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Seismotectonic and stress distribution in the central Chile subduction zone
Journal of South American Earth Sciences 15 (2002) 11±22
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Seismotectonic and stress distribution in the central Chile subduction zone M. Pardo a,*, D. Comte a, T. Monfret b a
Departamento de GeofõÂsica, Universidad de Chile, Blanco Encalada 2085, Casilla 2777, Santiago, Chile b UMR GeÂosciences Azur, IRD, 250 rue Albert Einstein, 06560 Valbonne, France Received 1 November 2001; accepted 1 November 2001
Abstract We determined the stress tensor from focal mechanisms and the shape of the downgoing Nazca Plate subducted beneath central Chile (26± 358S), based on accurately located hypocenters of local and teleseismic events, at three different segments de®ned by the rupture zones of great thrust earthquakes. Between latitudes 26±338S the slab ¯attens at intermediate depths and moves upwards probably related to the subduction of buoyant lithosphere associated to the Juan Fernandez Ridge (JFR). To the south the subduction is steeper and the slab penetrates into the mantle with a constant dip of 278 implying a sharp dip change around 338S. The slab seismicity reaches a maximum depth of 200 km south of 298S while to the north deep seismicity is observed at depths of 570±630 km, with a gap between 200 km and these depths. A remarkable along-strike variation in the seismic slab length is observed from 1100 km north of 298S, to 800 km between 30 and 328S and 350 km south of 338S. The seismicity suggests a thick oceanic crust related to the JFR subduction, furthermore this intermediate depth seismicity is more intense and more seismic moment is released than in the adjacent zones. The stress tensors show differences between the steep subduction and the ¯at slab zones, being more important at intermediate depths where the slab ¯attens. At shallow depths the stress is compressive due to plate convergence. At intermediate depths the slab is under tensional stress regime, related to the slab pull in the steep subduction zone and also to buoyant forces in the ¯at slab zone. The maximum depth of interplate coupling between Nazca and South American Plates is 60 km, similar to other places along the Chilean subduction zone. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Flat slab; Subduction zone; Nazca Plate; Central Chile
1. Introduction The seismicity and tectonic of central Chile is mainly characterized by the subduction of the oceanic Nazca Plate beneath the continental South American lithosphere, which takes place with a convergence rate of about 8 cm/yr in the N788E direction (DeMets et al., 1994). Several studies have shown that the shape of the oceanic downgoing slab in the region exhibits along-strike variations in the dip angle (Barazangi and Isacks, 1976; Cahill and Isacks, 1992; Jordan et al., 1983). These changes in dip, along with the maximum observed rupture length of great earthquakes, have suggested segmentation in the subduction zone (Swift and Carr, 1974; Barazangi and Isacks, 1976). Beneath central Chile and western Argentina (288S ±338S), the subducted Nazca Plate has a low dip angle (,108) being almost subhorizontal, and it extends eastward for hundreds of kilometers at a depth of ,100 km before reassuming its downward descent (e.g. Cahill and Isacks, 1992). To the south of 338S, the subduction of the oceanic slab has a dip * Corresponding author. Tel.: 156-2-696-6563; fax: 156-2-696-8686. E-mail address: [email protected] (M. Pardo).
angle of 258±308 (Fuenzalida et al., 1992), implying that a sharp change in dip occurs around 338S. Based on teleseismic hypocentral locations, Swift and Carr (1974) and Barazangi and Isacks (1976) interpreted a tear in the oceanic slab as the boundary between these segments. Later, based on teleseismic and local data it has been proposed a continuous ¯exure rather than a tear in the slab around 338S (Fuenzalida et al., 1992; Cahill and Isacks, 1992; Araujo and SuaÂrez, 1994). The data used to determine these models are mainly hypocenters reported by international agencies from stations at teleseismic distances. These hypocenters are usually mislocated due to a poor azimuthal coverage of the worldwide reporting stations. In some cases, magnitude and number of phases recorded have been used to ®lter the events in order to identify a subset of better-located earthquakes. As an example, Cahill and Isacks (1992) considered as reliable hypocenters those from the ISC catalog with at least 15 observations. The maximum depth of seismic coupling of the interplate contact, along segments of the Chilean subduction zone, have been analyzed by Tichelaar and Ruff (1991), Pacheco et al. (1993) and SuaÂrez and Comte (1993). They used
0895-9811/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 0895-981 1(02)00003-2
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M. Pardo et al. / Journal of South American Earth Sciences 15 (2002) 11±22
Fig. 1. Epicenters of the accurately located events in the region, from JHD procedure (dark circles) and from local data (open circles). The rupture lengths of the great earthquakes used to segment the zone are shown with vertical lines. The trace of three vertical cross-sections, width ^100 km, is shown on each segment. Bathymetry is contoured each 700 m. Isodepth contours, after Cahill and Isacks (1992), and the Chile±Argentina boundary line that coincides with the Andes peaks are shown as reference.
selected data from large interplate earthquakes and available local data, and proposed values of the maximum depth of coupling between 50 and 60 km. Considering the rupture zones of the great subduction earthquakes in 1922 (Atacama, Ms 8:3), 1943 (Illapel, Ms 7:9), and 1906 (Valparaiso, Ms 8:4) (Comte et al., 1986; Beck et al., 1998), we divided the central Chile zone into three segments (Fig. 1). They are, respectively, Segment-1, from 268S to 308S, corresponding to the southward beginning of the ¯at slab subduction; Segment-2, from 308S to 328S, where the slab is ¯at and subhorizontal at intermediate depths; and Segment-3, from 328S to 358S, where the dip of the slab is 258±308. In each segment we improved the local and teleseismic hypocenter locations, determined new focal mechanisms and considered the focal mechanisms reported by Harvard centroid moment tensor (HCMT). The purpose of this article is to analyze and compare the seismotectonic characteristics of these three segments in central Chile relative to the shape of the
subducted Nazca Plate, the associated stress ®eld and the geometry of the interplate coupled zone. 2. Tectonic setting The zone of study includes central Chile and western Argentina, where the Nazca Plate is subducting beneath the South American Plate at a rate of about 8 cm/yr. The subduction zone along the Chile trench is seismically very active, where magnitude over eight interplate underthrusting earthquakes have occurred along its entire length (Comte et al., 1986; Beck et al., 1998). The Andes Cordillera, formed mainly by plate compression, is narrow with an average elevation of 4000 m and peaks of over 6000 m. There is active volcanism to the south of 338S, while in the ¯at slab zone the volcanism terminated approximately 9±10 Ma ago (Kay et al., 1988). In the back-arc, the overriding continental plate is
M. Pardo et al. / Journal of South American Earth Sciences 15 (2002) 11±22
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Table 1 Source parameters of the earthquakes modeled using the waveform inversion and hypocenter relocated with JHD #
Date
O.T. (hh/mm)
Lat (8S)
Long (8W)
Depth (km)
mb
Mo 10 17 N m
Strike (8)
Dip (8)
Rake (8)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
80/07/13 80/07/19 81/03/23 81/04/01 81/05/09 81/11/07 82/10/26 82/11/18 83/01/10 83/10/21 83/12/15 84/02/03 85/01/18 85/03/23 85/03/25 85/04/09 85/06/10 87/05/19
06:20 11:52 19:28 18:03 09:50 03:29 03:24 20:39 12:32 08:14 04:22 08:29 15:00 14:36 05:14 01:56 15:37 12:56
33.45 29.13 33.59 27.41 26.55 32.23 29.76 31.21 27.40 30.69 33.11 29.56 29.48 33.22 34.23 34.04 28.0 30.37
70.20 69.72 71.94 63.36 64.82 71.39 71.31 65.79 63.38 69.20 70.21 71.07 70.82 72.23 72.19 71.47 66.84 71.63
101 101 27 553 15 56 66 171 556 101 99 54 80 23 27 51 154 44
5.6 6.1 5.8 5.9 5.5 6.1 5.6 5.5 5.6 5.5 6.0 5.7 5.8 5.6 6.1 6.3 5.8 5.6
5.3 18.9 13.8 56.8 0.7 260.2 4.5 16.1 4.5 2.5 18.5 3.4 44.6 2.3 37.4 558.1 46.4 26.0
199 331 371 45 296 294 174 337 344 183 252 14 326 340 313 342 311 338
80 33 19 7 50 15 89 63 43 66 25 46 68 39 20 22 59 30
117 248 113 240 146 40 78 286 275 258 246 99 16 107 51 89 2104 94
seismically active in the Sierras Pampeanas and Precordillera of western Argentina, and these have been the site of several large damaging earthquakes with maximum magnitude about 7.5, mainly associated to compression and crustal shortening. The total crustal shortening at the latitude of 308S has been estimated on approximately 150±170 km (Allmendinger et al., 1990). The principal bathymetric feature in the oceanic plate is the Juan Fernandez Ridge (JFR), a hot spot seamount chain that is subducted at the trench around 338S. The subduction of the JFR is well correlated with the sharp change in the dip of the downgoing slab around latitude 338S. It may be one of the principal factors to explain the ¯at slab subduction from about 288S ±338S (Cross and Pilger, 1982; Cahill and Isacks, 1992; Von Huene et al., 1997; Gutscher et al., 2000; YanÄez et al., 2001). It also may be considered the existence of the Challenger Fracture Zone in this region, and to the south, of the Chile ridge that extends from the junction of the Paci®c, Antarctic, and Nazca Plates, to the triple junction of the Antarctic, Nazca, and South American Plates (Tebbens et al., 1997). Several studies have related the geometry of the subducted oceanic slab with the upper-plate tectonics in central Chile±western Argentina (Jordan et al., 1983; Allmendinger et al., 1990; Cahill and Isacks, 1992; Fuenzalida et al., 1992). They correlated the ¯at subduction of this segment of Nazca Plate with different features in the overriding South America Plate: absence of Quaternary volcanism, crustal shortening in the back-arc, and a steady topographic rise from the coast to the Andes peaks. 3. Data The data used in this study are accurately determined
hypocenters obtained from local data recorded by permanent and temporary seismological stations in the region, and relocated hypocenters of events recorded at teleseismic distances by the worldwide seismological network. Also we used focal mechanisms from the HCMT catalog, and new focal mechanisms determined in this study from a long-period body wave inversion of earthquakes with magnitude mb $ 5.5 that occurred in the zone and were recorded by the global digital seismological network. 3.1. JHD hypocenters The hypocenter of earthquakes with magnitude mb $ 4.8, between 1964 and 1998, and recorded at teleseismic distances were relocated using the joint hypocenter determination method (JHD) (Dewey, 1971). The reason for relocating hypocenters from teleseismic data is the well-known observed mislocation between the hypocenters reported by the international agencies and the ones determined with local data. The JHD method determines relocated hypocenters relative to the hypocenters of the selected calibration events in the zone, applying time adjustments to the phase readings of P, pP, and S waves from the reporting seismological stations. These station-phases time adjustments are obtained from the variance of the different phase arrival times of the calibration events. Then, knowing the station-phases time adjustments, the hypocenter of all the events within the zone can be relocated using the single event location method (Dewey, 1971). The precision of each relocated hypocenter is estimated by computing error ellipsoids for the hypocentral coordinates at a 90% con®dence level. The data to perform the relocation procedure are the phase readings reported by the International Seismological
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M. Pardo et al. / Journal of South American Earth Sciences 15 (2002) 11±22
relative to the hypocenters obtained with the JHD procedure are on average 19 km with a standard deviation of 22.5 km. 3.2. Hypocenters determined from local data
Fig. 2. Vertical cross-sections of seismicity with depth, shown in Fig. 1. Origin is at the trench and oriented normal to it. The dark circles correspond to JHD locations and open circles to local data solutions. At the top of each pro®le, the projection of the coast is presented with a black inverted triangle, and the position of the Andes arc with a black (active volcanism) and gray (absence of Quaternary volcanism) triangle.
