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 Antofagas ta earthquake (M-W=8.0) have been inverted simultaneously for both hypocent er 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 h ydrophones. We performed a series of inversions with increasing complexity: I-D, 2-D, and 3-D. This approach was chosen to account for the heterogeneo us seismicity distribution and to obtain a smooth regional model in areas o f low resolution. Special efforts were made to assess the solution quality including standard resolution estimates and tests with synthetic travel tim es. The subducted plate is imaged between 20 and 50 km in depth as an eastw ard dipping high-V-P feature. High V-P/V-S ratios within the oceanic crust possibly indicate elevated fluid content. Underplating of material eroded c lose to the trench is found beneath the Mejillones Peninsula. The lower cru st 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 int erpreted as remants of magmatic intrusions. A zone of high V-P/V-S ratios i s found within the rupture area of the Antofagasta main shock, just above t he 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 migra tion from the subducted oceanic crust into the overlying lower crust.