Crustal Deformation in the South-Central Andes Backarc Terranes As Viewed from Regional Broad-Band Seismic Waveform Modelling

Geophys. J. Int. (2005) 163, 580–598 doi: 10.1111/j.1365-246X.2005.02759.x

Crustal deformation in the south-central Andes backarc terranes as viewed from regional broad-band seismic waveform modelling

∗ , Patricia Alvarado,1,2 Susan Beck,1 George Zandt,1 Mario Araujo2 3 and Enrique Triep4 1Geosciences Department, University of Arizona, Tucson, AZ, USA 2Departamento de Geof´ısicay Astronom´ıa,Universidad Nacional de San Juan, Argentina 3Instituto Nacional de Prevenci´onS´ısmica,Argentina 4Instituto Geof´ısicoSismol´ogicoFernando Volponi, Universidad Nacional de San Juan, Argentina

Accepted 2005 July 22. Received 2005 July 11; in original form 2004 December 10

SUMMARY The convergence between the Nazca and South America tectonic plates generates a seismically active backarc region near 31◦S. Earthquake locations deﬁne the subhorizontal subducted oceanic Nazca plate at depths of 90–120 km. Another seismic region is located within the continental upper plate with events at depths <35 km. This seismicity is related to the Precordillera and Sierras Pampeanas and is responsible for the large earthquakes that have caused major human and economic losses in Argentina. South of 33◦S, the intense shallow continental seismicity is more restricted to the main cordillera over a region where the subducted Nazca plate starts to incline more steeply, and there is an active volcanic arc. We operated a portable broad- band seismic network as part of the Chile–Argentina Geophysical Experiment (CHARGE) from 2000 December to 2002 May. We have studied crustal earthquakes that occurred in the back arc and under the main cordillera in the south-central Andes (29◦S–36◦S) recorded by the CHARGE network. We obtained the focal mechanisms and source depths for 27 (3.5 < M w < 5.3) crustal earthquakes using a moment tensor inversion method. Our results indicate mainly reverse focal mechanism solutions in the region during the CHARGE recording period. 88 per cent of the earthquakes are located north of 33◦S and at middle-to-lower crustal depths. The region around San Juan, located in the western Sierras Pampeanas, over the ﬂat-slab seg-

Seismology ment is dominated by reverse and thrust fault-plane solutions located at an average source depth of 20 km. One moderate-sized earthquake (event 02-117) is very likely related to the northern GJI part of the Precordillera and the Sierras Pampeanas terrane boundary. Another event located near Mendoza at a greater depth (∼26 km) (event 02-005) could also be associated with the same ancient suture. We found strike-slip focal mechanisms in the eastern Sierras Pampeanas and under the main cordillera with shallower focal depths of ∼5–7 km. Overall, the western part of the entire region is more seismically active than the eastern part. We postulate that this is related to the presence of different pre-Andean geological terranes. We also ﬁnd evidence for different average crustal models for those terranes. Better-ﬁtting synthetic seismograms result using a higher P-wave velocity, a smaller average S-wave velocity and a thicker crust for seismic ray paths travelling through the crust of the western Sierras Pampeanas (Vp = 6.2– 6.4 km s−1, Vp/Vs > 1.80, th = 45–55 km) than those of the eastern Sierras Pampeanas (Vp = 6.0–6.2 km s−1, Vp/Vs < 1.70, th = 27–35 km). In addition, we observed an apparent distribution of reverse crustal earthquakes along the suture that connects those terranes. Fi- nally, we estimated average P and T axes over the CHARGE period. The entire region showed P- and T-axis orientations of 275◦ and 90◦, plunging 6◦ and 84◦, respectively. Key words: Andes backarc, continental crust, earthquake-source mechanism, seismotectonics, subduction zone.

1INTRODUCTION Crustal seismicity in Argentina and Chile between 29◦S and 36◦S ∗ is related to the Andean deformation (Fig. 1). The region is Corresponding author: Gould Simpson Building #77, 1040 E 4th St., Tucson AZ 85721 USA. E-mail: [email protected]

580 C 2005 RAS Moment tensor inversion of Andean backarc crustal earthquakes 581

-1

Figure 1. PDE-NEIC seismicity during the CHARGE project seismic deployment. The CHARGE broad-band seismic instruments operated continuously between 2000 December and 2002 May. Contours represent approximately the depth to the top of the subducted slab by Cahill & Isacks (1992). We studied 27 shallow earthquakes that occurred in the main cordillera of the Andes and backarc region with magnitudes M > 4.0 and depths <60 km (see the text and Fig. 9).

characterized by the convergence between the oceanic Nazca plate (Dziewonski et al. 1981; Dziewonski & Woodhouse 1983). A com- − and the continental South America plate at a rate of 6.3 cm yr 1 pilation of these seismic data, including all available focal mecha- ◦ and an azimuth of 79.5 (Kendrick et al. 2003). Major along-strike nism solutions, can be found in Alvarado et al. (2004). However, changes in the Wadati-Benioff zone occur in this segment. The strike continental seismicity that occurs in the easternmost part over the of the Nazca plate changes in dip from subhorizontal to more in- flat-slab segment and south of 33◦Softhe Andean backarc is still ◦ clined in the vicinity of 33 S (Barazangi & Isacks 1976; Cahill & poorly known. Isacks 1992) (Fig. 1). Approximately 490 earthquakes of magni- Small- to moderate-sized crustal earthquakes are more abundant tude greater than 4.0 and focal depth <50 km have occurred in but difficult to study with global seismic data. Their study is im- the last 20 yr in the main cordillera and backarc region between portant in understanding the seismotectonic processes and seismic ◦ ◦ 29 and 36 S. In addition, large crustal earthquakes have caused hazards of the region. Unfortunately, these events are often not well many deaths and great devastation near San Juan and Mendoza recorded by global seismic networks. Regional broad-band portable (Argentina) in 1861, 1894, 1944, 1952 and 1977 (Castellanos 1944; seismic networks can help to characterize this seismicity by pro- Groeber 1944; INPRES 1977; Triep 1979; Volponi 1979; Bas- viding focal mechanism determinations and stress orientation es- tias et al. 1993). There is also a record of one shallow Ms = 6.9 timates, improving source-depth estimates, and permitting tests of earthquake that occurred in the main cordillera causing damage in seismic velocity models for the region. Chile in 1958 (Piderit 1961) (Fig. 2). Recently two crustal earth- In this paper, we present a regional study of the moderate-sized quakes with moment magnitudes of ∼6.4 have occurred in the main crustal seismicity in the Andean back arc between 29◦S and 36◦S ◦ ◦ Andean cordillera (PDE-NEIC location 34.93 S and 70.39 W, depth using broad-band seismic data collected during 18 months in the ◦ 16 km) and in Catamarca province (INPRES location 28.73 S and Chile–Argentina Geophysical Experiment (CHARGE). Complete ◦ 66.20 W, depth 30 km) in 2004 August to September. three-component seismograms recorded at regional distances were The backarc seismicity in the immediate eastern flank of the used to invert for the seismic moment tensor of 27 crustal earth- Andes since 1963 has been studied using teleseismic data and short- quakes. We also determined their focal depths. In addition, we ex- period seismic networks (Stauder 1973; Barazangi & Isacks 1976; plored average crustal seismic velocity structures for the region. INPRES 1977, 1985; Triep 1979, 1987; Chinn & Isacks 1983; These results are discussed in the context of previous geological Triep & Cardinalli 1984; Giardini et al. 1985; Kadinsky-Cade 1985; and geophysical studies. Langer & Bollinger 1988; Pujol et al. 1991; Smalley & Isacks 1990; Regnier et al.1992, 1994; Assump¸cão & Araujo 1993; Smalley et al. 2REGIONAL TECTONIC SETTING 1993; Langer & Hartzell 1996). More recently, the Harvard catalogue provided more than 30 focal mechanism solutions for the Most of western South America between 29◦S and 36◦Sismade up region since 1977 using the centroid moment tensor (CMT) method of pre-Carboniferous accreted terranes (Ramos et al. 1986, 2000;

C 2005 RAS, GJI, 163, 580–598 582 P. Alvarado et al.

Figure 2. Main geological provinces, terranes and historical earthquakes (represented by the year of occurrence and magnitude in parentheses) are taken from all geological and seismic references cited in the text.