Center (ISC) for events with magnitude mb $ 4.8 and recorded at 20 or more stations. In each of the three segments that we divided the central Chile subduction zone, we selected a set of 25 calibration events recorded also by local stations and with focal depth constrained using a long-period body wave inversion. For the ®rst segment (268S ±308S), we considered events from the 1973 swarm that occurred offshore, the largest events recorded by a temporary network of 38 stations deployed at the zone in 1998 between September 20 and November 30 (Comte et al., 2001), and eight modeled earthquakes in the zone (Table 1). For the second segment (308S±328S), aftershocks of the 1997 Punitaqui earthquake Mw 7:1 and the largest events in the region (Pardo et al., 2001) including three modeled events were considered (Table 1). For the third segment (338S±368S), the largest aftershocks of the 1985 San Antonio earthquake Mw 8:0 (Comte et al., 1986), and seven modeled earthquakes (Table 1) were used as calibration events. The solution was considered reliable when the error ellipsoid semi-axes were less than 15 km. All the events, which did not meet these criteria, were discarded so the ®nal relocated catalog is not complete. A ®nal subset of 1026 reliable hypocenters (^15 km) was obtained from an initial set of about 1250 events. The mislocation of the ISC hypocenters
Local data recorded by permanent and temporary seismological stations at the zone were considered to determine accurate hypocenters in central Chile. For the ®rst segment we used the hypocentral catalog from Comte et al. (2002) obtained from data of a temporary network of 38 stations deployed for 81 days in 1998. A joint hypocentral and body waves tomography was performed, so the locations were determined using a 3D velocity model. Applying convergence criteria, a ®nal set of 874 accurate hypocenters with an estimated mean error of ^5 km was obtained. In the second segment, we used data from a temporary network of 38 stations from IRIS-PASSCAL and Lithoscope-France, installed from November 15, 1999 to March 3, 2000 (Pardo et al., 2000). The database is still being processed, so here we use only locations of events during the last 45 days of 1999. The hypocenters were determined using a minimum 1D layered velocity model (Kissling et al., 1994) and several hypocenter location computer programs. We considered only the robust solutions obtained with the different techniques, which also satis®ed: a minimum of 15 phase readings, a root mean square error rms , 0.25, and estimated hypocentral errors of less than 10 km. A ®nal set of 1733 accurate located events (^10 km) was considered. In the third and southernmost segment, we used the hypocentral database for events between 1985 and 1995 (Pardo et al., 1997), obtained from a joint hypocentral and 3D body waves velocity model inversion (5785 events). The phase arrival times data to perform the inversion were obtained from: (1) the catalog of the Seismological Service, University of Chile (UCH); (2) a temporary network of eight stations deployed for one month after the 1985 earthquake (Mw 8:0; Comte et al., 1986); (3) a temporary network of 14 stations installed for two months between 32.58S and 348S (Fuenzalida et al., 1992); and (4) a network of 12 stations installed for 2 months between 348S and 35.58S during 1995 (Pardo et al., 1997). We also considered the catalog from UCH, 1996±1999, ®ltered by number of recording stations greater than 8, rms less than 0.25, and hypocentral errors less than 10 km, obtaining a set of 803 events that met this criteria. 3.3. Long-period body wave inversion The formal inversion of P, SV, and SH waveforms constrains the focal mechanism, the centroid focal depth, the seismic moment, and the source time function of the modeled earthquake. We used the method given by NaÂbelek (1984) to model 18 events that occurred between 1980 and 1987, with long-period seismograms recorded at teleseismic distances (258 # D # 908) from the global digital seismological network. The seismograms were ®ltered with a high-pass ®lter with
M. Pardo et al. / Journal of South American Earth Sciences 15 (2002) 11±22
15
cutoff frequency of 0.017 Hz (60 s) to eliminate long-period noise, and they were equalized to a peak magni®cation of 1500 at a distance of 408. All the modeled events are in the magnitude range 5.5 # mb # 6.2. Their source parameters are presented on Table 1, and the focal mechanism, waveform adjustment, and source time function are shown in Appendix A. 4. Seismicity and shape of the subducted slab
Fig. 3. Sketch of the shape of the subducted slab in the three considered segments along central Chile (S), presented on vertical cross-section normal to the trench. At the top, the distances from the trench to the coast (C), and to the Andes arc (A), are indicated. The distance C is the same for Segment-1 and Segment-2.
The epicentral distribution of the ®nal accurately located seismicity from local and JHD events is presented in Fig. 1. In order to analyze the shape of the subducted slab in the three segments, vertical cross-sections of the seismicity with depth were drawn (Fig. 2). The origin of all the crosssections is the trench, with a width of ^100 km and oriented in the direction perpendicular to the trench. A sketch of the Wadati±Benioff zone in each segment is shown in Fig. 3, assuming that it coincides with the top of the seismicity observed in each cross-section of Fig. 2.
Fig. 4. Same as Fig. 1 including focal mechanisms presented on lower hemispheric projection, from body wave inversion (gray) and HCMT (black). Dark quadrants indicate compressive ®rst motions.
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M. Pardo et al. / Journal of South American Earth Sciences 15 (2002) 11±22
Table 2 Stress tensor parameters. For the stress axes, at the top is the azimuth, clockwise from north, and at the bottom is the plunge. Principal axes of the stress tensor (s 1, s 2, s 3); tensor shape factor (R); score and number of used focal mechanisms (#FM) Central Chile Segment-1 Shallow depth (0±70 km) Intermediate depth (100±200 km) Deep Depth (570±630 km)
Segment-2 Shallow depth (0±70 km) Intermediate depth (100±200 km) Back-arc (0±45 km) Segment-3 Shallow depth (0±70 km) Intermediate depth (100±200 km)
s 1 (8)
s 2 (8)
s 3 (8)
266.6 23.5 297.7 83.2
171.7 7.8 149.4 4.2
63.9 58.2 59.1 2.6
80.2 50.7
345.8 2.5
253.5 32.0
257.1 20.2 81.8 82.6 97.7 4.0
166.3 7.0 320.8 2.7 188.3 5.0
54.5 62.3 230.5 4.6 329.1 81.2
265.0 0.4 355.5 46.2
175.0 0.3 205.1 32.6
52.0 89.3 103.8 12.4
4.1. Shallow depths (0±100 km) The shallow part of the subduction zone indicates an interplate geometry that initially dips with about 108 and gradually increases to 258 at a depth of 45 km (Figs. 2 and 3). This geometry is observed throughout the subduction zone in central Chile, and no lateral changes are observed at these depths, even at places where major bathymetric features, as the JFR, are subducted. The shape at the initial part of the subduction seems to be related only to the interplate collision, and appears to be independent of the age and relative convergence rate of the oceanic plate. Major changes in the shape of the subducted slab are observed below depths of 45 km (Figs. 2 and 3). At the southern Segment-3 (338S±358S), the slab penetrates into the mantle with an almost constant dip of 278, below depths of 70 km. To the north of 338S, the slab sharply bends in Segment-2, reaching a maximum dip of 408 at a depth of about 80 km, and from there the dip drastically diminishes to a ¯at slab shape at a depth of 100 km. In Segment-1 the subducted slab starts to be ¯at, and the change in the slab dip is more moderate, reaching about 308 at a depth of 70 km, and from there the slab ¯attens and continues with an almost subhorizontal trajectory at depths of ,100 km. 4.2. Intermediate depths (100±200 km) North of 338S, the slab is ¯at at depths around 100 km. In the northernmost Segment-1, the oceanic plate is subhorizontal to the east at a distance of about 200 km (250±
R
Score
# FM
21.49
0.897
18
1.61
0.979
14
2.04
0.998
10
20.77
0.887
33
2.59
0.904
19
21.36
0.968
13
20.10
0.960
52
1.48
0.995
9
450 km from the trench). The slab dip increases to 508 at a depth of 170 km and distance of 550 km from the trench (Figs. 2 and 3). In Segment-2 the ¯at slab extends for about 300 km (200±500 km from the trench), and then penetrates more into the mantle with a dip of 258 down to a maximum seismic depth of 200 km (Figs. 2 and 3). The Wadati± Benioff zone inferred for Segment-3 shows that the oceanic plate sinks into the mantle with an almost constant dip of 278, and no seismicity below depths of 160 km is detected (Figs. 2 and 3). 4.3. Depths .200 km Below depths of 200 km, seismicity is only observed at Segment-1 in clusters at depths between 570 and 630 km. The events occur in a narrow seismic zone that can be observed from 16.58S to 298S at these depths (Cahill and Isacks, 1992). These deep earthquakes are spatially separated from the intermediate depth events by a gap in seismicity along the slab (Figs. 2 and 3). The continuity of the slab through this aseismic zone was ®rst suggested on the basis of high-frequency waves from nearby earthquakes and transmitted through the seismicity gap region (Isacks and Barazangi, 1973). Later, modeling of wide-angle re¯ections from the upper surface of the Nazca Plate provided strong evidence for continuity of the downgoing slab beneath central and eastern Peru (James and Snoke, 1990), and it was also con®rmed by tomographic images for the Andean subduction zone (Engdahl et al., 1995).
