OFF-FAULT TENSILE CRACKS: A LINK BETWEEN GEOLOGICAL FAULT OBSERVATIONS, LAB EXPERIMENTS AND DYNAMIC RUPTURE MODELS

Earthquake ruptures may be idealized as shear cracks with tips that propagate along a pre-existing plane of weakness, the model fault. The stress concentration at the propagating tip is proportional to the velocity: the faster the tip propagates, the greater the stress concentration. As the velocity of a propagating mode II rupture tip (v_II) approaches the Rayleigh wave speed (c_R), the stress concentration can reach values large enough to exceed the strength of rocks near the fault, resulting in inelastic deformation in the so-called damage zone. We examine the local nature of the dynamic stress field in the vicinity of the tip of a semi-infinite sub-Rayleigh (slower than c_R) mode II crack with a velocity-weakening cohesive zone. We constrain the model using results from dynamic photo-elastic experiments, in which ruptures were nucleated spontaneously in Homalite-100 plates along a bonded, pre-cut, and inclined interface subject to a vertical uni-axial prestress. During the rupture propagation and adjacent to the cohesive zone, tensile cracks grew periodically along one side of the rupture interface at a roughly constant angle relative to the shear rupture interface. The occurrence and inclination of the tensile cracks are explained by the analytical model presented here. The observation of fast rupture velocities during earthquakes coupled with a strong velocity dependence of the stress-field surrounding a rapidly propagating rupture tip has important implications for geologists analyzing exhumed faults, giving them tools to interpret physical processes that happen during earthquakes on the order of seconds to microseconds. With slight modifications, the model presented here can be scaled to natural faults, providing diagnostic criteria for interpreting velocity, directivity, and static pre-stress state associated with past earthquakes on exhumed faults. Indirectly, this method also allows one to constrain the velocity weakening nature of natural ruptures, providing an important link between field geology, laboratory experiments, and seismology.