The magma chamber associated with the eruption of Miyakejima Volcano, Japan, since 2000, inferred from crustal deformation data (1983-2004).

Miyakejima Island, an active basaltic stratovolcano lying 170 km to the south of Tokyo, Japan, resumed its volcanic activity in late June 2000 after a quiescence period for 17 years since the last eruption in 1983. The activity changes its eruptive style and still continues with a significant volcanic gas emission from the summit caldera, which has forced the inhabitants of Miyakejima to evacuate from the island. In order to understand the magmatic activity and the gas emission, it is crucial to reveal the magma supply system beneath the island. For this purpose we elucidated locations and geometries of a magma chamber, intruded dikes, and a source related with the gas emission by using the crustal deformation data including GPS, tilt changes and leveling through the period after the 1983 eruption up to now. Following dike intrusions beneath the island at the initial stage of the activity (June 26 - 27, 2000), the observed crustal deformation showed a rapid contraction of the island due to a subsurface magma migration from a magma chamber to the west off the coast. To estimate the location and configuration of the magma chamber, we compared the deformation with various source models. We found that a source model consisting of a spherical deflation source about 8 km deep in the southwestern part of the island and a vertical dike-shaped deflation source on the spherical source is most appropriate for the crustal deformation (June 28 - mid September, 2000). The total amount of the contraction is ~0.4 km³; if it is added to the amount of collapsed summit, which is considered to have penetrated into the chamber [Geshi et al., 2002], the amount of withdrew magma is estimated to be ~1.0 km³. The amount is consistent with the amount of intruded magma at the west off the coast of Miyakejima [Nishimura et al., 2001]. An inflated source model with a similar configuration can explain the crustal deformation observed from 1983 to before the activity of 2000, suggesting the source had been accumulating magma before the activity. We infer that the source is the magma chamber of Miyakejima and the chamber consists of two parts: a shallow dike-shaped chamber ~4 km deep and a deep-seated spherical chamber ~9 km deep. Ueda et al. (2004) shows that the crustal deformation at the initial stage of the activity (June 26 - 27, 2000) can be interpreted by three dike-shaped inflation sources and one dike-shaped deflation source. They concluded that there existed two phases of magma movement; a small intrusion at a depth range of 1-3 km beneath the southwestern flank of the island from 18h30m to 21h on June 26, a large intrusion beneath the west coast, and deflation of a dike-shaped deflation source beneath the small intrusion, both of which started simultaneously at about 21h on June 26. The magma chamber model suggests that the former dike intruded from the upper bound of the dike-shaped portion of the magma chamber, and the latter intruded laterally from the chamber. Crustal deformation after October 2000 can be approximately explained by a spherical deflation source, about 3 km deep, beneath the summit. The source is located near the upper bound of the chamber, where we expect a conduit that connects the chamber with the gas vent on the summit. Since it deflates during the significant gas emission, the deflation source probably relate with the magma degassing process in the conduit.