Coal in the Powder River Basin of Wyoming and Montana (fig. 1) is greatly in demand. It can be economically mined from the surface, and the land is much easier to reclaim than land above underground mines that has been damaged by subsidence or by underground fires. Exploitation of the vast coals resources in the basin has caused a rapid influx of population into the area and an associated increase in building construction. The activity has also generated concern about possible regulation of development. With these facts in mind, the U.S Geological Survey has been investigating the geologic implications of development in the Powder River Basin. One area of study has been the effect of earth tremors caused by blasting in the large coal surface mines or by naturally occurring earthquakes.

The basin proper was classified by Algermissen and Perkins (1976) as belonging to a region of low seismic risk. Naturally occurring earthquakes are, therefore, mainly of scientific interest, unless of course there was an isolated event that occurred near a populated area or a surface mine.

We are not aware of any damage to people or to property caused by blasting in the coal surface mines even though thousands of tons of explosives are detonated each year in the basin. The maximum weight of an individual explosive charge and the time interval between blasts are regulated so that any nearby structures will not be damaged or the residents disturbed. Blasting, nevertheless, does produce seismic tremors that can be recorded over 200 kilometers away. In addition, at one mine, some very low order aftershocks were recorded relatively close to the source within 2 hours after blasting.