An experimental perspective on brittle tensile fragmentation during subshear rupture on bimaterial fault interfaces

Abstract:

Bimaterial faults, characterized by the juxtaposition of rock with different elastic properties across the fault interface, are common in all tectonic settings. Modeling predicts that during subshear rupture along such interfaces tension is enhanced nearly isotropically on the more competent side of the fault resulting in quasi-isotropic tensile loading. However, it remains to be seen whether or not rocks dynamically fractured by impulsive tension can be deciphered from similar damage that occurs under compression. Using a modified sample configuration for a Split Hopkinson Pressure Bar apparatus we investigate the fracture populations formed in rocks under isotropic tension as a function of strain rate. The sample design includes a rock disk bonded to two disks of more compliant material (e.g., lead). Under axial loading, radial strain in the compliant bounding material produces transversely isotropic tension in the rock sample. In contrast to other commonly used dynamic tension experimental techniques this modified sample configuration produces fracture arrays that can be used for comparison to field analogues. Experimentally formed fracture networks are used to validate and further enhance dynamic fragmentation models which can be used to predict the expected fracture density produced during subshear earthquake rupture. We perform experiments on a range of rock types including macro- (Westerly Granite) and micro-crystalline (diabase) intrusive rocks, welded tuff, and high porosity sandstone (Berea Sandstone). The influence of initial damage on the final fracture density of the rock is studied by heat treating specimens or performing successive loading tests. Our results show that under isotropic tensile loading conditions the strain rates required to fracture rock are below the threshold required for strain rate dependent fracturing under compressive loading. Therefore, fragmentation at strain rates below those investigated here (10⁰-10² s^-1) will result in similar fragment sizes and fracture densities to those produced in this study. These findings suggest that fragmentation under tensile loading can produce large distributed damage zones on the competent side of a bimaterial interface and is a viable mechanism for producing distributed pulverized zones up to 100 m from a fault.