[Abstract]

An earthquake swarm was observed near Hachijo Island, Japan, between 13 August and September 2002. Continuous GPS measurements at four sites on Hachijo Island detected the displacements accompanying the earthquake swarm. Eastward displacements of 2-6 cm were detected during the period of 13-16 August, after which the displacements tended toward the northwest. A dike intrusion model is proposed based on the observed ground deformation. The putative intrusion is located at 3.3 km depth under Hachijo-Fuji, and is a 2.2-m wide tensile dike of depth 3.3 km and is extended to 4.5 km deep. The dike intrusion model estimated from the ground deformation is consistent with a location and a dimension of the model based on the low-frequency earthquake mechanism proposed by Kumagai et al. (2003). We discuss three models of a dike intrusion, a point deflation, and a dike deflation from the eastward or southeastward displacements detected in the period of 16-22 August. From the ground deformation it is difficult to decide the priority of the models. According to the depth and volume changes of the estimated pressure source of three models, it is suggested that the deeper part of the first inflated dike turns to deflation on August 16.

Key words:

ground deformation, Hachijo Volcano, GPS measurements, dike intrusion model

e-mail:

kimata@seis.nagoya-u.ac.j