Upward Earthquake Swarm Migration in the Northeastern Noto Peninsula, Japan, Initiated From a Deep Ring‐Shaped Cluster: Possibility of Fluid Leakage From a Hidden Magma System.

This study describes an ongoing intense earthquake swarm in the crust of the northeastern Noto Peninsula, Japan, that began around the end of 2019. Fluid movement related to volcanic activity is often involved in earthquake swarms in the crust. However, no volcanic activity has occurred in this region since the Middle Miocene (15.6 Ma). This study investigates the cause of this earthquake swarm based on the spatiotemporal evolution of earthquake hypocenters and seismic reflectors. The hypocenter relocation of 10,940 earthquakes (M > 1) reveals that they are all crustal and migrated upward, activating a complex network of faults at depths shallower than 20 km. The initiation of this earthquake swarm occurred at a locally deep depth (z = 17 km), and the local hypocenter distribution shows a characteristic circular pattern. We find a distinctive S‐wave reflector in the immediate vicinity. A low‐velocity anomaly exists below the reflector, and a high helium isotope ratio and a low‐gravity anomaly were observed at the surface. These observations suggest that the current seismicity is associated with fluids released by ancient or possibly unrecognized modern magmatic activity, although no volcanic activity has been documented in this area for 15 million years. Significant crustal deformation was observed during this swarm and is probably related to the fluid movement and aseismic deformation that contributed to this earthquake swarm. The strongest M5.4 earthquake (as of October 2022) occurred near the migration front on the largest planar structure, leaving the shallow extension unruptured. Plain Language Summary: An intense earthquake swarm is currently occurring in the crust of the northeastern Noto Peninsula, Japan. Fluid movement related to volcanic activity is often involved in earthquake swarms in the crust, but no volcanic activity has occurred in this region since the middle Miocene (15.6 Ma). We investigate the cause of this earthquake swarm using precisely determined earthquake locations and seismic reflectors. We find that the earthquakes moved from deep to shallow depths via many planes, similar to earthquake swarms near volcanoes. The strongest M5.4 earthquake occurred near the migration front on the largest planar structure. Further earthquakes may be possible in the shallow part of this fault. We find a distinctive S‐wave reflector, suggesting a fluid source, in the immediate vicinity of the initiation point of this swarm. The local hypocenters show a circular pattern similar to the ring fault that forms above the magma reservoir. These observations may indicate that the current seismic activity is being impacted by fluids related to ancient or new hidden magmatic activity. The present results suggest that hidden magma‐induced structures and fluids can generate earthquakes even in areas where no volcanic activity has been observed for over 10 million years. Key Points: An intense seismic swarm continues in a non‐volcanic region, with upward earthquake migration through a complex network of faultsA deep S‐wave reflector located near the seismicity initiation point and above the low‐velocity anomaly may represent a fluid sourceThe significant surface deformation observed during this swarm was caused by aseismic deformation activated by fluid migration [ABSTRACT FROM AUTHOR]