Dalla Salda et al. 1992; Astini et al. 1995). Geological studies have uplifts of the western USA (Jordan & Allmendinger 1986). The documented how the major terrane sutures have experienced exten- western Sierras Pampeanas are composed principally of abundant sional and compressional deformation since the Jurassic (Ramos crystalline calcites, amphiboles, basic and ultrabasic rocks, and 1994; Rapela 2000; Ramos et al. 2002). In this seismic study we scarce granitic bodies (Caminos 1979). In contrast, the eastern also explore what control these recognized geological terranes might Sierras Pampeanas exhibit schists and gneisses of a mainly sedi- have on present Andean deformation. mentary origin, granitic rocks, some of which reach batholitic di- Fig. 2 shows the morphological and structural provinces and ter- mensions, and abundant migmatites and granulites (Kraemer et al. rane boundaries of our study area. These provinces have developed 1995; Varela et al. 2000). Proterozoic–Early Palaeozoic sutures sep- since ∼25 Ma in response to the subduction conditions that pro- arate the R´ıode la Plata, Pampia and Famatinia terranes in the eastern duced a progressive eastward migration of the orogenic front and Sierras Pampeanas (Aceñaloza& Toselli 1976). The western Sierras the volcanic arc (Ramos 1988; Kay et al. 1991). The Principal Pampeanas contain rocks as old as 1.1 to 1.0 Ga (McDonough et al. Cordillera,which forms the main part of the Andes, is composed of 1993) that correlate in age with the Grenville orogeny. These rocks Mesozoic marine deposits and the basement is involved in their are part of the Cuyania terrane, which also includes the Precordillera deformation. The northern and southern parts are characterized by (Dalla Salda et al. 1992; Ramos et al. 2000). The boundary between thick-skinned tectonics, while the central segment, with peaks with the Cuyania and Famitinia terranes is interpreted as a major Early elevations higher than 6700 m, is characterized by thin-skinned Palaeozoic suture (Dalla Salda et al. 1992; Astini et al. 1995; Ramos tectonics (Ramos et al. 1996). The Frontal Cordillera includes et al. 2002). However, the origin and evolution of these terranes are Palaeozoic–Triassic magmatic rocks that behaved as a rigid block still debated (Aceñaloza et al. 2002). during the Andean deformation (Ramos et al. 1996). The Pre- South of 33◦S, a very active volcanic arc (Fig. 1) coincides cordillera forms the foothills of the Andes between 29◦S and 33◦S. with a decreasing age of the subducted plate and hence a higher It is a fold and thrust belt developed on a Palaeozoic carbonate plat- geothermal gradient (Yañez& Cembrano 2004). Crustal seismicity form (Baldis et al. 1982). East of the Andes, the Sierras Pampeanas is very active in the foreland over the flat-slab segment (Smalley province is composed of a series of crystalline basement-cored & Isacks 1990; Regnier et al. 1992; Smalley et al. 1993; Gutscher Precambrian–Early Palaeozoic rocks uplifted and tilted by com- et al. 2000). The Precordillera and Sierras Pampeanas have pro- pression during the formation of the Andes and separated by broad duced large crustal earthquakes (Costa et al. 2001; INPRES 2005) and relatively undeformed basins (Jordan et al. 1983; Jordan 1995). with the most destructive having magnitude ∼7.0 and occurring These uplifts are considered a modern analogue to the Laramide near San Juan in 1944 (Castellanos 1944; Groeber 1944) and 1977

C 2005 RAS, GJI, 163, 580–598 Moment tensor inversion of Andean backarc crustal earthquakes 583

(INPRES 1977; Volponi 1979; Kadinsky-Cade 1985; Costa et al. Table 1. Crustal velocity structures used in the SMTI. ◦ 2000) (Fig. 2). In contrast, south of 33 Svery shallow (depth Thickness Vp Vp/Vs Density < − − 20 km) earthquakes are restricted to the high cordillera (km) (km s 1)(grcm3) (Barrientos et al. 2004) (Figs 1 and 2). Model 1 10 5.88 1.70 2.70 3DATAANDREGIONAL WAVEFORM 35 6.20 1.70 2.85 Half-space 8.15 1.80 3.30 MODELLING Model 2 The CHARGE network consisted of 22 seismic stations with 10 45 6.20 1.80 2.85 STS2 and 10 Guralp-esp sensors from IRIS-PASSCAL (USA), and Half-space 8.15 1.80 3.30 two Guralp-40T sensors from the Instituto Nacional de Prevención S´ısmica (INPRES) and the Universidad Nacional de San Juan (Argentina), respectively. They were deployed in two transects from in the fitting of the data and consequently in the seismic moment the Chilean coast to central Argentina at about 30◦S and 36◦S with tensor results. Finally, we compared synthetic and observed data several stations in between (Fig. 1). Stations JUAN, USPA, HUER aligned at the first P-wave arrival for each station. This reduces the and LLAN were installed coincident with INPRES sites. The rest of dependence on location uncertainties, origin time and the chosen the instruments were deployed inside a temporary vault located on seismic velocity structure. Thus, from 75 earthquakes of magnitudes hard-rock sites. The instruments operated continuously recording at M ≥ 4.0 and depths <50 km that occurred during the CHARGE 40 samples per second for 1.5 yr (2000 December–2002 May). In operation period (Fig. 1) we chose the 27 events with the best that period, we recorded 75 shallow earthquakes having body wave locations. magnitudes M ≥ 4.0 including four events of 5.0 < M < 6.0, with In determining the seismic moment tensor, we used two seismic locations in the backarc and main Andean cordillera region (Fig. 1). velocity structures for the region, summarized in Table 1. These We performed a linear least-squares seismic moment tensor in- structures are based on receiver function analysis (Shearer 2002; version (SMTI) for 27 crustal earthquakes that were well recorded Gilbert et al. 2005) and Pn studies (Fromm et al. 2004) that also using at least four CHARGE stations. We used available global used the CHARGE data and forward modelling in this study.Model 1 data from station PEL (Geoscope) in Chile to study three events represents the eastern Sierras Pampeanas, and Model 2 character- located near Mendoza. We also determined the best focal depth. We izes the western Sierras Pampeanas, Precordillera and Cordillera will refer to the events by the year and Julian day of occurrence. terranes. The main difference in the models is the Vp/Vs ratio in The SMTI technique (Randall et al. 1995) consists of modelling the crust (especially Vs). Therefore, for cases involving ray paths in the complete three-component seismic-displacement records at re- both terranes we combined both structures to obtain a better fit to gional distances to obtain the seismic moment tensor for a point the data. source. We considered epicentral distances up to 727 km. We used a bandpass filter between 15 and 50 s for larger earth- Accurate seismogram modelling depends upon reliable seismic quakes and 15–30 s for smaller events to compare synthetic and ob- locations and a seismic velocity structure. We used the epicentre in- served data. As a result, we obtained the seismic moment tensor and formation determined by the Argentine seismic network (INPRES) corresponding best double-couple fault-plane solution for a point operating in the region. Data from more than 23 seismic stations source that had the smallest amplitude misfit. We also estimated that use short-period, broad-band and acelerometer sensors were the compensated linear vector dipole (CLVD) component for each used to constrain the hypocentres. The determination encompasses moment tensor. This parameter measures how different the source more than 10 phases with more than two S-wave arrival times. The is from a pure double-couple solution. Thus CLVD = 0 per cent earthquake locations were calculated using HYPO71 (Lee & Valdes means that the moment tensor is 100 per cent a pure double-couple, 1985) and a simplified crust-upper mantle model. Horizontal epi- producing a dislocation which in theory has a geometry identical central errors are smaller than 10 km. Vertical hypocentre errors to the faulting. A discussion about how non-double-couple com- are in general larger, but we determined the focal depth using the ponents affect the solution using the same SMTI technique can be complete broad-band regional waveforms in the SMTI. found in Mancilla et al. (2002). The amplitude-misfit normalized In order to assess the effect of epicentral mislocations on our error is the sum of the square difference between synthetic and ob- focal mechanisms, we performed several tests using the epicentre served waveforms, divided by the sum of the squares of all observed determinations. Before the inversion, the north-south and east-west seismogram amplitudes considered in the inversion. The seismic seismic components at each CHARGE seismic station were ro- displacements are not linearly dependent on the focal depth. There- tated into the orthogonal radial and tangential components using the fore, we performed a grid search for the source depth, producing INPRES epicentre information. For each event, we produced plots synthetic seismograms at a series of fixed hypocentral depths. of the horizontal particle motion at each station for a ∼1.5-s time Fig. 3 shows the results for event 02-117 (MD = 5.3) that oc- window around the first P-wave arrival. For a correct epicentre lo- curred northeast of San Juan on 2002 April 27. We modelled this cation, there should be no energy on the tangential component, and event at ten CHARGE stations for all three components of displace- horizontal particle motion should be maximum in the radial com- ment using different trial depths. The stations used relative to the ponent. We found that horizontal hypocentral determination errors epicentre show good azimuthal coverage (Fig. 3c). The results of <6kmproduced more than 90 per cent of the seismic energy concen- the focal depth versus normalized amplitude misfit, CLVD compo- trated in the radial direction with respect to the tangential direction. nent and focal mechanism are shown in Fig. 3(a). The amplitude- As another way to test the impact of epicentral mislocations, we misfit error curve shows a single minimum around the best depth determined the focal mechanisms with and without data from the (21 km). This is also coincident with a minimum for the CLVD closer CHARGE stations. Data from closer stations are more im- (0 per cent). The focal mechanism solution remains robust for the pacted by epicentral mislocation because the mislocation is a larger depth range investigated with the inversions. Fig. 3(b) shows the percentage of the total path length. Hence it introduces larger errors fit of the data and synthetic for the best depth using a bandpass

C 2005 RAS, GJI, 163, 580–598 584 P. Alvarado et al.

