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Ogasawara H.,Ritsumeikan University | Kato H.,3D Geoscience | Hofhann G.,AnglogoldAshanti | Roberts D.,AnglogoldAshanti | And 6 more authors.
48th US Rock Mechanics / Geomechanics Symposium 2014 | Year: 2014

Downsizing an overcoring diameter to 60 mm (BX), the Compact Conical-ended Borehole Overcoring technique (BX CCBO) proved suitable for adverse conditions and difficult access from surface in deep levels South African gold mines. BX CCBO in-situ stress measurements have been carried out at seven sites at depths of 1.0 to 3.4 km, the measured maximum principal stresses, a ranging between 76 and 146 MPa. In South African gold mines, such a number of in-situ stress measurements have never been camed out previously in the σ1 and depth ranges relevant to the described studies. The in-situ stress measurements allowed us to confirm that an assumption on far-field (initial) stress gradient was acceptable with only slight modifications. An improved assumption on far-field stress gradient allowed us to better constrain both the stress and strength on the M>2 earthquake faults with σ1 ranging between 100 and 177 MPa. The stress range was also confirmed by analyzing breakouts and core discings ins hole drilled through a M12.1 hypocenter. At one site where σ1 was 100 11 OMPa, a BX CCBO was compared with a CSIRO cell in a good agreement, although some issues were found to address regarding BX CCBO measurements. Copyright © 2014 ARMA, American Rock Mechanics Association. Source


Naoi M.,Kyoto University | Nakatani M.,University of Tokyo | Igarashi T.,University of Tokyo | Otsuki K.,Tohoku University | And 12 more authors.
Journal of Geophysical Research B: Solid Earth | Year: 2015

We observed very small repeating earthquakes with -5.1 ≤ Mw ≤ -3.6 on a geological fault at 1 km depth in a gold mine in South Africa. Of the 851 acoustic emissions that occurred on the fault during the 2 month analysis period, 45% were identified as repeaters on the basis of waveform similarity and relative locations. They occurred steadily at the same location with similar magnitudes, analogous to repeaters at plate boundaries, suggesting that they are repeat ruptures of the same asperity loaded by the surrounding aseismic slip (background creep). Application of the Nadeau and Johnson (1998) empirical formula (NJ formula), which relates the amount of background creep and repeater activity and is well established for plate boundary faults, to the present case yielded an impossibly large estimate of the background creep. This means that the presently studied repeaters were produced more efficiently, for a given amount of background creep, than expected from the NJ formula. When combined with an independently estimated average stress drop of 16 MPa, which is not particularly high, it suggests that the small asperities of the presently studied repeaters had a high seismic coupling (almost unity), in contrast to one physical interpretation of the plate boundary repeaters. The productivity of such repeaters, per unit background creep, is expected to increase strongly as smaller repeaters are considered (∝ Mo -1/3 as opposed to Mo -1/6 of the NJ formula), which may be usable to estimate very slow creep that may occur on intraplate faults. ©2015. American Geophysical Union. All Rights Reserved. Source


Naoi M.,University of Tokyo | Naoi M.,Kyoto University | Nakatani M.,University of Tokyo | Kgarume T.,South African Council for Scientific and Industrial Research | And 18 more authors.
Journal of Geophysical Research B: Solid Earth | Year: 2015

Three months of acoustic emission (AE) monitoring in a South African gold mine down to Mw -5 revealed a newly emergent planar cluster of 7557 events -3.9 ≤ Mw ≤ -1.8 (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. ©2015. American Geophysical Union. All Rights Reserved. Source


Naoi M.,University of Tokyo | Nakatani M.,University of Tokyo | Otsuki K.,Tohoku University | Yabe Y.,Tohoku University | And 9 more authors.
Tectonophysics | Year: 2015

Acoustic Emissions (AE) down to MW~. -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 MW -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. © 2015 Elsevier B.V. Source


Moriya H.,Tohoku University | Naoi M.,University of Tokyo | Naoi M.,Kyoto University | Nakatani M.,University of Tokyo | And 9 more authors.
International Journal of Rock Mechanics and Mining Sciences | Year: 2015

We applied advanced mapping techniques to 291 230 acoustic emission (AE) events as small as around M -4 that were recorded over 50 days by an ultra-high resolution network close to the active front of a tabular mining stope being advanced northward at 1. km depth in the Cooke 4 Gold Mine in South Africa. We first applied joint hypocenter determination (JHD) to improve absolute locations, and then applied the double-difference relative location algorithm to the JHD output. These steps resolved the seemingly continuous, dense cloud of AEs that extend about 20. m ahead of the stope front into several discrete, steeply dipping tabular clusters a few meters thick and 10-30. m in dip extent, separated by quiet intervals a few meters thick. The clusters have a strike parallel to the stope face and a dip of about 65°, resembling commonly observed large shear fractures along the plane of maximum shear (Ortlepp shears). In general, the activity of the clusters changed in similar ways as the stope face advanced, but each cluster remained stationary and the gaps between clusters were impressively quiet. This study demonstrates that high-resolution AE mapping can delineate the formation of large structures of localized damage in the highly stressed intact rock mass ahead of the stope face, a process that may culminate in hazardous seismic events. © 2015 Elsevier Ltd. Source

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