Steady activity of microfractures on geological faults loaded by mining stress

Abstract

Acoustic Emissions (AE) down to M W~-4 were recorded at a site 1 km beneath the surface in the Cooke 4 Mine, South Africa. Several planar AE clusters with lateral extent of 10-100 m were identified. Most of them were located several tens of meters away from the mining front, and exhibited steady activity during the analysis period of about two months. Some of the clusters coincided with mapped faults. The planar-cluster AEs were sharply aggregated within a thickness of several decimeters, likely delineating the fracture interface of the fault and its higher-order morphology such as branches, bends, and stepovers. The composite focal mechanism evaluated for each cluster was consistent with slip events on the fracture interface. These results imply that numerous shear microfractures occur steadily on a natural fault surface subjected to a mining-related stress increase. The planar clusters consist of very small AEs (99.7% were smaller than M W -2), exhibiting high b-values much exceeding unity. This contrasts with the more usual b-values of the stope-cluster AEs, which were aggregated within 20 m of the mining front and exhibited a more scattered distribution. The size distribution of microfractures on a fracture interface may directly reflect fine-scale irregularities of the interface. On the other hand, many other mapped faults near the planar AE clusters were not accompanied by AE activities, despite the fact that these quiet faults were subjected to a similar stress history. The presence or absence of AE activities on a fault may reflect different states of the fault, including stress and strength.

Migration of earthquakes with a small stress drop in the Tanzawa Mountains, Japan

“…One possibility would be that the errors were underestimated. Another possibility is that the earthquakes took place on a couple of parallel planes (e.g., Naoi et al 2015). Unfortunately, it is quite difficult to investigate which would be the most plausible explanation from our results.…”

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

“…8a-d) of Clusters B-D lack narrow, dominant traces that likely delineates the macroscopic fracture surface of preexisting discontinuity. 26 The location accuracy of the events in Clusters B-D is almost the same as that of the events in Cluster N (i.e., better than 30 cm in the N-S direction) because they are similarly situated with respect to the observation network. Therefore, the observation that Clusters B-D have a thickness of 2-4 m must be real and not the result of location error.…”

“…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.…”

Study on the law mining stress evolution and fault activation under the influence of normal fault

“…In the horizontal direction, the trapezoidal distribution load is 0.5 times that of the vertical direction; Y direction displacement is restricted before and after the Y boundary, and the Z direction displacement is restricted at the bottom surface, and the other surface displacement is set free. The fault zone is simulated with 1 m wide weak and soft zone (Naoi et al, 2015). In the process of simulation, the Mohr-Coulomb failure strength criterion is adopted (Jiang et al, 2014).…”

“…The characteristics of stress change near fault were studied using numerical simulation (Islam et al, 2009). By monitoring the AE activity of the fault zone, the influence of mining activity on the fault zone was analyzed (Naoi et al, 2015).…”