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.

The Effect of Dynamic Stress Cycling on the Compressive Strength of Rocks

“…In samples where the critical length is not reached, the first K I term remains dominant and greater stresses are required to overcome K IC . One interesting extension of these results is to the role of rapid stress cycling in the fragmentation of rocks under tensile stresses (Griffith et al, 2018;Xu & Ben-Zion, 2017). This process is further complicated by the extreme rate sensitivity of dynamic fracture initiation and propagation toughness to macroscopic loading rate (Bhat et al, 2012;Zhang & Zhao, 2014), resulting in nonlinear relationships between scalar measures of peak macroscopic stress and strain rate and the ultimate compressive strength of rocks.…”

Variation of Hydraulic Properties Due to Dynamic Fracture Damage: Implications for Fault Zones

“…Alternatively, transient permeability measured on pulverized samples during the first confining pressure cycle may represent permeability after a high strain rate pulverization event under tensile loading conditions, after which the stress field is restored—equivalent to an increase in confining pressure during our measurements. However, the microstructure of fragmented rock in radial isotropic tension and pulverized rock in compression may not be similar at similar strain rates (Griffith et al, ).…”

“…The strain rate threshold increases as a function of confining pressure (Yuan et al, ) and decreases by progressive accumulation of fracture damage through successive high strain rate loadings (Aben et al, ; Doan & D'Hour, ). Other loading mechanisms proposed for off‐fault pulverization, such as tensile dynamic loading (Griffith et al, ) and decompression of pore fluid saturated rock (Mitchell et al, ), rely on strain rates that are about 2 orders of magnitude lower (1 s −1 ) than those necessary for pulverization in compression but still far exceed conventional laboratory deformation strain rates (10 −5 s −1 ) and tectonic strain rates. Pulverization in compression is expected to occur in a narrow band (at most a few meters wide) around the fault core, based on strain rate predictions from rupture models (Aben et al, ; Griffith et al, ; Xu & Ben‐zion, ) (Figure ).…”

“…Other loading mechanisms proposed for off‐fault pulverization, such as tensile dynamic loading (Griffith et al, ) and decompression of pore fluid saturated rock (Mitchell et al, ), rely on strain rates that are about 2 orders of magnitude lower (1 s −1 ) than those necessary for pulverization in compression but still far exceed conventional laboratory deformation strain rates (10 −5 s −1 ) and tectonic strain rates. Pulverization in compression is expected to occur in a narrow band (at most a few meters wide) around the fault core, based on strain rate predictions from rupture models (Aben et al, ; Griffith et al, ; Xu & Ben‐zion, ) (Figure ). It is important to note that high strain rate fracture damage is expected outside the pulverized band as well (Aben et al, ) (Figure ), although difficult to distinguish from quasi‐static damage when complete overprinting has occurred.…”

“…The peak strain rate decays with distance from the fault. Damage zone rock close to the fault experiences strain rates above the pulverization threshold (10 2 s −1 in compression (Aben et al, ; Doan & Gary, ), 10 1 s −1 for isotropic tension (Griffith et al, )); further from the fault strain rates are sufficient for dynamic fracturing. Quasi‐static failure may occur at even larger distance from the fault, or dynamic stresses fall below the quasi‐static peak strength of the rock.…”