Magma intrusion and discharge process at the initial stage of the 2000 activity of Miyakejima, Central Japan, inferred from tilt and GPS data

The 2000 eruptive activity of Miyakejima island began with an earthquake swarm and crustal deformation that was clearly observed by continuous observations of ground tilt and GPS on Miyakejima. Based on the crustal deformation data, we estimated the magma migration process at the initial stage (18:30 LT on 2000 June 26-06:00 LT on 2000 June 27) of the activity. The activity in 2000 has been characterized by a collapse-caldera formation and no fissure eruption on the flank of the island, which is markedly different from the style of the recent eruptions in 1940, 1962 and 1983. We constructed a source model that approximately explains the crustal deformation data. The model consists of four dykes, of which three are intruding dykes and the other is a contracting dyke. We can infer the magma migration process at the initial stage from the model as follows. At 18:30 LT on June 26, the magma began to ascend from a dyke-shaped magma chamber and intrude at the southwestern flank of Miyakejima. In the intrusion, the total volume of intruded magma was similar to 4 x 10(6) m(3). At around 21:00 LT, a relatively large volume of magma began to intrude as a dyke beneath the west coast of Miyakejima. The dyke then propagated laterally towards the northwest. The amount of magma intruded from 2 1: 00 LT to 0 1: 00 LT was similar to 40 x 10(6) m(3). This large intrusion caused a discharge of magma from the magma chamber. The northwestward propagation of the dyke and the contraction of the chamber continued thereafter. The discharge of magma from the chamber beneath Miyakejima probably starved the first intrusion that had been ascending towards the southwestern flank of the island, resulting in the collapse-caldera formation after the initial stage. The style of the 2000 eruptive activity relative to recent activity has primarily been changed by the subsurface discharge of magma towards the northwest.