Magmatic system of the Kikai submarine caldera, SW Japan, imaged by passive seismic tomography

The eruption of the Kikai caldera, located south of Kyushu in southwestern Japan, at 7.3 ka, was the largest volcanic eruption globally during the Holocene. Because of its offshore nature and sparse observational networks, the location and spatial extent of its current magma supply system remains unknown. We performed seismic tomography using data obtained from amphibious seismic observations to clarify the current magma supply system of the Kikai caldera based on its seismic velocity structure. A low-velocity zone was imaged in the crust just below the region inside the caldera rim within a 0–30 km depth. At a depth of 10 km, the P-wave velocity reduction rate was significant and the Vp/Vs ratio was low, suggesting the dominant presence of volatiles as supercritical fluid upwelling from the deep magma reservoir. Low S-velocity zones imaged at depths of 20–25 km were interpreted as deep magma reservoirs in the lower crust. Another low S-velocity region imaged at a depth of 15–20 km, approximately 20 km northeast of the caldera center, suggested the presence of another deep magma reservoir. The existence of multiple deep magmatic reservoirs is consistent with the interpretation of petrological studies of past eruptive activities. Furthermore, a low S-velocity region was imaged in the mantle wedge just above the area where intraslab earthquakes were active. Dehydration embrittlement in the slab mantle causes intraslab earthquakes and the generated fluids may flow from the slab into the mantle wedge, thereby forming a partial melting zone.