Fine Structure of the Subducting Slab and the 2022 M 7.4 Fukushima-Oki Intraslab Earthquake

The 16 March 2022 M 7.4 Fukushima-Oki earthquake is the largest one among forearc intraslab earthquakes in Japan since 2000. These subcoast events can cause severe damage to the local society because of their proximity to inhabited areas. However, their generating mechanism is still not clear. Here, we present 3D high-resolution seismic tomography of the source zone of four large intraslab events (M ≥ 7.0) during 2003-2022 in northeast Japan, which is obtained by inverting high-quality arrival-time data recorded at both onshore and offshore seismic stations. Aftershocks of the subcoast intraslab earthquakes are mainly distributed in gaps of high-velocity bodies with high Poisson's ratio and at the upper ∼ 20 km depth of the subducting Pacific slab. Our results indicate that the four large intraslab events were caused by rupturing of buried hydrated faults that formed at the outer rise and dehydration embrittlement on the fault planes.