Volcano-tectonic seismicity at Unzen Volcano, Japan, 1985–1999

The eruption of Unzen Volcano in southwestern Japan began on November 17, 1990, and an extrusion of a dacite lava dome lasted from May 20, 1991, until early February 1995. The seismic network operated by the Shimabara Earthquake and Volcano Observatory of Kyushu University (SEVO) located about 4000 volcano-tectonic earthquakes during the period from 1985 to 1999. The seismicity was greatest in the period between one year before the eruption and the start of the lava dome extrusion. The hypocenters were mostly distributed within a 12×20 km area on the western side of the volcano. An inclined seismic–aseismic boundary shallows eastward to the summit, probably reflecting the thermal structure beneath the volcano. Within 4 km west from the summit, however, the seismicity is very low. It was suggested that the high-temperature, ductile and low-Q bodies exist there. We obtained focal mechanism solutions for about 90 earthquakes. Among them, orientations of P-axes for the earthquakes occurring on the upper side of the inclined seismic–aseismic boundary suggest a pressure source located below this boundary. Although most of the focal mechanisms have T-axes nearly horizontal in the N–S direction, consistent with the N–S extensional tectonic stress, several earthquakes whose focal mechanisms are inconsistent with the tectonic stress were observed within a horizontal distance of 5 km from the summit. This is because the pressurization of volcanic gas or magma beneath the summit produced a strong local stress in the summit surroundings.