Dynamic Rupture Along a Material Interface With Creation of Off-fault Damage

Recent geological observations along several large strike slip faults show clear asymmetry in the damage pattern of fault zone rocks, with one side having considerably more damage than the other (Dor et al., 2004). The observed asymmetry implies that ruptures along these faults propagate preferentially in one direction. A preferred propagation direction is a predicted outcome of rupture along an interface that separates different elastic media (e.g., Weertman, 1980; Andrews and Ben-Zion, 1997; Ben-Zion and Huang, 2002). Such ruptures produce dynamic dilation at the tip that propagates in the direction of slip on the more compliant side of the fault, and dynamic compression at the opposite tip. Consequently, rupture along a material interface evolves to a unidirectional wrinkle-like pulse that propagates in the direction of slip on the compliant side of the fault. In addition, wrinkle-like ruptures along a material interface produce strongly asymmetric fault-normal motion near the propagating tip, with larger motion on the compliant side. Rupture along a material interface has the following two competing mechanisms for creation of off-fault damage. (1) Anelastic deformation on the extensional quadrant in the preferred propagation direction, which for a bi-material configuration is on the stiffer side, as was calculated by Andrews (2004) for rupture in a homogenous solid. (2) Anelastic deformation on the more compliant side due to the asymmetric motion across the fault. Mechanism (1) is favored by low normal stress, small contrast of material properties across the fault, and large difference between the static and kinetic coefficients of friction. Mechanism (2) is favored by the opposite set of conditions. In this study we attempt to quantify the conditions for which the above two mechanisms are active. Previous analytical and numerical studies found that wrinkle-like ruptures between two purely elastic materials diverge for a broad range of conditions (e.g., Adams, 1995; Ben-Zion, 2001; Ranjith and Rice, 2002; Adda-Bedia and Amar, 2003). Another goal of our study is to find whether the creation of off-fault damage will provide a natural length scale for the wrinkle-like pulse and a mechanism that stabilizes the divergence in the purely elastic case. The results will provide an improved understanding of ruptures along a material interface, and important input for inferring from field observations the preferred propagation direction of ruptures along faults.