Lithological controls on fault damage zone development by coseismic tensile loading

Abstract

During subshear earthquake rupture opposing sides of the fault experience transient impulses of tensile and compressive peak mean normal stresses exceeding the strength of the rock resulting in off-fault damage. This damage is expected to be influenced by lithology and occur preferably on the nominally tensile side of ruptures because rocks are weaker in tension than compression. Whereas the static tensile strength of many rocks is known, our capacity to relate parameters used to quantify brittle deformation in fault damage zones commonly measured in the field, including fracture density and fragment size, to earthquake rupture conditions is poor. This is due to a lack of experimental data relating tensile strength, fracture density, and fragment size to lithology under transient loading conditions. To relate brittle damage to coseismic tensile loading conditions, we use a modified sample configuration for a Split Hopkinson Pressure Bar (SHPB) designed to induce tensile rock fragmentation. Experiments cover a range of rock types including granite, diabase, welded tuff, and sandstone. All crystalline rocks fail predominantly via mode-I fracture, while sandstone fails through localized dilation bands, distributed pore space expansion, and less common mode-I fractures. Measurements of fracture density, fragment size, and porosity change are compared to tensile strength and strain rate at failure. We derive empirical relationships for (1) the transition from fracture to dilation banding as a function of initial porosity and (2) the strain rate dependence of tensile strength. These empirical relationships are used to estimate the rock strength during earthquake rupture on the side of the fault loaded under tension for comparison to predicted stress decay with distance from a fault to estimate the lithology dependent extent and nature of coseismic fault damage. Our experiments offer new insights into the lithological controls on the formation of asymmetric fault damage zones by dynamic rupture.