Recent earthquake activity unlocks the mysteries of the Wasatch Fault

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Ground motion recordings over the past decade reveal how ground shaking dampens and amplifies beneath the urban corridor in Utah.

Written by Lauren Koenig, Ph.D. Candidate, Michigan State University (@Lauren_A_Koenig)

Citation: Koenig, K. , 2021, Recent earthquake activity unlocking the mysteries of the Wasatch Fault, Temblor, http://doi.org/10.32858/temblor.184

An image of Mount Nebo, the highest mountain in the Wasatch Range in Utah, taken from southwest Navi on I-80. Credit: Cory Maylett, CC BY-SA 3.0

The Wasatch Fault, which stretches down central Utah, has been relatively calm for the past millennia. So, when the Salt Lake City area was hit by the 5.7 Magna earthquake in 2020, Morgan Mosheti, a geophysicist with the US Geological Survey’s Center for Geohazard Sciences, saw an opportunity. With more than 2,300 aftershocks, this pivotal earthquake has provided a set of data that can help researchers understand the geological hazards of one of the world’s longest and most active faults. Moschetti’s work is revealing new details about how the Earth’s seismic movements from the Wasatch Fault move through the region when earthquakes strike. He and his team found that ground shaking caused by earthquakes did not match the predictions of standard models, which means that risk models may need an update for the Wasatch Front.

An error that goes through

The Wasatch Fault Zone extends approximately 240 miles (390 kilometers) from central Utah to northern Idaho and has been divided into 10 individual fault segments. The area is classified as a “normal” fault zone, which means that one side is declining in relation to the other – in this case, the west side is declining downward relative to the east side. Most of Utah’s population lives on the western side of the fault zone, where deep sedimentary basins amplify seismic waves.

Extension, or horizontal expansion of the Earth’s crust, produces “normal” sloping faults in western and central Utah that move more vertically compared to the horizontal movement of the San Andreas slip fault in California. Credit: “Laying Roots in Earthquake Country,” Utah Geological Survey

The fault zone can produce earthquakes of magnitude 7.5 and is one of the highest sources of earthquake risk within the contiguous United States. “It’s a rather fascinating area because there is geological evidence of large earthquakes in relatively recent time periods, but there is relatively little seismic activity there now,” Moschetti says. Since the next major earthquake is expected to strike in 50 years, new information about the Wasatch fault is necessary to mitigate seismic damage in this densely populated region.

Ground shaking from the Wasatch fault eases faster than expected

Moschetti and his team compiled a database of ground motion records from 59 earthquakes since 2010—about half of which came from the Magna earthquake sequence—and identified regional features that influence earthquake risk. They published their findings in a new study in Seismology Research Letters showing that ground-wave ripples from small and medium-sized earthquakes weaken, or weaken, more quickly than standard models of the Wasatch Fault predict. These standard models are based on seismic records from places like California. But in Utah, according to Moschetti’s findings, the amplitude of the Earth’s shaking fades more quickly as the waves travel away from the source of the rupture in a process called near-source attenuation.

The Wasatch Fault extends approximately 240 miles (390 kilometers) from Malad City, Idaho, to Fayette, Utah. Credit: “Laying Roots in Earthquake Country,” Utah Geological Survey

“There are many things that can lead to higher than expected attenuation rates,” says James Holt, a seismologist at the University of Utah Seismological Stations (UUSS) who was not involved in this study. “The combined effects of fault structure, source material properties, and local source conditions — such as geological structure — have a significant impact on how seismic energy propagates near the rupture.”

These complexities, combined with a lack of seismic data close to the source, make it difficult for scientists to determine the exact details of radiated seismic energy, such as the attenuation process. However, the vast amount of data produced by the 2020 Magna earthquake may allow researchers to develop a specific ground motion model of the Wasatch fault or modify standard models to account for near-source attenuation.

“This is an important contribution because many ground motion model data sets simply do not contain data close to the source,” Holt says. “Improving the accuracy of ground motion models in this critical region close to the source … will certainly improve downstream hazard calculations and reduce seismic risk in densely populated urban areas such as Salt Lake City.”

While these findings could inform how scientists assess earthquake risk in the region, Moschetti says it’s unclear whether these same effects would be observed for larger and potentially devastating earthquakes. “These observations should not affect how anyone thinks about preparing for the magnitude 7+ earthquakes in Utah,” Moschetti says.

In addition to risk assessments

While researchers know that seismic waves are strongly amplified in deep sedimentary basins such as those in the Salt Lake Valley, Moschetti’s work also reveals new details about the relationship between basin depth and ground motion, which is useful for site-specific risk assessment, Holt says. . The team found that waves amplify strongly in the deepest part of the basin just west of Salt Lake City and become more choppy at the basin edge near downtown. “If you really have an important [building or other human-made structure]You’ll be interested to see what the local geological structure under your feet can do to increase the local seismic energy that comes in,” says Holt.

Salt Lake City is located in the basin of the Greater Salt Lake – the bottom of ancient Lake Bonneville. Earth movement caused by earthquakes moves rapidly through this vast sedimentary basin. Credit: Garrett, CC BY 2.0

An earthquake occurs approximately every 300 years by one of the central segments of the Wasatch fault zone, but earthquakes in several segments are “late”. Since this fault is a known risk, it is one of the most closely monitored faults in the world, with a network of seismic devices collecting data at all times. The relatively small Magna earthquake was a rare opportunity to catch the bug in action.

The proximity of the 2020 earthquake to an urban area is of particular value to the seismic risk community. “We can use the new ground motion information to develop better building codes,” said Kathryn Weeden, a research scientist at USS University who was not involved in the study. “New information about the Wasatch Fault and the Salt Lake Valley is making us all safer.”

references

Moschetti, M. P., Churchwell, D., Thompson, E. M., Rekoske, J. M., Wolin, E., & Boyd, OS (2021). Seismic wave propagation and trough amplification at the Wasatch, Utah front. Seismic search letters. https://doi.org/10.1785/0220200449

Utah Seismic Safety Authority (2008). Putting Roots in Earthquake Country – Handbook of Earthquakes in Utah. Salt Lake City, Utah: Utah Geological Survey. Retrieved from https://www.utah.gov/beready/documents/roots_earthquake_low.pdf

Wald, L (2015). How large are earthquakes and how frequent are they on an earthquake fault? Retrieved from https://www.usgs.gov/natural-hazards/earthquake-hazards/science/how-big-and-how-frequent-are-earthquakes-wasatch-fault?qt-science_center_objects=0#

Wong, I. G., Lund, W. R., Duross, C., Thomas, P., Arabasz, W., Crone, A. J.,… & Rowman, S. (2016). Earthquake probability for the Wasatch Front region of Utah, Idaho, and Wyoming. Utah Geological Survey.

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