Cutoff depths of shallow earthquakes beneath the active volcanoes in Japan and their implication for heat source of magma reservoirs

Abstract Seismic tomography, electromagnetic surveys, and ground deformation analyses have been conducted to locate magma reservoirs and to understand the magma supply system beneath active volcanoes. Magma reservoirs are heat sources; therefore, high temperatures in the crust beneath the active volcanoes are expected to prevent brittle failures in shallow depths. The present study systematically examines the cutoff depths of shallow earthquakes around 61 active volcanoes in Japan, defined as volcanoes that have erupted within 10,000 years and/or currently show significant thermal activities, using a unified earthquake catalog. The results show that approximately 95% of the active volcanoes indicate shallower cutoff depths beneath the summits of active volcanoes compared with the surrounding crust. Similar characteristics are recognized for calderas. These results strongly suggest that active volcanoes and calderas are affected by the heat sources of the magma reservoirs. However, Quaternary volcanoes that are not classified as the active volcanoes show deeper cutoff depths, suggesting weak thermal activity. To locate the magma reservoirs, spatial changes in the cutoff depths of shallow earthquakes around these volcanoes are compared to the temperature distributions in the crust, which are numerically calculated from the thermal diffusion process by a heat source. These systematic investigations of the cutoff depths of shallow earthquakes around active volcanoes can be used to evaluate and locate magma reservoirs. Graphical Abstract