Hebgen Lake fault scarp in 1959. (Photo by J.R. Stacy | USGS)
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. 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. Along some sections of the faults, the lidar-based and field-measured throws agreed well.
Along other fault sections, however, the lidar-derived fault throws were up to three times larger than those measured in 1959. This discrepancy indicates that the lidar data are also detecting fault throws from one or two prehistoric earthquakes in addition to the 1959 earthquake. Other sections of the faults, specifically the ridge section of the Red Canyon fault, showed only 1959 movement. Researchers argue that prehistoric scarps that likely existed were not preserved on the steep slopes in the talus and debris that characterize this fault section.
While mapping the fault scarps soon after the 1959 earthquake, USGS geologists noted several gaps where scarps did not form for short distances along the surface trace of the Red Canyon and Hebgen faults. These gaps are also observed in the lidar data and appear to reflect structural complexities in the near-surface fault geometry. Several other gaps in scarp formation were observed in the lidar data where USGS geologist documented post-earthquake surface offsets. Apparently at some locations, scarps have been obliterated by 55 years of erosion and/or human disturbance of the land surface.
The lidar-derived fault throw along the entire surface rupture zone of the 1959 earthquake was used to estimate a moment magnitude—the magnitude of the earthquake based on the amount of fault slip multiplied by the area over which slip occurred—of 7.1 ± 0.2. Although not as accurate as a seismologically determined magnitude, this independent determination based on surface displacement agrees well with the reported magnitude of the 1959 earthquake and provides an excellent tool for estimating magnitudes of prehistoric earthquakes, for which no seismic data are available.
The new lidar data, acquired 55 years after the earthquake, reveal new details about the 1959 earthquake scarp complexity and extent. These new data also show that, despite the lack of technology at the time, geologists made remarkably good observations and interpretations of the 1959 earthquake right after it occurred. Without the benefits of GPS, computers with mapping software, and other technologies we now take for granted, and using only air photos and 1:62,500 (15-minute) scale topographic maps, the geologists created excellent maps of the 1959 earthquake fault scarps. Post-event studies of future surface-rupturing earthquakes in the Intermountain West will benefit from the suite of modern technology-based tools and improve our understanding of major earthquakes in ways not dreamt possible by geologists a half-century ago.
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.
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