Abstract

In December 1971, the Idaho National Engineering Laboratory deployed a network of short-period high-gain vertical seismographs to monitor earthquake activity within a designated study area (43.0° to 44.5°N, 111.5° to 114.0°W) centered on the eastern Snake River Plain. In the 11 yr prior to the 28 October 1983 Borah Peak, Idaho, earthquake, there were 102 events located within the study area with a duration magnitude (M) range of 0.7 ≦ M ≦ 3.2. Three microearthquakes (M ∼ 1) were detected on the eastern Snake River Plain. Seventy-five microearthquakes were located northwest of the eastern Snake River Plain, primarily in the Birch Creek Valley-Beaverhead Mountains region. During the 11-yr period that the Idaho National Engineering Laboratory seismograph stations have been active, there were only two events located inside the study area within 35 km of the epicenter of the Borah Peak, Idaho, earthquake. In the 2.6 yr prior to the Borah Peak, Idaho, earthquake, there were no foreshocks with magnitudes ≧ 1 observed within a 20 km radius of the epicenter. Future large earthquakes in the Lost River Range, Lemhi Range, and segments of the Beaverhead Mountains may also occur without significant foreshock activity.

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