Steep intraplate 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

Abstract Western Kanagawa, located in eastern Japan, has repeatedly experienced M-8 class megathrust earthquakes, referred to as Kanto earthquakes, within a tectonic setting characterized by the NNW-ward subduction of Philippine Sea plate. 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 destructive earthquake generation in this area, it is imperative to determine the precise location and geometry of this Mw 5.0 earthquake sequence: whether it occurred on the plate interface, in the continental arc, or within the subducting plate. To tackle this question, we conducted following analyses: (1) machine-learning-based event detection using PhaseNet and GaMMA, (2) hypocenter relocation with phase picks and waveform cross-correlations, and (3) hypocenter clustering for identifying rectangular fault planes with hypocenter locations and their point-cloud normal vectors. Our analysis revealed the presence of three fault planes, including two parallel steep SSE-dipping planes consistent with the steeper nodal plane of the focal mechanisms for the mainshock and its largest aftershock. These planes lie approximately 1.5 km beneath the plate interface model, indicating intraplate reverse faulting within the subducting Izu–Bonin arc as this earthquake sequence, as opposed to interplate thrusting or reverse faulting in the continental arc. The extracted fault planes with varying strikes and high non-double-couple components, both indicative of complex faulting, suggest that loading stress is more complex than that of simple subduction. The focal area is in the collision–subduction transition: collisional compression and weakly coupled plate interface. Complex stress conditions, including both plate coupling and arc–arc collision, may have contributed to the earthquake generation. These findings would help assess the future occurrence of Kanto earthquakes and comprehend seismotectonics in the Izu collision zone. Graphical Abstract