A Tensile Origin for Fault Rock Pulverization

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 10(2) s(-1), pointing to a coseismic origin; however, strain rates during earthquake rupture 100m 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 similar to 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.