Long‐Living Earthquake Swarm and Intermittent Seismicity in the Northeastern Tip of the Noto Peninsula, Japan.

The factors controlling earthquake swarm duration are remain unclear, especially in the long‐living ones. A severe earthquake swarm struck the tip of the Noto peninsula, Japan. Ten M > 4.0 earthquakes occurred, and the sequence has continued more than 4 years. We investigated the spatiotemporal characteristics of the swarm using relocated hypocenters to elucidate the factors causing this long duration. The swarm consists of four seismic clusters—northern, northeastern, western, and southern—the latter of which began first. Diffusive hypocenter migrations were observed in the western, northern, and northeastern clusters with moderate to low diffusivities, implying a low‐permeability environment. Rapid diffusive migration associated with intermittent seismicity deep within the southern cluster suggests the presence of a highly pressurized fluid supply. We conclude that the nature of this fluid supply combined with intermittent seismicity from the southern cluster and a low‐permeability environment are the key causes of this long‐living swarm. Plain Language Summary: Earthquake swarms are sequences of several earthquakes occurring in a concentrated area over a given period. Unlike other major earthquakes, which have one main shock and several subsequent aftershocks, swarms lack a clear mainshock event. The causes of long‐lasting earthquake swarms are not sufficiently understood. In the northeastern tip of the Noto Peninsula in Japan, more than 20,000 earthquakes occurred between May 2018 and June 2022, including 10 events over magnitude 4.0. To understand the controlling factors of this long‐living earthquake swarm, we investigated the spatiotemporal characteristics of the swarm using high‐resolution relocated hypocenter locations. The hypocenters of the swarm are spatially separated in four clusters and initiated from the southern cluster. We also observed a diffusive pattern in hypocenter distribution, which is typical of earthquake swarms surrounding volcanoes or fluid injection wells, implying the existence of fluid as a driving factor of the swarm. In the southern cluster specifically, we found several intermittent seismic activities with rapid diffusive changes in hypocenter distribution, suggesting the presence of a highly pressurized, deep‐source fluid supply. The intermittent fluid supply from the southern cluster toward the others and the relatively low‐permeability environment are key factors in the longevity of this earthquake swarm. Key Points: An energic and long‐living earthquake swarm has been observed in the northeastern tip of the Noto peninsula, JapanObserved diffusive hypocenter migrations suggest that pore fluid pressure migration is a driving factor of the swarmIntermittent seismicity at the bottom of the initial cluster suggests that a geyser‐like fluid supply is a key factor in swarm longevity [ABSTRACT FROM AUTHOR]