Stress Fields due to Strike-slip Faults, Tensile Faults and Cylindrical Soft Bodies, and Related Seismic Activities.

Abstract

We made numerical computations of the change in the fracture stress due to the generation of vertical strike-slip faults and vertical/horizontal tensile faults and the induction of round soft bodies (two-dimensional) in an elastic medium. These sources simulate, respectively, shallow intraplate earthquakes, magma intrusion into dikes/sills in volcanic regions, and volcanoes/magma reservoirs which are more easily subject to shear and volumetric deformations than the surrounding areas. The fracture stress is defined as the summation of shear stress and a certain amount of normal stress and indicates the effective stress which contributes to generating fracture. We present some examples of seismic activities which seem to have occurred in accordance with the simulation, i.e., cases where earthquakes occurred in regions where the fracture stress is theoretically expected to increase. By the formation of vertical strike-slip faults the fracture stress increases in regions extending from the fault edge both tangential to and normal to the fault strike. In the latter case, the increase in the fracture stress appears on the sides of the fault where the medium is subject to dilatation and the frictional stress on existing conjugate faults may decrease. It is often observed that aftershock activities expand in the direction of the fault strike or a conjugate fault appears from the end of the main fault and runs into the dilatational region. The former example is the aftershock activities following the Tangshan earthquake in 1976 and the typical example of the latter case is the largest aftershock occurrence following the western Nagano earthquake in 1984. When vertical tensile faults are generated, the fracture stress increases especially in regions extending obliquely from the fault end, while the stress is suppressed on both sides of the fault. Some seismic activities associated with the eruption of Izu-Oshima volcano in 1986 and the marine eruption off the east coast of the Izu Peninsula in 1989 may be related with these characteristic stress changes. When horizontal tensile faults are generated deep under the free surface, the ground surface over the fault heaves up and the fracture stress usually increases there. It is well known that the seismicity in and around the Izu Peninsula became quite active after the 1974 Izu-Hanto-Oki earthquake and the ground upheaval was also observed. When a round soft body (two-dimensional) is put in a compressive stress field, it shrinks more in the direction of maximum pressure axis than in its orthogonal direction. Then, the fracture stress increases at both sides of the soft body normal to the maximum pressure axis and decreases at the other sides when it is compared with that of the stress field without the soft body. The distribution of the fracture stress around an expanding source may be characterized by reversing the above result. There exist seismic activities which seem to be related with these stress changes, e.g., earthquakes occurred before the 1962 Miyakejima eruption to the west of the island and swarm activities began in the region to the southeast a few years before the 1979 eruption of Ontake volcano.