Figure 3. Results of the regional seismic moment tensor inversion (SMTI) for event 02-117. (a) Amplitude-misfit error between synthetic and observed seismic displacements versus source depth (curve with the focal mechanisms). The minimum occurs at a depth of 21 km. We ran an inversion every 1 km testing also the CLVD percentage in the solution (dashed line curve). The minimum in the CLVD component coincides with the minimum in the amplitude-misfit error curve. Note how stable the focal mechanism solution is for the depth range explored. (b) Matches between synthetic and observed waveforms for the best depth (21 km). Epicentral distance and azimuth of the stations used are indicated below the station name. Waveforms were bandpass filtered between 15 and 50 s. (c) Location map of broad-band stations used in the SMTI showing the ray paths relative to the epicentre. For comparison, we present the Harvard-CMT solution.

= × 15 filter between 15 and 50 s. The SMTI solution is M 0 5.81 depth (25 km) that produced the solution M 0 = 2.01 × 10 Nm; 16 ◦ ◦ ◦ 10 Nm;M w = 5.1; depth = 21 km; fault plane 1: strike = 34 , M w = 4.2; depth = 25 km; fault plane 1: strike = 31 , dip = 31 , ◦ ◦ ◦ ◦ dip = 40 , rake = 98 ;fault plane 2: strike = 204 , dip = 50 , rake = rake = 124◦;fault plane 2: strike = 173◦, dip = 65◦, rake = 72◦. ◦ 83 .Weused the Aki & Richards (1980) convention for each fault- We also present the SMTI results for event 02-005 (MD = 4.8) plane solution, which gives the azimuth of the fault, the dip of the with an epicentral location near the Barrancas anticline, the struc- fault considering the block located at the right when looking at the ture on which the 1985 (M w = 5.9) earthquake is thought to have azimuthal direction, and the rake, which is the angle of displace- occurred as discussed in Section 5.3 (Figs 2 and 5). The best results ment measured in the fault plane from the azimuthal direction. The show the minimum in the amplitude-misfit error curve as a function 16 Harvard-CMT solution for this event (M 0 = 8.02 × 10 Nm; of the source depth occurs for a minimum in the CLVD curve at ◦ M w = 5.2; depth = 49.3 km; fault plane 1: strike = 341 , dip = 26 km depth (Fig. 5a). 10 CHARGE stations were used in this in- ◦ ◦ ◦ ◦ 41 , rake = 31 ;fault plane 2: strike = 227 , dip = 71 , rake = version (Fig. 5b). Station AREN recorded the largest amplitudes ◦ 127 ) has a similar magnitude with a strike-slip component for the of the three-component seismic displacements used. In order to test focal mechanism and a deeper source for the centroid located south- how much this station dominates the SMTI we ran the inversion tak- west of the INPRES location. Even though our focal mechanism is ing out data from AREN. We found similar results (M 0 = 1.18 × 16 ◦ different from the CMT solution, the P-axis orientation does not 10 Nm;M w = 4.7; depth = 26 km; fault plane 1: strike = 61 , ◦ significantly change within 8 (Table 2). Modelling higher frequen- dip = 22◦, rake = 175◦;fault plane 2: strike = 156◦, dip = 88◦, cies (as using the regional SMTI technique) than those considered rake = 68◦). Fig. 5(c) shows the synthetic-observed data compari- using the CMT inversion method should give us a better estimate son for the best depth (26 km). of the depth of the event. There are only two CMT solutions in the region during the CHARGE network operation period that we can 4SENSITIVITY TO CRUSTAL MODELS compare with (events 02-117 and 01-285). Overall, both solutions have small differences (their focal mechanisms are mainly reverse The seismic modelling for the source depends to second order or mainly strike-slip solutions), although the CMT depths tend to on the seismic velocity structure used to calculate the Green’s be larger (Table 2). functions (Sileny 2004). In order to test the sensitivity of the solution Fig. 4 shows another example for event 02-107 (MD = 4.4) located to the chosen crustal model we selected two earthquakes (events north of event 02-117. We modelled this earthquake using the closer 01-138b and 02-117) with epicentral locations in the eastern and stations and data filtered with a bandpass between 15 and 30 s. western Sierras Pampeanas, respectively. For each seismic event, We present these results using Model 1 and Model 2 separately. we performed a grid search for the crustal thickness, P-wave crust The focal mechanism solutions do not change significantly around velocity, and crustal Vp/Vs ratio using the SMTI method described the best depth. However, a better fit is obtained using the seismic previously. velocity structure described in Model 2 (Fig. 4a). This reflects the First, we constructed the Green’sfunctions for event 01-138b with fact that most of the seismic ray paths travelled in the western Sierras magnitude M w = 4.3 located near Córdoba(Table 2 and Fig. 6c) Pampeanas for the stations considered in the SMTI (Fig. 4b). We for a set of simple average models consisting of a crustal layer show in Fig. 4(c) the synthetic and observed data match for the best over a half-space. The models had a crustal thickness varying from

C 2005 RAS, GJI, 163, 580–598 Moment tensor inversion of Andean backarc crustal earthquakes 585 2 2 2 2 1 2 2 2 2 2 1 2 2 2 2 2 2 2 ustal 1–2 1–2 Model Cr column INPRES in wn local 30 14 13 14 22 16 20 13 23 27 25 14 15 15 18 20 14 10 11 25 17 sho #comp the s from ﬁtting rm 0.237 0.264 0.469 0.438 0.445 0.184 0.394 0.410 0.389 0.241 0.303 0.569 0.239 0.221 0.154 0.530 0.516 0.200 0.280 0.331 are ram D g M w 5.1 4.0 3.5 3.6 3.8 4.3 3.8 3.5 4.0 4.4 4.1 3.8 4.8 4.0 4.0 3.7 3.6 3.9 5.3 4.1 4.0 M seismo (stat.) the D y magnitude b 5.1 3.9 4.0 4.2 4.0 4.5 3.4 4.1 4.1 5.3 4.2 4.2 4.8 4.3 4.2 4.1 3.8 4.0 4.2 4.5 M en v s gi M 5.6 3.4 3.0 4.0 4.4 3.2 3.2 3.3 3.2 3.3 duration rors er and Nm 000 300 800 400 950 209 334 569 574 229 966 512 405 269 743 km), 12 (with 2990 1590 1060 1230 1510 1060 56 10 20 105 Seismic 6 moment w 10 ≤ M ror 2 62 85 84 47 18 83 79 84 71 63 85 78 67 77 Pl 37 53 81 15 80 74 (er -axis T 3 23 13 99 31 Az magnitude 168 118 328 343 209 103 143 162 292 292 359 210 207 202 178 305 location 6 5 4 6 9 6 7 4 3 5 3 2 3 2 3 Pl 23 19 26 25 27 11 moment solution -axis P epicentral 80 14 96 91 87 76 Az 259 262 294 100 116 311 290 110 101 145 114 308 113 326 135 moment, time, 15 mechanism 56 89 96 33 19 95 82 91 96 58 83 91 86 88 97 83 75 66 12 79 77 47 69 94 74 72 78 147 104 Slip 103 106 103 171 − 198 130 138 173 104 119 169 104 ocal Origin seismic F 75 73 41 46 57 64 50 40 49 71 53 83 39 51 39 71 26 49 53 41 41 19 64 44 52 42 51 48 46 48 46 43 76 27 57 49 83 81 48 42 78 36 W. Dip ◦ –71 e ◦ 36 19 22 23 15 45 15 14 20 34 60 34 16 98 55 64 301 185 149 204 251 133 344 115 218 198 204 276 193 181 190 238 231 236 215 218 356 353 334 314 191 222 151 mechanism, Strik and S focal ◦ 5 7 6 3 17 15 21 22 20 20 14 16 22 15 10 13 20 14 –36 km km), ◦ depth 15–20 (13–17) (18–24) (13–15) (21–24) (20–30) (10–20) (20–32) (20–30) Modelled 29 2.5 ≤ een ror ed) 5 betw (er 10 20 16 30 40 23 11 51 10 33 80 19 20 33 10 15 15 54 km 65.9 22.3 depth (ﬁx Location depth backarc ree g 67.73 64.93 68.02 69.56 67.87 69.04 69.815 68.999 67.873 69.012 67.729 67.956 66.857 69.080 64.435 66.963 69.495 67.214 68.015 67.534 68.935 de − − − − − − Modelled − − − − − − − − − − − − − − − Andean Longitude . the gue. of study ree es g 31.0 catalo 34.82 32.72 33.16 33.286 31.596 33.194 31.247 33.211 31.756 32.072 29.337 33.021 31.787 31.383 32.113 33.459 33.346 31.533 33.131 29.474 − de − − − this Latitude − − − − − − − − − − − − − − − − − thquak NEIC from ear the are ime T GMT from Origin 04:21:31 14:25:03 17:16:26 02:27:35 11:14:18 hr:mm:ss 13:15:9.1 00:37:35.2 13:10:13.3 04:38:23.3 22:37:24.3 01:17:50.4 01:11:50.4 11:36:26.8 20:18:40.2 04:21:37.9 is 22:57:11.06 15:00:26.10 05:40:06.10 11:06:19.99 00:49:27.15 06:13:12.60 intraplate s columns M of three y yr/ da Date last month/ 01/10/12 01/09/17 01/07/08 01/06/28 01/06/20 01/05/18 01/05/18 01/05/10 01/12/15 01/05/07 01/12/14 01/03/15 01/12/05 01/02/09 01/11/29 01/02/06 01/11/24 00/12/25 01/11/12 00/12/21 arameters Magnitude P the . 2 gue. and e l b y -Julian ) ms), ∗ ( 01-285 01-260 01-189 01-179 01-171 01-138b 01-138a 01-130 01-349 01-127 01-348 01-074 01-339 01-040 01-333 01-037 01-328 00-360 01-316 Da catalo r yr Ta Date 00-356

