Dynamic fracturing by successive coseismic loadings leads to pulverization in active fault zones

Abstract

Previous studies show that pulverized rocks observed along large faults can be created by single high‐strain rate loadings in the laboratory, provided that the strain rate is higher than a certain pulverization threshold. Such loadings are analogous to large seismic events. In reality, pulverized rocks have been subject to numerous seismic events rather than one single event. Therefore, the effect of successive “milder” high‐strain rate loadings on the pulverization threshold is investigated by applying loading conditions below the initial pulverization threshold. Single and successive loading experiments were performed on quartz‐monzonite using a Split Hopkinson Pressure Bar apparatus. Damage‐dependent petrophysical properties and elastic moduli were monitored by applying incremental strains. Furthermore, it is shown that the pulverization threshold can be reduced by successive “milder” dynamic loadings from strain rates of ~180 s−1 to ~90 s−1. To do so, it is imperative that the rock experiences dynamic fracturing during the successive loadings prior to pulverization. Combined with loading conditions during an earthquake rupture event, the following generalized fault damage zone structure perpendicular to the fault will develop: furthest from the fault plane, there is a stationary outer boundary that bounds a zone of dynamically fractured rocks. Closer to the fault, a pulverization boundary delimits a band of pulverized rock. Consecutive seismic events will cause progressive broadening of the band of pulverized rocks, eventually creating a wider damage zone observed in mature faults.

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

“…et al (2016), in which load paths had longer dwell times. This is in contrast to an assumption by Aben et al (2016) that K I is linearly related to applied stress, as would be the case for a crack uniformly loaded by an applied tensile stress (e.g., Irwin, 1957;Tada et al, 1973). Part of this complexity results from the inherently strain hardening nature of microcrack growth under compressive loads before cracks reach a critical length (Ashby & Sammis, 1990;Bhat et al, 2012;Xu & Ben-Zion, 2017).…”

“…Reduced dynamic rock strength, coupled with damage accumulated over multiple earthquake cycles (Aben et al, 2016), likely enhances pulverized fault damage zone formation. This reduction may be attributed to the achievement of a critical wing crack length during the first stress cycle where neighboring crack interaction drives unstable propagation during the subsequent cycle.…”

“…In experiments studying the effect of repeated seismic ruptures on rock pulverization, Aben et al (2016) subjected quartz monzonite samples to successive high strain rate loading events using a split Hopkinson pressure bar (SHPB). In these tests, a twofold reduction in the critical strain rate required to fragment quartz monzonite from that required in a single load cycle was observed (Aben et al, 2016). In these tests, a twofold reduction in the critical strain rate required to fragment quartz monzonite from that required in a single load cycle was observed (Aben et al, 2016).…”

Experimental postseismic recovery of fractured rocks assisted by calcite sealing

“…A detailed description of the method and the specifications of this apparatus are provided in Aben et al []. The rock was subjected to successive loadings at relatively low stress (i.e., below the pulverization threshold) to ensure better control on the amount of damage, similarly to the experiments by Aben et al []. Such experiments result in a pervasive sample‐wide fracture network without losing the cohesion of the rock and without accumulating of large amounts of shear along the fractures [ Aben et al , ].…”

From slow to fast faulting: recent challenges in earthquake fault mechanics

“…Pulverization under high strain rates has been reproduced in the laboratory using Hopkinson impact bars to create short pulses of intense stress on rock samples [24,[46][47][48]. In a recent example of such experimental work, Barber & Griffith [49] argue that the surface energy represents a substantial proportion of the total mechanical energy under extreme loading conditions, possibly an energy sink comparable to the amount of frictional work on a seismic fault.…”