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Potchefstroom, South Africa

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

Van Aswegen G.,Institute of Mine Seismology | Stander M.,Open House Management Solutions
Journal of the Southern African Institute of Mining and Metallurgy | Year: 2012

The geometry and morphology of a set of low-angle fractures around a stope in a deep Witwatersrand gold mine are explained in terms of extension fractures forming under variable conditions of stress. Primary extension fractures (E1) form some distance ahead of an advancing stope along the σ1, σ2 plane. With stope advance, a couple of these fractures end up in a stress regime conducive to transpressional shear and a secondary set of extension fractures (E2) is formed at a high angle to the primary fractures. i.e. at a low angle to the stope. As the E2 fractures are undermined, they migrate into a stress regime of transtensional shear and a third set of extension fractures (E3) may develop between E2 fractures. These have sigmoidal shapes, being parallel to the E2 fractures at the E2 discontinuity where σ3 is negative, and curved through the unfractured rock between E2 fractures where σ3 is positive at the instant of fracturing. The fractures all display fractographic features characteristic of dynamic extension failure with striae indicative of the direction of rupture propagation and the local, instantaneous orientation of σ1. © The Southern African Institute of Mining and Metallurgy, 2012. Source

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