C 2005 RAS, GJI, 163, 580–598 586 P. Alvarado et al. 2 2 2 2 1 2 ustal 1–2 Model Cr 30 10 18 21 15 30 36 29 (stat.) #comp s rm 0.316 0.190 0.171 0.207 0.300 0.254 0.310 w 5.1 4.0 4.2 4.0 4.0 4.7 5.2 4.5 M D 5.3 3.9 4.4 3.9 4.0 4.8 4.3 M s M 4.8 3.2 3.9 2.7 4.1 3.5 Nm 100 800 200 966 12 1210 2010 1070 5860 58 11 80 Seismic moment 10 83 11 65 75 Pl 82 43 50 88 -axis T 72 51 85 45 44 Az 145 176 178 5 4 8 2 Pl 30 18 39 17 solution -axis P Az 299 242 277 280 287 266 291 257 15 mechanism 98 69 31 83 72 86 68 92 95 88 124 175 Slip 109 − 210 127 ocal F 50 65 40 53 50 77 31 88 43 37 43 71 61 22 41 47 Dip e 34 17 31 21 61 204 173 194 204 156 356 349 227 279 341 166 Strik 21 25 18 15 26 17 km 49.3 (5–9) depth (16–21) Modelled 16 10 27 km 12.3 13.3 23.6 15.4 depth Location

ree Figure 4. Results of the SMTI for event 02-107. (a) Curves with focal mech- g 67.6 68.61 67.78 67.827 64.090 67.965 66.175 66.859 − de anisms showing the amplitude-misfit errors between synthetic and observed − − − − − − − Longitude seismograms versus hypocentral depth for two velocity structure models (Table 1). The minimum is observed for a CLVD = 7 per cent (dashed line) at a source depth of 25 km using Model 2 (western Sierras Pampeanas). (b) ree g 31.65 30.36 31.72 30.998 30.314 30.908 33.313 32.124 Location map of the stations used in the SMTI showing ray paths from the de − − − Latitude − − − − − epicentre to the CHARGE stations. (c) Synthetic and observed waveforms for the best fit (25 km) using a bandpass filter of 15-30 s. Epicentral distances centroid. and azimuth of the stations used are shown below the station name. the ime T GMT for Origin 25 to 65 km in steps of 5 km, a P-wave crustal velocity varying from 23:53:53 21:35:47 hr:mm:ss 02:11:09.2 11:13:51.5 04:23:14.8 23:53:58.8 12:02:57.3 09:16:27.3 5.8 to 6.6 km s−1 in steps of 0.2 km s−1, and a Vp/Vs ratio fixed − .) at 1.70. We maintained the P-wave velocity at 8.15 km s 1 for the solutions y half-space with a Vp/Vs ratio of 1.80. We ran an inversion matching yr/ da Date

month/ synthetic and observed seismograms using a series of source depths 02/04/27 02/04/17 02/03/15 02/03/10 02/01/19 02/01/05 01/12/18 Continued

( for each crustal model. We used a bandpass ﬁlter between 15 and ard-CMT . v 2 30 s. Fig. 6(a) shows the results for the minimum normalized misﬁt e l

Har errors obtained for each model. Accepting misﬁt errors with rms b y -Julian ) )

∗ ∗ < < ( 02-117 02-107 02-074 02-069 02-019 Da yr 02-005 Ta Date 01-352 ( 0.25 constrains the crustal thickness to be 45 km and P-wave

C 2005 RAS, GJI, 163, 580–598 Moment tensor inversion of Andean backarc crustal earthquakes 587

1.85 in steps of 0.05. Again, changes in Vp/Vs ratio correspond to S-wave crustal velocity variations only. No modifications were made in the half-space described above. Fig. 6(b) displays the results of the inversions obtained using each seismic model. A crustal thickness of <44 km and Vp/Vs values of 1.65–1.70 produce misfit errors with rms <0.25. Therefore, models using a low crustal Vp of 6.0–6.2 km s−1,a low Vp/Vs ratio of 1.65–1.70, and a crustal thickness corresponding to a normal crust (30 km), or at least no thicker than 45 km, predict the best results. Nevertheless, the seismic ray paths for the earthquake considered in this grid search (event 01-138b) recorded at the CHARGE seismic stations used in the inversion test travelled mostly through the eastern Sierras Pampeanas (Figs 2 and 6c). The focal mechanism solutions did not change significantly around the best depth (Fig. 6e) and remained stable for the entire range of parameters tested. The same grid-search process was carried out for event 02-117 with magnitude M w = 5.1 and epicentre location in the western Sierras Pampeanas (Table 2 and Fig. 3). For this test, we only considered stations located in the western part of the CHARGE network region (compare Fig. 7c to Fig. 3c). We started testing crustal models with P-wave crustal velocity varying from 5.80 to 6.60 km s−1 in steps of 0.2 km s−1, Vp/Vs = 1.80, and thickness varying from 25 to 65 km in steps of 5 km. We maintained the mantle parameters Vp = 8.15 km s−1 and Vp/Vs = 1.80. We ran an inversion for each model and a set of fixed source depths using a bandpass filter of 15–50 s. Fig. 7(a) shows a map of the SMTI minimum amplitude-misfit error results for each crustal model. The best fit (>75 per cent) occurs around a crustal Vp = 6.40 km s−1 and a thickness of 45–52 km. Fig. 7(b) shows SMTI results for a grid search around Vp/Vs and crustal thickness for the same event. We tested models fixing the crustal P-wave velocity at 6.20 km s−1 and varying the crustal S-wave velocity to obtain Vp/Vs ratios in the range 1.60–1.85 in steps of 0.05. The crustal thickness varied from 25 to 65 km in steps of 5 km. The mantle parameters were the same as before. In contrast to the test results for the event in the eastern Sierras Pampeanas (Fig. 6b), the best results for event 02-117 occur when considering a crustal structure with Vp/Vs ratio greater than 1.80 and thickness between 40 and 57 km. Fig. 7(d) shows examples of the amplitude-misfit error and CLVD versus source depth for a set of 45-km-thick crust models. We conclude that the best results are observed using a higher Vp/Vs ratio (1.80–1.85) if the ray paths travel in the western Sierras Pampeanas geological terranes. Figure 5. Results of the SMTI for event 02-005. (a) Amplitude-misfit er- Fig. 8 shows seismic ray paths for variations in S-wave velocity rors between synthetic and observed seismograms versus hypocentral depth used in the SMTI of the 27 earthquakes here studied. The P-wave (curve with focal mechanisms). The minimum with a CLVD = 0 per cent velocity that we used in all inversions was practically the same for (dashed line curve) occurs at a depth of 26 km. (b) Map of the stations used both models (Table 1). However, based on the SMTI results (Figs 6a, in the SMTI showing the ray paths from the epicentre. (c) Synthetic and d, e and 7a), it seems that Vp is smaller in the eastern than in the observed seismic waveforms for the best depth (26 km) using a bandpass western part of the region. Fig. 8 shows that most of the ray paths are filter of 15–40 s. Epicentral distances and azimuth of the stations used are sampling the western geological terranes. However, even with less shown below the station name. sampling in the eastern Sierras Pampeanas the results indicate that Model 1 is more appropriate for this region. The last two columns crustal velocities to be ∼6.0–6.2 km s−1. The misfit increases rapidly in Table 2 show the total number of seismic components and the (>75 per cent) using a high crustal Vp and/or a thickened crust crustal model used in the SMTI of each event, respectively. (Figs 6a, d and e). We did not modify the structure for the upper mantle in this The second step consisted of fixing Vp at 6.0 km s−1 and initiating grid-search analysis. Previous studies in the Altiplano using the agrid search around crustal thickness and Vp/Vs ratio using the same technique and band-period ranges of 15–50 s and 15–100 s SMTI for the same earthquake. This means that only the S-wave demonstrated that the SMTI technique is less sensitive to changes velocity and crustal thickness varied. We ran an inversion for each in the upper mantle seismic velocities (Swenson et al. 2000). model of crustal thickness varying the thickness from 25 to 65 km Our results are in agreement with the Vp/Vs estimates based on in steps of 5 km and varying Vp/Vs ratio varying from 1.60 to receiver function analysis (Gilbert et al. 2005) and crustal thickness

