Crustal structure beneath earthquake swarm in the Noto peninsula, Japan

Abstract

We carry out seismic tomography study to reveal three-dimensional (3D) seismic velocity structures in the Noto peninsula, Japan, where swarm-like seismic activity started in December 2020. The obtained results reveal a highly heterogeneous structure in the crust. The most striking feature is the existence of a low-velocity anomaly in the lower crust beneath the Noto earthquake swarm. Although the data set used in this study cannot resolve the upper mantle structure, previous regional tomographic studies suggest that a low-velocity anomaly exists at depths of 50–150 km around the Noto peninsula. We thus infer that fluids are supplied to the crustal level from the upper mantle low-velocity anomaly, which is interpreted as a fluid-rich region. The upward migration of the large volume of fluids over a very short time scale probably induces frequent aseismic slip along individual crustal faults, and numerous earthquakes are subsequently triggered as a result of stress loading by aseismic slip. Since major active faults exists at a shallower extension of the hypocenters of the Noto earthquake swarm, we consider that the Noto earthquake swarm occurs along pre-existing fault planes. Dense temporary seismic observations will highlight a smaller-scale (5–10 km) 3D seismic velocity model and finer hypocenter distribution, which provide additional information for better understanding of the generation mechanisms of the Noto earthquake swarm.