Generation of microcracks by dynamic shear rupture and its effects on rupture growth and elastic wave radiation

Laboratory and field observations suggest that dynamically propagating earthquake faults generate a large number of tensile microcracks in their vicinity, which will contribute to the formation of fault zones. Intense interactions are expected to occur between such microcracks and a dynamically propagating fault, which will complicate the fault growth. Near-field seismic waves will also be affected by the generation of microcracks. We numerically study how such tensile microcracks are generated and how dynamic growth of a macroscopic shear rupture and near-field elastic waves are affected by the distribution of generated microcracks. It is essential to consider a large number of microcracks in such studies, so that it is impractical to consider each microcrack individually from the viewpoint of computation time and memory. We overcome this difficulty by representing the microcrack distribution by anisotropic properties of the overall elastic coefficients on the basis of Hudson's (1980) study. Our simulations show that the decrease in the microcrack density is approximated well by a logarithmic function of the distance from the rupture plane. Microcracks on the dilational side of the rupture plane are shown to make larger angles to the rupture plane than on the compressive side. These are consistent with field and laboratory observations. It is also shown that dynamically generated microcracks tend to reduce the rupture-tip sheer stress. This implies that the dynamic growth of a shear rupture is more decelerated when microcracks are generated than when the shear rupture is isolated in an isotropic and homogeneous medium. If the dynamic rupture growth is arrested suddenly, an abrupt expansion of the distribution zone of microcracks is shown to occur near the arrested rupture tip. Aftershocks are expected to cluster in this zone because of the shear stress enhancement there and the high density of distributed microcracks, which facilitates aftershock occurrence due to dynamic coalescence of microcracks. Our simulations also show that the component of radiated displacement waves perpendicular to the rupture plane is much more affected by the generation of microcracks than the parallel component.