C 2005 RAS, GJI, 163, 580–598 588 P. Alvarado et al.

−1

−1 0.25

−1

0.85

0.25 −1

0.70

−1

−1 −1

−1

Figure 6. Grid search results obtained for event 01-138b using the SMTI technique. (a) Synthetic and observed data misfit errors for different crustal Vp and crustal thickness combinations. We maintained crustal Vp/Vs = 1.70, mantle Vp = 8.15 km s−1 and Vp/Vs = 1.80. Dots represent different crustal model parameters used in (e). (b) Synthetic and observed data misfit errors for different crustal Vp/Vs ratio and crustal thickness pairs maintaining crustal Vp = 6.0 km s−1. The mantle parameters were the same as in (a). (c) CHARGE stations used in the inversions with respect to the event location. (d) Synthetic and observed seismogram fit for the best depth using the model with crustal Vp = 6.00 km s−1 and thickness = 40 km. (e) Amplitude-misfit errors versus hypocentral depth for different crustal Vp model parameters showed as dots in (a) for a 40-km-thick crust. Dashed lines are the CLVDcomponent in the solution. Note how the focal mechanism solution does not significantly change around the best depth (∼5 km). from Pn studies (Fromm et al. 2004) that also used CHARGE data. Chilenia paths (Ramos 1994). Our estimates in the western terranes Previous studies using a dense local network (1987–1988 PANDA are in close agreement with those results, although our surface- experiment) in the region around Sierra Pie de Palo (Fig. 2) cov- wave-based average crustal models do not discriminate between the ering an area of roughly 150 km north–south by 100 km east–west properties of the Pampeanas and Eastern Precordillera terranes. showed the westward increase of the Moho depth between the Sierra Pie de Palo (48–55 km) and the Precordillera (55–60 km) from 5RESULTS AND DISCUSSION P-and S-wave converted phases (Regnier et al. 1994). Interpreta- tions of JHD PANDA station corrections also predict higher crustal Our results for the events in this study are listed in Table 2 and shown velocities (>6.4 km s−1) near Sierra Pie de Palo than to the west of in Fig. 9. The table includes the SMTI results for both fault-plane so- the Eastern Precordillera (Pujol et al. 1991). These studies found dif- lutions, the orientation (azimuth and plunge) of the P- and T-axis, the ferences in the crustal structure between the Cuyania and Chilenia seismic moment, the moment magnitude and the amplitude-misfit terranes that we cannot resolve because we have very few pure error. Values for Ms and MD are from the global NEIC and local

C 2005 RAS, GJI, 163, 580–598 Moment tensor inversion of Andean backarc crustal earthquakes 589

−1 0.55

−1 −1 0.30

0.55

0.30

Figure 7. Grid search results obtained for event 02-117 using the SMTI technique. (a) Maps of the synthetic and observed data misﬁt errors for each crustal Vp and crustal thickness pairs explored. Mantle parameters are as in Fig. 6. (b) Same as (a) but now varying Vp/Vs ratio and maintaining crustal Vp = 6.20 km s−1. Dots show the crust models used in the inversions shown in (d). (c) Ray paths, seismic station and epicentre location used in all of the inversions shown in (a) and (b). Note that the ray paths are travelling in the western geological terranes for this grid searching. The same event, analysed with more station-components, is shown in Fig. 3. (d) Amplitude-misﬁt error and CLVD component (dashed line) versus source depth for different crustal Vp/Vs models (dots in diagram b).

INPRES catalogues, respectively. Even though in theory single best depth coincident with a minimum or an acceptable (<20 per three-component station waveforms are sufficient to estimate the cent) CLVD component. moment tensor solution and focal depth, we limit the minimum num- In some cases, we observed a broad minimum instead of a more ber of stations to four (Fig. 8). The more stable solutions, however, local minimum in the amplitude-misfit error curve around the best are related to higher numbers of station/components used (see the depth. However, the focal mechanism solutions did not change for number of used components (column #comp) in Table 2). We also the depth-range explored. In general, the depth curves were broader observed that the CLVDcomponent depends on azimuthal coverage. for events located outside of the CHARGE network with magnitudes Hence we present solutions with a minimum misfit error around the <3.9. For example, event 00-360 in Table 2 shows a minimum at

C 2005 RAS, GJI, 163, 580–598 590 P. Alvarado et al.

Figure 8. Ray paths showing the different seismic velocity models (Table 1) used during the inversion of the 27 crustal earthquakes in this study. Crustal S-wave velocity variations are responsible for Vp/Vs changes. Higher Vp/Vs ratios produced better ﬁts for ray paths mainly travelling in the western geological terranes (see Fig. 2) while lower Vp/Vs ratios seem to better characterize the Eastern Sierras Pampeanas.

20 km depth, but errors are still acceptable for a depth-range of 20– thicker Andean crust than in the eastern shields for this region (Feng 30 km. Higher frequencies are required to constrain the depth better. et al. 2004). However, at higher frequencies this is complicated because the so- Earthquakes with epicentre locations in the western Sierras lution becomes more sensitive to the complex shallow structure of Pampeanas show focal mechanisms with pure thrust or reverse fault- the crust. Nevertheless, our determinations are still very useful to plane solutions. These earthquakes are at depths of between 15 and discriminate between crustal and deeper or intraslab seismic events, 25 km, which represent middle-to-lower crustal levels. The only ex- and to characterize their focal mechanism solution. We obtained ception is event 01-348, which we will discuss later. In the eastern good depth constraints for 80 per cent of the crustal earthquakes Sierras Pampeanas, we find strike-slip fault-plane solutions (events studied with an estimated error ≤2.5 km. 02-019 and 01-138b) at shallower crustal depths. We were still able Overall, we find the backarc has mainly reverse or thrust focal to model event 02-074 (MD = 4.0) located ∼50 km north of Córdoba mechanism solutions with average P-axis and T-axis oriented at az- using CHARGE data at epicentral distances up to 727 km with poor imuths of 275◦ and 90◦, and plunges of 6◦ and 84◦, respectively. azimuthal coverage. Its solution shows a reverse focal mechanism However, the earthquakes could be representing different styles and a source depth of ∼18 km. This example shows that it is possible of deformation as reflected by crustal seismicity variations as we to estimate the moment tensor for M ∼ 4.0 events with locations discuss below. outside the network using the SMTI technique. Event 01-285 lo- A larger number of events modelled in this study have locations in cated in the main cordillera had the largest magnitude (Ms = 5.6) of the western terranes Cuyania and Famatina (Figs 1, 2, 9, 10 and 11). the studied crustal earthquakes. Its focal mechanism solution indi- The Pampia terrane does not show seismic activity for earthquakes cates strike-slip motion at a very shallow depth (5 km). The region of magnitudes larger than 4.0 during the CHARGE time period. around Mendoza showed deeper crustal earthquakes in the range However, palaeoseismic studies along Quaternary faults point out of 10–26 km and reverse focal mechanisms with some strike-slip significant earthquakes that have occurred in the southern part of this components. The southernmost located earthquake we modelled is terrane (Costa & Vita-Finzi 1996; Costa et al. 2000). Interestingly, event 01-074, which occurred on 2001 March 15 in the eastern flank we also observed crustal earthquakes further east at about 800 km of the Andean cordillera with a moderate magnitude (MD = 4.9). from the trench near Córdobain central Argentina related to the R´ıo Our results indicate a pure-thrust fault-plane focal mechanism and de la Plata terrane (events 02-074, 01-138b and 01-339). Another avery shallow depth of 3 km for this event. Notably, the events distinction already mentioned was observed in the seismic velocity located south of 33◦Sinthe high cordillera showed very shallow structure for the region. We could fit the observed data better when depths (<5 km) during the CHARGE period. using an S-wave seismic velocity that was 6 per cent lower in the crust (40–57 km thickness) in the western Sierras Pampeanas than in the 25- to 45-km-thick crust of the eastern Sierras Pampeanas. 5.1 Western Sierras Pampeanas–Sierra Pie de Palo This result could be related to a different composition or a more and Valle Fertil´ Megashear fractured crust in the west or a combination. Regional surface-wave The region around the Sierra Pie de Palo (Fig. 2) is of particular tomographic studies also show lower S-wave velocities beneath the interest because of the last large damaging (Ms = 7.4) earthquake in

