On the Activity of Conjugate Faults System

Abstract

The activity of pre-existing conjugate faults system is considered in relation to the regional stress field. Fault movement is assumed to be like a stick slip, and the magnitude of effective shear stress on each fault surface is compared.
It is derived that neither of the two faults is more a ctive provided the coefficient of friction on the faults ti has not changed from the coefficient of inner friction at the time the conjugate faults system had been formed, even if the stress field has changed since then. When p>p0, the fault which makes a smaller angle to the maximum pressure axis is more easily activated, and when p<p0, the reverse is deduced.
The residual stress field after the activity of either of the con j u gate faults is also considered and the relation between the principal axes of initial stress field and those of the residual stress field is obtained. From the result it is concluded that, when u>u0, the very fault which activates first is more active successively, but when p<u0, there is a possibility that the conjugate faults move one another.
Although this is still a preliminaly research, th e above formulated idea is applied to the Matsushiro earthquake swarm and the seismicity of the Izu Peninsula. It is found that the fault motion associated with these activities can be explained by this idea. But further study will be required for applications of this theory to geology.