Rapid climate change has shaken the Las Vegas Fault

The fault beneath North Las Vegas has now been proven to have tectonic origins – with ramifications for seismic hazards in Las Vegas. New research reveals when and how the error slipped and what caused it to move.

Written by Megan Sever, science writer and editor (@ megansever4)

Quote: Siver, M, 2020, Rapid Climate Change Shaking the Las Vegas Fault Tumblr, http://doi.org/10.32858/temblor.111

The Eglington Fault Scarp, shown here (looking west at Tule Springs National Fossil Family Monument), is cut through thick carbonate hoods that reveal past climate changes in the Las Vegas Valley. Credit: Kathleen Springer, USGS

Las Vegas is famous for its veil of secrecy. Despite this, scientists have lifted this veil on at least one piece of Vegas history: seismic activity on the fault that lies beyond the northern part of the city. In a new study, researchers have found that the error has slipped drastically in the past. They cut back the dates of the last time you slipped and pinpoint the cause: Rapid climate change in the Las Vegas Valley driven by climate change half a world away.

Geoff Bigatti and Kathleen Springer of Research Geology at the United States Geological Survey stand on the D2 carbon bedspread of the Las Vegas Formation at Tule Springs National Fossil Family Memorial. Bed D2 reveals that the water level has dropped from 10 to 33 meters (33 to 108 feet) in just two decades. Credit: Alan O’Neill, Retired, NPS

Differential pressure or seismic activity?

The Eglington Fault, like many others under Las Vegas, has been known for years. Shaima Abdel Halim, a doctoral student at the University of Nevada, Las Vegas, who studies the dangers of the Las Vegas Valley but was not involved in the new research, says faults – evidence of Earth’s movement – can be seen all over the Las Vegas Valley. . But for most of these faults, this displacement was thought to be the result of a process called differential compression, whereby the ground falls on one side of the fault or accumulates more on the other due to groundwater changes, such as the lowering of water levels, Abdel Halim says. Eglington was the only fault considered to have been driven by tectonic changes, such as continental expansion, but even the movement on Eglington was often attributed to pressure.

Kathleen Springer and Jeff Bigatti, both of the United States Geological Survey (USGS), found otherwise. For two decades, they have been exploring the Las Vegas Formation, chronicling the sediments and recreating the entire chronology of the formation that lies at the surface and below the Las Vegas Valley, says Springer. The team reports in Geology that the formation – silt, carbonate, and other sedimentary rocks – records more than half a million years of expanding and shrinking wetlands, desert springs, and groundwater ecosystems. The Eglington Fault cuts through the Las Vegas Formations.

Restrict movement dates

One of the units of formation is a former swamp topped with a 1 to 1.5 meter (3 to 5 feet) thick carbon cap that is “very diffuse” across the Las Vegas Valley – “You can walk on this surface for days,” Pigati says. Springer says the hat was “deformed” by the Ellington Fault after it was deposited and hardened about 27,000 years ago. Pigati and Springer measured a displacement of 4.2 meters (13.8 ft) along the fault.

To further restrict when the movement occurred, the pair dated another moorland layer above the carbonate cap, 24,000 years ago, which in turn is topped by another carbonate cap. The different carbonate layers define these sudden and extreme changes in the groundwater in the valley. About 23,300 years ago, the groundwater level began to decline and decreased between 10 and 33 meters (33 to 108 feet) over a maximum of 300 years, but most likely over two decades, Springer says. Like other expansion / contraction events of the wetlands in the Las Vegas formation, this drop corresponds precisely to the Dansgaard-Oeschger event, a sudden climate shift in the Northern Hemisphere recorded in Greenland ice samples showing a warming of Greenland. And this, Springer says, happened when the Eglington Fault slipped.

The overall carbonate cover for a D2 bed from the Las Vegas Formation at Tule Springs National Fossil Family Memorial in the center and foreground. Las Vegas scale in the background. It is possible that the decrease in the water table – changing the pressure load above the Ellington Fault – may have caused the fault to slip. Credit: Eric Scott, Cal State University, San Bernardino

Earth weight reduces stress

After calculating stress changes at different dipping angles, coefficient of friction values ​​and estimates of water level drop, Springer and Bigatti determined the rift when the ground weight – and thus the stress load – suddenly decreased as groundwater levels decreased. The process is analogous to faults resulting from removing glaciers or glaciers, says Bigatti, but this process has not been seen in the groundwater system. “It must have been more than a coincidence that the fault broke at the same time that the water table dropped to 33 meters,” says Springer. Obviously, Pigati notes, the fault must be primed for it to be triggered by stress reduction.

Although Springer and Pigati determined when the fault slid – between 23,300 and 19,500 years ago – they could not definitively say whether it slid slowly over that time, at a rate of about 1.1 mm (0.04 in) per year, or in one or several earthquakes Larger . There is no evidence that the rift triggered an earthquake in 300,000 years before or since, but, of course, it could slip at any time. Thus, the team says, these findings alter earthquake risk in the Las Vegas Valley. At the very least, they say, it shows that Eggington is indeed a tectonic fault capable of causing earthquakes.

“This is a beautiful piece of science,” says Craig Dipolo, a seismologist at the Nevada Office of Mines and Geology who was not involved in the study. Springer and Bigatti have gone out of “an enormous effort to have that level of detail” in both dating and climate change, and their accounts are robust, says DePolo. “I find their timing very convincing.”

More than 2.2 million people live in the Las Vegas Valley and nearly 50 million people visit it every year. The new research indicates that the valley has greater seismic risks than previously thought. Credit: LasVegasLover, CC BY-SA 3.0

Implications for Earthquake Hazards in Las Vegas

The idea that faults in the Las Vegas Valley are not seismic faults has been “entrenched” for decades, Dipolo says, leading to the prevailing view that there is no seismic hazard in Las Vegas. He says it would “be nice” if there were no earthquake hazards in the city, but unfortunately that’s not what the data indicate.

Springer and Pigati’s work will be incorporated into the upcoming USGS National Seismic Hazard Model (NSHM) update for the Las Vegas Valley, “but it is too early to say how the results will affect risk estimates,” says Rich Briggs, a USGS research geologist who was not involved in The new study. Briggs says the NSHM update, slated for release in 2023, “will balance new information like this with current measurements and models to arrive at revised estimates of the likelihood of Earth’s shaking.”

One of the remaining questions about the Ellington Fault, Dipolo says, is what scientists consider the slip rate or risk return rate – is it over 300,000 years old, 20,000 years old, or something else? It’s linked to the nearby Decatur rift and near other active seismic zones called the California Wash and Walker Lane Fault Zone, he says, and both could also affect seismic activity and thus the rate of hazard return in Eglington. Also, did any other cracks slide into the canyon at the same time as the Eglington? These are important questions for determining error risks.

Curious about earthquake risks? Check it out at Temblor

Further reading

Springer, KB, and Pigati, JS, 2020, Climate Driven Displacement on the Eglington Rift, Las Vegas, Nevada: Geology, v. 48, p. 574-578, https://doi.org/10.1130/G47162.1

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