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Earthquakes felt two nights in a row in SE Wyoming

CHEYENNE — Earthquakes were recorded two nights in a row almost exactly 24 hours apart along the Continental Divide in southeastern Wyoming this week.

The first incident, a 3.9 magnitude earthquake that occurred at 2:14 a.m. Friday, had an epicenter near Antelope Hills, a rural subdivision between Saratoga and Encampment in Carbon County. Its depth was 15.4 km.

Residents in the towns of Saratoga, Encampment and Riverside were within a MMI 3.5 shaking intensity zone, according to the USGS.

Laramie was within a third zone, MMI 2.5, in which “weak” or no tremors at all could have been felt.

According to the USGS, MMI stands for “Modified Mercalli Intensity, which estimates the shaking intensity from an earthquake at a specific location by considering its effects on people, objects, and buildings.”

The second event was a 3.2 magnitude earthquake six miles southeast of Saratoga at 1:06 a.m. Saturday. Its depth was 10.5 km.

“The second one was an aftershock,” Yaareb Altaweel, a Denver-based geophysicist with the USGS National Earthquake Information Center, said early Saturday morning. “An aftershock is still an earthquake. It is just a way to categorize and link earthquakes together.”

A small earthquake before a bigger one is called a foreshock, he explained.

There were three other earthquakes recorded in the same area last September, Altaweel said, but there is no pre-existing fault line in the area.The Continental Divide, however, runs through Carbon County.

“The entire area is under stress from the oceanic crust going under the continental crust,” he explained. “The stress has to go somewhere.”

While there isn’t one magnitude above which damage will occur, according to the USGS, fallout from an earthquake usually depends on other variables, like the distance from the earthquake, the type of soil a building is on and building construction.That said, damage does not usually occur until an earthquake magnitude reaches somewhere above 4 or 5.

Usually, stress between the oceanic crust and continental crust would move through pre-existing fault lines, Altaweel said. But, at times, new fault lines can begin to appear as earthquakes.

Because of multiple smaller events now recorded along the Continental Divide in Carbon County, smaller events could begin occurring more often, although Altaweel said that the USGS does not make earthquake forecasts.

“There is no prediction of earthquakes,” Altaweel said. “ We look at historical data, but I can’t say (it’s a) forecast. It is not a forecast. But based on statistics, the likelihood of a bigger earthquake is slim. But the likelihood of a smaller earthquake is much higher.”

Although the two events occurred at nearly the same time two nights in a row, Altaweel said that was a coincidence.

“I was the one who published the second earthquake (on the USGS website), and when I put it on the map to see where it was, I saw the first one.The first thing that came to mind was, ‘OK, it’s an aftershock. That’s normal.’The timing is just a coincidence,” Altaweel said.

This story was published on June 4, 2023.