Source Mechanisms of Laboratory Earthquakes During Fault Nucleation and Formation

Abstract

Seismic data provides key information on the physics of the fracture process ranging from fracture nucleation, crack growth, and damage accumulation, to crack coalescence and strain localization. Several micromechanical models have been developed over the years which seek to describe failure modes (e.g., Ashby & Hallam, 1986;Kemeny & Cook, 1991), taking into account pore-emanant fracturing (Baud et al., 2014), sliding wing cracks (Baud et al., 2014), friction effects (McClintock, 1962), and pore collapse (Zhu et al., 2010). To link these models to geophysical signatures recorded at the field scale, controlled laboratory rock deformation experiments equipped with dense microseismic arrays have become a routinely used tool (e.g., Benson et al., 2007;Fazio et al., 2017;Lockner et al., 1992). Here, fault growth may be considered analogous to the field scale development of earthquake rupture generating acoustic emission (AE), which is a wellused analog to tectonic earthquakes due to the scale invariance of these processes (Hanks, 1992;Hatton et al., 1994;Hudson & Kennett, 1981). The inclusion of AE sensors is now a routine laboratory rock physics method in the investigation of fault zone structure with the added benefit of a controlled environment.There is an extensive literature reporting the evolution of fracture mechanisms inferred from the analysis of AE (e.g., tensile, shear, or compaction) that occur as damage propagates (e.g., Stanchits et al., 2006;Zang et al., 1998). Triaxial rock deformation experiments on fine-grained granites suggest that this process is tensile dominated (Cox & Scholz, 1988), whereas a higher proportion of shear-components are found in coarser-grained materials (Lei et al., 1992). This hypothesis is further supported by new observations linking macroscopic shear fracture to microcrack development prior to the yield point (Lei et al., 2000), highlighting the occurrence of tensile fracturing at the front of a shear process zone. These scenarios can

Rate Effect on the Direct Shear Behavior of Granite Rock Bridges at Low to Subseismic Shear Rates

“…At present, the quantitative constraint of this ratio in the laboratory experiment remains challenging (Chen & Spetzler, 1993; Cheng & Wong, 2018). The technique of acoustic emission (AE) could help quantitatively characterize the evolution of microcracks with different cracking mechanisms (King et al., 2021; Lei et al., 2004; Stanchits et al., 2006) and will be included in our future work.…”

Acoustic Emissions in Rock Deformation and Failure: New Insights from Q-Statistical Analysis

“…fabric, porosity, grain size, and cementation) different lithologies such the Darley Dale Sandstone (DDS, porosity 14%) and Alzo Granite (AG, porosity < 1%). The AE data sets have been analysed in terms of source mechanisms time and spatial evolution before the rupture from some of the authors [16]. AE source mechanisms analysis has evidenced single fracture nucleation for AG and multiple fracture nucleation for DDS due to single or multiple competing dilatant and compactant regions.…”

“…AE source mechanisms analysis has evidenced single fracture nucleation for AG and multiple fracture nucleation for DDS due to single or multiple competing dilatant and compactant regions. Fracture growth and propagation appears controlled by the different confinements, with increasing pressure controlling the time evolution and size of dilatant and compactant regions, eventually controlling cracks coalescence into macroscopic fractures [16].…”

“…For AG, a single damage cycle of crack nucleation and growth is sufficient to propagate fractures and develop the planar localization leading to dynamic failure. Whilst in DDS it can take multiple cycles of nucleation for coalescence to take place due to interacting mechanisms induced by multiple fracture nucleation sites [16]. Thus, the DDS shows a much clear premonitory phase, or foreshock, before a critical damage threshold which would allow the coalescence into a larger scale deformation structure.…”

“…Here, fault growth may be considered analogous to the field scale development of earthquake rupture generating acoustic emissions, which is a well-used analog to tectonic earthquakes due to the scale invariance of these processes [13][14][15]. Source mechanisms evolution with respect varying effective pressure can also be inferred from AE, allowing to determine the effect of the increasing lithostatic pressure on the fracturing mechanisms and failure mode [16].…”