Velocity Structure and Seismotectonics prior to the 2010 Chile Earthquake (Mw 8.8) in the Maule Region from an Amphibious Seismological Network

The project SFB574 “Volatiles and Fluids in Subduction Zones” aims to better understand the processes and quantities involved in the exchange of fluids in convergent margins. The current phase concentrates on the accretionary margin of south-central Chile, a region that was affected after our data mining phase in 2008 by the rupture of the great 2010 Maule Earthquake (Mw 8.8). Within the project, an “amphibious” network of 15 ocean bottom seismometers and 22 land stations was operated from April to October 2008 along 350 km from the outer-rise to the magmatic arc. Additional readings from 11 permanent stations of the Chilean Seismological Service were included in the database improving the coverage to the north and south. One of the goals of the project is to gain a detailed image of the structure of the crust and upper mantle and the seismogenic zone by analyzing precise local earthquake locations and combined passive and active source seismic tomographic images. To achieve precise earthquake locations and to serve as an initial model for local earthquake tomography, we derived a P- and S-wave minimum-1D model using a very high-quality subset of 260 events with ~4000 P-wave and ~2000 S-wave arrivals. Most of the ~1500 earthquakes recorded over the six-month period were originated within the subducting slab down to ~140 km depth, with a higher concentration beneath the main cordillera, at depths of 80-100 km. We observe for the first time with a local network a double Benioff zone in this area. Fewer events were generated at the outer-rise, at depths of ~20-40 km, closely following the NE-SW trend of the oceanic plate faulting. The upper-plate seismicity occurred mainly beneath the main cordillera, and within the margin backstop offshore and SW of Pichilemu coastal town, where high activity was clustered in a ~1200 km2 area, between 10-30 km depth. The sparse interplate microseismicity nucleated from ~45 up to ~10 km depth and the upper limit is in good agreement with 2D refraction velocity modeling of the shallow part of the margin. The lack of seismicity supports the models reporting nearly complete interplate locking before the great earthquake. From the relocated catalog, a subset of events has been selected for the 3D inversion of seismic velocities. We review the tomographic results, together with source mechanisms, to present valuable insights into the structure and stress distribution in the Maule region before the 2010 earthquake.