ダイラタンシーによる間隙水圧の変動を考慮した地震発生過程のモデル化

Abstract

Dilatancy and pore fluid pressure changes in a fault zone significantly affect the mode of the nucleation process, and are key factors for understanding the physical mechanisms of some precursory phenomena. We investigate slip processes with a model of a vertical fluid-infiltrated strike-slip fault in a 3-D elastic half space using a Dietrich/Ruina rate-and state-dependent friction law and an evolution equation for plastic porosity. Due to dilatancy, porosity is thought to increase with slip velocity. In this model plastic porosity is assumed to depend logarithmically on the state variable in the friction law. The results of numerical simulations show that porosity in the fault zone increases with increasing slip velocity in the nucleation zone. Then, pore fluid pressure in the fault zone drops significantly, resulting in dilatant hardening. With an increasing dilatancy coefficient, a larger pore fluid pressure drop is observed during the nucleation process, which in turn increases the size of the nucleation zone. In the case of considerably larger dilatancy coefficients, the fault system becomes stable. The various modes of slip processes are reproduced by the spatial variation of dilatancy coefficients. We examine changes in pore-fluid pressure on the fault near the surface. When a slow event occurs in a deeper region of a seismogenic zone in the later stage of an earthquake cycle, it is observed that slip velocity on the fault near the surface increases significantly. Slip velocity also increases slightly around one year before the main rupture begins. These changes in slip velocity lead to a significant drop in the pore-fluid pressure on the fault near the surface.