M. Pardo et al. / Journal of South American Earth Sciences 15 (2002) 11±22
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of Fig. 4. They are presented in Table 2, and in Fig. 5 on back-hemispheric lateral projection. 5.1. Stress at shallow depths (0±70 km)
Fig. 5. Same as Fig. 2, showing the focal mechanisms from body wave inversion (gray) and from HCMT (black) on lateral back-hemispheric projection onto the vertical cross-section. In the same projection are shown the calculated stress tensors for the zones indicated by a rectangle in each pro®le.
5. Stress tensor in the region The stress tensors along the considered segments were obtained from the reported and newly determined focal mechanisms of events in the zone (Fig. 4, Table 1, and Appendix A). We calculate the stress tensor that best ®ts the focal mechanisms in a region using the method of Rivera and Cisternas (1990). The orientation of the stress tensor, given by the Euler angles, and its shape factor R sz 2 sx= sy 2 sx referred to the geographical coordinates x; y; z NS; EW; vertical; are calculated based on the fault plane solutions from the focal mechanisms. A set of tensors is selected in a random way in the tensor space, and for each tensor a score is determined as the sum of the scalar products between theoretical and observed slip vectors for all the considered mechanisms. The score ranges from 0 to 1, depending if none or all of the slip vectors from the observed focal mechanisms are ®tted with the calculated stress tensor, so it indicates the quality of the stress tensor solution. Then the best-®tted tensor, with highest score, is considered as solution. The shape factor R indicates that the stress regime within the considered zone is related to compression R , 0; to strike-slip faulting 0 , R , 1; or to extension R . 1: We calculate the best-®tted stress tensor at the three segments in central Chile, along the vertical cross-sections
Along and around the interplate zone, at depths less than 70 km, the shape factor R , 0 of the obtained stress tensors in all central Chile indicates a compressive regime (Table 2). The orientation of s 1 in all the segments is roughly in the convergence direction between Nazca and South American Plates. At Segment-3 zone, s 1 is horizontal whereas in Segment-1 and Segment-2 its plunge is about 238. The s 3 axis is vertical in Segment-3, but with a plunge of near 608 and azimuth N558E±658E in the ¯at zone segments (Table 2, Fig. 5). The focal mechanisms in this zone are mainly thrust and reverse, but some normal faulting events within the oceanic plate are observed with an average focal depth greater than the interplate compressive events (Fig. 5). We also determined the stress tensor at the back-arc in Segment-2, where seismicity clusters with maximum depths of 50 km and reported magnitude up to Mw 7:4; in the Argentinean Sierras Pampeanas. The associated focal mechanisms are mainly reverse, with west-dipping fault planes. The stress tensor for this zone indicates compression with horizontal s 1 nearly in EW direction, s 2 axis oriented NS and s 3 vertical (Table 2, Fig. 5), which is in agreement with the observed deformation in a thick-skinned style and crustal shortening (Jordan et al., 1983; Allmendinger et al., 1990). 5.2. Stress at intermediate depths (100±200 km) At these depths, the slab is under tension R . 1 (Table 2). At Segment-3, south of 338S, the s 3 axis is oriented along the slab with a plunge of 128 and is clearly associated to slab pull forces. While at the ¯at slab zone, where the stress tensor is similar in Segment-1 and Segment-2, the s 3 axis is almost horizontal in N508E±608E direction (Table 1, Fig. 5). In this region s 1 is vertical, and at Segment-3 it is oriented NS with a plunge of 468. In general the focal mechanisms indicate normal faulting with average T-axis along the subduction, but there are some with strike-slip component or T-axis with component normal to the subduction, especially at Segment-2 (Figs. 4 and 5). 5.3. Stress at deep depths (570±630 km) At the zone of deep events in Segment-1, depths of 570± 630 km, the obtained stress tensor is related with extension, but with the s 1 axis parallel to the slab dip and a plunge of 518. The s 2 axis is almost horizontal in NS direction and s 3 axis is nearly perpendicular to the slab (Table 2, Fig. 5). The focal mechanisms in this region show mainly normal to vertical faulting, with P-axis along the subduction (Fig. 4).
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M. Pardo et al. / Journal of South American Earth Sciences 15 (2002) 11±22
5.4. Maximum depth of interplate coupling The depth extent of the seismogenic interplate contact can be estimated from the maximum depth of the shallow thrust earthquakes and the depth of transition between compressive to tensional events (SuaÂrez and Comte, 1993). From the focal mechanisms in the region (Figs. 4 and 5), the maximum depth of thrust and reverse mechanism events that can be associated with interplate faulting are 60, 62, and 51 km, respectively, in each segment. The minimum depth of downdip normal mechanism events, with T-axis along the subducted slab, is 66 km at Segment-1, 68 km at Segment-2, and 60 km at Segment-3. Hence the maximum depth of interplate coupling can be estimated in about 60 km in the entire central Chile zone (Fig. 5). This result is in agreement with previous results at selected places along the Chilean subduction zone (SuaÂrez and Comte, 1993; Tichelaar and Ruff, 1991; Pacheco et al., 1993). In Northern Chile (188S±268S) the maximum depth extent of the coupled zone is 60 ^ 10 km (Comte and SuaÂrez, 1993); therefore this depth seems to be almost constant along the whole subduction zone in Chile.
6. Discussion and conclusions The shape of the subducted Nazca Plate beneath central Chile, inferred from the top of the seismicity in the zone, shows that there are important along-strike variations in the dip angle, with a sharp transition around 338S from ¯at to a 278-dipping slab at intermediate depths. The steeper subduction at Segment-3 permits the existence of an asthenospheric wedge that can generate active volcanism in the Andes. The ¯at slab geometry observed north of 338S precludes the existence of this type of wedge, hence active volcanism is absent in this zone. The thickness of the seismic zone at the brittle top of the slab, at intermediate depths, is about 30 km in Segment-1 and Segment-3, while at Segment-2 it is about 50 km, with the uncertainties associated to the hypocenter locations. This implies that a thicker oceanic crust is present at the zone of the JFR subduction. This anomalous crust below the subducted seamount chain can be associated to thermal metamorphism related to the passage of the slab over the hot spot that origins the JFR and to isostatic compensation. Furthermore, this anomalous crust might increase the buoyancy of the slab, which contributes to the slab ¯attening. In Segment-2 the intermediate depth seismicity clusters along the subducted JFR. At these depths, besides the normal faulting events with T-axis along the slab, there are some earthquakes that exhibit strike-slip mechanisms or T-axis with NS component, which cannot be explained by only slab pull forces. These mechanisms might be related to reactivation of pre-existing faults in the thick oceanic crust, created near the trench and outer-rise by the interaction of the JFR subduction (Kirby et al., 1996).
Relative to the adjacent segments, the intermediate depth seismicity in Segment-2 is more intense and the seismic moment released is more important, as evidenced by the larger number of occurrence of JHD locations and HCMT solutions of earthquakes with magnitude mb . 5 (Figs. 4 and 5). Considering only the events with focal mechanisms in the cross-sections for each segment (Fig. 5) that occurred between 1977 and 1999, the accumulative seismic moment (10 18 N m) is 8.54 in Segment-2, compared to 7.11 in Segment-1 and 1.83 in Segment-3, for a number of events of 19, 18, and 10, respectively. At the back-arc and shallow depths in Segment-2, there are clusters of mainly reverse-faulting events in the Sierras Pampeanas region, with west-dipping fault plane and maximum depth of 50 km, related to crustal shortening and to thick-skinned deformation (Figs. 4 and 5). Another interesting observation in the ¯at slab zone between 278S and 338S is the westward shift on about 50 km of the highest peaks of the Andes boundary between Chile and Argentina (Fig. 1). The maximum shift in this narrow Andean zone is about 100 km between 318S and 328S, a zone that coincides with the subduction of the JFR beneath the continent. A remarkable along-strike variation in the length of the seismogenic part of the slab is also observed. The seismic slab length diminished to the south from about 1100 km at Segment-1, to 800 km at Segment-2, and 350 km at Segment-3. This observation suggests that Segment-2 ¯at slab zone could be a transition between two different plate segments of the actual subducted Nazca Plate. A northern segment corresponds to old oceanic plate generated at the East Paci®c Rise, and a southern segment of younger oceanic lithosphere generated at the Chile Rise, which is closer to the continent. However, it is important to point out that the existence of the seismicity gap below depths of 200 km at Segment-1, where the slab is thought to be continuous down to depth of 650 km, could imply that the intraslab seismicity does not image the deeper portion of the slab. The rheology and thermal conditions of the slab at depths below 200 km could inhibit the occurrence of earthquakes, at least down to 500 km depth, so the estimation of the length of the slab from its seismicity should only be a lower bound. The interplate zone is under compression due to plate interaction, with s 1 stress axis oriented along the convergence direction and a similar shape throughout central Chile, at least down to depths about 45 km. The fact that the s 1 stress axes exhibit a plunge of about 238 in Segment-1 and Segment2, while it is horizontal in Segment-1, implies a more ef®cient transmission of compressive stress to the overriding continental plate in the ¯at slab zone (Table 2 and Fig. 5). This contributes to explain the observations of intense continental deformation, crustal shortening, and shallow seismicity at the back-arc in the ¯at slab zone. There are not enough focal mechanisms to estimate the
M. Pardo et al. / Journal of South American Earth Sciences 15 (2002) 11±22
Fig. 6. Body waveform inversion of the events modeled in this study
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M. Pardo et al. / Journal of South American Earth Sciences 15 (2002) 11±22
Fig. 7. Body waveform inversion of the events modeled in this study.