C 2005 RAS, GJI, 163, 580–598 Moment tensor inversion of Andean backarc crustal earthquakes 591

Figure 9. Results of the SMTI showing lower hemisphere projection focal mechanisms and source depths in parentheses (Table 2). Dark quadrants are compressional motions. Observe the distribution of the crustal seismicity with respect to the presence of geological terranes and their boundaries. Location of the seismic data showed in the cross-section in Fig. 11 is indicated by the rectangle. Brackets at 31◦S indicate the location of the geological proﬁle shown in Fig. 11.

1977 linked to different interpretations of its causal fault (Fig. 10) we think that it is important to briefly put our results in the context (Kadinsky-Cade et al. 1985; Langer & Bollinger 1988; Langer & of known structures in and around the Sierra Pie de Palo (Figs 10 Hartzell 1996). This Precambrian block is one of the westernmost and 11). ranges of the Sierras Pampeanas and the most seismically active Event 02-117 (Figs 3, 9 and 10) is very intriguing because its in this geological province (Figs 1 and 2). Previous seismic studies epicentre is in the vicinity of the Valle Fértil major reverse fault that show along-strike variation of structures in this area (Langer & borders the western side of this north-northwest trending range and Bollinger 1988; Regnier et al. 1992) making it difficult to associate is responsible of its uplift (Jordan & Allmendinger 1986; Snyder microseismicity with single planar faults (Smalley & Isacks 1990). et al. 1990) (Figs 2 and 10). This 600-km-long fault zone has rocks The 3-D structural geometry of this range is very complex, and characteristic of an ophiolitic belt (Ramos et al. 2000). It seems to its relationship with the adjacent areas is interpreted as different have generated several earthquakes recorded by the global seismo- blocks bounded by nearly north-trending thrust faults with important logical network (Stauder 1973; Chinn & Isacks 1983; Dziewonski E–W transverse megafractures (Bastias & Weidmann 1983; Perez et al. 1988), which also detected the moderate-size (M w = 5.1) event & Martinez 1990; Regnier et al. 1992; Zapata & Allmendinger 02-117. However, the SMTI fault-plane trends of our solution and 1996; Zapata 1998) (Fig. 10). Ramos et al. (2002) suggested an the depth suggest it may be associated with the main north-northeast alternative interpretation for the southernmost block of the Sierra oriented axis of the Sierra Pie de Palo and other non-exposed struc- Pie de Palo, a passive roof duplex related to a basement mid-crustal tures located to the north of this range (Zapata & Allmendinger wedge. We found earthquake sources deeper than 15 km for seven 1996; Zapata 1998). In fact, seismic reflection interpretations at the events with epicentral locations to the east of the major axis of the intersection of the Sierra Pie de Palo and the next range to the east, main anticlinorium of the Sierra Pie de Palo. Almost all of the fault- Sierra Valle Fértil, predict a buried anticline as the northern ter- plane solutions have trends oriented parallel to this axis and indicate mination of the Sierra Pie de Palo named the North Pie de Palo pure thrust or reverse faulting (Fig. 10). anticline (Fig. 10). We speculate that event 02-117 may be related Given uncertainties in the earthquake locations, it is very specu- to the fault that bounds the North Pie de Palo anticline to the west. lative to relate any one event to one structure. However, because of This evidence has two implications: the Sierra Pie de Palo western the increasing interest in relating regional seismicity with structural fault branches from the Sierra de Valle Fértil fault (Fig. 10, inset) as system or crustal volumes associated with structures in active zones, predicted by Zapata (1998). Second, this structure correlated with

C 2005 RAS, GJI, 163, 580–598 592 P. Alvarado et al.

Figure 10. Major thrust faults around the Sierra Pie de Palo according to the geological references cited in the text and earthquakes in this study with maximum epicentre-location errors of ±6km(Table 2). Approximate locations of the historical large crustal earthquake in 1944, and the foreshock (F) and mainshock (M) of the Ms = 7.4 1977 earthquake are also shown. Fractures and faults divide the Sierra Pie de Palo in several blocks. Event 02-117 (M w = 5.1) at 21 km depth (Fig. 3) is likely related with the northern continuation of this oldest Pampean range in the buried North Pie de Palo anticline (Zapata 1998) (see the vertical projection of this event in the inset). Projections and labels for the seismic events are the same as in Fig. 9. the northernmost part of a suture between the Precordillera and the studies by Regnier et al. (1992) also deﬁne a cluster of seismicity western Sierras Pampeanas (Fig. 10) of Grenville age (Zapata 1998) around 5 km depth for this area. However, a potential occurrence of is seismically active. event 01-348 along the west-dipping fault-plane solution cannot be Another moderate-sized earthquake (event 01-348; MD = 5.3) ruled out, suggesting its relationship with a shallower d´ecollement occurred in the southeast of the Sierra Pie de Palo (Fig. 10) on under Sierra Pie de Palo like that modelled by Ramos et al. 2001 December 14. Its focal mechanism solution indicates a very (2002). shallow west-dipping plane or a vertical east-dipping plane and There are other events (00-356, 01-107, 01-333, 01-352 and 01- 6kmfocal depth. Previous large events in this area like the second 328) located near the terrane boundaries that separate Cuyania from shock of the 1977 (Ms = 7.3) earthquake (Kadinsky-Cade 1985) the Famatina and Pampia terranes. They also separate the region and the 1941 (M = 6.7) earthquake (INPRES 2005) have large in terms of seismicity. Very few crustal earthquakes of magnitudes location uncertainties and do not show a clear correlation with ex- >4.0 had locations in the Pampia terrane during the CHARGE pe- posed faults (INPRES 1977; Volponi 1979; Langer & Bollinger riod (Figs 1, 2, 9, 10 and 11). 1988; Langer & Hartzell 1996). Although a smaller earthquake, Deformation rates based on moment tensor summation (Kostrov event 01-348, is very shallow so we might expect it to correlate with 1974) using the Harvard-CMT solutions for events (M w > 5.0) that an exposed fault. This earthquake could be related to some contin- occurred in the last 27 yr around the Pie de Palo range (Fig. 2) predict uation to the southwest of active faults seen on the east side of the 31.8 mm yr−1 rate of crustal shortening (Siame et al. 2002, 2005). southern block of the Sierra Pie de Palo known as the Nikizanga fault This estimate results from multiplying the horizontal component system (Ramos & Vujovich 1995; Ramos et al. 2002) (Fig. 10). The of the maximum shortening strain rate (oriented in the direction east-dipping Nikizanga fault showed a 10 km surface rupture during N93◦E) times the width of 93 km for the seismically deformed the large deep-crustal earthquake of 1977, and for this reason it is volume of 123 × 93 × 30 km3,which covers the Sierra Pie de Palo. interpreted as a secondary fault (Costa et al. 2000). Local seismicity This calculation is dominated by the 1977 earthquake sequence.

C 2005 RAS, GJI, 163, 580–598 Moment tensor inversion of Andean backarc crustal earthquakes 593

Figure 11. SMTI results in a vertical projection along a cross-section around 31◦S (see Fig. 9 for location) including: (a) GPS velocities (yellow circles) around this latitude from Brooks et al. (2003), and (b) Geological interpretation modiﬁed from Regnier et al. (1992), Ramos et al. (2002) and Siame et al. (2002). The western Sierras Pampeanas are more seismically active than the eastern basement blocks. Focal depths with errors of ±2.5 km are in good agreement with the proposed d´ecollementsunder Sierra de Pie de Palo based on geological studies. Note the different horizontal and vertical scales used in the geological cross section but not for the fault-plane focal-mechanism projections.

