions recorded during the Northridge earth- – 20 km depths. The former imply that crustal shortening affect- quake, Bull. Seismol. Soc. Am. 86, no. 1B, S93 S99. Abrahamson, N. A., W. J. Silva, and R. Kamai (2014). Summary of the ing the central Andes would be concentrated at the transition at ASK14 ground motion relation for active crustal regions, Earthq. depth between the PC and the FC at depth. 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