The effect of asymmetric damage on dynamic shear rupture propagation I: No mismatch in bulk elasticity

High-speed digital photography was used to study rupture propagation on the interface between transparent damaged and undamaged photoelastic plates. Bilateral ruptures were nucleated on pre-machined faults at an angle α to the uniaxial loading axis. Stress concentration at the crack tips produced fringes in polarized laser light that allowed their positions to be measured in successive photos. We found that fracture damage introduces a strong asymmetry in propagation speed different from that expected due to the lower elastic stiffness in the damaged material alone. When the tensile lobe of a rupture tip propagated through the damaged material the velocity of that rupture was reduced or stopped. By contrast, when the compressive lobe of a rupture tip passed through the damage, the velocity of that rupture was unaffected by the damage. A physical interpretation is that passage of a tensile lobe through the damage expends energy by lowering the normal stress on pre-existing cracks thus allowing frictional sliding along the crack surfaces. When the compressive lobe of the rupture passes through the damage, compressive stresses prevent sliding, only minor energy is dissipated, and the damage has almost no effect on the velocity. This effect can produce asymmetric propagation for earthquake ruptures on slip surfaces near the edge of a highly damaged fault zone.