We calculated the earthquake moment tensor sum with the same quakes in the area during different periods of time (1.5 and 27 yr), technique using the eight closest crustal events to the Sierra Pie de are ∼35◦ and ∼11◦ clockwise rotated with respect to GPS orienta- Palo in the 1.5 yr CHARGE period. These events are located in tion observations. This might indicate that other tectonic processes an area of approximately 90 × 38 km2 with a seismogenic depth are occurring besides crustal earthquakes that affect geodetic mea- range of 19 km (Figs 9 and 10). In this particular volume, the main surements. However, it could also indicate that even though the main contraction axis is oriented along an azimuth 297◦ and has a plunge shortening orientation is that observed by GPS data, the pre-existing of 1◦,whereas the minimum contraction axis has an azimuth of fractures and faults associated to the Sierra Pie de Palo (Fig. 10) are 46◦ and a plunge of 86◦. The rate of crustal deformation along the controlling the moderate-to-large earthquake deformation. maximum shortening strain orientation over the width of 38 km and the 1.5 yr period is about 0.45 mm yr−1. As expected, when comparing Siame et al.’s (2005) estimation 5.2 Eastern Sierras Pampeanas with our estimate of earthquake crustal shortening, we find that their Focal mechanisms for this region are either strike-slip or reverse rate of deformation is nearly two orders of magnitude greater. This fault-plane solutions (Figs 9 and 11). The strike-slip events have is not surprising given the Ms = 7.4 1977 earthquake (Fig. 2). We very shallow hypocentres. Since our determinations provide the first believe that our estimate is less likely related to the 1977 earthquake seismic source characterizations in this region, we consider it im- sequence since event 02-117 on 2002 April 27 (Table 2 and Figs 3, 9 portant to discuss their tectonic implications in order to exploit their and 10) in our calculation is the largest magnitude >5.0 earthquake seismic information. that occurred in this area over a period of 14 yr. However, neither Our results for the largest size (MD = 4.8) earthquake in this re- estimate is likely to represent the long-term rates. gion, event 01-138b (Table 2) that occurred close to Soto, Córdoba, Interestingly, the GPS velocity vectors observed in the vicinity of indicate a shallow depth (7 km) and a strike-slip solution (Figs 6 the Sierra Pie de Palo and the Sierra Valle Fértil over a time span of and 9). No movement along strike-slip Quaternary faults has been ◦ ∼3–5 yr show a decay towards the east along an azimuth of ∼82 recognized in the region (Costa et al. 2001). However, interpreta- (Brooks et al. 2003) (Fig. 11), which agrees with the plate conver- tions of the orogenic evolution mainly during the Pampean cycle of ◦ gence direction (N79.5 E-Kendrick et al. 2003). Our estimate for the Sierras de Córdobadescribe eastward dipping, NW–SE trend- the maximum shortening axis orientation and that determined by ing, sinistral wrench faults for the studied region (Baldo et al. 1996). Siame et al. (2005), both based on the moment-tensor sum of earth- This observation is coincident with the movement along the N16◦W

C 2005 RAS, GJI, 163, 580–598 594 P. Alvarado et al. nodal plane of the SMTI solution for event 01-138b. Tectonicstudies (MD = 4.2) at similar depths might be related to this décollement by Kraemer et al. (1995) and Martino et al. (1995) also find evidence system (Table 2 and Fig. 9). of north-south trending shear zones related to the suture between R´ıo One intriguing (MD = 4.8) earthquake that we modelled in this de la Plata and Pampia terranes (Fig. 2). Their studies also suggest region is event 02-005. The focal mechanism has a very low an- the reactivation of old faults since the Pliocene. Thus it is likely that gle southeast-dipping fault plane or a high-angle west-dipping fault the historic 1908 (M = 6.5) earthquake that occurred very close plane. Also, it has the deepest depth estimate (26 km) in our study to the event 01-138b epicentre and affected the same population (Table 2 and Fig. 5). Its epicentral location lies below the gentle centres (INPRES 2005) was generated by similar structures in the domes of the Cuyo basin (Figs 2 and 9). Based on seismic re- region. These findings are important in helping to identify struc- flection data, a tectonic suture in the Cuyania terrane between the tures that generate seismicity at very shallow depths that might af- Precordillera and the western Sierras Pampeanas basements has fect cities like Córdoba,which has one of the larger populations in been proposed further north (∼100 km) at similar middle-to-lower Argentina. crustal depths (Cominguez & Ramos 1991). Perhaps the very low We also constrained two reverse fault-plane focal mechanisms angle fault-plane solution of event 02-005 is related to the same for events 01-339 and 02-074 with magnitudes of ∼4.0 and depths proposed suture providing evidence of its present activity. This co- of 16–18 km, respectively (Fig. 9). They represent the easternmost incides with a major change in the geometry of the subducted Nazca Andean compression acting over a region where the flat slab finally plate near this latitude where the plate changes from a near horizontal resumes its inclination to the east (Cahill & Isacks 1992) (Fig. 1). dip to a steeper dip of 30◦ (Fig. 1). Magnetotelluric studies revealed a strong contrast in resistivity for The seismic moment tensor solutions for this region give average crustal structures that could be responsible for the uplift of the P- and T-axis orientations of azimuth 82◦ and 156◦, and plunge of Sierras de Córdobaalong east-dipping faults (Booker et al. 2004). 11◦ and 55◦, respectively. The same studies also predict their extension into deeper levels (40 km). 5.4 High Cordillera Event 01-349 (MD = 4.1) in the northern region is close to the Sierra Velasco (Figs 2 and 9) and has a dip-slip source mechanism. Shallow seismicity that occurs in the high Cordillera is absent north Both fault-plane trends are parallel to the main NE-trend of the east- of 33◦S for magnitude 4.0 and larger events (Fig. 1). An interesting ern front of this range. This deformation contributes to the uplift of concentration of crustal seismicity began in 2001 October around the basement of the eastern Sierras Pampeanas, sometimes gener- Tupungatito volcano (Fig. 2), generating 80 aftershocks in approx- ating larger reverse focal mechanisms such as the recent Catamarca imately one month (Barrientos et al. 2004). This location marks earthquake (2004 September 7, Harvard-CMT solution: depth = the beginning of the crustal seismicity to the south. Coincidentally, ◦ ◦ 20 km; M w = 6.1; fault plane 1: strike = 216 , dip = 37 , rake = the downgoing plate changes to a steeper subduction angle, an ac- 80◦;fault plane 2: strike = 48◦, dip = 54◦, rake = 97◦) that occurred tive volcanic arc starts to the south, and the Precordillera and the ∼90 km northeast of event 01-349. basement-cored faulting of the Sierras Pampeanas are absent in Our results in the eastern Sierras Pampeanas are consistent with the foreland (Figs 1 and 2) (Ramos et al. 2002). We modelled the GPS observations. According to Brooks et al. (2003) the crustal seis- main shock (Ms = 5.6) of this sequence (event 01-285) that oc- micity deformation observed far away from the trench contributes to curred on 2001 October 14 southwest of Cordóndel Plata in the recover the elastic loading component, which is the entire GPS ve- Frontal Cordillera (Fig. 2). This was the largest continental crust locity field observed in the eastern Sierras Pampeanas (green curve event recorded during the CHARGE experiment. Its focal mecha- in Fig. 11a). Their study assumes that the locking of the converg- nism corresponds to a near-vertical strike-slip fault-plane solution ing Nazca and South America plates as far as 600 km to the west with a very shallow focal depth of 5 km (Fig. 9). The CMT focal generates this elastic component. mechanism solution, constrained for a fixed depth of 15 km, is very similar (Table 2). Another larger (M w = 6.5) earthquake (event 04-241 in Fig. 9) with an epicentre located 190 km southwest of event 01-285 oc- 5.3 Mendoza curred on 2004 August 28, generating 85 aftershocks in 2 days. The Mendoza has been the site of several destructive earthquakes Harvard-CMT focal mechanism solution also indicates a strike-slip 18 (Fig. 2). The 1985 (M w = 5.9) earthquake took place ∼15 km south solution (M 0 = 7.18 × 10 Nm;M w = 6.5; depth = 16 km; of the city of Mendoza in the Barrancas anticline (Fig. 2) above blind fault plane 1: strike = 21◦, dip = 61◦, rake =−178◦;fault plane 2: thrust faults (INPRES 1985; Triep 1987; Chiaramonte et al. 2000). strike = 290◦, dip = 88◦, rake =−29◦). If this earthquake and event The region is characterized by oil fields hosted by the Triassic Cuyo 01-285 are being generated by the same major strike-slip Andean basin hydrocarbon reserves (Moratello 1993; López-Gamundi& structure, we infer that the most likely ruptured plane is very approx- Astini 2004). Tectonic analysis based on seismic imaging suggests imate to the NE-striking nodal plane of both solutions. A detailed that the faults involve the basement and the productive traps could be study including the aftershocks detected for this earthquake follow- the result of the reactivation of older structures (Dellapé&Hegedus ing a methodology similar to that used by Fromm et al. (2005) may 1993; Ramos et al. 1996). lead to a determination of the activated fault plane. At this latitude, the Precordillera loses its surface expression, Although not in an organized pattern, recent GPS data show a and the Cuyo rift basin as well as a series of low-elevationed anti- slight clockwise rotation of the backarc vectors with respect to the clines continue to the south (Fig. 2) with less east–west contraction forearc vectors relative to the stable cratonic South America (Brooks (Ramos et al. 1996; Brooks et al. 2000). An estimate of a shal- et al. 2003). Studies in other segments (18◦S–28◦S and 39◦S–43◦S) low décollementat about 15 km has been identified to the west of of the Andean cordillera have documented strike-slip faults more the Cuyo basin (Fig. 2) using balanced cross-sections and seismic than 1000 km long (Armijo & Thiele 1990; Hervé1994). The strike- reflection data (Kozlowski et al. 1993; Ramos et al. 1996). The west- slip faults in the High Cordillera are also related to the location of dipping thrust fault-plane solutions for events 01-127 and 01-189 an active volcanic arc and a more inclined position of the subducted

