Inelastic off-fault deformation and pulverization in the lab: Tensile fragmentation of crystalline vs. granular sedimentary rocks in response to isotropic tension

W. A. Griffith, Zachary D. Smith, Michael Braunagel, Thomas Marren, & Sasha Larocque

Submitted August 14, 2019, SCEC Contribution #9570, 2019 SCEC Annual Meeting Poster #161 (PDF)

Poster Image: 
The presence of pulverized (highly fragmented, but weakly strained) zones extending 100-200 m from major strike slip faults, including the San Andreas Fault, have been attributed to impulsive compressive stresses associated with propagating earthquake rupture tips. However, theoretical and experimental evidence suggests that such zones may be formed on the transient tensile side of passing ruptures. These pulverized damage zones represent long-lived inelastic off-fault deformation that affect fault dynamics throughout the seismic cycle. We explore the tensile origin of pulverized fault rocks associated with major strike slip faults through a modified Split-Hopkinson Pressure Bar (SHPB) experiment that induces 2D isotropic tension. In the experiments, a sandwich sample configuration is used in which a rock disk is bonded between two cylinders composed of more compliant material such as lead or polycarbonate. Axial shortening during experiments results in radial and circumferential tension in the rock disk due to radial expansion of the compliant end materials. Experiments on both porous granular (sandstone) and crystalline (granite) rocks enable us to evaluate variations in tensile stress based on the rock type. We validate strain and strain rate histories collected on SHPB strain gauges using high speed photography and digital image correlation. Our modified SHPB experiments on Westerly Granite show that at strain rates of 25s-1 to 170s-1, the rock fails by an isotropic pattern of polygonal fractures. Under similar conditions, deformation of Berea Sandstone is accommodated by distributed grain boundary failure and pore space expansion, therefore preventing fragmentation by fracture growth. These results explain asymmetric off-fault damage observed in the field where crystalline rocks 100-200 m from the core of the fault are pulverized, but adjacent porous sedimentary rocks appear to be relatively undeformed.

Key Words
inelastic deformation, pulverization, dynamic rupture, fault damage zone, fault and rupture mechanics

Citation
Griffith, W. A., Smith, Z. D., Braunagel, M., Marren, T., & Larocque, S. (2019, 08). Inelastic off-fault deformation and pulverization in the lab: Tensile fragmentation of crystalline vs. granular sedimentary rocks in response to isotropic tension. Poster Presentation at 2019 SCEC Annual Meeting.


Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)