A fault that lies beneath north Las Vegas has now been shown to have tectonic origins — with implications for seismic hazards in Las Vegas. New research reveals when and how much the fault slipped and what caused it to move.
By Megan Sever, science writer and editor (@megansever4)
Citation: Sever, M., 2020, Rapid climate change caused Las Vegas fault to shake, Temblor, http://doi.org/10.32858/temblor.111
Las Vegas is known for its veil of secrecy. Now though, scientists have lifted that veil on at least one piece of Vegas history: the seismic activity on a fault that underlies the northern portion of the city. In a new study, researchers found that the fault has slipped significantly in the past. They narrowed down the dates of when it last slipped and determined why: a rapid climatic change in the Las Vegas Valley driven by climate change half a world away.
Differential compaction or seismic activity?
The Eglington Fault, like several others beneath Las Vegas, has been known for years. Fault scarps — evidence of land movement — can be seen throughout the Las Vegas Valley, says Shaimaa Abdelhaleem, a doctoral student at the University of Nevada, Las Vegas, who studies the hazards of the Las Vegas Valley but was not involved in the new research. But for most of these faults, this displacement was thought to have been caused by a process called differential compaction, in which the land on one side of a fault falls or compacts more than another due to subsurface land changes, like the water table going down, Abdelhaleem says. The Eglington was the only fault even considered to have been moved by tectonic changes, like continental stretching, but even movement on the Eglington has been more often ascribed to compaction.
Kathleen Springer and Jeff Pigati, both of the U.S. Geological Survey (USGS), found otherwise. For a couple of decades, they have been exploring the Las Vegas Formation, dating the sediments and re-establishing the full chronology of the formation that lies both at the surface and beneath the Las Vegas Valley, Springer says. The team reported in the journal Geology that the formation — silts, carbonates and other sedimentary rocks — records more than half a million years of expanding and contracting wetlands, desert springs and groundwater ecosystems. The Eglington Fault cuts through the Las Vegas Formation.
Constraining the dates of movement
One of the units of the formation is a former marsh topped by a 1- to 1.5-meter-thick (3 to 5 feet) carbonate cap that is “extremely pervasive” through the Las Vegas Valley — “you can walk on that surface for days,” Pigati says. The cap was “warped” by the Eglington Fault after it was deposited and hardened about 27,000 years ago, Springer says. Pigati and Springer measured a 4.2-meter-high (13.8 feet) displacement along the fault.
To further constrain when movement happened, the pair dated another marsh bed above the carbonate cap, from 24,000 years ago, which in turn is topped by another carbonate cap. These different carbonate layers demarcate abrupt and extreme groundwater changes in the valley. About 23,300 years ago, the groundwater level started dropping and it fell between 10 and 33 meters (33 to 108 feet) over a maximum of 300 years, but more likely over the course of a couple of decades, Springer says. Like other wetland expansion/contraction events in the Las Vegas Formation, this drop corresponds precisely with a Dansgaard-Oeschger event, an abrupt climate shift in the Northern Hemisphere recorded in Greenland ice cores that shows warming in Greenland. And this, Springer says, is when the Eglington Fault slipped.
Land weight decreases stress
After calculations of stress changes at different dip angles, coefficient of friction values and water-table drop estimates, Springer and Pigati determined the fault slipped when the weight of the land — and thus the stress load — suddenly decreased as the groundwater levels fell. The process is analogous to faults triggered by removal of glaciers or pluvial lakes, Pigati says, but this process has not been seen in a groundwater system. “It has to be more than coincidence that the fault broke at the same time the groundwater table dropped up to 33 meters,” Springer says. Obviously, Pigati notes, the fault had to be primed for it to be triggered by the stress decrease.
Though Springer and Pigati determined when the fault slipped — between 23,300 and 19,500 years ago — they cannot say definitively whether it slipped slowly over that time, at a rate of about 1.1 millimeters (0.04 inches) per year, or in one or several bigger quakes. There is no evidence the fault produced an earthquake in the 300,000 years preceding or since then, but, of course, it could slip at any time. Thus, the team says, these findings change the earthquake hazard for the Las Vegas Valley. At the very least, they say, it shows that the Eglington is indeed a tectonic fault capable of producing earthquakes.
“This is a nice piece of science,” says Craig dePolo, an earthquake geologist at the Nevada Bureau of Mines and Geology who was not involved in the study. Springer and Pigati put “a tremendous amount effort in getting that level of detail” in both the dating and the climate changes and their math is solid, dePolo says. “I find their timing extremely compelling.”
Implications for Las Vegas seismic hazard
The idea that the faults in the Las Vegas Valley are not seismic faults has “been entrenched” for decades, which has led to the prevailing view that there is no seismic hazard in Las Vegas, dePolo says. It “would be great” if there were no earthquake hazards in the city, he says, but unfortunately that’s just not what the data are suggesting.
Springer and Pigati’s work will be incorporated into the next update of the USGS National Seismic Hazard Model (NSHM) for the Las Vegas Valley, “but it’s too early to say how the results will affect hazard estimates,” says Rich Briggs, a research geologist at USGS who was not involved in the new study. The NSHM update, which is scheduled to come out in 2023, “will balance new information like this with existing measurements and models to arrive at revised estimates of the probability of ground shaking,” Briggs says.
One remaining question about the Eglington Fault, dePolo says, is what scientists consider the slip rate or hazard return rate — is it more than 300,000 years or is it 20,000 years or something else? It is connected with the nearby Decatur Fault and near other active seismic areas called the California Wash and the Walker Lane Fault Zone, he says, each of which could also affect seismicity and thus the hazard return rate on the Eglington. Also, did any other faults in the valley slip at the same time as the Eglington? These are important questions for determining the fault’s hazard.
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Springer, K.B., and Pigati, J.S., 2020, Climatically driven displacement on the Eglington fault, Las Vegas, Nevada: Geology, v. 48, p. 574–578, https://doi.org/10.1130/G47162.1