M. Pardo et al. / Journal of South American Earth Sciences 15 (2002) 11±22
stress tensor at the downdip zone between 70 and 100 km. However, analysis of the sequence of the 1997 Punitaqui earthquake Mw 7:1 that occurred at 68 km depth within the oceanic plate in Segment-2 (Pardo et al., 2002) shows that this zone exhibits compression along the slab associated with ¯exural stress due to the slab bending. Below depths of 45 km and north of 338S, the slab bends and ¯attens to an almost subhorizontal trajectory at depths of 100 km, continuing for 200 km at Segment-1 and 300 km in Segment-2 before reassuming its descent into the mantle. While in Segment-3, south of 338S, the slab sinks in the mantle with an almost constant dip of 278 (Fig. 3). At intermediate depths the slab is under a tensional stress regime, with s 3 axis along the slab in Segment-3 related to slab pull, while it is almost horizontal in N508±608E direction at Segment-1 and Segment-2. This means that the main force acting over the slab in the subhorizontal subduction region may be the result of the slab pull and a NS extension related to the slab ¯attening due to buoyant forces. The maximum depth of interplate coupling is estimated to be about 60 km in the entire central Chile zone. This result agrees with those obtained in other places along the Chilean subduction zone, therefore this depth seems to be almost constant along the whole subduction zone in Chile. Acknowledgements We thank the Seismological Service of the University of Chile for the data from their local network, and to IRISPASSCAL and Lithoscope-France for providing the instruments used in the 1999 temporary network. Most of the plots were generated using the GMT software. We thank A. Eisenberg for fruitful discussions. D. James and an anonymous referee provided valuable comments and helpful reviews. This work was partially supported by grants FONDECYT 1981145 and 1990355, F. Andes C-13563 and IRD-France. Appendix A Body waveform inversion of the events modeled in this study. The numbers at the top are related to Table 1. The focal mechanism of P (Top), SV (Middle), and SH (Bottom) are presented on lower hemispheric projection. The observed and the synthetic waveform are shown with solid and dashed lines, respectively. On the left top of each event the source time function is presented. References Allmendinger, R.W., Figueroa, D., Snyder, D., Beer, J., Mpodozis, C., Isacks, B., 1990. Foreland shortening and crustal balancing in the Andes at 308S latitude. Tectonics 9, 789±809. Araujo, M., SuaÂrez, G., 1994. Geometry and state of stress of the subducted
21
Nazca Plate beneath central Chile and Argentina: evidence from teleseismic data. Geophys. J. Int. 116, 283±303. Barazangi, M., Isacks, B., 1976. Spatial distribution of earthquakes and subduction of the Nazca Plate beneath South America. Geology 4, 686±692. Beck, S., Barrientos, S., Kausel, E., Reyes, M., 1998. Source characteristics of historical earthquakes along the central Chile subduction zone. J. S. Am. Earth Sci. 11, 115±129. Cahill, T., Isacks, B., 1992. Seismicity and shape of the subducted Nazca Plate. J. Geophys. Res. 97, 17503±17529. Comte, D., SuaÂrez, G., 1993. Comment on `Seismic coupling along the Chilean subduction zone' by B.W. Tichelaar and L.R. Ruff. J. Geophys. Res. 98, 15825±15828. Comte, D., Eisenberg, A., Lorca, E., Pardo, M., Ponce, L., Saragoni, R., Singh, S.K., SuaÂrez, G., 1986. The 1985 central Chile earthquake: a repeat of previous great earthquakes in the region? Science 233, 449±453. Comte, D., Haessler, H., Dorbath, L., Pardo, M., Monfret, T., Lavenu, A., Pointoise, B., Hello, Y., 2002. Seismicity and stress distribution around Copiapo, Northern Chile region using off-shore and inland locally recorded data. Phys. Earth Planet. Int. in press. Cross, T., Pilger, R., 1982. Controls of subduction geometry, location of magmatic arcs, and tectonics of arc and back-arc regions. Bull. Geol. Soc. Am. 93, 545±562. DeMets, C., Gordon, R.G., Argus, D.F., Stein, S., 1994. Effect of recent revisions to the geomagnetic reversal time scale on estimate of current plate motions. Geophys. Res. Lett. 21, 2191±2194. Dewey, J.W., 1971. Seismicity studies with the method of joint hipocenter determination. PhD Thesis. University of California, Berkeley. Engdahl, E.R., Van der Hilst, R.D., Berrocal, J., 1995. Imaging of subducted lithosphere beneath South America. Geophys. Res. Lett. 22, 2317±2320. Fuenzalida, A., Pardo, M., Cisternas, A., Dorbath, L., Dorbath, C., Comte, D., Kausel, E., 1992. On the geometry of the Nazca Plate subducted under central Chile (32±34.58S) as inferred from microseismic data. Tectonophysics 205, 1±11. Gutscher, M., Spakman, W., Bijwaard, H., Enghdal, R., 2000. Geodynamics of ¯at subduction: seismicity and tomographic constraints from the Andean Margin. Tectonics 19, 814±833. Isacks, B., Barazangi, M., 1973. High-frequency shear waves guided by a continuous lithosphere descending beneath Western South America. Geophys. J. R. Astron. Soc. 33, 129±139. James, D., Snoke, J.A., 1990. Seismic evidence of the deep slab beneath Central and Eastern Peru. J. Geophys. Res. 95, 4989±5001. Jordan, T.E., Isacks, B., Allmendinger, R., Brewer, J., Ramos, V., Ando, C., 1983. Andean tectonics related to geometry of the subducted Nazca Plate. Geol. Soc. Am. Bull. 94, 341±361. Kay, S.M., Maksaev, V., Moscoso, R., Mpodozis, C., Nasi, C., Gordillo, C.E., 1988. Tertiary magmatism in Chile and Argentina between 288S and 338S: correlation of magmatic chemistry with changing Benioff zone. J. S. Am. Earth Sci. 1, 21±38. Kirby, S., Enghdal, R., Denlinger, R., 1996. Intermediate-depth intraslab earthquakes and arc-volcanism as physical expressions of crustal and uppermost mantle metamorphism in subducting slabs. In: Bebout, G., Scholl, D., Kirby, S., Platt, J. (Eds.). Subduction: Top to Bottom. Geophysical Monograph 96American Geophysical Union, pp. 195±214. Kissling, E., Ellsworth, W.L., Eberhart-Phillips, D., Kradolfer, U., 1994. Initial reference models in seismic tomography. J. Geophys. Res. 99, 19635±19646. NaÂbelek, J., 1984. Determination of earthquake source parameters from inversion of body waves. PhD Thesis. Mass. Inst. Technol., Cambridge. Pacheco, J.F., Sykes, L.R., Scholz, C.H., 1993. Nature of seismic coupling along simple plate boundaries of the subduction type. J. Geophys. Res. 98, 14133±14169. Pardo, M., Comte, D., Monfret, T., Vera, E., 1997. TomografõÂa en Chile Central: variaciones en el espacio y el tiempo de la velocidad de ondas P
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en la zona de ruptura del terremoto del 3 de Marzo de 1985. VIII Congreso GeoloÂgico Chileno 3, pp. 1795±1799. Pardo, M., Monfret, T., Vera, E., Eisenberg, A., Gaffet, S., Lorca, E., 2000. Red sismoloÂgica temporal en Chile Central (308S±328S): datos locales para estudio sismotectoÂnico de la regioÂn. IX Congreso GeoloÂgico Chileno 2, pp. 464±467. Pardo, M., Comte, D., Monfret, T., Boroschek, R., Astroza, M., 2002. The October 15, 1997 Punitaqui earthquake Mw 7:1 : a destructive event within the subducting Nazca Plate in central Chile. Tectonophysics, V.345, pp. 199±210. Rivera, L., Cisternas, A., 1990. Stress tensor and fault plane solutions for a population of earthquakes. Bull. Seismol. Soc. Am. 80, 600±614. SuaÂrez, G., Comte, D., 1993. Comment on `Seismic coupling along the Chilean subduction zone' by B.W. Tichelaar and L.R. Ruff. J. Geophys. Res. 98, 15825±15828.
