ABSTRACT
A near source observation of mining induced earthquakes was conducted at 2.6 km beneath the ground in a deep gold mine in South Africa. About 20,000 events with hypocenter distances shorter than 200 m were recorded by using a high sampling rate and wide dynamic range observation system. Small phases preceding the large amplitude P-onsets were found in the seismograms for the 6 largest events whose seismic moments ranged from 5.0 × 1010 to 1.4 × 1011Nm. Duration of the small initial phase found in the present study is comparable with those reported previously for natural earthquakes with comparable seismic moment that were recorded at 7 to 11 km from their hypocenters. On the other hand, the waveform of the small initial phase is much complicated and rather resembles those of the moderate to large earthquakes.
