A Tensile Origin for Fault Rock Pulverization

Abstract

The origin of highly fragmented, but weakly strained rocks found along major strike-slip faults has been enigmatic since their first recognition. These so-called pulverized rocks occur up to 100 m away from the principal slip zone of seismogenic faults around the world. Previous dynamic compression experiments have suggested that rock pulverization occurs at strain rates on the order of 102 s−1, pointing to a coseismic origin; however, strain rates during earthquake rupture 100 m from faults is expected to be 4 orders of magnitude smaller. We present evidence from new modified Split-Hopkinson Pressure Bar experiments that instead supports a tensile origin for coseismic rock pulverization. In the new experimental configuration, the axial compressive load from the Split-Hopkinson Pressure Bar induces radially isotropic tension in a Westerly Granite disk bonded between two lead cylinders. The isotropic tensile state of stress results in the formation of polygonal fracture arrays that bound axis-parallel columnar fragments. The tensile strength of Westerly Granite measured at strain rates between ~5 and 50 s−1 bridges the gap between low strain rate and shock strengths reported previously, supporting an interpretation that highly fragmented rocks may form in a state of isotropic tension. The resulting fragment size is independent of strain rate and instead appears to be controlled by elastic strain energy, a strong function of material strength, and fracture toughness. Our results provide a solution to the strain rate-distance scaling problem between laboratory experiments and field observations of pulverized rocks and also explain the asymmetric distribution of pulverized fault rocks about strike-slip faults.