Swift, S., Carr, M., 1974. The segmented nature of the Chilean seismic zone. Phys. Earth Planet. Int. 9, 183±191. Tebbens, S.F., Cande, S.C., Kovacs, L., Parra, J.C., LaBrecque, J.L., Vergara, H., 1997. The Chile ridge: a tectonic framework. J. Geophys. Res. 102, 12035±12059. Tichelaar, B.W., Ruff, L.R., 1991. Seismic coupling along the Chilean subduction zone. J. Geophys. Res. 96, 11997±12022. CONDOR Scientists, Von Huene, R., CorvalaÂn, J., Flueh, E.R., Hinz, K., Korstgard, J., Ranero, C.R., Weinrebe, W., 1997. Tectonic control of the subducting Juan Fernandez Ridge on the Andean margin near Valparaiso, Chile. Tectonics 16, 474±488. YanÄez, G., Ranero, C., Von Huene, R., DõÂaz, J., 2001. Magnetic anomaly interpretation across the southern central Andes (328±348S): the role of the Juan Fernandez Ridge in the late Tertiary evolution of the margin. J. Geophys. Res. 106, 6325±6435.
www.elsevier.com/locate/jsames
Seismotectonic and stress distribution in the central Chile subduction zone M. Pardo a,*, D. Comte a, T. Monfret b a
Departamento de GeofõÂsica, Universidad de Chile, Blanco Encalada 2085, Casilla 2777, Santiago, Chile b UMR GeÂosciences Azur, IRD, 250 rue Albert Einstein, 06560 Valbonne, France Received 1 November 2001; accepted 1 November 2001
Abstract We determined the stress tensor from focal mechanisms and the shape of the downgoing Nazca Plate subducted beneath central Chile (26± 358S), based on accurately located hypocenters of local and teleseismic events, at three different segments de®ned by the rupture zones of great thrust earthquakes. Between latitudes 26±338S the slab ¯attens at intermediate depths and moves upwards probably related to the subduction of buoyant lithosphere associated to the Juan Fernandez Ridge (JFR). To the south the subduction is steeper and the slab penetrates into the mantle with a constant dip of 278 implying a sharp dip change around 338S. The slab seismicity reaches a maximum depth of 200 km south of 298S while to the north deep seismicity is observed at depths of 570±630 km, with a gap between 200 km and these depths. A remarkable along-strike variation in the seismic slab length is observed from 1100 km north of 298S, to 800 km between 30 and 328S and 350 km south of 338S. The seismicity suggests a thick oceanic crust related to the JFR subduction, furthermore this intermediate depth seismicity is more intense and more seismic moment is released than in the adjacent zones. The stress tensors show differences between the steep subduction and the ¯at slab zones, being more important at intermediate depths where the slab ¯attens. At shallow depths the stress is compressive due to plate convergence. At intermediate depths the slab is under tensional stress regime, related to the slab pull in the steep subduction zone and also to buoyant forces in the ¯at slab zone. The maximum depth of interplate coupling between Nazca and South American Plates is 60 km, similar to other places along the Chilean subduction zone. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Flat slab; Subduction zone; Nazca Plate; Central Chile
1. Introduction The seismicity and tectonic of central Chile is mainly characterized by the subduction of the oceanic Nazca Plate beneath the continental South American lithosphere, which takes place with a convergence rate of about 8 cm/yr in the N788E direction (DeMets et al., 1994). Several studies have shown that the shape of the oceanic downgoing slab in the region exhibits along-strike variations in the dip angle (Barazangi and Isacks, 1976; Cahill and Isacks, 1992; Jordan et al., 1983). These changes in dip, along with the maximum observed rupture length of great earthquakes, have suggested segmentation in the subduction zone (Swift and Carr, 1974; Barazangi and Isacks, 1976). Beneath central Chile and western Argentina (288S ±338S), the subducted Nazca Plate has a low dip angle (,108) being almost subhorizontal, and it extends eastward for hundreds of kilometers at a depth of ,100 km before reassuming its downward descent (e.g. Cahill and Isacks, 1992). To the south of 338S, the subduction of the oceanic slab has a dip * Corresponding author. Tel.: 156-2-696-6563; fax: 156-2-696-8686. E-mail address: [email protected] (M. Pardo).
angle of 258±308 (Fuenzalida et al., 1992), implying that a sharp change in dip occurs around 338S. Based on teleseismic hypocentral locations, Swift and Carr (1974) and Barazangi and Isacks (1976) interpreted a tear in the oceanic slab as the boundary between these segments. Later, based on teleseismic and local data it has been proposed a continuous ¯exure rather than a tear in the slab around 338S (Fuenzalida et al., 1992; Cahill and Isacks, 1992; Araujo and SuaÂrez, 1994). The data used to determine these models are mainly hypocenters reported by international agencies from stations at teleseismic distances. These hypocenters are usually mislocated due to a poor azimuthal coverage of the worldwide reporting stations. In some cases, magnitude and number of phases recorded have been used to ®lter the events in order to identify a subset of better-located earthquakes. As an example, Cahill and Isacks (1992) considered as reliable hypocenters those from the ISC catalog with at least 15 observations. The maximum depth of seismic coupling of the interplate contact, along segments of the Chilean subduction zone, have been analyzed by Tichelaar and Ruff (1991), Pacheco et al. (1993) and SuaÂrez and Comte (1993). They used
0895-9811/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 0895-981 1(02)00003-2
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M. Pardo et al. / Journal of South American Earth Sciences 15 (2002) 11±22
Fig. 1. Epicenters of the accurately located events in the region, from JHD procedure (dark circles) and from local data (open circles). The rupture lengths of the great earthquakes used to segment the zone are shown with vertical lines. The trace of three vertical cross-sections, width ^100 km, is shown on each segment. Bathymetry is contoured each 700 m. Isodepth contours, after Cahill and Isacks (1992), and the Chile±Argentina boundary line that coincides with the Andes peaks are shown as reference.
selected data from large interplate earthquakes and available local data, and proposed values of the maximum depth of coupling between 50 and 60 km. Considering the rupture zones of the great subduction earthquakes in 1922 (Atacama, Ms 8:3), 1943 (Illapel, Ms 7:9), and 1906 (Valparaiso, Ms 8:4) (Comte et al., 1986; Beck et al., 1998), we divided the central Chile zone into three segments (Fig. 1). They are, respectively, Segment-1, from 268S to 308S, corresponding to the southward beginning of the ¯at slab subduction; Segment-2, from 308S to 328S, where the slab is ¯at and subhorizontal at intermediate depths; and Segment-3, from 328S to 358S, where the dip of the slab is 258±308. In each segment we improved the local and teleseismic hypocenter locations, determined new focal mechanisms and considered the focal mechanisms reported by Harvard centroid moment tensor (HCMT). The purpose of this article is to analyze and compare the seismotectonic characteristics of these three segments in central Chile relative to the shape of the
subducted Nazca Plate, the associated stress ®eld and the geometry of the interplate coupled zone. 2. Tectonic setting The zone of study includes central Chile and western Argentina, where the Nazca Plate is subducting beneath the South American Plate at a rate of about 8 cm/yr. The subduction zone along the Chile trench is seismically very active, where magnitude over eight interplate underthrusting earthquakes have occurred along its entire length (Comte et al., 1986; Beck et al., 1998). The Andes Cordillera, formed mainly by plate compression, is narrow with an average elevation of 4000 m and peaks of over 6000 m. There is active volcanism to the south of 338S, while in the ¯at slab zone the volcanism terminated approximately 9±10 Ma ago (Kay et al., 1988). In the back-arc, the overriding continental plate is
M. Pardo et al. / Journal of South American Earth Sciences 15 (2002) 11±22
13
Table 1 Source parameters of the earthquakes modeled using the waveform inversion and hypocenter relocated with JHD #
Date
O.T. (hh/mm)
Lat (8S)
Long (8W)
Depth (km)
mb
Mo 10 17 N m
Strike (8)
Dip (8)
Rake (8)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
80/07/13 80/07/19 81/03/23 81/04/01 81/05/09 81/11/07 82/10/26 82/11/18 83/01/10 83/10/21 83/12/15 84/02/03 85/01/18 85/03/23 85/03/25 85/04/09 85/06/10 87/05/19
06:20 11:52 19:28 18:03 09:50 03:29 03:24 20:39 12:32 08:14 04:22 08:29 15:00 14:36 05:14 01:56 15:37 12:56
33.45 29.13 33.59 27.41 26.55 32.23 29.76 31.21 27.40 30.69 33.11 29.56 29.48 33.22 34.23 34.04 28.0 30.37
70.20 69.72 71.94 63.36 64.82 71.39 71.31 65.79 63.38 69.20 70.21 71.07 70.82 72.23 72.19 71.47 66.84 71.63
101 101 27 553 15 56 66 171 556 101 99 54 80 23 27 51 154 44
5.6 6.1 5.8 5.9 5.5 6.1 5.6 5.5 5.6 5.5 6.0 5.7 5.8 5.6 6.1 6.3 5.8 5.6
5.3 18.9 13.8 56.8 0.7 260.2 4.5 16.1 4.5 2.5 18.5 3.4 44.6 2.3 37.4 558.1 46.4 26.0
199 331 371 45 296 294 174 337 344 183 252 14 326 340 313 342 311 338
80 33 19 7 50 15 89 63 43 66 25 46 68 39 20 22 59 30
117 248 113 240 146 40 78 286 275 258 246 99 16 107 51 89 2104 94
seismically active in the Sierras Pampeanas and Precordillera of western Argentina, and these have been the site of several large damaging earthquakes with maximum magnitude about 7.5, mainly associated to compression and crustal shortening. The total crustal shortening at the latitude of 308S has been estimated on approximately 150±170 km (Allmendinger et al., 1990). The principal bathymetric feature in the oceanic plate is the Juan Fernandez Ridge (JFR), a hot spot seamount chain that is subducted at the trench around 338S. The subduction of the JFR is well correlated with the sharp change in the dip of the downgoing slab around latitude 338S. It may be one of the principal factors to explain the ¯at slab subduction from about 288S ±338S (Cross and Pilger, 1982; Cahill and Isacks, 1992; Von Huene et al., 1997; Gutscher et al., 2000; YanÄez et al., 2001). It also may be considered the existence of the Challenger Fracture Zone in this region, and to the south, of the Chile ridge that extends from the junction of the Paci®c, Antarctic, and Nazca Plates, to the triple junction of the Antarctic, Nazca, and South American Plates (Tebbens et al., 1997). Several studies have related the geometry of the subducted oceanic slab with the upper-plate tectonics in central Chile±western Argentina (Jordan et al., 1983; Allmendinger et al., 1990; Cahill and Isacks, 1992; Fuenzalida et al., 1992). They correlated the ¯at subduction of this segment of Nazca Plate with different features in the overriding South America Plate: absence of Quaternary volcanism, crustal shortening in the back-arc, and a steady topographic rise from the coast to the Andes peaks. 3. Data The data used in this study are accurately determined
hypocenters obtained from local data recorded by permanent and temporary seismological stations in the region, and relocated hypocenters of events recorded at teleseismic distances by the worldwide seismological network. Also we used focal mechanisms from the HCMT catalog, and new focal mechanisms determined in this study from a long-period body wave inversion of earthquakes with magnitude mb $ 5.5 that occurred in the zone and were recorded by the global digital seismological network. 3.1. JHD hypocenters The hypocenter of earthquakes with magnitude mb $ 4.8, between 1964 and 1998, and recorded at teleseismic distances were relocated using the joint hypocenter determination method (JHD) (Dewey, 1971). The reason for relocating hypocenters from teleseismic data is the well-known observed mislocation between the hypocenters reported by the international agencies and the ones determined with local data. The JHD method determines relocated hypocenters relative to the hypocenters of the selected calibration events in the zone, applying time adjustments to the phase readings of P, pP, and S waves from the reporting seismological stations. These station-phases time adjustments are obtained from the variance of the different phase arrival times of the calibration events. Then, knowing the station-phases time adjustments, the hypocenter of all the events within the zone can be relocated using the single event location method (Dewey, 1971). The precision of each relocated hypocenter is estimated by computing error ellipsoids for the hypocentral coordinates at a 90% con®dence level. The data to perform the relocation procedure are the phase readings reported by the International Seismological
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relative to the hypocenters obtained with the JHD procedure are on average 19 km with a standard deviation of 22.5 km. 3.2. Hypocenters determined from local data
Fig. 2. Vertical cross-sections of seismicity with depth, shown in Fig. 1. Origin is at the trench and oriented normal to it. The dark circles correspond to JHD locations and open circles to local data solutions. At the top of each pro®le, the projection of the coast is presented with a black inverted triangle, and the position of the Andes arc with a black (active volcanism) and gray (absence of Quaternary volcanism) triangle.
