1990 年 99 巻 2 号 p. 153-165
The Off-East Izu Peninsula has been a realm of active swarm earthquakes since the end of 1978. Earthquake swarms with a magnitude of around 5 have occurred once or twice a year in a belt extending in a NW-SE direction. The trend of the seismic belt coincides with the regional pressure axis inferred from many active strike-slip faults. The seismic belt is situated in the Fuji Volcanic Belt that is characterized by active volcanoes and high geotherm. The purpose of this study is to detect a mode of horizontal crustal deformation that is expected to occur before and after an earthquake swarm.
A new automatic electronic distance measuring device was manufactured for this investigation. The device uses an electronic distancemeter as a sensor and is controlled by a personal computer. A measuring line was laid between Ito on the east coast of the Izu Peninsula and the Hatsushima Island in Sagami Bay. The distance between the two points is about 9.7 km. Laser beam is shot from Ito and reflected from Hatsushima. Measurements are carried out every 10 minutes. The observation was started on April 14 of 1989. Although seismicity was low and no change in distance was observed toward the end of June, a new earthquake swarm commenced to occur on June 30 in the region covered by the measuring line. Day by day earthquakes increased in number and magnitude and eventually a large earthquake with a magnitude of 5.5 struck the region on July 9.
The earthquake resulted from right-lateral strike-slip faulting with a trend of N 86 °W. In accordance with seismicity the distance on the measuring line gradually increased and attained to an elongation of 22.4 cm just after the occurrence of the large earthquake. Right-lateral slip of 30 cm along the fault and opening of 10 cm across the fault were estimated on the basis of the amount of elongation and another data of 16 cm elongation that was observed on the different measuring line by the Geographical Survey Institute. A submarine volcanic eruption occurred in the central part of the fault four days after the appearance of the fault. But no considerable elongation was observed on the measuring line.
Aftershocks clustered in the tension region of the right-lateral strike-slip fault. This fact as well as the opening across the fault and the volcanic eruption suggests the increase of pressure of pore-filling liquid within the crust that results from a convection current of thermal water heated by magma. The author concludes that the present earthquake swarm is a phenomenon of shear failure due to decreasing effective stress in concurrence with increasing pore pressure in the rock.