Temperature- and fluid-controlled seismicity in the Beppu graben, Kyushu, Japan

The Beppu graben is located at the eastern end of the Beppu–Shimabara rift zone in the southwest Japan subduction zone. The focal distribution of the earthquakes that have occurred in this graben shows that the cut-off depth of the earthquakes increases with the distance from the Mt. Tsurumi volcano, which is considered to reflect the temperature structure affected by a high-temperature magma body under the volcano. In this study, the temperature structure in this graben is estimated, which shows that the cut-off depth coincides with the isotherm of 350–400 °C and that the depth and diameter of the magma body are 7–9 km and 1–2 km, respectively. The obtained cut-off temperature agrees with that of the brittle–ductile transition of the crust, and the earthquakes are probably triggered by the fluids migrated from the magma body into the brittle crust under active stretching. There is an active seismic cluster in the eastern flank of the volcano near the southern border fault of this graben; the focal distribution of the seismic cluster shows a northward-dipping planar shape. In the beginning of the seismic burst in the seismic cluster, the hypocenters migrated eastward, suggesting the infiltration of fluids through a fault. The seismic cluster is located in an area with a higher subsidence rate in this graben. The seismicity was probably caused by the incursion of magma-derived fluids into an active fault with a high dislocation rate.