Dynamic nucleation process of shallow earthquake faulting in a fault zone

Two distinct phases are commonly observed at the initial part of seismograms of large shallow earthquakes: low‐frequency and low‐amplitude waves following the onset of a P wave ( P 1 ) are interrupted by the arrival of the second impulsive phase P 2 enriched with high‐frequency components. This observation suggests that a large shallow earthquake involves two qualitatively different stages of rupture at its nucleation. We propose a theoretical model that can naturally explain the above nucleation behaviour. The model is 2‐D and the deformation is assumed to be anti‐plane. A key clement in our model is the assumption of a zone in which numbers of pre‐existing cracks are densely distributed; this cracked zone is a model for the fault zone. Dynamic crack growth nucleated in such a zone is intensely affected by the crack interactions, which exert two conflicting effects: one tends to accelerate the crack growth, and the other tends to decelerate it. The accelerating and decelerating effects are generally ascribable to coplanar and non‐coplanar crack interactions, respectively. We rigorously treat the multiple interactions among the cracks, using the boundary integral equation method (BIEM), and assume the critical stress fracture criterion for the analysis of spontaneous crack propagation. Our analysis shows that a dynamic rupture nucleated in the cracked zone begins to grow slowly due to the relative predominance of non‐coplanar interactions. This process radiates the P 1 phase. If the crack continues to grow, coalescence with adjacent coplanar cracks occurs after a short time. Then, coplanar interactions suddenly begin to prevail and crack growth is accelerated; the P 2 phase is emitted in this process. It is interpreted that the two distinct phases appear in the process of the transition from non‐coplanar to coplanar interaction predominance.