STRUCTURAL INDICATORS OF FRACTURE VELOCITY DURING JOINTING, FAULTING AND EARTHQUAKES

We present structural indicators for propagation velocity of faults and joints in experiment and field, and discuss mechanisms that form these indicators. Fracture propagation velocities may be divided into a quasi-static regime during which acceleration forces are small and may be ignored, and a dynamic regime during which acceleration forces may not be ignored. The transition between these two regimes is at propagation velocity of approximately 0.35 the shear wave velocity of the host material. Experimental works of tensile, brittle fracturing demonstrated, during the last few decades, that this transition velocity is characterized by structural transition from a simple, smooth fracture to a complex, branched one. We present (a) experimental results of fracturing of rock-like plates subjected to plate-parallel tension; (b) field analysis of shatter-cones which are tensile fractures that develop during impacts; and (c) analysis of earthquake rupture zones observed at focal depth. We found that tensile fractures that propagate in the dynamic regime have branched, tree-like hierarchic geometry with relatively rough surfaces. Their propagation velocity can be measured from v-shaped striations that develop on fracture surfaces due to fracture-front-waves. These striations indicate that shatter-cones propagate at extreme velocity of 0.9-0.98 the maximum possible velocity. The structure of rupture zones associated with four earthquakes (m=2.2 to 4.7) was examined at focal depth in deep gold mines, South Africa. We found zones of intense fracturing and brecciation that range in width from a few millimeters to a few tens of meters. The rupture zones are characterized by rock pulverization forming “rock powder” layers with cumulative thickness up to 30 millimeter for a single event. While similar structures could develop by large displacement in the quasi-static regime, these rupture zones were formed by small displacement during a single earthquake. Thus, these structures are interpreted as reflecting the extreme deformation conditions at the process zone of dynamically propagating earthquakes.