Coseismic crustal seismic velocity changes associated with the 2024 Mw 7.5 Noto earthquake, Japan

The 2024 Mw 7.5 Noto earthquake, Japan, was preceded by an intense seismic swarm thought to be driven by upward fluid migration. Crustal seismic velocities vary with external perturbations caused by earthquakes, and the presence of pressurized fluids in the crust amplifies the resulting coseismic velocity change. Hence, we characterize subsurface fluid by measuring the coseismic velocity change associated with the 2024 mainshock. For this purpose, we perform multi-frequency-band ambient noise seismic interferometry using data from permanent and temporary seismic stations. Significant coseismic velocity drops are observed, with an average decrease of about 0.5% inside the Noto peninsula, reaching 0.6 to 0.8% in the regions near the coseismic slip peaks. The observed velocity drops inside the peninsula correlate well with the modeled static-stress-change-induced velocity drops and Peak Ground Velocity (PGV) and Acceleration (PGA) as proxies of dynamic stress change. However, their respective contribution to the observed coseismic velocity drop remains unclear because of the similarities in their spatial pattern. Outside the Noto Peninsula, the observed velocity drops average around 0.1%, which is predominantly attributed to dynamic stress changes from passing waves because modeled static stress changes are negligible at these great distances. Although the addition of temporary stations significantly increases the resolution of the velocity drop measurements in the pre-mainshock swarm zone, our results exhibit no large velocity drop anomaly in this region, suggesting that the amount of pressurized fluids in the shallow crust down to ∼ 2.5 km depth is not anomalously large. This implies that the upward migration of fluids preceding the mainshock is likely confined to greater depths.