seismicity near the slip maximum of the 1960 mw 9.5 ... the black arrow indicates the convergence...
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Seismicity near the slip maximum of the 1960 Mw 9.5 Valdivia earthquake (Chile): Plate interface lock and reactivation of the subducted Valdivia Fracture Zone
Yvonne Dzierma,1 Martin Thorwart,1 Wolfgang Rabbel,1 Claudia Siegmund,1
Diana Comte,2 Klaus Bataille,3 Paula Iglesia,4 and Claudia Prezzi4
Received 10 October 2011; revised 5 April 2012; accepted 26 April 2012; published 23 June 2012.
 Understanding the processes behind subduction-related hazards is an important responsibility and major challenge for the Earth sciences. Few areas demonstrate this as clearly as south-central Chile, where some of the largest earthquakes in human history have occurred. We present the first observation of local seismicity in the Villarrica region (39�–40�S), based on a temporary local network of 55 stations installed from the Chilean coast into the Argentinian back-arc for one year. While consistent with the Chilean national catalog (SSN), our results allow us to observe smaller magnitudes with a completeness of about 2.0 and image the geometry of the Wadati-Benioff Zone from the Chile Trench down to 200 km. Offshore, a gap in interplate seismicity is observed in the region of the 1960 Valdivia earthquake slip. Above the interface, two offshore seismicity clusters possibly indicate ongoing stress relaxation. In the subducting Nazca Plate, we find a prominent seismicity cluster along the extrapolated trace of the oceanic Valdivia Fracture Zone (VFZ). The seismicity cluster is observed between 70 and 130 km depth and comprises mainly strike-slip events. It indicates weakening and reactivation of the major VFZ by dehydration of oceanic crust and mantle. Interpreting the subducted VFZ section as a localized reservoir of potential fluid release offers an explanation for the Villarrica volcanic complex that is located above the reactivated VFZ and shows the highest volcanic activity in South America. Crustal seismicity is observed near Puyehue volcano, which recently started to erupt (June 2011).
Citation: Dzierma, Y., M. Thorwart, W. Rabbel, C. Siegmund, D. Comte, K. Bataille, P. Iglesia, and C. Prezzi (2012), Seismicity near the slip maximum of the 1960 Mw 9.5 Valdivia earthquake (Chile): Plate interface lock and reactivation of the subducted Valdivia Fracture Zone, J. Geophys. Res., 117, B06312, doi:10.1029/2011JB008914.
 Strong tsunami generating earthquakes belong to the most hazardous geodynamic processes occurring along the world’s plate margins. The South-Central Chilean subduc- tion zone is known for having produced some of the largest observed earthquakes - among them, the greatest instrumen- tally recorded earthquake, the 1960 Mw 9.5 Valdivia event [Barrientos and Ward, 1990; Cifuentes, 1989] - and has recently ruptured again during the 2010 Mv 8.8 Maule
earthquake and aftershock sequence [Delouis et al., 2010]. In addition to a particularly high potential for large inter- seismic strain accumulation, this region is home to some of South America’s most active volcanoes, Llaima and Villarrica [Stern, 2004].  The geometry and geophysical properties of this
subduction system are well constrained north of 39�S and south of 41�S, but not in between, where the 1960 coseismic slip was greatest (between 39�S and 40�S). Although broad similarities with the north and south adjacent regions are observed, the region between 39�S and 40�S (but not between 40�S and 41�S) is found to be very different in geomorphology and gravity [Hackney et al., 2006; Rehak et al., 2008; Alasonati Tasarova, 2007], and has been spec- ulated to present a different stress regime. On the basis of the gravity measurements, it was hypothesized that plate coupling in this segment of the subduction zone is stronger than in the adjacent regions [Hackney et al., 2006]. Con- versely, GPS measurements suggest that plate locking is reduced between the incoming Valdivia and Mocha frac- ture zones [Moreno et al., 2011]. This apparent controversy may actually be due to the different time scales considered: the gravity signature will be stable over geological times,
1SFB 574, Institute of Geosciences, Department of Geophysics, Christian-Albrechts University Kiel, Kiel, Germany.
2Departamento de Geofísica, Universidad de Chile, Santiago, Chile. 3Departamento de Ciencias de la Tierra, Universidad de Concepción,
Concepción, Chile. 4CONICET, Departamento de Ciencias Geológicas, Facultad de Ciencias
Exactas y Naturales, Universidad de Buenos Aires, Buenos Aires, Argentina.
