Historical Earthquake Activity
The historical catalog from 1872 to 2007 shows earthquakes of magnitude (M) 2.0 and greater occurred primarily in the Basin and Range (mountains and valleys). This dramatically contrasts the small number of earthquakes that have occurred in the Snake River Plain. Earthquakes occur along normal faults that formed the Basin and Range as a result of the tensional stresses in the earth’s crust. This process has occurred for about 16 million years and is still active today. Two large historic earthquakes, the 1983 surface-wave magnitude (Ms) 7.3 Borah Peak, Idaho and 1959 Ms 7.5 Hebgen Lake, Montana earthquakes ruptured along such normal faults (where the mountains move up and the valleys drop down). Since crustal extension in this region occurs slowly over millions of years, repeat times of earthquakes with M > 7.0 are on the order of thousands of years. For example, the earthquake that occurred prior to the 1983 Borah Peak earthquake occurred 5,000 to 7,000 years earlier.
The historical earthquake catalog also shows that the Snake River Plain is seismically quiet (or aseismic) relative to the surrounding active Basin and Range region. The largest earthquake that may have occurred within the Snake River Plain is the 1905 M 5.7 Shoshone earthquake. The November 11, 1905, Shoshone earthquake occurred before there was instrumental monitoring in Idaho and, because its location was based on felt reports, it may have an error of 100 km or more. Modified Mercalli Intensity (MMI) zones that were assigned based on damage reports documented at the time of the earthquake indicate the epicenter may be to the south of the Snake River Plain. Because the official epicenter is located within the Snake River Plain, INL seismic design criteria include ground motion contributions from an earthquake similar in size to 1905 Shoshone earthquake occurring at INL.