An image of a magma intrusion process inferred from precise hypocentral migrations of the earthquake swarm east of the Izu Peninsula

Abstract

Earthquake swarms in the area east of the Izu Peninsula, Central Japan have been active and have been repeated intermittently since 1978 after 40 years of quiescence. The activities were always accompanied by crustal deformations, which were well modelled by dyke intrusions. To study the process of a dyke intrusion, precise hypocentres of the latest activity occurring in 1998 are obtained in this paper using waveform similarity, by which new images of volcanic processes have been successfully acquired at several volcanoes. The relocated hypocentres are mainly aligned on a thin vertical plane with a circular shape at a depth of 3–7 km, and there is an aseismic area at the centre. The normal direction to the plane coincides well with the direction of tectonic extensional stress around the hypocentral area and matches theoretical models well. At the beginning of the activity, a small fraction of the events occurred at greater depth, where hypocentres align on a vertical line and migrate upward at a rate of 1 km h−1. The migration rate in this stage agrees well with a theoretical model of buoyancy-driven dyke propagation. After 1 day from the beginning, earthquakes began to occur around the aseismic area, and hypocentres spread on the thin circular plane described above. During this activity, the hypocentres seemed to migrate downwards and upwards from the centre. The earthquakes are caused by shear fracture at the tip of the dyke and the migration of hypocentres reflects that the dyke expands downward and upward from the neutrally buoyant layer that is located at the centre of the swarm. In this paper, we also propose a process of magma intrusion based on the precise migration of hypocentres, and show that the 1998 activity is composed of a few simple volcanic processes: magma rising by buoyancy, it staying at a neutral buoyancy point and spreading outward from the neutral point with elastic fracture by an inside excess magma pressure.