三宅島2000年噴火前後の地殻変動と変動源の推定 : 1983年噴火後から2001年5月までの収縮・膨張源一

We analyzed crustal deformation data measured by leveling and continuous GPS on and around Miyakejima from 1983 to 2001. The observed deformation of Miyakejima shows its inflation before the 2000 eruption. It is modeled by an inflation (Mogi) source located at a depth of 9.5 km, 2 km southwest of the summit crater. The estimated volume increase of the inflation source was 0.2 km3 during the period between the 1983 eruption and the 2000 eruption. The shrinking and subsidence of Miyakejima after the eruption until the middle of 2001 characterize the deformation associated with the 2000 eruption except for two periods. One exception occurred just after the start of the earthquake swarm on June 26, 2000. The other occurred just after the summit collapse on July 8, 2000. The shrinking and the subsidence are explained by two deflation (Mogi) sources, that is, a shallow source at a depth of 2.9 km and a deep source at a depth of 9.5 km. Although both the deep and the shallow sources deflated from June to August 2000, only the shallow source deflated at a rate of 6.1 × 104m3/day after September 2000, when the summit crater started to emit volcanic gas. The observed expansion of Miyakejima just after the summit collapse is modeled by an inflation source at a depth of 1-2 km beneath the collapsed caldera. This inflation might be due to the filling of the vacant space beneath the summit with the material of the collapsed summit, as was suggested by FURUYA et al, (2001), filled