AN EXPERIMENTAL STUDY OF THE EFFECTS OF FAULT ZONE STRUCTURE ON EARTHQUAKE RUPTURES

The interaction between a dynamic mode II fracture on a fault plane and off-fault damage has been studied using high-speed photography. Fracture damage was created in photoelastic Homalite plates by thermal shock in liquid nitrogen and rupture velocities were measured by imaging fringes at the tips. Three cases were studied: an interface between two damaged Homalite plates, an interface between damaged and undamaged Homalite plates, and the interface between damaged Homalite and undamaged polycarbonate plates. Ruptures on the interface between two damaged Homalite plates travel at sub-Rayleigh velocities (even after the elastic wave velocities have been corrected for the damage) indicating that energy is lost by sliding on off-fault fractures, even though no new damage is created. Propagation on the interface between damaged and undamaged Homalite is asymmetric. Ruptures propagating in the direction for which the compressional lobe of their crack-tip stress field in the damage (which we term the ‘C’ direction) are unaffected by the damage. In the opposite ‘T’ direction, the rupture velocity is significantly slower than the velocity in undamaged plates at the same load. Specifically, transitions to supershear observed using undamaged plates are not observed in the ‘T’ direction. Propagation on the interface between damaged Homalite and undamaged polycarbonate exhibits the same asymmetry, even though the elastically “favored” ‘+’ direction coincides with the ‘T’ direction in this case. The scaling properties of the interaction between the crack-tip field and off-fault damage are explored using an analytic model for a non-singular slip-weakening slip-pulse formulated by Rice et al. (Bull. Seism. Soc. Am., 2005) and verified using a direct measurement of the interaction range using damage zones adjacent to the fault having a range of widths.