Fault-Specific Earthquake Sources
The faults closest to INL facilities are the Lost River, Lemhi, and Beaverhead faults. They are normal faults and are located along the base of the mountains. Geologic, geophysical, and geodetic investigations indicate that these faults terminate at the same locations where the mountains end along the western boundary of INL. Potential earthquakes along these normal faults have been studied in detail for INL probabilistic seismic hazards assessments to estimate the following:
- Maximum earthquake magnitudes
- Distances to INL facilities
- Dates of when the last earthquakes occurred
- Intervals of repeat times or how often earthquakes have occurred in the past.
The results of paleoseismic investigations (or fault trenching) indicate that the normal faults are capable of generating earthquakes of magnitude 6.6 to 7.2. Earthquakes of this size occur because normal faulting earthquakes rupture fault segments 20 to 30 km (12.4 to 18.6 miles) long and at depths of about 15 km (9.3 miles) where crustal rocks have higher strengths, resulting in greater energy release over larger fault areas. Similar rupture dimensions were observed during the 1983 Borah Peak earthquake, which ruptured the central segment of the Lost River fault.
The most recent earthquakes occurred more than 15,000 years ago. Soil samples from fault trenches excavated across the Lost River and Lemhi fault segments closest to INL indicate that the most recent earthquakes occurred 15,000 to 24,000 years ago. The most recent earthquake on the southern segment of the Beaverhead may have occurred 30,000 years ago. Because of the longer recurrence intervals (or repeat times), these fault segments contribute more to the ground motion levels at a return period of 10,000 years.