Role of geometric complexities and off-fault damage in dynamic rupture propagation

To analyze the effect of fault branches on dynamic rupture propagation we numerically simulated the observed dynamic slip transfer from the Denali to Totschunda faults during the Mw 7.9, November 3, 2002, Denali fault earthquake, Alaska and show that the theory and methodology of Poliakov et al. [2002] and Kame et al. [2003] is valid for the 2002 Denali fault event. To understand the effect of fault branch length on dynamic rupture propagation we analyze earthquake ruptures propagating along a straight "main" fault and encountering a finite-length branch fault. We show finite branches have the tendency of stopping or re-nucleating rupture on the main fault depending on their length in addition to the parameters singled out by Kame et al. [2003]. We also illustrate branch-related complexities in rupture velocity and slip evolution. We illustrate the effect of backward branches (branches at obtuse angle to the main fault with the same sense of slip as the main fault) and propose a mechanism of backward branching. As a field example we simulate numerically, using a two-dimensional elastodynamic boundary integral equation formulation incorporating slip-weakening rupture, the backward branching phenomenon observed during the Landers 1992 earthquake.; To characterize the effect of supershear ruptures on off-fault materials we extend a model of a two-dimensional self-healing slip pulse, propagating dynamically in steady-state with slip-weakening failure criterion, to the supershear regime and show that there exists a non-attenuating stress field behind the Mach front which radiates high stresses arbitrarily far from the fault (practically this would be limited to distances comparable to the depth of the seismogenic zone). We apply this model to study damage features induced during the 2001 Kokoxili (Kunlun) event in Tibet. We also study the 3D effects of supershear ruptures by simulating bilateral ruptures on a finite-width vertical strike-slip fault breaking the surface of an elastic half-space, and focus on the wavefield in the near-source region. We provide numerical evidence for the existence of Rayleigh Mach fronts, in addition to shear Mach fronts. We conclude that radiating Mach waves of three-dimensional supershear ruptures do transmit large-amplitude ground motions and stresses far from the fault. The amplitudes along the shear Mach front would be moderated at distances greater than the fault width by decay with distance due to geometrical spreading. However, in an ideally elastic material, we do not expect any geometrical attenuation along the Rayleigh Mach front.