EFFECTS OF A LOW-VELOCITY ZONE ON A DYNAMIC RUPTURE

Dynamic-crack earthquake simulations generally assume that the crustal material surrounding faults is laterally homogeneous. Tomographic and near-fault seismic studies indicate that the crust near faults is ins tead comprised of rocks of varying material velocities. We have tested the effects of adding material-velocity variation to simulations of s pontaneously propagating earthquakes. We used two-dimensional plane st rain conditions coupled with a slip-weakening fracture criterion and e xamined earthquakes on faults that bisect finite-width low-velocity zo nes embedded in country rock and earthquakes on faults that bound two different velocity materials. When a fault bisects a low-velocity zone , the normal stress remains unchanged, but both the rupture velocity a nd slip-velocity pulse shape are perturbed. The presence of the low-ve locity zone induces high-frequency oscillations in the slip function n ear the rupture front. When the fault is on the edge of the low-veloci ty zone, the oscillations are more pronounced, and repeated sticking a nd slipping can occur near the rupture front. For the slip-weakening ( velocity-independent) friction model, however, the temporary sticking does not lead to permanent arrest of slip, and slip duration is still controlled by the overall rupture dimension. When an earth quake ruptu res a fault juxtaposing a lower-velocity material against a higher-vel ocity material, the normal stress across the fault near the crack tip is perturbed. The sign of the normal stress perturbation depends on th e direction of rupture, leading in some cases to a directional depende nce of rupture velocity. When slip is accompanied by stress reduction, a positive feedback develops between the normal and shear stress chan ges, as previously noted by Andrews and Ben-Zion (1997), resulting in an apparently unavoidable grid-size dependence in computation of stres s change near the rupture front. Numerical experiments indicate, howev er, that the rupture velocity is insensitive to this zone size depende nce, which is highly localized immediately behind the crack tip. The f actors controlling the rupture velocity in the simulations, including directional dependence, are further elucidated by a new analytical sol ution for rupture of an asperity on a frictionless interface.