Caldera Records: Lidar Helps Understand Deadly 1959 Hibjin Earthquake | wild montana

Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators at the Yellowstone Volcano Observatory. This week’s contribution is from Mike Stickney, director of the Office of Seismic Studies at the Montana Bureau of Mines and Geology.

Major earthquakes that rupture to the Earth’s surface and form faults are rare in West Intermountain during historical times. For an earthquake to cause a surface rupture, it must have a magnitude of 6.5 or more.

Only three historical earthquakes have produced surface faults in the West Intermountain. The 1934 Hansel Valley earthquake of magnitude 6.6 near Salt Lake, the magnitude 7.3 earthquake of Hebgen Lake, Montana, the 1959 earthquake, and the 6.9 magnitude earthquake in Idaho in 1983.

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Therefore, the surface-rupturing earthquake presents a rare opportunity to study a modern counterpart to the hundreds of prehistoric earthquakes that formed faults crisscrossing the West Intermountain. Recent technological advances are providing new insights into the details of earthquake geology and fault gap formation, even decades after they were formed.

In 2014, airborne lidar data was collected along the Hibjin Red Canyon Faults, the primary faults that erupted in the 1959 Lake Hibjin earthquake. Geologists have used this data to augment previous studies of these amazing faults. By creating more than 440 detailed topographic profiles across fault walks, they measured the shapes of the outcrops and calculated the amount of slip along the faults. The slip measurements (known as fault throwing) were compared to field measurements collected by USGS geologists shortly after the 1959 earthquake.

Along some of the fault sections, the lidar-based and field-measured throws agreed well. However, along other fault sections, lidar-derived fault throws were three times greater than those measured in 1959. This discrepancy indicates that lidar data also detect fault throws from one or two prehistoric earthquakes as well as to the 1959 earthquake.

Other fault sections, specifically the ridge section of the Red Valley Fault, only showed the 1959 movement. The researchers argue that the prehistoric outcrops, which likely existed, were not preserved on the steep slopes of the ankle and debris that characterizes this fault section.

While mapping faults shortly after the 1959 earthquake, USGS geologists noticed many gaps where outcrops did not form for short distances along the surface impact of the Red Canyon faults and Hepgen faults. These gaps are also observed in the lidar data and appear to reflect structural complexities in the geometry of the near-surface fault. Several other gaps in scar formation were noted in the lidar data as a USGS geologist documented surface compensation after the earthquake. Apparently, in some locations, the cliffs have been wiped out due to 55 years of erosion and/or human disturbance on the Earth’s surface.

The lidar-derived fault throw along the entire surface rupture zone of the 1959 earthquake was used to estimate the magnitude of the moment—the magnitude of the earthquake based on the amount of slip multiplied in the region where the slip occurred—from 7.1 ± 0.2. Although not as accurate as the magnitude determined by earthquakes, this independent determination based on surface displacement is in good agreement with the reported magnitude of the 1959 earthquake and provides an excellent tool for estimating the magnitudes of prehistoric earthquakes, for which no seismic data are available.

New lidar data, obtained 55 years after the earthquake, reveals new details about the complexity and rareness of the 1959 earthquake. This new data also shows that despite the lack of technology at the time, geologists provided remarkably good observations and explanations of the earthquake 1959 immediately after its occurrence. Without the benefits of GPS, computers with mapping software, and other technologies we now take for granted, and using only aerial photographs and topographic maps at a scale of 1:62500 (15 minutes), geologists have created excellent maps of earthquake fault faults in general. 1959.

Post-event studies of future surface-burst earthquakes at Intermountain West will take advantage of our state-of-the-art toolkit based on technology and improve our understanding of major earthquakes in ways geologists could not have dreamed of half a century ago.

For more information, see the scientific paper describing the lidar survey of the Hibgen Lake rifting event: Johnson, KL, Nissen, E. Montana earthquake from airborne lidar terrain; Journal of Geophysical Research: Solid Earth, 123,8229-8248. https://doi.org/10.1029/2017JB015039.

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