Major earthquakes that rupture up to the ground surface and form fault scarps are rare occurrences in the Intermountain West during historical times. For an earthquake to generate surface rupture it typically needs to have a magnitude of 6.5 or greater.
Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week’s contribution is from Mike Stickney, director of the Earthquake Studies Office at the Montana Bureau of Mines and Geology.
Only three historical earthquakes have produced surface ruptures in the Intermountain West; the magnitude 6.6 Hansel Valley earthquake in 1934 near the Great Salt Lake, the magnitude 7.3 Hebgen Lake, Montana, earthquake in 1959, and the magnitude 6.9 Borah Peak, Idaho, earthquake in 1983.
So, a surface-rupturing earthquake presents a rare opportunity to study a modern analog of the hundreds of prehistoric earthquakes that formed fault scarps that crisscross the Intermountain West. Recent technological developments are providing new insights into the details of earthquake geology and fault scarp formation, even decades after they formed.
In 2014, airborne lidar data were collected along the Hebgen Red Canyon faults, which are the primary faults that ruptured in the 1959 Hebgen Lake earthquake. Geoscientists used these data to augment previous studies of these spectacular fault scarps. By construction of more than 440 detailed topographic profiles across the fault scarps, they measured the shapes of the scarps and calculated the amount of slip along the faults. These fault-slip measurements (known as fault throw) were compared with field measurements collected by USGS geologists shortly after the 1959 earthquake.
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