Local earthquake tomography of shallow subduction in north Chile: A combined onshore and offshore study

Selected travel time data from the aftershock series of the great Antofagasta earthquake (M-W=8.0) have been inverted simultaneously for both hypocenter locations and three-dimensional V-P and V-P/V-S structure. The data were collected with a dense 44-station seismic network including ocean bottom hydrophones. We performed a series of inversions with increasing complexity: I-D, 2-D, and 3-D. This approach was chosen to account for the heterogeneous seismicity distribution and to obtain a smooth regional model in areas of low resolution. Special efforts were made to assess the solution quality including standard resolution estimates and tests with synthetic travel times. The subducted plate is imaged between 20 and 50 km in depth as an eastward dipping high-V-P feature. High V-P/V-S ratios within the oceanic crust possibly indicate elevated fluid content. Underplating of material eroded close to the trench is found beneath the Mejillones Peninsula. The lower crust of the overlying plate is characterized by an average V-P of 6.8-8.9 km/s and an average to low V-P/V-S ratio. Large areas of anomalously high V-P are found in the lower crust south of the city of Antofagasta; they are interpreted as remants of magmatic intrusions. A zone of high V-P/V-S ratios is found within the rupture area of the Antofagasta main shock, just above the subducted slab. Its location within the region of highest stress release from the main Shock suggests that the main shock rupture causes the high V-P/V-S ratio. The high V-P/V-S ratio could indicate postseismic fluid migration from the subducted oceanic crust into the overlying lower crust.