Seismic events miss important kinematically governed grain scale mechanisms during shear failure of porous rock

Abstract

Catastrophic failure in brittle, porous materials initiates when smaller-scale fractures localise along an emergent fault zone in a transition from stable crack growth to dynamic rupture. Due to the rapid nature of this critical transition, the precise micro-mechanisms involved are poorly understood and difficult to image directly. Here, we observe these micro-mechanisms directly by controlling the microcracking rate to slow down the transition in a unique rock deformation experiment that combines acoustic monitoring (sound) with contemporaneous in-situ x-ray imaging (vision) of the microstructure. We find seismic amplitude is not always correlated with local imaged strain; large local strain often occurs with small acoustic emissions, and vice versa. Local strain is predominantly aseismic, explained in part by grain/crack rotation along an emergent shear zone, and the shear fracture energy calculated from local dilation and shear strain on the fault is half of that inferred from the bulk deformation.

Precursory Signals to Injection Induced Fault Reactivation in the Laboratory Using Active Ultrasonic Monitoring Methods

“…While the general pattern of fluid injection is still visible (Figure 7d), the more subtle trend of fault reactivation is lost by the more complicated slip along the rough fault plane (Figures 8b and 8d). Cartwright‐Taylor et al., 2022 described crack rotation with antithetic slip as an additional mechanism for local stress rotation and slip, allowing shear along more unfavorably orientated faults, which among fracturing and crushing of the grains influence the scattering, therefore causing a more incoherent K compared to sliding along the smooth fault surface, where we won't expect this fault rotation and antithetic slip.…”

“…For induced seismicity, forecasting can be done using probabilistic models (Király‐Proag et al., 2016; Langenbruch et al., 2018), which can include various production scenarios (Dempsey & Suckale, 2017). In laboratory settings, frictional sliding experimental studies have been performed using passive acoustic monitoring (Cartwright‐Taylor et al., 2022; Guglielmi et al., 2015; Noël et al., 2019; Ye & Ghassemi, 2020), mostly to investigate fault mechanics and often to target the onset of the first small and precursory slip events of the pre‐seismic phase. However, robust, and reliable predicting of fault failure and the resulting earthquake has proven to be a challenging task (Geller, 1997; Hough, 2009; Kagan & Jackson, 1991; Pritchard et al., 2020), even for experimental faults under controlled laboratory settings (Main & Meredith, 1989).…”

“…Generally speaking, stress changes can be inferred by analyzing the change in acoustic or seismic velocity (Cartwright‐Taylor et al., 2022; Xie et al., 2018). For intact rocks, it has been shown that seismic velocities change in response to stress, for example, due to compression of the rocks (Barnhoorn et al., 2018; Winkler & Nur, 1979).…”

Deformation evolves from shear to extensile in rocks due to energy optimization

“…Indeed, a recent analysis highlights that acoustic emissions may not detect important kinematic deformation events during triaxial compression [21]. Moreover, comparing the numbers of acoustic emissions classified as extensile or shear may be misleading because the frequency-magnitude distribution of acoustic emissions in many experiments is similar to the Gutenberg-Richter relation for earthquakes [3], and so a population of acoustic emissions at a given time includes a wide range of event magnitudes.…”

Effect of the loading condition on the statistics of crackling noise accompanying the failure of porous rocks

“…Acoustic emissions (AE) are the primary source of information about the microscopic processes of fracturing [5][6][7][8] providing us with valuable data about the temporal and spatial evolution of the ensemble of micro-cracks including the ability to discriminate between microcracking events dominated either by local tension or shear [9]. In a recent breakthrough, the evolution of the spatial structure of damage can now be seen directly by synchrotron micro-computer tomography imaging of live tests [10,11]. The acoustic 'crackling noise' generated during the compressive failure of rocks in laboratory experiments has been found to exhibit scale-free statistics similar to earthquakes, consistent with the universality of cracking phenomena across a broad range of length scales [2,4,[12][13][14][15][16].…”