First results from seismicity and local earthquake tomography in the Villarrica-Valdivia region, South-Central Chile

DZIERMA, Y.; THORWART, M.; SIEGMUND, C.; RABBEL, W.; COMTE, D.; BATAILLE, K.; IGLESIA LLANOS, MARÍA P.; PREZZI, CLAUDIA BEATRIZ

Pucón

Workshop; SFB 574 Subduction Workshop; 2010

The 1960 Valdivia earthquake initiated near 38°S and propagated southward to about 46°S where the Chile Rise meets the trench. Slip associated with the rupture was of the order of 5–10 m, but reached almost 40 m at 39.3°S, adjacent to the very active Villarrica volcano. In addition to possible earthquake-induced effects on the volcanic activity in this region, the distribution and activity of the volcanoes is intricately linked to N–S, NW–SE and NE–SW trending faults and lineaments. The region north of Villarrica and the rupture zone of the Valdivia earthquake have been studied extensively. However, previous work focused on the parts of the plate interface adjacent to the epicentre and did not cover the area of maximum slip (39–42°S). Seismic studies of this region have also been hampered in the past because this region has long been considered a seismic gap due to the scarcity of local earthquakes registered in this region both by international networks (IRIS-NEIC) and the Chilean National Seismological Service (SSN); this is consistent with a highly-coupled (locked) plate interface upon which significant strain can accumulate and later release in a great earthquake. To investigate the geophysical and structural characteristics of this less well-explored part of the Chilean margin, the seismology subproject A2 deployed 55 seismic stations between 39° and 40°S, covering the area from the coast to the back-arc in Argentina. The network was installed in November 2008 and ran for one full year, consisting of 40 shortperiod seismometers and 15 broad-band stations. The registered local seismicity is significantly higher than expected, with more than 1000 observed events out of which about 300 were localized in the area covered by the network. The plate interface where the 1960 slip occurred is free of seismicity. At greater depths, the Wadati-Benioff-Zone can be clearly observed down to depths of about 160 km with a dip angle of about 30°; some indication for a double Benioff-zone at shallower depths (ca. 60 km) is also found. In the continental crust, seismicity is observed related to local fault structures and the active volcanoes. Some clusters of seismicity are located offshore, possibly delineating fault structures in the incoming oceanic plate. In the vicinity of the trench, a large number of events is recorded, and focal mechanisms will be presented, along with the first results from local earthquake tomography.his region has long been considered a seismic gap due to the scarcity of local earthquakes registered in this region both by international networks (IRIS-NEIC) and the Chilean National Seismological Service (SSN); this is consistent with a highly-coupled (locked) plate interface upon which significant strain can accumulate and later release in a great earthquake. To investigate the geophysical and structural characteristics of this less well-explored part of the Chilean margin, the seismology subproject A2 deployed 55 seismic stations between 39° and 40°S, covering the area from the coast to the back-arc in Argentina. The network was installed in November 2008 and ran for one full year, consisting of 40 shortperiod seismometers and 15 broad-band stations. The registered local seismicity is significantly higher than expected, with more than 1000 observed events out of which about 300 were localized in the area covered by the network. The plate interface where the 1960 slip occurred is free of seismicity. At greater depths, the Wadati-Benioff-Zone can be clearly observed down to depths of about 160 km with a dip angle of about 30°; some indication for a double Benioff-zone at shallower depths (ca. 60 km) is also found. In the continental crust, seismicity is observed related to local fault structures and the active volcanoes. Some clusters of seismicity are located offshore, possibly delineating fault structures in the incoming oceanic plate. In the vicinity of the trench, a large number of events is recorded, and focal mechanisms will be presented, along with the first results from local earthquake tomography.