Seismic source analysis of two anomalous earthquakes in Northern Chile

Northern Chile is a seismically very active region driven by subduction of the Nazca plate beneath the South American plate. Anomalous focal mechanisms were reported by international agencies for two recent earthquakes with magnitudes larger than Mw 6. The September 11, 2020, Mw 6.2 Loa River earthquake occurred in the forearc under the Coastal Cordillera and had a strike-slip mechanism, while most common regional seismicity is characterised by NS-oriented, trench-parallel, thrust mechanisms, consistent with the subduction geometry. The June 3, 2020, Mw 6.8 San Pedro de Atacama earthquake occurred at intermediate depth with a normal faulting mechanism. In this case, the anomalous behavior involves the NW-SE striking of the focal mechanism, which deviate from the typical NS orientation in the region. In this study we reconstruct the rupture geometry for these two earthquakes by means of moment tensor inversion, rupture directivity analysis and aftershock distribution. Seismological results are discussed in the frame of the spatial and temporal distribution of local seismicity, which are analyzed using clustering techniques. Our results show that both earthquakes reveal the activation of seismogenic structures in the continental crust or within the subducting oceanic crust close to the subduction interface. The Loa River earthquake occurred at the base of the continental crust, at the contact with the slab. This finding highlights the role of seismicity associated with crustal faults in the South American plate, which poses a secondary seismic hazard for the region, alongside the seismicity directly associated with the subduction zone. The San Pedro de Atacama earthquake, in turn, occurred within the subducting slab. Its anomalous mechanism may indicate a local anomaly in the geometry of the subducting slab.