The Borah Peak, Idaho Earthquake of October 28, 1983—Summary

Abstract

The Borah Peak, Idaho earthquake of October 28, 1983 occurred at 8:07 a.m. Mountain Daylight Time. This earthquake, which had a Richter magnitude of 7.3, was the largest earthquake to occur in Idaho in recorded history (since 1872). The epicenter was located approximately 30 km northwest of the town of MacKay (Fig. 1), in central Idaho (see also Taylor, et al). The earthquake shook eight northwestern states. Some minor damage to buildings was reported as far away as Salt Lake City, Utah and Boise, Idaho. The epicenter is located in the center of large Zone 2 seismic area, as designated in the 1982 Uniform Building Code. This area was an area of low historic seismicity and had previously been designated a Zone 3 area in the 1979 UBC. Two small Idaho communities are located some distance beyond the ends of the fault scarp. MacKay is approximately 21 km (13 mi.) southeast of the earthquake epicenter and 24 km (15 mi.) from the southern end of the scarp. Challis is located approximately 60 km (40 mi.) north-northwest of the epicenter and 19 km (12 mi.) beyond the northern end of the surface faulting (Taylor et al). Challis has the largest population which varies from 1,000 to 2,500 depending on the time of the year. MacKay is the second largest, with a population of approximately 500. The balance of the 4,000 to 5,000 people that live in the area live on ranches and in small mining communities.