Seismicity and velocity structure of the Southern Chilean subduction zone (between 37° and 39°S) revealed by the TIPTEQ local seismic network

The largest earthquakes are being generated at convergent plate boundaries, where oceanic plates subduct beneath other tectonic plates. Understanding the factors leading to these earthquakes in the coupling zone of convergent margins and their interrelation with surface deformation are the main aims of the international and interdisciplinary research initiative TIPTEQ (From The Incoming Plate To megaThrust EarthQuake Processes). High resolution images of the seismogenic zone and the forearc structure form the base for identifying the processes involved. Within this project a large temporary seismological network was installed in southern Chile between November 2004 and October 2005, covering the forearc between 37° and 39°~S. In this region the Mw=9.5 1960 Chile earthquake, the worldwide largest instrumentally ever recorded earthquake, nucleated. The network consisted of up to 120 digitally recording and continuously running seismic stations equipped with short period sensors. The onshore network was complemented by 10 ocean bottom seismometers/hydrophones. In total we observed 544 local earthquakes (with local magnitudes between 1 and 5.2) and numerous regional and teleseismic events. A high-quality subset of 213 local earthquakes with 14.754 P and S onset times (occurring in the interseismic cycle) was used for a simultaneous inversion for the 1-D velocity model and precise earthquake locations. By relocating artificial shots we estimated the accuracy of the earthquake hypocenter to about 1~km horizontally and 500~m vertically. In a second step, locations and 1-D model were used for the simultaneous inversion for the 3-D velocity structure and hypocenters (tomography). The events are found in the crustal forearc, the downgoing plate, and at the interface between both plates. Crustal events along NW and NNW striking, deep-reaching faults reflect the interseismic transpressional deformation of the forearc crust due to the subduction of the Nazca plate. The transverse Lanalhue fault zone is seismically active and seems to accomplish differential lateral stresses between subduction zone segments. Many events situated in an internally structured, planar seismicity patch at 20 to 40~km depth near the coast indicate a stress concentration at the plate's interface (seismogenic zone) at 38°~S which might in part be induced by the fragmented forearc structure.