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Seismicity near the slip maximum of the 1960 Mw 9.5 Valdiviaearthquake (Chile): Plate interface lock and reactivationof 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 importantresponsibility and major challenge for the Earth sciences. Few areas demonstrate this asclearly as south-central Chile, where some of the largest earthquakes in human historyhave occurred. We present the first observation of local seismicity in the Villarrica region(3940S), based on a temporary local network of 55 stations installed from the Chileancoast into the Argentinian back-arc for one year. While consistent with the Chilean nationalcatalog (SSN), our results allow us to observe smaller magnitudes with a completenessof about 2.0 and image the geometry of the Wadati-Benioff Zone from the Chile Trenchdown to 200 km. Offshore, a gap in interplate seismicity is observed in the region of the1960 Valdivia earthquake slip. Above the interface, two offshore seismicity clusterspossibly indicate ongoing stress relaxation. In the subducting Nazca Plate, we find aprominent seismicity cluster along the extrapolated trace of the oceanic Valdivia FractureZone (VFZ). The seismicity cluster is observed between 70 and 130 km depth andcomprises mainly strike-slip events. It indicates weakening and reactivation of the majorVFZ by dehydration of oceanic crust and mantle. Interpreting the subducted VFZ sectionas a localized reservoir of potential fluid release offers an explanation for the Villarricavolcanic complex that is located above the reactivated VFZ and shows the highest volcanicactivity in South America. Crustal seismicity is observed near Puyehue volcano, whichrecently 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 reactivationof the subducted Valdivia Fracture Zone, J. Geophys. Res., 117, B06312, doi:10.1029/2011JB008914.
 Strong tsunami generating earthquakes belong to themost hazardous geodynamic processes occurring along theworlds plate margins. The South-Central Chilean subduc-tion zone is known for having produced some of the largestobserved earthquakes - among them, the greatest instrumen-tally recorded earthquake, the 1960 Mw 9.5 Valdivia event[Barrientos and Ward, 1990; Cifuentes, 1989] - and hasrecently 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 ofSouth Americas most active volcanoes, Llaima and Villarrica[Stern, 2004]. The geometry and geophysical properties of this
subduction system are well constrained north of 39S andsouth of 41S, but not in between, where the 1960 coseismicslip was greatest (between 39S and 40S). Although broadsimilarities with the north and south adjacent regions areobserved, the region between 39S and 40S (but notbetween 40S and 41S) is found to be very different ingeomorphology and gravity [Hackney et al., 2006; Rehaket al., 2008; Alasonati Tasarova, 2007], and has been spec-ulated to present a different stress regime. On the basis ofthe gravity measurements, it was hypothesized that platecoupling in this segment of the subduction zone is strongerthan in the adjacent regions [Hackney et al., 2006]. Con-versely, GPS measurements suggest that plate locking isreduced between the incoming Valdivia and Mocha frac-ture zones [Moreno et al., 2011]. This apparent controversymay 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 Geofsica, Universidad de Chile, Santiago, Chile.3Departamento de Ciencias de la Tierra, Universidad de Concepcin,
Concepcin, Chile.4CONICET, Departamento de Ciencias Geolgicas, 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.(firstname.lastname@example.org)
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 instantaneouslocking. In any case, the marked contrast between this segmentand 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 earthquakesregistered in this region both by international networks(IRIS-NEIC, http://earthquake.usgs.gov/earthquakes/) andthe Chilean National Seismological Service (SSN, http://ssn.dgf.uchile.cl/). However, while this region may actually beseismically quiet due to plate-locking at the interface fol-lowing the last great earthquake [Hu et al., 2004;Khazaradzeet al., 2002], the observed lack of seismicity may also beartificially induced by station coverage. We here presentresults from a temporal local network of 55 stations installedbetween 39S and 40S, covering the area from the Chileancoast 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 andstrain buildup along the plate interface. Recently, Morenoet al.  have suggested that the 1960 slip interface islocked and building up stress for the next large earth-quake and also suggested that locking is only partial alongthe trace of the subducting Valdivia Fracture Zone. We testthis hypothesis by investigating the local seismicity distri-bution along the 1960 slip interface. Second, we aim toresolve the deep structure of theWadati-Benioff-Zone, whichis unconstrained in this region because the internationalcatalog data does not delineate a dipping slab even whenconsidered over several decades. The intermediate depth
seismicity along the Wadati-Benioff Zone is an importantindicator for dehydration reactions of the subducting platecrust and mantle, which in turn feed the volcanic arc. Sincethe study region is home to one of the most active volcanoesof the Chilean Southern Volcanic Zone, a primary goal of ourwork is to investigate the dehydration of the downgoing slaband possible influences of the subducting fracture zones onfluid release and pathways. Third, the regional character ofthis study will allow us to determine the relevance of activefault zones geologically observed in the overriding plate andthe activity of the volcanic centers.
2. Tectonic Framework and Previous Studies
 In the study region (Figure 1), the Nazca Plate subductsobliquely (77.5) beneath the South American Plate with aconvergence velocity of 73.7 mm/yr [DeMets et al., 2010].The downgoing slab is cut by several transform faults, themost prominent of which is the Valdivia Fracture Zone(VFZ) impinging on the trench at 40S. The VFZ constitutesthe boundary between 26 Ma oceanic crust produced at theEast Pacific Rise and younger (1820 Ma) crust produced atthe 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 Liquie-OfquiFault Zone (LOFZ), a 1100 km long trench-parallel slip sys-tem [Cembrano et al., 2000; Rosenau et al., 2006]. Togetherwith a number of WNW-trending fault zones in the overridingplate (most prominently in this area, the Mocha-VillarricaFault Zone, MVFZ [Melnick and Echtler, 2006]), this systemof active faults is a major determinant for the position of theactive 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 - NeuqunBasin. Red contour lines indicate the slip distribution of the 1960 M = 9.6 earthquake for 5 m, 15 m, 25 mand 35 m [Barrientos and Ward, 1990]. Black lines show crustal faults (CF) [Melnick and Echtler, 2006].Major faults: LOFZ - Liquie-Ofqui-fault zone, MVFZ -Mocha-Villarrica-fault zone, LFZ - Lanalhue faultzone, MFZ - Mocha fracture zone and VFZ - Valdivia fracture zone. Main volcanoes: V - Villarrica, L -Llaima, P - Puyehue-Cordn Caulle. Major cities shown: Te - Temuco, Va - Valdivia, Os - Osornoand Ba - Bariloche. The black arrow indicates the convergence rate of 73.7 mm/y and direction of theNazca 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 overridingplate is segmented into three main units (from west to east):the Coastal Cordillera, Central Valley, and Main Cordillera,where the active volcanic arc is located [see, e.g., Charrieret al., 2007]. From surface geology observations, theCentral Valley seems to be absent in the Villarrica regionbetween 39 and 40S. In addition to the atypical surfacegeology, this region is found to exhibit an unusual negativeBouguer gravity