Abstract

We investigate the source parameters of picoseismic and nanoseismic events (Mw>-4.1) recorded with a high-sensitivity seismic network at the Mponeng gold mine in South Africa to gain new insights into the scaling of small seismic events. The Japanese–German Underground Acoustic Emission Research in South Africa (JAGUARS) network, composed of one three-component (3C) accelerometer (sensitivity 50 Hz to 25 kHz) and 8 acoustic emission (AE) sensors, (sensitivity 1 kHz to 180 kHz) is located at a depth of 3268 m and covers the limited volume of approximately 300×300×300 m. The AE sensors are calibrated with respect to the 3C accelerometer in the frequency band 400 Hz–17 kHz. The waveform data of two datasets are analyzed; (1) the aftershock sequence of an Mw 1.9 event that occurred approximately 30 m from our network, and (2) the postblasting activity recorded during working days, located at a distance >90 m from the network at the exploitation level. For the analysis we applied spectral fitting and spectral ratio methods. The calculated values of Mw range from -0.8 down to -4.1 with corner frequencies 0.8 kHz–13.6 kHz (source sizes from 8 cm to 1.3 m). We observe static stress drops ranging from 1 MPa to 10.0 MPa with apparent stresses of 0.01 MPa–1.00 MPa. Stress drops are independent of the moment, suggesting self-similar behavior in the analyzed magnitude range -4.1<Mw<-0.8.

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