Tilt and Strain Change During the Explosion at Minami-dake, Sakurajima, on November 13, 2017

Abstract We herein proposed an alternative model for deformation caused by each eruption at Sakurajima, which have been previously interpreted as being due to a Mogi-type source beneath Minami-dake. On November 13, 2017, a large explosion with a plume height of 4,200 m occurred at Minami-dake. During the three minutes following the onset of the explosion (November 13, 2017, 22:07–22:10 (Japan standard time (UTC+9); the same hereinafter), phase 1, a large strain change was detected at the Arimura observation tunnel (AVOT) located approximately 2.1 km southeast from the Minami-dake crater. After the climax of the explosion (November 13, 2017, 22:10–24:00), phase 2, a large deflation was detected at every monitoring stations due to the continuous Strombolian eruption. Subsidence toward Minami-dake was detected at five out of six stations whereas subsidence toward the north of Sakurajima was detected at the newly installed Komen observation tunnel (KMT), located approximately 4.0 km northeast from the Minami-dake crater. The large strain change at AVOT during phase 1 can be explained by a very shallow deflation source beneath Minami-dake at 0.1 km below sea level (bsl). For phase 2, a deeper source beneath Minami-dake at a depth of 3.3 km bsl deflated in addition to the shallow source beneath Minami-dake, which turned inflationary after the deflation obtained during phase 1. However, this model cannot explain the tilt change of KMT. Adding a spherical deflation source beneath Kita-dake at a depth of 3.2 km bsl can be explain the tilt and strain change at KMT and the other stations. The Kita-dake source was also found in a previous study of long-term ground deformation events. Not only the deeper Minami-dake source M D but also the Kita-dake source deflated due to the Minami-dake explosion.