Local Seismicity And Seismo-Tectonic Structure Of The Aysén Region, Southern Chile

The largest recorded earthquakes have all taken place along subduction margins (e.g. Chile, 1960; Andaman-Sumatra, 2004; Chile, 2010). A better understanding of the stress distribution involved in these tectonic environments could lead to better prediction of the occurrence and maximum possible magnitudes of earthquakes at given segments of a subduction zone. The study area of the present research, the Aysén Region, is located at the southern end of the subduction zone between the Nazca and South American plates, in front of the area where the Chile Ridge is currently being subducted below the Chile triple junction. Another major tectonic feature in the Aysén Region is the Liquiñe-Ofqui Fault Zone, a +1000 km long strike-slip fault that accommodates part of the trench-parallel component of the oblique subduction between the Nazca and South American plates. On the 21st of April 2007, a Mw=6.2 earthquake shook the Aysén Fjord, Southern Chile, following a seismic sequence of more than three months in an unprecedented episode for this region characterized by null or low seismicity. In order to study the aftershocks' activity of this main event, we installed and kept a seismic network of 15 short-period stations in the area around for an interval of nearly seven months. We characterized the seismogenic structure of the zone by calculating a minimum 1-D local velocity model and obtaining precise hypocentres locations. We also calculated fault plane solutions based on first motion polarities and P/SV amplitude ratios. The P-wave velocity model shows velocities varying between 4.05 and 5.18 km/s for the upper 5 km and high Vp/Vs ratio suggesting possible fracturing and presence of fluids. The Moho discontinuity would be localized at ~23 km depth. Aftershock events are located mainly between 5 and 10 km depth and disposed in two alignments: a ~N-S-trending alignment that matches with the trace of the Liquiñe-Ofqui fault and a NE-trending which could correspond to an structure not previously seen. Selected focal mechanisms show a strong strike-slip faulting which coincides with the nature of the Liquiñe-Ofqui fault. We are currently analysing an archive of 509 local events recorded by a network of 20 short-period and 40 broadband seismometers between 2004 and 2005 in the whole Aysén Region. This dataset will allow us to test our velocity model, to extend our area of research to a regional context and, therefore, to describe the local seismicity and seismo-tectonic structure of the Aysén Region.