In the Sunnyside mining district, Utah, coal is mined under thick and variable overburden which is locally weakened by faults and other structural discontinuities. Stress changes and local stress concentrations produced by mining under these conditions often cause sudden and violent ruptures in the coal and surrounding rock mass. The strain energy released by this type of failure, which can produce shock waves and may discharge coal and rock with explosive force, is often a serious threat to life and property. These releases of strain energy are called bumps or bounces by miners if they occur in the coal, and rock bursts if they occur in the surrounding rock mass. Many of these releases are so violent that they generate seismic waves that can be felt, or at least detected by seismic instruments, miles from the site of the rupture, whereas others are smaller and can be detected only by those sensitive seismic instruments within a few thousand feet of the site of the rupture.
In 1969 and 1970, about 27,000 and about 15,000 earth tremors, respectively, were recorded by the five-station seismic monitoring network that is located at the surface and encompasses most of the mine workings in the district. Of these totals, 512 and 524 earth tremors, respectively, were of sufficient magnitude (greater than 1. 5 on the Richter scale) so that the hypocenters could be accurately located. In 1968 about 20,000 tremors were recorded, with 281 large enough to plot, but in 1967 over 50,000 were recorded, of which 540 were plotted.
In this report we discuss the way in which seismic activity, geology, and mining are related or seem to be related for the period 1967 through 1970, with emphasis on the period 1969-70. We also suggest certain mining procedures which, based on studies during the period, might increase the safety and efficiency of mining operations in the Sunnyside district. A complete tabulation of the larger magnitude earth tremors which occurred during 1969-70 and descriptions of how they relate to mining are given in the appendix. Similar tabulations and descriptions for 1967 and 1968 are available in earlier reports (Barnes and others, 1969; Dunrud and others, 1970).