2019 M6.7 Yamagata-Oki earthquake in the stress shadow of 2011 Tohoku-Oki earthquake: Was it caused by the reduction in fault strength?

Earthquake occurrence in the stress shadow provides a unique opportunity for extracting information about the physical processes behind earthquakes because it highlights processes other than the ambient stress change in earthquake generation. In this study, we examined the fault structure and the spatiotemporal distribution of the aftershocks of the 2019 M6.7 Yamagata-Oki earthquake, which occurred in the stress shadow of the 2011 M9.0 Tohoku-Oki earthquake, to better understand the earthquake generation mechanism. The detection and hypocenter relocation of the aftershocks led to the delineation of three planar earthquake structures consitent with their focal mechanisms. The results suggest that individual aftershocks were caused by a slip on these macroscopic planar structures. Aftershock hypocenters rapidly migrated upward from the deeper part of the major plane (fault), similar to the recent earthquake swarm sequences following the Tohoku-Oki earthquake in the upper plate. Moreover, we investigated the temporal evolution of the surface strain rate distribution in the source region using GNSS data. The east-west contraction strain rate in the source region of the Yamagata-Oki earthquake, with an E-W compressional reverse fault mechanism, changed to an E-W extension as a result of the Tohoku-Oki earthquake, and it continued until the occurrence of the Yamagata-Oki earthquake. The upward hypocenter migrations, together with the earthquake occurrence in the stress shadow and in the E-W extension strain rate field, suggest that a reduction in the fault strength due to uprising fluids contributed to the occurrence of this earthquake sequence. Localized aseismic deformations, such as aseismic creeps, beneath the fault zone may also have contributed to the earthquake occurrence. The results support the hypothesis that aseismic processes in the deeper part of the fault play crucial roles in the occurrence of shallow intraplate earthquakes.