High-Resolution Strain Monitoring During M>2 Events in a South African Deep Gold Mine in Close Proximity to Hypocentres

Abstract

The authors document strain changes up to ~10 -4 associated with two M>2 events 2.4 km deep at distances less than ~100 m from a strainmeter. This corresponds to a ~7 MPa stress change recorded only within the hypocentral area. This change was recorded with a sensitive, wide-dynamic-range, Ishii strainmeter. A 15-m hole was drilled subparallel to the fault strike, in order to continuously monitor slip-driving shear and normal strains with a sampling frequency of 25 Hz. Two M>2 events took place around the fault. For both events, relaxation in the maximum principal stress at a rate of 10 -6 /week was observed for several days prior to the main shock. In one of the two events, foreshocks were concentrated in the last several tens of seconds, accompanied by strain steps and logarithmic post-seismic deformation. However, no acceleration in deformation was observed, even with a resolution of 1/10,000 of a coseismic step for each event.

Small slow‐strain steps and their forerunners observed in gold mine in South Africa

“…The reef on the western side of this fault (chain line in Figure 1a) had been mined when an M3.4 earthquake occurred on this fault. G. van Aswegen (one of the present authors) anticipated another event on this fault, associated with so‐called eastern mining (solid line in Figure 1a), so we drilled a 15‐m hole subparallel to the fault strike from the tunnel and installed the strainmeter about 10 m from this fault (a diamond in Figure 1a) [ Ishii et al , 2000; Ogasawara et al , 2001, 2005; Ogasawara and the Research Group for Semi‐controlled Earthquake‐generation Experiments in South African Deep Gold Mines , 2002]. As anticipated, the mining of the reef on the eastern side of the fault induced seismicity.…”

“…The strainmeter accommodates strain larger than 10 −4 and detects strain as subtle as 10 −9 (see more details in the work of Ogasawara et al [2001, 2005] and Ogasawara and the Research Group for Semi‐controlled Earthquake‐generation Experiments in South African Deep Gold Mines [2002]). The strainmeter has mechanical impulses and step responses that looks like a poorly‐damped oscillation lasting for periods of 1 to 2 seconds.…”

“…We recorded more than 2000 strain steps (10 −9 ∼ 10 −4 ), including a 5.7 × 10 −6 instantaneous strain step (associated with an Mw0.2 earthquake in September 2, 2001), a 7 × 10 −5 (Mw2.9; Feb. 4, 2003), and a 10 −4 (Mw2.0; Apr. 12, 2003) [ Takeuchi , 2005; Ogasawara et al , 2005].…”

“…Long‐term strain changes and seismicity within 250m of our strainmeter at the Bambanani mine (after Ogasawara et al [2005]), and the occurrence of LDR (Little Dynamic Response) events, which is central in this article, as detailed in Section 3, with steps of >10 −7 . Connected blue dots show cumulative number of LDR events highlighted in this study.…”

“…In order to observe earthquake forerunners, the Research Group for Semi‐controlled Earthquake‐generation Experiments in South African deep gold mines (SeeSA) [ Iio and Fukao , 1992; Ogasawara and the Research Group for Semi‐controlled Earthquake‐generation Experiments in South African Deep Gold Mines , 2002] attempted to continuously monitor strain changes with a resolution of 24 bit 25 Hz at the Bambanani mine, Welkom, beginning in 1998 [ Ishii et al , 2000]. An Ishii four‐component borehole strainmeter [ Ishii et al , 1997] was installed at the mine, and many strain steps (<10 −4 ) were recorded associated with the seismicity (<Mw 2.9) within 100 m from the strainmeter [ Takeuchi , 2005; Ogasawara et al , 2005]. So we checked whether these steps were preceded by accelerations in strain.…”