Rupture of solidified ancient magma that impeded preceding swarm migrations led to the 2024 Noto earthquake

An intense earthquake swarm lasting ~3 years ultimately led to the 2024 <i>M</i>_w (moment magnitude) 7.5 earthquake in the Noto Peninsula, Japan. The spatial complexities in swarm evolution and earthquake rupture have been observed, but the factors controlling these complexities remain unclear. Using high-resolution subsurface imaging with dense seismic observation, we identified a high-velocity body collocated with the major slip zone, which the preceding swarm avoided. The spatial distribution and absolute velocity of the high-velocity body and the adjacent ring-shaped swarm cluster indicate that the high-velocity body is a solidified ancient magma. It initially acted as an impermeable barrier to the fluid migrations that triggered swarm earthquakes, eventually rupturing as an asperity of the 2024 earthquake. Our observation suggests that the heterogeneity in fault zone permeability, originating from ancient volcanic activity (>15 million years ago), controlled the present-day swarm evolution and the large earthquake generation.