The results are presented of an investigation of the foci of Witwatersrand earth tremors by means of 5 small mechanical seismographs. Timing accuracy of about 0.05 sec. was obtained by the use of synchronous electric clocks at each station, the time marks from which were correlated daily by means of radio time signals. The instruments are described, and an account of the locating procedure is given. The average velocities for the area were found to be Vp = 5.40 ± .07 km/sec., Vs = 3.19 ± .03 km/sec. The energy in a large tremor was estimated to be of the order of 1018 ergs.
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