7t h Int ernat ional Symposi um on Andean Geodynamics (ISAG 2008, Nice), Ext ended Abst ract s: 160-163 160 Active tectonics in the Central Chilean Andes: 3D tomography based on the aftershock sequence of the 28 August 2004 shallow crustal earthquake Diana Comte 1 , Marcelo Farías 1,2 , Reynaldo Charrier 2 , & Andrea González 2 1 Dept. Geofísica, Universidad de Chile, Santiago, Chile (dcomte@dgf.uchile.cl ) 2 Dept. Geología, Universidad de Chile, Santiago, Chile (mfarias@dgf.uchile.cl ) KEYWORDS : crustal seismicity, tectonics, Central Andes, 3D tomography Introduction Most of the seismological research in the Andes has been mainly oriented to the detection and understanding of the seismicity associated with the large thrust-fault earthquakes that characterize the subduction environment that governs tectonics in this region. However, the growing number of stations in the permanent seismological network and the deployment in the last years of temporary networks in different regions of the country have allowed the detection of intense crustal seismicity beneath the Chilean forearc-arc region. For instance, the temporary seismic network deployed along the Las Leñas and Pangal river valleys (34°25'S), between January and May 2004, permitted to better constrain the abundant shallow intra-continental seismicity previously detected in that region. Although most of the seismicity is randomly distributed in the region, several microearthquakes occur along the trace of the major El Diablo - El Fierro fault-system. This fault, recognized between 33°30' and 35°15’S, is located at or close to the eastern contact between Mesozoic and Cenozoic deposits in the Principal Cordillera and, locally, below active volcanoes , and is considered to have participated in the development (extension) and tectonic inversion of a widely extended (>600 km long) Cenozoic extensional basin along the Principal Cordillera. The associated seismic activity implies that this structure is still active and participates in the present-day adjustments of the Andean crust. Further south, at 35°S, a Mw=6.5 strike-slip shallow (<10 km) earthquake occurred on August 28, 2004, generating moderate damage in the region, reaching a maximum intensity VI MM. The Seismological Service of the University of Chile deployed a local network to monitoring the aftershock sequence. A 3D detailed Vp and Vs velocities determination was obtained along the aftershock area of the 2004 earthquake; results show an essentially NS distribution reaching depths lower than 15 km. This behaviour is in agreement with that observed further north, in the Las Leñas - Pangal region. The 2004 shallow earthquake is the second one recorded by local networks in Chile, the previous one occurred in the northern Chile forearc in 2001 (Mw=6.2) (Farías et al., 2005). The 2004 shallow earthquake is similar to the major intraplate Las Melosas earthquake (Mw=6.9) occurred on September 4, 1958, possibly associated with the El Diablo - El Fierro fault- system. The occurrence of the 2004 earthquake offers the possibility to analyze this seismicity from a seismotectonic point of view, in order to understand the present-day crustal adjustments. Retour au Sommaire Sommaire / Contents / Contenido