Source Parameters of the Mw 5.7 Pica Crustal Earthquake in Northern Chile

Abstract On 10 September, 2008, an Mw 5.7 earthquake occurred under the Central Valley of northern Chile near the town of Pica at a depth of ∼33 km within the continental crust of the South America plate. We find this earthquake to be a high stress-drop, reverse-oblique event that generated unusually high ground accelerations of up to 0.67g. Overall, its observed ground motion intensities are considerably larger than those predicted by ground motion models, particularly at short periods. The source properties inferred through waveform modeling indicate reverse-oblique fault motion on a ∼75 km2 plane dipping to the northeast, which is corroborated by the located aftershock distribution. Stress-drop values of the mainshock and larger aftershocks were estimated through S-wave spectrum modeling, with values up to ∼250 MPa for the mainshock. The event occurred in a cold section of the continental crust under the Central Valley, and its fault kinematics and orientation are consistent with the dominant style of faulting and stress field under the neighboring Coastal Cordillera. Although our recurrence analysis shows that crustal events in the region occur at a lower rate than interplate and inslab events, crustal events of similar or higher magnitude than the Pica earthquake have occurred, on average, approximately once every three years in northern Chile, which could pose an important hazard to nearby populations or critical infrastructure.