Controls on the formation of pulverized off-fault rocks: Laboratory investigations using Arkansas Novaculite

Abstract:

A number of control parameters (i.e., strain rate, peak stress, number of load cycles) have been proposed to govern the formation of pulverized off-fault rocks (POFR) during earthquakes. Recent descriptions of fracture damage associated with high strain rate experiments on rock suggest that the portion of the work budget consumed in creating new fracture surfaces is fundamentally dependent on the loading rate. As POFR exhibit high fracture density, understanding this dependence is critical for constraining the processes responsible for their formation. The transition from weakly to highly fragmented (i.e., pulverized) is thought to be controlled by microcrack dynamics, which are sensitive to loading rate, but also material heterogeneity and pre-existing flaw distribution. Arkansas Novaculite is mineralogically homogeneous and nearly flaw free above the length scale of its sub-micron grain size, providing us with an ideal rock to evaluate continuum-based models of fragmentation. We have performed a series of dynamic compression tests on Arkansas Novaculite using a split-Hopkinson pressure bar. Our preliminary experimental results suggest that pulse shape and amplitude, both of which dictate the stress, strain rate, and total strain, exert a more fundamental control on the transition from localized fracture to pervasive fragmentation than any of these three latter parameters in isolation. Damage created across the transitional loading rate regime is characterized using BET surface area analysis, micro-CT scanning, and optical and scanning electron microscopy, and portions of the work budget partitioned to create new fracture surfaces are evaluated by measurements of fracture surface area on the post-mortem specimens. We show a dramatic increase in dynamic strength of Arkansas Novaculite, which highlights the importance of inherent flaws on the fragmentation process, consistent with the predictions of high strain rate fragmentation models.