Quasi‐static slip patch growth to 20 m on a geological fault inferred from acoustic emissions in a South African gold mine

Abstract

Three months of acoustic emission (AE) monitoring in a South African gold mine down to M w À5 revealed a newly emergent planar cluster of 7557 events À3.9 ≤ M w ≤ À1.8 (typical rupture radius of 6-70 cm) that expanded with time to reach a size of 20 m on a preexisting geological fault near an active mining front 1 km beneath the ground. It had a sharply defined, planar configuration, with hypocenters aggregated within a thickness of only several decimeters. We infer that the zone defines an aseismic slip patch on the fault, wherein the individual AEs represent failures of very small asperities being loaded by the aseismic slip. Additional support for the interpretation was obtained by analyzing composite focal mechanisms and repeating events. The patch expansion over 2 months was likely quasistatic because all individual AEs ruptured much smaller areas than the cluster size at the corresponding time. The b values dropped gradually from 2.6 to 1.4, consistent with a significant increase in shear stress expected of the mining style. Another cluster with similar characteristics emerged later on a neighboring part of the same fault and grew to a 10 m extent in the last weeks of the study period. The quasi-static expansion of inferred localized slow-slip patches to sizes of 10-20 m suggests that the critical crack length on natural faults can be at least as large, much exceeding the decimeter range derived from laboratory stick-slip experiments on saw-cut rocks.

Delineation of large localized damage structures forming ahead of an active mining front by using advanced acoustic emission mapping techniques

“…8(c) and (d)). The continuity of these dominant traces in slice view is also good in the strike direction, and it holds over the cluster's entire extent of $20 m. 25 These features suggest that Cluster N delineates a macroscopic fracture surface itself. Indeed, a corresponding geological fault was identified at the outcrop during mining excavation, and therefore Cluster N was interpreted as onfault AE activity.…”

“…Indeed, a corresponding geological fault was identified at the outcrop during mining excavation, and therefore Cluster N was interpreted as onfault AE activity. 25 In contrast, Clusters B-D define tabular zones of significant (2-4 m) thickness, and their AE distributions do not exhibit the degree of continuity seen in Cluster N. (Cluster A had insufficient events for detailed analysis.) More importantly, the section views ( Fig.…”

“…9(g)) and grew with time. 25 Cluster D, the last and northernmost cluster in the stope-front cloud, emerged only in the last time interval (Fig. 9(h)), several meters ahead of the mining front at the time.…”

“…They most likely represent activity on preexisting geological faults and are discussed elsewhere. 25,26 The RMS residuals of the travel times, calculated for all final source locations determined by JHD (Fig. 4), provided a measure of the quality of the computed source locations for a given velocity model.…”

Steady activity of microfractures on geological faults loaded by mining stress

“…The origin in the x-y plane corresponds to the center of the AE network, and the origin in the z-coordinate corresponds to the level of the horizontal tunnel (gray lines) at 1 km depth. Blue planes with bold black top edges: approximation planes visually fitted for planar AE alignments (Plane VI corresponds to Plane E in Naoi et al, 2015). Red, green, magenta, and cyan hatches: mined-out stopes.…”