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1983 Borah Peak, Idaho Earthquake

The 1983 surface-wave magnitude (Ms) 7.3 Borah Peak, Idaho earthquake ruptured the central portion of the NW-trending Lost River Fault, producing a 36-km (22-mile) long surface fault scarp with a maximum of 4.2 m (14 ft) height. The earthquake hypocenter was located at a depth of about 16 km (or 10 miles). The Borah Peak mainshock and aftershocks defined a planar normal fault dipping 49 degrees to the southwest.

This earthquake caused substantial damage to unreinforced masonry buildings in the towns of Challis and Mackay, Idaho and resulted in two deaths in Challis, Idaho. The INL Site was located in Modified Mercalli Intensity Zone VI during the earthquake.

Inspections of existing INL facilities following the earthquake revealed no structural or component damage that would compromise structural integrity at INL facilities, including the Advanced Test Reactor (ATR). Ground motions from the Borah Peak earthquake exceeded the 0.01 g trip threshold level and triggered the ATR Plant Protective System, which automatically notified the reactor and it safely shut itself down.

At the time, 13 of 15 operating strong-motion accelerographs recorded ground motions from the Borah Peak earthquake. The strong-motion accelerographs were located 89 to 110 km (55 to 68 miles) from the earthquake’s epicenter. Peak horizontal accelerations recorded at INL ranged from 0.022 to 0.078 g for basement and free-field sites. The tri-axial acceleration data were digitized, corrected, and processed to produce acceleration, velocity, and displacement time-histories and response and Fourier spectra. The INL free-field acceleration data are contained in the Pacific Earthquake and Engineering Research Center.

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