Faulting of Rocks in a Three-Dimensional Stress Field by Micro-Anticracks

Abstract

Nucleation and propagation of a shear fault is known to be the result of interaction and coalescence of many microcracks. Yet the character and rate of the microcracks' interactions, and their dependence on the three-dimensional stress state are poorly understood. Here we investigate formation of microcracks during sandstone faulting under 3D-polyaxial stress fields by analyzing multi-stationary acoustic waveforms. We show that in a true three-dimensional stress state (a) faulting forms in a orthorhombic pattern, and (b) the emitted acoustic waveforms from microcracking carry a shorter rapid slip phase. The later is associated with microcracking that dominantly develops parallel to the minimum stress direction. Our results imply that due to inducing the micro-anticracks, the three-dimensional (3D) stress state can quicken dynamic weakening and rupture propagation by a factor of two relatively to simpler stress states. The results suggest a new nucleation mechanism of 3D-faulting with implications for earthquakes' instabilities, as well as the understanding of avalanches associated with dislocations.

Microscopic Evolution of Laboratory Volcanic Hybrid Earthquakes

“…Similar multistages relaxations have been reported in recorded dynamic slip profiles and stresses measured in rock friction experiments (Fig. S2)2426272829.…”

“…The first intriguing result of the analysis is that we find that ultrasound excitations possess patterns of temporal evolution of network parameters that are universal among recorded events22232425. The appearance of universal patterns in any measure of excited signals shows the robustness of the collective process in the source(s) against the much faster scattering processes.…”

“…From this perspective, the system relaxes in multiple stages and time scales rather than single time scale. In general, the aforementioned phases are typical for dry events and simple friction tests conducted under dry conditions2223242526 (Fig. S2).…”

A Directional Crack Damage Memory Effect in Sandstone Under True Triaxial Loading

“…More recently, Browning et al () confirmed the presence of the Kaiser effect during true triaxial stressing experiments but concluded that the effect is more accurately a damage memory rather than a stress memory effect. In general, the state of stress in the crust is triaxial (i.e., σ 1 > σ 2 > σ 3 ; Zoback & Zoback, ), and results from experiments that recreate these more realistic stress conditions have noted significant effects on rock strength (Haimson & Chang, ), crack densities, crack orientations (Browning et al, ; Ghaffari et al, ), and fluid flow characteristics (Nasseri et al, ). A Kaiser effect has also been observed at active volcanoes that are subject to cyclic deformation.…”

Acoustic characterization of crack damage evolution in sandstone deformed under conventional and true triaxial loading

“…In some cases these new cracks have a highly anisotropic orientation distribution, and the resulting changes in acoustic wave velocities are therefore also highly anisotropic. This anisotropy can be measured using an array of velocity transducers and by measuring P and S wave velocities concurrently [Schubnel et al, 2006;Ghaffari et al, 2014;Nasseri et al, 2014;Brantut, 2015]. While elastic opening and closure of microcracks is, by definition, reversible, inelastic processes will result in permanent changes to the void space (pores and cracks), and therefore permanent changes in the wave velocities [Han, 2016].…”