C 2005 RAS, GJI, 163, 580–598 Moment tensor inversion of Andean backarc crustal earthquakes 595

Nazca plate segments. Interestingly, those faults do not show high Mendoza. They are probably associated with brittle behaviour on seismic activity (Siame et al. 1997) as it is observed in the 33◦S– deeper décollementsthat are related to reactivation of old faults 36◦S Andean segment. and terrane sutures. Although strike-slip focal mechanisms are not Reverse and thrust faults are the main mechanisms that ac- dominant features in the region, we found two earthquakes with very count for shortening in the Principal and Frontal Cordillera over shallow strike-slip solutions, one in the eastern Sierras Pampeanas the steep subduction zone (Kozlowski et al. 1993; Ramos et al. and the other in the high Cordillera. The event in the Cordillera 1996; Cristallini & Ramos 2000). However, the slightly oblique as well as another crustal earthquake that occurred in 2004 in its convergence direction between the Nazca and South America plates vicinity could be accommodating strike-slip deformation produced could also provide a potential source for right-lateral slip parallel to by the slightly oblique plate convergence. the Andes (Siame et al. 1997). Previous workers have recognized Our tests of different average seismic velocity models for the crust Palaeozoic strike-slip movements along a main fault that limits the indicate that a higher P-wave seismic velocity (6.2–6.4 km s−1), a Principal and Frontal Cordilleras (Fuentes et al. 1993). The same lower S-wave seismic velocity (Vp/Vs = 1.80–1.85) and a thicker structure is believed to have been reactivated as a thrust fault that crust (45–52 km) represent the western Sierras Pampeanas, while a uplifted the Frontal Cordillera as a block (Kozlowski et al. 1993). Vp of 6.0–6.2 km s−1,aVp/Vs ratio of 1.65–1.70, and a thickness The possible occurrence of event 01-285 along this major structure of 27–37 km characterize the eastern Sierras Pampeanas. Exposed would be an indication of a modern reactivation of this structure basement rocks in the eastern and western regions also exhibit dif- as a right-lateral strike-slip fault. In addition, other strike-slip de- ferent compositions. The western Sierras Pampeanas have a more formation in the High Cordillera has been recognized constraining mafic composition with very little granitic rocks compared to the focal mechanisms in the Chilean side (Alvarado 1998). The 1958 eastern Sierras Pampeanas. The high seismic activity and different (Ms = 6.9) Las Melosas, Chile, earthquake is another example of a seismic properties in the western Sierras Pampeanas terrane may be strike-slip seismic source in the Cordillera (Piderit 1961; Barrientos due to its different basement composition, to a more fractured crust et al. 2004) (Fig. 2). in the western part, or to a combination of these factors over the flat We also modelled another moderate-sized earthquake, event subduction region. 01-074 (M w = 4.8), that occurred at the orogenic front of the Prin- cipal Cordillera (Figs 2 and 9). Its location lies 25 km east of the Sosneado volcano in the Malargüefold and thrust belt (Fig. 2). This ACKNOWLEDGMENTS thick-skinned deformation was developed by tectonic inversion of a pre-Andean rift system during late Cenozoic times (Ramos et al. We are especially grateful to Robert Fromm, who passed away in 1996). Our focal mechanism solution indicates a shallower west- July 2004, for his generosity and helpful assistance with scripts dipping or a more inclined east-dipping fault plane and a very shal- and constructive suggestions. We thank the GSAT group at the low focal depth of 3 km. The main dip-slip movement found is University of Arizona for fruitful discussions and Rick Bennett consistent with the active structures recognized in the area. for his help in interpreting GPS data. Mauro Saez helped us Previous seismic studies cited above and our analysis show that with the preparation of the figures of this paper. National Science moderate earthquakes occur at very shallow depths in this region. Foundation grant EAR-9811878 supported the project CHARGE. Therefore, seismic studies can have important implications in the The instruments used in the field program were provided by the evaluation of seismic hazards in Mendoza, as other authors have PASSCAL facility of the Incorporated Research Institutions for already indicated (Bastias et al. 1993; Zamarbide & Castano 1993; Seismology (IRIS) through the PASSCAL Instrument Center at Moreiras 2004). the New Mexico Institute of Mining and Technology. Process- ing of the seismic data was performed using Seismic Analysis Code SAC2000 software. Maps were generated using Graphic Mapping Tools software. We also used seismological data re- 6CONCLUSIONS trieved from the International Seismological Centre, Harvard-CMT, Seismic moment tensor inversions using regional broad-band and INPRES on-line bulletins. We acknowledge our gratitude CHARGE waveforms were determined for 27 moderate-sized to the Universidad Nacional de San Juan and the Instituto Na- (3.5 < M w < 5.3) earthquakes located in the Andean backarc crust cional de PrevenciónS´ısmicaof Argentina, the Departamento de over the flat-slab segment (30◦S–33◦S) and in the high Cordillera Geof´ısica at the Universidad de Chile (Chile), and all personnel south of 33◦S. The SMTI solutions show mainly reverse or thrust for their support to the CHARGE project (www.geo.arizona.edu/ events with average P- and T-axis orientations of azimuth 275◦ and CHARGE). 90◦, and plunge 6◦ and 84◦, respectively. This is also in agreement We are very grateful for the valuable help during the field work with previous results around San Juan from the PANDA experiment in Argentina from Ministerio de Obras y Servicios Públicos, San (Regnier et al. 1992). Juan Government; Escuadrón25 ‘Jáchal’(San Juan) and Escuadrón The basement of the Cuyania terrane had the highest seismic 29 ‘Malargüe’(Mendoza) from Gendarmer´ıa Nacional; Embalse activity during the 2000–2002 CHARGE deployment. The Valle Las Pichanas and familia Farroni, DIPAS-ReparticiónCruz del Fértil fault, which is also a terrane boundary, separates the region Eje (Córdoba);Parque Nacional Ischigualasto-Valle de la Luna in terms of level of seismic activity. Very limited crustal seismicity (Ischigualasto, San Juan); Dique Nivelador Pachimoco (San Juan); of M > 4.0 occurs in the Pampia terrane over the flat subduction Municipalidad and Secretar´ıade Turismo de Malargüe(Mendoza); region. However, this terrane transfers deformation into the adjacent Puesto ‘Los Rincones’ (familia Ontiveros, Ischigualasto, San Juan), cratonic areas. We observed reverse-fault crustal earthquakes in the ‘Puesto Las Peñas’(familia Pérez,Malargüe,Mendoza), ‘Puesto R´ıo de la Plata terrane as far as 800 km east of the oceanic trench. Vega-Miranda’ (familia Moyano, Malargüe, Mendoza), ‘Puesto La This is also observed in the historical seismicity of the region. Niebla’ (familia Zambrano, Malargüe,Mendoza). Most of the reverse or thrust earthquakes occurred at mid-crustal We thank the editor John Beavan, and two anonymous reviewers focal depths (>21 km) in the western Sierras Pampeanas and near for their comments to improve this paper.

C 2005 RAS, GJI, 163, 580–598 596 P. Alvarado et al.

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