Slip rupture processes on velocity-weakening faults have been found in simulations to occur by two basic modes, the expanding crack and self-healing modes. In the expanding crack mode, as the rupture zone on a fault keeps expanding, slip continues growing everywhere within the rupture. In the self-healing mode, rupture occurs as a slip pulse propagating along the fault, with cessation of slip behind the pulse, so that the slipping region occupies only a small width at the front of the expanding rupture zone.

We discuss the determination of rupture mode for dynamic slip between elastic half-spaces that are uniformly prestressed at background loading level τb0 outside a perturbed zone where rupture is nucleated. The interface follows a rate and state law such that strength τstrength approaches a velocity-dependent steady-state value τss(V) for sustained slip at velocity V, where dτss(V)/dV ≦ 0 (velocity weakening). By proving a theorem on when a certain type of cracklike solution cannot exist, and by analyzing the results of 2D antiplane simulations of rupture propagation for different classes of constitutive laws, and for a wide range of parameters within each, we develop explanations of when one or the other mode of rupture will result. The explanation is given in terms of a critical stress level τpulse and a dimensionless velocity-weakening parameter T that is defined when τb0 ≥ τpulse. Here τpulse is the largest value of τb0 satisfying τb0 − (μ/2c)V ≤ τss(V) for all V > 0, where μ is the shear modulus and c is the shear wave speed. Also, T = [−dτss(V)/dV]/(μ/2c) evaluated at V = Vdyna, which is the largest root of τb0 − (μ/2c)V = τss(V); T = 1 at τb0 = τpulse, and T diminishes toward 0 as τb0 is increased above τpulse.

We thus show that the rupture mode is of the self-healing pulse type in the low-stress range, when τb0 < τpulse or when τb0 is only slightly greater than τpulse, such that T is near unity (e.g., T > 0.6). The amplitude of slip in the pulse diminishes with propagation distance at the lowest stress levels, whereas the amplitude increases for τb0 above a certain threshold τarrest, with τarrest < τpulse in the cases examined. When τb0 is sufficiently higher than τpulse that T is near zero (e.g., T < 0.4 in our 2D antiplane simulations), the rupture mode is that of an enlarging shear crack.

Thus rupture under low stress is in the self-healing mode and under high stress in the cracklike mode, where our present work shows how to quantify low and high. The results therefore suggest the possibility that the self-healing mode is common for large natural ruptures because the stresses on faults are simply too low to allow the cracklike mode.

First Page Preview

First page PDF preview
You do not currently have access to this article.