Boettcher, Margaret

1983 Borah Peak Earthquake, M 7.3

We unfortunately did not have enough time to visit the reportedly amazing fault scarp produced by 1983 Borah Peak Earthquake. However, we still discussed this event, the largest recorded earthquake in Idaho, at Craters of the Moon.

On October 28, 1983, a magnitude 7.3 earthquake shook the northwestern United States. The epicenter was located just south of Challis, Idaho where the earthquake triggered a landslide that subsequently collapsed a schoolhouse killing two children. Twenty-four years earlier, at the nearby Hebgen Lake in Montana, a M7.5 earthquake occurred. During our visit to Hebgen Lake region, we stood on the pile of primarily gneiss rubble (21 million cubic meters) that slid down the canyon wall at an estimated rate of 160 km/hour, taking the lives of 26 campers, damming the Madison River, and creating Earthquake Lake.

Scarp of the Madison Slide, formed as a result of the M7.5 Hebgen Lake earthquake in 1959 in southern Montana.

These two large earthquakes are located with in the parabolic zone of seismicity surrounding the Snake River Plain. This is one of the most seismically active regions in the United States. Many faults have been mapped in the region and in particular a trench was dug in 1976 across a Holocene fault scarp. The Borah Peak Earthquake in 1983 produced a 34km long surface rupture, which crossed the 1976 trench. This provided an excellent opportunity to study various properties of large earthquakes in the region. Many of the same fault strands, last active about 10,000 years ago, were reactivated in the 1983 earthquake. The similarity of the displacements and ground breakage patterns indicate that the recent Borah Peak earthquake was very similar in magnitude and character to the most recent prehistoric event.

The similarity in magnitude and extent of the 1983 Borah Peak Earthquake and its prehistoric predecessor supports the Characteristic Earthquake Model. This model predicts large, recurrent earthquakes. The Characteristic Earthquake Model has been suggested for regions of long lived fault systems (e.g. San Andreas), while the Gutenberg-Richter Model, in which more earthquakes occur in every smaller magnitude interval, is most often thought to characterize less well developed fault zones. The indication that some faults in the Snake River Plain behave according to the Characteristic Earthquake Model provides insight into the processes of earthquake faulting in general.

There were many aftershocks following the 1983 event. Twenty earthquakes of M>4 occurred and hundreds of smaller earthquakes were recorded in the months following the mainshock. No foreshocks greater than M2 occurred in the preceding two months. The 1983 Borah Peak Earthquake resulted in many landslides and rock-falls. The zone of deformation spanned a 35km wide region, but the effects of the earthquake were felt hundreds of km away from Calgary in Canada, to Salt Lake City, Utah. Eruption rates of geysers were altered in regions of Yellowstone National Park and liquefaction in sediment fill created craters 75 feet in diameter. Four elk hunters witnessed the earthquake rupture and production of the fault scarp.