Analysis of dynamic behavior of interactive fault system and estimate of high frequency strong motion

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The critical slip displacement D_c is defined as the amount of relative slip required for the shear stress to decrease to the residual stress level. It is indicated that there is a discrepancy between laboratory measurements of D_c and values obtained from modeling earthquakes. Our theoretical analysis has shown that the existence of a thick fault zone for natural earthquake is likely to contribute this discrepancy.We also theoretically studied the dynamic nucleation of an earthquake in a fault zone in which small preexisting cracks are densely distributed. Dynamic rupture nucleated in such a zone is significantly influenced by the crack interactions. In general, crack interactions cause two conflicting effects : One is to promote crack growth, and the other to decelerate it. These two are due to coplanar and non-coplanar crack interactions, respectively. Hence if non-coplanar interactions are predominant at the initial state, the crack growth rate is very slow at its onset as shown in our simulations. Observation of seismic quiescence before a large earthquake implies that the assumption that non-coplanar crack interactions prevail at initial state will be reasonable.