Extensional aspects of earthquake induced ruptures determined by an analysis of fracture bifurcation

Numerical and experimental investigation of symmetric fracture bifurcation (Kalthoff, 1972), has shown that for forks with small branch angles α<α_c, where α_c is approximately 14°, the propagation of the branches tends to enlarge the angle. For forks with larger branch angles, α>α_c, the propagation of the branches tends to diminish the angle. Forks with the critical angle α_c will propagate in their original direction. Kalthoff theorized that the branch angle changes as a function of K_I/K_II, where K_I and K_II are the stress intensity factors for tensile and shear (sliding) modes, respectively, and K_I is considerably larger than K_II. In this study I test the hypothesis that this fracture mechanic theory applies to the analysis of fault bifurcation in the crust, particularly in cases of rapid fracture. Fractures produced during the 1968 earthquake at the Coyote Creek fault in California are intensively branched and an example of rapid rupture. The angular behaviour of the branching ruptures in eight forks follows Kalthoffs theory unusually well. This implies that fracture at the surface was dominated by the tensile mode. Additional observations that support this implication are: series of prominent ruptures which show openings (of 20-30 mm per rupture), the symmetrical and bilateral forking, the high-intensity and angular shapes of individual branches, the opening of grabens associated with several bifurcations, lack of bifurcation in the southern break of the Coyote Creek fault, and the patterns of en echelon fractures which reflect mixed mode surfacial rupture. Hence, contrary to previous interpretations, according to field evidence and fracture mechanic theory, the fault bifurcation and opening along the Coyote Creek fault are not compatible with local tension caused by the primary shear. Fracture probably occurred by different mechanical modes at depth and at the surface. While faulting may have originated by shear at depth, rupture at the surface was dominated by far-field tension associated with NE-SW extension in South California. The present model predicts the directions of fracture propagation along the fault.