Analysis of the Aftershocks of the Mw 6.7 19 January 2019 Coquimbo-La Serena Earthquake: Implications for Wedge Dynamics and Intraplate Deformation

Abstract:

In December 2018, a network of 88 seismic stations was deployed in northern Chile to investigate the boundary between the M8.4 2015 Illapel and M8.5 1922 Copiapo earthquakes. On 19 January, 2019 the M6.7 Coquimbo-La Serena earthquake occurred within the southern part of this network, which recorded an abundance of aftershock activity. This serendipitous cornucopia of data allowed us to investigate the unusual nature of this event and the subsurface structure of the wedge above the Nazca plate. Body wave arrival times and first motion polarities were estimated using the Regressive ESTimator (REST) (Comte et al., 2019). Application of quality threshold filters resulted in a reference data set of 10270 events with 197814 P and 194920 S wave arrival times. Following the procedures of Comte et al (2019) we generated a 3D model of Vp, Vs, and Vp/Vs with which we relocate hypocenters and determine focal mechanisms. Combining these results with Harvard and USGS CMT solutions, we find that the ambient seismicity in the subducted Nazca plate occurs in a well-defined double seismic zone separated by about 20 km to at least 90 km depth. The main shock, and most of its aftershocks, are located in a small (~10 km scale) volume in the middle of the double seismic zone at about 63 km depth. The main shock mechanism is normal but with a significant (40%-60%) non-double couple component. Focal mechanisms of other events are predominantly thrust in the upper zone, normal in the lower zone, and normal in the main aftershock cluster between the zones. The upper seismic zone is associated with strong gradients in Vp and Vs and high Vp/Vs (~1.85). The aftershock region has unusually low Vp (< 7 km/s), Vs (< 4 km/s), and Vp/Vs (~1.65). These low wavespeeds extend up the slab to depths as shallow as 30 km. We infer that the upper seismic zone is hydrated and close to the top of the subducted Nazca plate, whereas the aftershock region between the double seismic zones is dehydrated, with the main shock caused by dehydration reactions resulting in volumetric collapse. Nakajima et al (2011) found similar intraplate low wavespeeds associated with an analogous M7.1 aftershock of the 2011 Tohoku earthquake. Wavespeeds above the Nazca plate are similar to those determined by Comte et al (2019) for Illapel, and we infer that the same type of wedge mechanics likely extends to the north.