2001年箱根群発地震活動に伴った傾斜変動と圧力源の時間変化

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Remarkable changes were observed by tiltmeters installed in the Hakone caldera during the 2001 intense swarm activity. Using the records of the tiltmeters as well as the GPS data acquired by the Geographical Survey Institute (GSI), we investigate the pressure source which produced the crustal deformation. We divide the period from May 23 to September 9 into five stages based on the changes in the direction and the rate of tilt at Komagatake Station. The overall model during the whole period is composed of a Mogi-source at a depth of 7km (volumetric expansion: 7.1×10^6m^3) and two shallow open cracks (depth range 0-0.2km; volumetric expansion 0.15-0.51×10^6m^3). The tilt changes in each stage, including that in the first stage at the time when the swarm activity had not yet commenced, is well explained by the expansion of the same cracks, which is considered to show the validity of our pressure models. The strike of the crack in owakidani and that near Komagatake are WNW-ESE and NW-SE, respectively. The S-wave splitting analysis (Honda and Tanada, 2006) indicates existence of cracks in those directions around the source region, and many fissures in the NW-SE direction are observed on the surface of the field (Kobayashi et al., 2006). However, local earthquakes are not distributed along the direction, but the epicentral area extends from south to north as a whole. This feature differs from that of the swarm activity occurring in the region east-off Izu Peninsula, in which extension of hypocentral distribution is estimated to correspond to the zone of dike intrusion. We consider that up-rising steam or volcanic gases compelled to open the pre-existing cracks to produce the crustal deformation in the 2001 Hakone swarm activity. It is plausible that the two-crack pressure source of our model represents a set of many small cracks or the largest one in the source region.
2009-10-31

2001年箱根群発地震活動に伴った傾斜変動と圧力源の時間変化

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