Center (ISC) for events with magnitude mb $ 4.8 and recorded at 20 or more stations. In each of the three segments that we divided the central Chile subduction zone, we selected a set of 25 calibration events recorded also by local stations and with focal depth constrained using a long-period body wave inversion. For the ®rst segment (268S ±308S), we considered events from the 1973 swarm that occurred offshore, the largest events recorded by a temporary network of 38 stations deployed at the zone in 1998 between September 20 and November 30 (Comte et al., 2001), and eight modeled earthquakes in the zone (Table 1). For the second segment (308S±328S), aftershocks of the 1997 Punitaqui earthquake Mw 7:1 and the largest events in the region (Pardo et al., 2001) including three modeled events were considered (Table 1). For the third segment (338S±368S), the largest aftershocks of the 1985 San Antonio earthquake Mw 8:0 (Comte et al., 1986), and seven modeled earthquakes (Table 1) were used as calibration events. The solution was considered reliable when the error ellipsoid semi-axes were less than 15 km. All the events, which did not meet these criteria, were discarded so the ®nal relocated catalog is not complete. A ®nal subset of 1026 reliable hypocenters (^15 km) was obtained from an initial set of about 1250 events. The mislocation of the ISC hypocenters
Local data recorded by permanent and temporary seismological stations at the zone were considered to determine accurate hypocenters in central Chile. For the ®rst segment we used the hypocentral catalog from Comte et al. (2002) obtained from data of a temporary network of 38 stations deployed for 81 days in 1998. A joint hypocentral and body waves tomography was performed, so the locations were determined using a 3D velocity model. Applying convergence criteria, a ®nal set of 874 accurate hypocenters with an estimated mean error of ^5 km was obtained. In the second segment, we used data from a temporary network of 38 stations from IRIS-PASSCAL and Lithoscope-France, installed from November 15, 1999 to March 3, 2000 (Pardo et al., 2000). The database is still being processed, so here we use only locations of events during the last 45 days of 1999. The hypocenters were determined using a minimum 1D layered velocity model (Kissling et al., 1994) and several hypocenter location computer programs. We considered only the robust solutions obtained with the different techniques, which also satis®ed: a minimum of 15 phase readings, a root mean square error rms , 0.25, and estimated hypocentral errors of less than 10 km. A ®nal set of 1733 accurate located events (^10 km) was considered. In the third and southernmost segment, we used the hypocentral database for events between 1985 and 1995 (Pardo et al., 1997), obtained from a joint hypocentral and 3D body waves velocity model inversion (5785 events). The phase arrival times data to perform the inversion were obtained from: (1) the catalog of the Seismological Service, University of Chile (UCH); (2) a temporary network of eight stations deployed for one month after the 1985 earthquake (Mw 8:0; Comte et al., 1986); (3) a temporary network of 14 stations installed for two months between 32.58S and 348S (Fuenzalida et al., 1992); and (4) a network of 12 stations installed for 2 months between 348S and 35.58S during 1995 (Pardo et al., 1997). We also considered the catalog from UCH, 1996±1999, ®ltered by number of recording stations greater than 8, rms less than 0.25, and hypocentral errors less than 10 km, obtaining a set of 803 events that met this criteria. 3.3. Long-period body wave inversion The formal inversion of P, SV, and SH waveforms constrains the focal mechanism, the centroid focal depth, the seismic moment, and the source time function of the modeled earthquake. We used the method given by NaÂbelek (1984) to model 18 events that occurred between 1980 and 1987, with long-period seismograms recorded at teleseismic distances (258 # D # 908) from the global digital seismological network. The seismograms were ®ltered with a high-pass ®lter with
M. Pardo et al. / Journal of South American Earth Sciences 15 (2002) 11±22
15
cutoff frequency of 0.017 Hz (60 s) to eliminate long-period noise, and they were equalized to a peak magni®cation of 1500 at a distance of 408. All the modeled events are in the magnitude range 5.5 # mb # 6.2. Their source parameters are presented on Table 1, and the focal mechanism, waveform adjustment, and source time function are shown in Appendix A. 4. Seismicity and shape of the subducted slab
Fig. 3. Sketch of the shape of the subducted slab in the three considered segments along central Chile (S), presented on vertical cross-section normal to the trench. At the top, the distances from the trench to the coast (C), and to the Andes arc (A), are indicated. The distance C is the same for Segment-1 and Segment-2.
The epicentral distribution of the ®nal accurately located seismicity from local and JHD events is presented in Fig. 1. In order to analyze the shape of the subducted slab in the three segments, vertical cross-sections of the seismicity with depth were drawn (Fig. 2). The origin of all the crosssections is the trench, with a width of ^100 km and oriented in the direction perpendicular to the trench. A sketch of the Wadati±Benioff zone in each segment is shown in Fig. 3, assuming that it coincides with the top of the seismicity observed in each cross-section of Fig. 2.
Fig. 4. Same as Fig. 1 including focal mechanisms presented on lower hemispheric projection, from body wave inversion (gray) and HCMT (black). Dark quadrants indicate compressive ®rst motions.
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M. Pardo et al. / Journal of South American Earth Sciences 15 (2002) 11±22
Table 2 Stress tensor parameters. For the stress axes, at the top is the azimuth, clockwise from north, and at the bottom is the plunge. Principal axes of the stress tensor (s 1, s 2, s 3); tensor shape factor (R); score and number of used focal mechanisms (#FM) Central Chile Segment-1 Shallow depth (0±70 km) Intermediate depth (100±200 km) Deep Depth (570±630 km)
Segment-2 Shallow depth (0±70 km) Intermediate depth (100±200 km) Back-arc (0±45 km) Segment-3 Shallow depth (0±70 km) Intermediate depth (100±200 km)
s 1 (8)
s 2 (8)
s 3 (8)
266.6 23.5 297.7 83.2
171.7 7.8 149.4 4.2
63.9 58.2 59.1 2.6
80.2 50.7
345.8 2.5
253.5 32.0
257.1 20.2 81.8 82.6 97.7 4.0
166.3 7.0 320.8 2.7 188.3 5.0
54.5 62.3 230.5 4.6 329.1 81.2
265.0 0.4 355.5 46.2
175.0 0.3 205.1 32.6
52.0 89.3 103.8 12.4
4.1. Shallow depths (0±100 km) The shallow part of the subduction zone indicates an interplate geometry that initially dips with about 108 and gradually increases to 258 at a depth of 45 km (Figs. 2 and 3). This geometry is observed throughout the subduction zone in central Chile, and no lateral changes are observed at these depths, even at places where major bathymetric features, as the JFR, are subducted. The shape at the initial part of the subduction seems to be related only to the interplate collision, and appears to be independent of the age and relative convergence rate of the oceanic plate. Major changes in the shape of the subducted slab are observed below depths of 45 km (Figs. 2 and 3). At the southern Segment-3 (338S±358S), the slab penetrates into the mantle with an almost constant dip of 278, below depths of 70 km. To the north of 338S, the slab sharply bends in Segment-2, reaching a maximum dip of 408 at a depth of about 80 km, and from there the dip drastically diminishes to a ¯at slab shape at a depth of 100 km. In Segment-1 the subducted slab starts to be ¯at, and the change in the slab dip is more moderate, reaching about 308 at a depth of 70 km, and from there the slab ¯attens and continues with an almost subhorizontal trajectory at depths of ,100 km. 4.2. Intermediate depths (100±200 km) North of 338S, the slab is ¯at at depths around 100 km. In the northernmost Segment-1, the oceanic plate is subhorizontal to the east at a distance of about 200 km (250±
R
Score
# FM
21.49
0.897
18
1.61
0.979
14
2.04
0.998
10
20.77
0.887
33
2.59
0.904
19
21.36
0.968
13
20.10
0.960
52
1.48
0.995
9
450 km from the trench). The slab dip increases to 508 at a depth of 170 km and distance of 550 km from the trench (Figs. 2 and 3). In Segment-2 the ¯at slab extends for about 300 km (200±500 km from the trench), and then penetrates more into the mantle with a dip of 258 down to a maximum seismic depth of 200 km (Figs. 2 and 3). The Wadati± Benioff zone inferred for Segment-3 shows that the oceanic plate sinks into the mantle with an almost constant dip of 278, and no seismicity below depths of 160 km is detected (Figs. 2 and 3). 4.3. Depths .200 km Below depths of 200 km, seismicity is only observed at Segment-1 in clusters at depths between 570 and 630 km. The events occur in a narrow seismic zone that can be observed from 16.58S to 298S at these depths (Cahill and Isacks, 1992). These deep earthquakes are spatially separated from the intermediate depth events by a gap in seismicity along the slab (Figs. 2 and 3). The continuity of the slab through this aseismic zone was ®rst suggested on the basis of high-frequency waves from nearby earthquakes and transmitted through the seismicity gap region (Isacks and Barazangi, 1973). Later, modeling of wide-angle re¯ections from the upper surface of the Nazca Plate provided strong evidence for continuity of the downgoing slab beneath central and eastern Peru (James and Snoke, 1990), and it was also con®rmed by tomographic images for the Andean subduction zone (Engdahl et al., 1995).