Corresponding author: M. Thorwart, SFB 574, Institute of Geosciences, Department of Geophysics, Christian-Albrechts University Kiel, Otto-Hahn-Platz 1, D-24118 Kiel, Germany. (email@example.com)
©2012. American Geophysical Union. All Rights Reserved. 0148-0227/12/2011JB008914
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 117, B06312, doi:10.1029/2011JB008914, 2012
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whereas the GPS measurements look at the instantaneous locking. In any case, the marked contrast between this segment and the surrounding regions make it a key region for under- standing subduction-related processes along this plate margin.  This region has not been subject to regional seismo-
logical studies because it was believed to exhibit no signifi- cant seismic activity, due to the scarcity of local earthquakes registered in this region both by international networks (IRIS-NEIC, http://earthquake.usgs.gov/earthquakes/) and the Chilean National Seismological Service (SSN, http://ssn. dgf.uchile.cl/). However, while this region may actually be seismically quiet due to plate-locking at the interface fol- lowing the last great earthquake [Hu et al., 2004;Khazaradze et al., 2002], the observed lack of seismicity may also be artificially induced by station coverage. We here present results from a temporal local network of 55 stations installed between 39�S and 40�S, covering the area from the Chilean coast to the back-arc in Argentina.  The main aims of the study are: first, to verify if the
local and regional seismicity in this region is really subdued, which has important implications for seismic locking and strain buildup along the plate interface. Recently, Moreno et al.  have suggested that the 1960 slip interface is locked and building up stress for the next large earth- quake and also suggested that locking is only partial along the trace of the subducting Valdivia Fracture Zone. We test this hypothesis by investigating the local seismicity distri- bution along the 1960 slip interface. Second, we aim to resolve the deep structure of theWadati-Benioff-Zone, which is unconstrained in this region because the international catalog data does not delineate a dipping slab even when considered over several decades. The intermediate depth
seismicity along the Wadati-Benioff Zone is an important indicator for dehydration reactions of the subducting plate crust and mantle, which in turn feed the volcanic arc. Since the study region is home to one of the most active volcanoes of the Chilean Southern Volcanic Zone, a primary goal of our work is to investigate the dehydration of the downgoing slab and possible influences of the subducting fracture zones on fluid release and pathways. Third, the regional character of this study will allow us to determine the relevance of active fault zones geologically observed in the overriding plate and the activity of the volcanic centers.
2. Tectonic Framework and Previous Studies
 In the study region (Figure 1), the Nazca Plate subducts obliquely (77.5�) beneath the South American Plate with a convergence velocity of 73.7 mm/yr [DeMets et al., 2010]. The downgoing slab is cut by several transform faults, the most prominent of which is the Valdivia Fracture Zone (VFZ) impinging on the trench at 40�S. The VFZ constitutes the boundary between �26 Ma oceanic crust produced at the East Pacific Rise and younger (18–20 Ma) crust produced at the Chile Rise in the south [Tebbens et al., 1997].  As a result of oblique subduction, a large crustal forearc
sliver has formed, bounded to the east by the Liquiñe-Ofqui Fault Zone (LOFZ), a 1100 km long trench-parallel slip sys- tem [Cembrano et al., 2000; Rosenau et al., 2006]. Together with a number of WNW-trending fault zones in the overriding plate (most prominently in this area, the Mocha-Villarrica Fault Zone, MVFZ [Melnick and Echtler, 2006]), this system of active faults is a major determinant for the position of the active volcanic centers [Cembrano and Lara, 2009].
Figure 1. Tectonic overview of the investigation area. Geologic units: CC - Coastal Cordillera, CP - Coastal Platform, CV - Central Valley, PG - Paleozoic Granites, MC - Main Cordillera and NB - Neuquén Basin. Red contour lines indicate the slip distribution of the 1960 M = 9.6 earthquake for 5 m, 15 m, 25 m and 35 m [Barrientos and Ward, 1990]. Black lines show crustal faults (CF) [Melnick and Echtler, 2006]. Major faults: LOFZ - Liquiñe-Ofqui-fault zone, MVFZ -Mocha-Villarrica-fault zone, LFZ - Lanalhue fault zone, MFZ - Mocha fracture zone and VFZ - Valdivia fracture zone. Main volcanoes: V - Villarrica, L - Llaima, P - Puyehue-Cordón Caulle. Major cities shown: Te - Temuco, Va - Valdivia, Os - Osorno and Ba - Bariloche. The black arrow indicates the convergence rate of 73.7 mm/y and direction of the Nazca Plate relative to the South American Plate.
DZIERMA ET AL.: SEISMICITY IN THE VALDIVIA REGION, CHILE B06312B06312
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 From a morphotectonic point of view, the overriding plate is segmented into three main units (from west to east): the Coastal Cordillera, Central Valley, and Main Cordillera, where the active volcanic arc