Shallow crustal rupture in a major MW 7.5 earthquake above a deep crustal seismic swarm along the Noto Peninsula in western Japan

A damaging M_W 7.5 earthquake struck the western coast of Japan along the Noto Peninsula on January 1, 2024. The initiation of large shallow earthquakes along the Noto Peninsula, particularly above deeper long-duration patchy seismic swarms, presents an unusual seismic phenomenon that warrants in-depth investigation of their interactions. The 2024 earthquake nucleated with an initial low average rupture velocity of 0.5–1.0 km s^-1 near the up-dip end of a long-lasting seismic swarm that commenced in November 2020. Analysis of dense seismic, geodetic, and tsunami observations provides good resolution of large shallow slip in the crust below the peninsula and extending offshore to the northeast, revealing a heterogeneous slip distribution characterized by bilateral two-stage rupture expansion during the faulting. Up to 8 m of slip occurred in several patches along ∼150 km of the southeastward-dipping thrust fault, which extends to near the seafloor along the northwest side of the peninsula. Up to 5 m of uplift occurred along the peninsula's northwestern coast. Up-dip fluid migration appears to have weakened the shallow fault prior to failure and influenced the initial slow rupture expansion, highlighting the need to monitor the evolution of worldwide swarms.