M. Pardo et al. / Journal of South American Earth Sciences 15 (2002) 11±22
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of Fig. 4. They are presented in Table 2, and in Fig. 5 on back-hemispheric lateral projection. 5.1. Stress at shallow depths (0±70 km)
Fig. 5. Same as Fig. 2, showing the focal mechanisms from body wave inversion (gray) and from HCMT (black) on lateral back-hemispheric projection onto the vertical cross-section. In the same projection are shown the calculated stress tensors for the zones indicated by a rectangle in each pro®le.
5. Stress tensor in the region The stress tensors along the considered segments were obtained from the reported and newly determined focal mechanisms of events in the zone (Fig. 4, Table 1, and Appendix A). We calculate the stress tensor that best ®ts the focal mechanisms in a region using the method of Rivera and Cisternas (1990). The orientation of the stress tensor, given by the Euler angles, and its shape factor R sz 2 sx= sy 2 sx referred to the geographical coordinates x; y; z NS; EW; vertical; are calculated based on the fault plane solutions from the focal mechanisms. A set of tensors is selected in a random way in the tensor space, and for each tensor a score is determined as the sum of the scalar products between theoretical and observed slip vectors for all the considered mechanisms. The score ranges from 0 to 1, depending if none or all of the slip vectors from the observed focal mechanisms are ®tted with the calculated stress tensor, so it indicates the quality of the stress tensor solution. Then the best-®tted tensor, with highest score, is considered as solution. The shape factor R indicates that the stress regime within the considered zone is related to compression R , 0; to strike-slip faulting 0 , R , 1; or to extension R . 1: We calculate the best-®tted stress tensor at the three segments in central Chile, along the vertical cross-sections
Along and around the interplate zone, at depths less than 70 km, the shape factor R , 0 of the obtained stress tensors in all central Chile indicates a compressive regime (Table 2). The orientation of s 1 in all the segments is roughly in the convergence direction between Nazca and South American Plates. At Segment-3 zone, s 1 is horizontal whereas in Segment-1 and Segment-2 its plunge is about 238. The s 3 axis is vertical in Segment-3, but with a plunge of near 608 and azimuth N558E±658E in the ¯at zone segments (Table 2, Fig. 5). The focal mechanisms in this zone are mainly thrust and reverse, but some normal faulting events within the oceanic plate are observed with an average focal depth greater than the interplate compressive events (Fig. 5). We also determined the stress tensor at the back-arc in Segment-2, where seismicity clusters with maximum depths of 50 km and reported magnitude up to Mw 7:4; in the Argentinean Sierras Pampeanas. The associated focal mechanisms are mainly reverse, with west-dipping fault planes. The stress tensor for this zone indicates compression with horizontal s 1 nearly in EW direction, s 2 axis oriented NS and s 3 vertical (Table 2, Fig. 5), which is in agreement with the observed deformation in a thick-skinned style and crustal shortening (Jordan et al., 1983; Allmendinger et al., 1990). 5.2. Stress at intermediate depths (100±200 km) At these depths, the slab is under tension R . 1 (Table 2). At Segment-3, south of 338S, the s 3 axis is oriented along the slab with a plunge of 128 and is clearly associated to slab pull forces. While at the ¯at slab zone, where the stress tensor is similar in Segment-1 and Segment-2, the s 3 axis is almost horizontal in N508E±608E direction (Table 1, Fig. 5). In this region s 1 is vertical, and at Segment-3 it is oriented NS with a plunge of 468. In general the focal mechanisms indicate normal faulting with average T-axis along the subduction, but there are some with strike-slip component or T-axis with component normal to the subduction, especially at Segment-2 (Figs. 4 and 5). 5.3. Stress at deep depths (570±630 km) At the zone of deep events in Segment-1, depths of 570± 630 km, the obtained stress tensor is related with extension, but with the s 1 axis parallel to the slab dip and a plunge of 518. The s 2 axis is almost horizontal in NS direction and s 3 axis is nearly perpendicular to the slab (Table 2, Fig. 5). The focal mechanisms in this region show mainly normal to vertical faulting, with P-axis along the subduction (Fig. 4).
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M. Pardo et al. / Journal of South American Earth Sciences 15 (2002) 11±22
5.4. Maximum depth of interplate coupling The depth extent of the seismogenic interplate contact can be estimated from the maximum depth of the shallow thrust earthquakes and the depth of transition between compressive to tensional events (SuaÂrez and Comte, 1993). From the focal mechanisms in the region (Figs. 4 and 5), the maximum depth of thrust and reverse mechanism events that can be associated with interplate faulting are 60, 62, and 51 km, respectively, in each segment. The minimum depth of downdip normal mechanism events, with T-axis along the subducted slab, is 66 km at Segment-1, 68 km at Segment-2, and 60 km at Segment-3. Hence the maximum depth of interplate coupling can be estimated in about 60 km in the entire central Chile zone (Fig. 5). This result is in agreement with previous results at selected places along the Chilean subduction zone (SuaÂrez and Comte, 1993; Tichelaar and Ruff, 1991; Pacheco et al., 1993). In Northern Chile (188S±268S) the maximum depth extent of the coupled zone is 60 ^ 10 km (Comte and SuaÂrez, 1993); therefore this depth seems to be almost constant along the whole subduction zone in Chile.
6. Discussion and conclusions The shape of the subducted Nazca Plate beneath central Chile, inferred from the top of the seismicity in the zone, shows that there are important along-strike variations in the dip angle, with a sharp transition around 338S from ¯at to a 278-dipping slab at intermediate depths. The steeper subduction at Segment-3 permits the existence of an asthenospheric wedge that can generate active volcanism in the Andes. The ¯at slab geometry observed north of 338S precludes the existence of this type of wedge, hence active volcanism is absent in this zone. The thickness of the seismic zone at the brittle top of the slab, at intermediate depths, is about 30 km in Segment-1 and Segment-3, while at Segment-2 it is about 50 km, with the uncertainties associated to the hypocenter locations. This implies that a thicker oceanic crust is present at the zone of the JFR subduction. This anomalous crust below the subducted seamount chain can be associated to thermal metamorphism related to the passage of the slab over the hot spot that origins the JFR and to isostatic compensation. Furthermore, this anomalous crust might increase the buoyancy of the slab, which contributes to the slab ¯attening. In Segment-2 the intermediate depth seismicity clusters along the subducted JFR. At these depths, besides the normal faulting events with T-axis along the slab, there are some earthquakes that exhibit strike-slip mechanisms or T-axis with NS component, which cannot be explained by only slab pull forces. These mechanisms might be related to reactivation of pre-existing faults in the thick oceanic crust, created near the trench and outer-rise by the interaction of the JFR subduction (Kirby et al., 1996).
