Source Parameters of Picoseismicity Recorded at Mponeng Deep Gold Mine, South Africa: Implications for Scaling Relations

We investigate the source parameters of picoseismic and nanoseismic events (M-w >-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 x 300 x 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 M-w 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 M-w 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 < M-w <-0.8.