Focal depths of Witwatersrand tremors*

Abstract

Earth tremors originating at a single Witwatersrand mine were recorded by six vertical seismometer stations all within about 10,000 ft. (3.0 km.) of the foci. A triggered system of recording permitted the first arrivals to be timed to 5 milliseconds, giving a location accuracy of about 250 ft. (75 m.). All the 100 foci determined were in close proximity to the position of active mining, or slightly above it, and had a mean depth of 7,500 ft. (2.3 km.). Eighty per cent of the first movements observed were downward, i.e., cataseismic. The implications of these results are discussed.