Constraints on the source fault of the 2024 Mw 6.2 earthquake off the western Noto Peninsula, Japan, based on ocean bottom seismometer observations

Abstract

The Mw 7.5 earthquake that struck the northern Noto Peninsula, central Japan, on 1 January 2024 markedly perturbed the regional stress field. Subsequently, on 26 November 2024, an Mw 6.2 thrust earthquake occurred offshore of the western Noto Peninsula. Although multiple submarine active faults have been mapped in this region, the specific geometry of the source fault has remained poorly constrained. To address this issue, we conducted temporary seismic observations using ocean-bottom seismometers and onshore stations during July and August 2025. A high-precision hypocenter relocation revealed a clear westward-dipping aftershock distribution with a dip of approximately 45° at depths of 5–10 km. This seismogenic plane coincides with the downward projection of the steeper (~ 60°), shallow segment of the Hakui-oki West Fault (HWF) previously imaged by seismic reflection surveys. Our results provide observational evidence that the Mw 6.2 earthquake most likely ruptured the deeper portion of the HWF, and the discrepancy in dip angles suggests that the fault may exhibit a listric geometry. This interpretation is consistent with reverse reactivation of an inherited normal-fault structure formed during Miocene rifting. These findings highlight the importance of incorporating depth-dependent listric geometries into offshore fault models to better represent tectonic deformation in inversion settings.