The January 2019 (Mw 6.7) Coquimbo Earthquake: Insights from a Seismic Sequence within the Nazca Plate

Abstract

On 20 January 2019, the Chilean cities of Coquimbo and La Serena were shaken by an intraplate earthquake of Mw 6.7 located at 70 km depth. High peak ground acceleration values and macroseismic intensities were reported. The mainshock was followed by more than 150 aftershocks higher than ML 2.5, a seismic sequence completely recorded by local stations. Using a 3D velocity model, we precisely located the seismicity. The aftershocks were located some 20 km above and shifted from the mainshock but still inside the Nazca plate. We also performed moment tensor inversion of nine events obtaining mostly normal-fault focal mechanisms and kinematic inversions using the elliptical-patch approach. We found that the mainshock broke an approximated zone of 6 km by 8 km, propagated upward in the northwest direction and away from the aftershock area. The rupture inverted from accelerograms containing up to 1 Hz was characterized with a high stress drop of 7.51 MPa and a short seismic source time function of only 3 s duration. Supplemental Content:Tables and figures describing some characteristics of the Mw 6.7 Coquimbo earthquake and the Chilean subduction zone.