GEOLOGIC MAPPING IN THE RUBY DAM AREA, SOUTHWEST MONTANA, WITH EMPHASIS ON EARTHQUAKE HAZARDS

Western Montana, to few people’s knowledge, is a very seismically active region and dams are particularly vulnerable to earthquakes. Ruby Reservoir Dam, located near Alder, MT, is a high hazard dam because of the amount of water it impounds and the loss of life that would occur if it were to fail. Since, Western Montana is seismically active, site specific geology of the newer sediment along with geophysical methods were done around Ruby Reservoir to try and locate younger faults that have not been previously mapped. If there are any young faults located, then a seismic susceptibility analysis will be performed on the dam to determine if any earthquake hazards could occur.

During the 2001 field season data was collected by mapping the region and also with geophysical surveys. To date a geologic map of the area is being compiled at a 1:12,000 scale and the geophysical data is being interpolated. There were no surface expressions found to suggest any Quaternary faulting in the area but the geophysical data, when completely interpolated, could show faulting just below the surface, which could have impacts on the dam.

The presentation would include: what field methods were used (mapping and geophysical methods), purpose of the research, previous work done in the area, stability of the Ruby Dam, and anything else that would apply.