断層間での破壊の乗り移り 応力が深さに依存する場合についての考察

Abstract

Numerical simulations have revealed that a rupture jumps from one fault to another near the Earth's surface. We show that a rupture jump occurs at a deep portion under depthdependent stress. We investigated rupture behaviour at a gap region between two faults, by calculating spontaneous rupture processes on two parallel strike-slip faults in a 3-D elastic medium. Depth-dependent stresses introduced from the results of hydraulic fracturing experiments at the KTB site are assumed. The differential stress is assumed to be proportional to depth. The rupture processes under depth-dependent and uniform stress conditions are compared : similar rupture processes on the first fault, but different ones on the second fault are obtained. Under the depth-dependent stress condition, an upward rupture on the first fault can trigger a rupture at a deep portion of the second fault, because strength is reduced at shallower depths. Near the Earth's surface, a rupture on the second fault can be triggered by only a P-wave radiated from the first fault. Strength and stress drop heterogeneities on faults cause a rupture jump at a deep portion and the complexity of the rupture process. Our results suggest that the stress increase with depth is an important factor.