Continuous Monitoring of Crustal Activity in East Izu Peninsula by Automatic Electronic Distance Measurement.

Abstract

The horizontal crustal deformation accompanying the east Izu swarm earthquakes of July 1989 was observed by the automatic electronic distance measurement. The amount of elongation of the measuring line of length 9.7km attained to 22.4cm. The earthquake with a magnitude of 5.5 resulted from right-lateral faulting with a strike of N86°W. Right-lateral strike-slip of 30cm along the fault and opening of 10cm across the fault were estimated on the basis of the amount of elongation and another data observed 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. Probably the eruption resulted from the deep-seated faulting and the observed opening across the fault plane. The crustal deformation of the east Izu area is characterized by a volumetric expansion that includes horizontal dilatation and upheaval. A speculative interpretation on the geologic processes that are occurring in the crust of the region is presented in the following. The volumetric expansion was brought about by the increase of pressure of pore-filling fluid in the crust because the east Izu area is a region of high geotherm in the Fuji volcanic belt that is abnormally activated during the past ten years. The earthquake swarm occurred as shear failure due to the decrease of effective stresses in the rock. Thermal water heated by magma in the depth of the crust will move upward in the shape of convection current and exert an additional pressure on the wall rock as a result of expansion at the level where it is coverted from a critical state of water to an ordinary gas and liquid phase.