The Effect of Off-Fault Damage on Earthquake Rupture Velocity: An Experimental Study

Fault zone fracture damage is expected to lower the propagation velocity of earthquake ruptures for two reasons. First, the limiting Rayleigh speed of the rupture [Vr = 0.92 Vs for Mode II or Vr = Vs for Mode III] is lowered to the extent that the damage lowers the shear wave speed Vs. Second, the rupture speed is further reduced because the fracture energy is increased in the fault-tip process zone. This extra energy is expended by frictional slip on existing crack damage and by the creation of new damage. In this study we simulated fault zones in birefringent homalite plates by thermally shocking [in liquid N2] strips of variable widths parallel to the slip surface. The slip-plane normal was at an angle of 25 degrees to the uniaxial loading axis. A shear rupture was nucleated in the stressed plate by exploding a wire that produced a 1 cm long stress-free patch along the slip surface. A laser and polarizers were used to generate transient fringe patterns that were recorded by high-speed digital cameras and used to find the shear and rupture speeds. Rupture speed was measured as a function of the half-width w of the of the damage zone for w = 0.5, 1, and 2 cm, and for cases where there was no damage [w = 0] and where the entire plate was damaged. As the damage zone half-width increased we observed that the rupture speed, S-wave velocity, and Vr/Vs decreased. The decrease of Vr/Vs with increasing w shows that the decreased rupture speed was not due solely to the decrease in shear speed, but was also due to an increased contribution to the fracture energy from the damage zone. A plot of Vr as a function of w yields a value of R0* [a measure of the spatial extent of Coulomb slip] near 3 cm, in agreement with the predicted value for homalite based on the analysis of Rice et al. [BSSA, 2005].