Relative to the adjacent segments, the intermediate depth seismicity in Segment-2 is more intense and the seismic moment released is more important, as evidenced by the larger number of occurrence of JHD locations and HCMT solutions of earthquakes with magnitude mb . 5 (Figs. 4 and 5). Considering only the events with focal mechanisms in the cross-sections for each segment (Fig. 5) that occurred between 1977 and 1999, the accumulative seismic moment (10 18 N m) is 8.54 in Segment-2, compared to 7.11 in Segment-1 and 1.83 in Segment-3, for a number of events of 19, 18, and 10, respectively. At the back-arc and shallow depths in Segment-2, there are clusters of mainly reverse-faulting events in the Sierras Pampeanas region, with west-dipping fault plane and maximum depth of 50 km, related to crustal shortening and to thick-skinned deformation (Figs. 4 and 5). Another interesting observation in the ¯at slab zone between 278S and 338S is the westward shift on about 50 km of the highest peaks of the Andes boundary between Chile and Argentina (Fig. 1). The maximum shift in this narrow Andean zone is about 100 km between 318S and 328S, a zone that coincides with the subduction of the JFR beneath the continent. A remarkable along-strike variation in the length of the seismogenic part of the slab is also observed. The seismic slab length diminished to the south from about 1100 km at Segment-1, to 800 km at Segment-2, and 350 km at Segment-3. This observation suggests that Segment-2 ¯at slab zone could be a transition between two different plate segments of the actual subducted Nazca Plate. A northern segment corresponds to old oceanic plate generated at the East Paci®c Rise, and a southern segment of younger oceanic lithosphere generated at the Chile Rise, which is closer to the continent. However, it is important to point out that the existence of the seismicity gap below depths of 200 km at Segment-1, where the slab is thought to be continuous down to depth of 650 km, could imply that the intraslab seismicity does not image the deeper portion of the slab. The rheology and thermal conditions of the slab at depths below 200 km could inhibit the occurrence of earthquakes, at least down to 500 km depth, so the estimation of the length of the slab from its seismicity should only be a lower bound. The interplate zone is under compression due to plate interaction, with s 1 stress axis oriented along the convergence direction and a similar shape throughout central Chile, at least down to depths about 45 km. The fact that the s 1 stress axes exhibit a plunge of about 238 in Segment-1 and Segment2, while it is horizontal in Segment-1, implies a more ef®cient transmission of compressive stress to the overriding continental plate in the ¯at slab zone (Table 2 and Fig. 5). This contributes to explain the observations of intense continental deformation, crustal shortening, and shallow seismicity at the back-arc in the ¯at slab zone. There are not enough focal mechanisms to estimate the
M. Pardo et al. / Journal of South American Earth Sciences 15 (2002) 11±22
Fig. 6. Body waveform inversion of the events modeled in this study
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Fig. 7. Body waveform inversion of the events modeled in this study.
M. Pardo et al. / Journal of South American Earth Sciences 15 (2002) 11±22
stress tensor at the downdip zone between 70 and 100 km. However, analysis of the sequence of the 1997 Punitaqui earthquake Mw 7:1 that occurred at 68 km depth within the oceanic plate in Segment-2 (Pardo et al., 2002) shows that this zone exhibits compression along the slab associated with ¯exural stress due to the slab bending. Below depths of 45 km and north of 338S, the slab bends and ¯attens to an almost subhorizontal trajectory at depths of 100 km, continuing for 200 km at Segment-1 and 300 km in Segment-2 before reassuming its descent into the mantle. While in Segment-3, south of 338S, the slab sinks in the mantle with an almost constant dip of 278 (Fig. 3). At intermediate depths the slab is under a tensional stress regime, with s 3 axis along the slab in Segment-3 related to slab pull, while it is almost horizontal in N508±608E direction at Segment-1 and Segment-2. This means that the main force acting over the slab in the subhorizontal subduction region may be the result of the slab pull and a NS extension related to the slab ¯attening due to buoyant forces. The maximum depth of interplate coupling is estimated to be about 60 km in the entire central Chile zone. This result agrees with those obtained in other places along the Chilean subduction zone, therefore this depth seems to be almost constant along the whole subduction zone in Chile. Acknowledgements We thank the Seismological Service of the University of Chile for the data from their local network, and to IRISPASSCAL and Lithoscope-France for providing the instruments used in the 1999 temporary network. Most of the plots were generated using the GMT software. We thank A. Eisenberg for fruitful discussions. D. James and an anonymous referee provided valuable comments and helpful reviews. This work was partially supported by grants FONDECYT 1981145 and 1990355, F. Andes C-13563 and IRD-France. Appendix A Body waveform inversion of the events modeled in this study. The numbers at the top are related to Table 1. The focal mechanism of P (Top), SV (Middle), and SH (Bottom) are presented on lower hemispheric projection. The observed and the synthetic waveform are shown with solid and dashed lines, respectively. On the left top of each event the source time function is presented. References Allmendinger, R.W., Figueroa, D., Snyder, D., Beer, J., Mpodozis, C., Isacks, B., 1990. Foreland shortening and crustal balancing in the Andes at 308S latitude. Tectonics 9, 789±809. Araujo, M., SuaÂrez, G., 1994. Geometry and state of stress of the subducted
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Nazca Plate beneath central Chile and Argentina: evidence from teleseismic data. Geophys. J. Int. 116, 283±303. Barazangi, M., Isacks, B., 1976. Spatial distribution of earthquakes and subduction of the Nazca Plate beneath South America. Geology 4, 686±692. Beck, S., Barrientos, S., Kausel, E., Reyes, M., 1998. Source characteristics of historical earthquakes along the central Chile subduction zone. J. S. Am. Earth Sci. 11, 115±129. Cahill, T., Isacks, B., 1992. Seismicity and shape of the subducted Nazca Plate. J. Geophys. Res. 97, 17503±17529. Comte, D., SuaÂrez, G., 1993. Comment on `Seismic coupling along the Chilean subduction zone' by B.W. Tichelaar and L.R. Ruff. J. Geophys. Res. 98, 15825±15828. Comte, D., Eisenberg, A., Lorca, E., Pardo, M., Ponce, L., Saragoni, R., Singh, S.K., SuaÂrez, G., 1986. The 1985 central Chile earthquake: a repeat of previous great earthquakes in the region? Science 233, 449±453. Comte, D., Haessler, H., Dorbath, L., Pardo, M., Monfret, T., Lavenu, A., Pointoise, B., Hello, Y., 2002. Seismicity and stress distribution around Copiapo, Northern Chile region using off-shore and inland locally recorded data. Phys. Earth Planet. Int. in press. Cross, T., Pilger, R., 1982. Controls of subduction geometry, location of magmatic arcs, and tectonics of arc and back-arc regions. Bull. Geol. Soc. Am. 93, 545±562. DeMets, C., Gordon, R.G., Argus, D.F., Stein, S., 1994. Effect of recent revisions to the geomagnetic reversal time scale on estimate of current plate motions. Geophys. Res. Lett. 21, 2191±2194. Dewey, J.W., 1971. Seismicity studies with the method of joint hipocenter determination. PhD Thesis. University of California, Berkeley. Engdahl, E.R., Van der Hilst, R.D., Berrocal, J., 1995. Imaging of subducted lithosphere beneath South America. Geophys. Res. Lett. 22, 2317±2320. Fuenzalida, A., Pardo, M., Cisternas, A., Dorbath, L., Dorbath, C., Comte, D., Kausel, E., 1992. On the geometry of the Nazca Plate subducted under central Chile (32±34.58S) as inferred from microseismic data. Tectonophysics 205, 1±11. Gutscher, M., Spakman, W., Bijwaard, H., Enghdal, R., 2000. Geodynamics of ¯at subduction: seismicity and tomographic constraints from the Andean Margin. Tectonics 19, 814±833. Isacks, B., Barazangi, M., 1973. High-frequency shear waves guided by a continuous lithosphere descending beneath Western South America. Geophys. J. R. Astron. Soc. 33, 129±139. James, D., Snoke, J.A., 1990. Seismic evidence of the deep slab beneath Central and Eastern Peru. J. Geophys. Res. 95, 4989±5001. Jordan, T.E., Isacks, B., Allmendinger, R., Brewer, J., Ramos, V., Ando, C., 1983. Andean tectonics related to geometry of the subducted Nazca Plate. Geol. Soc. Am. Bull. 94, 341±361. Kay, S.M., Maksaev, V., Moscoso, R., Mpodozis, C., Nasi, C., Gordillo, C.E., 1988. Tertiary magmatism in Chile and Argentina between 288S and 338S: correlation of magmatic chemistry with changing Benioff zone. J. S. Am. Earth Sci. 1, 21±38. Kirby, S., Enghdal, R., Denlinger, R., 1996. Intermediate-depth intraslab earthquakes and arc-volcanism as physical expressions of crustal and uppermost mantle metamorphism in subducting slabs. In: Bebout, G., Scholl, D., Kirby, S., Platt, J. (Eds.). Subduction: Top to Bottom. Geophysical Monograph 96American Geophysical Union, pp. 195±214. Kissling, E., Ellsworth, W.L., Eberhart-Phillips, D., Kradolfer, U., 1994. Initial reference models in seismic tomography. J. Geophys. Res. 99, 19635±19646. NaÂbelek, J., 1984. Determination of earthquake source parameters from inversion of body waves. PhD Thesis. Mass. Inst. Technol., Cambridge. Pacheco, J.F., Sykes, L.R., Scholz, C.H., 1993. Nature of seismic coupling along simple plate boundaries of the subduction type. J. Geophys. Res. 98, 14133±14169. Pardo, M., Comte, D., Monfret, T., Vera, E., 1997. TomografõÂa en Chile Central: variaciones en el espacio y el tiempo de la velocidad de ondas P
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Swift, S., Carr, M., 1974. The segmented nature of the Chilean seismic zone. Phys. Earth Planet. Int. 9, 183±191. Tebbens, S.F., Cande, S.C., Kovacs, L., Parra, J.C., LaBrecque, J.L., Vergara, H., 1997. The Chile ridge: a tectonic framework. J. Geophys. Res. 102, 12035±12059. Tichelaar, B.W., Ruff, L.R., 1991. Seismic coupling along the Chilean subduction zone. J. Geophys. Res. 96, 11997±12022. CONDOR Scientists, Von Huene, R., CorvalaÂn, J., Flueh, E.R., Hinz, K., Korstgard, J., Ranero, C.R., Weinrebe, W., 1997. Tectonic control of the subducting Juan Fernandez Ridge on the Andean margin near Valparaiso, Chile. Tectonics 16, 474±488. YanÄez, G., Ranero, C., Von Huene, R., DõÂaz, J., 2001. Magnetic anomaly interpretation across the southern central Andes (328±348S): the role of the Juan Fernandez Ridge in the late Tertiary evolution of the margin. J. Geophys. Res. 106, 6325±6435.