Microtremor sequences and tilting in a deep mine

Foreshock-aftershock sequences of two mine tremors, of magnitudes 1.5 and 1.2 were studied using a highly-sensitive seismic array and a tiltmeter in the source regions of the tremors. The tremors occurred at a depth of about 3.2 km in a deep gold mine adjacent to the array of six geophones and within 120 m of the tiltmeter. The seismic coverage of the sequences was complete down to magnitudes of −3.5; the tilt resolution ranged from 6 × 10−8 radians to 6 × 10−7 radians at the times of the sequences. In the first hour after each event 135 to 140 micro-aftershocks were recorded with a rate of occurrence that is generally proportional to the rate of tilt, diminishing with time, t, after the mainshock according to A(1+kt), where A and k are constants. A careful study of the records of microtremors and tilt before the main stocks revealed no indications of impending failure. Before the tremor of M = 1.5 the seismicity was at its normal ambient level of about 4 events per hour. Although the seismicity was unusually high before the event of M = 1.2 at about 30 events per hour, there was no aspect of this microshock activity suggesting that a larger tremor would follow. Similarly, analysis of the tilt records failed to produce any evidence of an instability developing before either of the tremors. The magnitude distributions, described by Log N = a − b M, for the microshocks (−3.5 ≦ M <O) are consistent with those reported previously for shocks with M ≧ O. Previous results indicating that b is stable to changes in the aseismic strain rate, θ˙, and that a = log θ˙ + K, were extended to much higher strain rates in this study.