Intraslab reverse faulting adjacent to the hypocenter of the 1923 Kanto earthquake: The Mw 5.0 western Kanagawa earthquake in eastern Japan on 9 August 2024

Western Kanagawa in eastern Japan is a region under complex tectonics with seismic activity. The northward subduction of Philippine Sea plate has historically caused M-8 class megathrust earthquakes, known as Kanto earthquakes. On 9 August 2024, an Mw 5.0 reverse-fault earthquake occurred near the epicenter of the latest Kanto earthquake in 1923. For future evaluation of the earthquake generation in this area, it is imperative to determine the precise location of this earthquake’s faulting: whether it occurred on the plate interface, an active fault, or within the slab. To tackle this question, we conducted a machine-learning-based workflow of (1) phase picking using PhaseNetWC, (2) hypocenter relocation with phase picks and waveform cross-correlations, and (3) extraction of rectangular fault planes through hypocenter clustering of positions and point-cloud normal vectors. Our result exhibited five fault planes. We obtained a significant plane dipping steeply to the south, consistent with the steeper nodal plane of the mainshock focal mechanism. This extracted plane, being 2 km deeper than the slab surface, demonstrates that the Mw 5.0 earthquake occurred as an intraslab earthquake on a steep reverse fault, as opposed to thrusting on the PHS surface or reverse faulting on an active fault in the forearc crust. Given the high stress rate on the plate interface in this area, complex stress states may have given rise to the occurrence of the steep reverse intraslab earthquake beneath highly coupled plate interface.