Active Tectonic in the Southern Central Andes, a Recent Example: the 28 August, 2004 Shallow Mw=6.5 Earthquake

Seismic registration with the permanent local seismic network in central Chile and a 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 seisms occur along the trace of the major Chacayes-Yesillo fault system. This fault, recognized between 34° 45' and 34° 30'S, is located at or next to the eastern contact between Mesozoic and Cenozoic deposits in the Principal Cordillera, 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 apparently associated seismic activity suggests that this structure is still active and participates in the present-day adjustments of the Andean crust. Recorded data show the complexity of the fault system. Several, almost vertical lineaments with depth distributions shallower than 10 km can be observed. 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 location of this earthquake coincides with the trace of another major fault (El Fierro Fault), that also separates Mesozoic from Cenozoic units (basin bounding fault?). Temporary seismic stations were deployed to follow in detail the evolution of the abundant aftershocks. Analyses show an essentially NS distribution reaching depths lower than 15 km. This behaviour is in agreement with that observed to the north in 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). 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 Chacayes-Yesillo 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.