“Anomalous” earthquakes have plagued Utah for decades. Geologists are finally close to an answer

The small town of Randolph, Utah (population 467) was struck by a deep 3.8 magnitude earthquake in the early morning hours of February 24, 1979, but, mysteriously, none of its residents noticed enough to bother filing a report. Geologists monitoring the quake at the University of Utah’s seismic stations were puzzled. The modest roar of the episode was within the range that typically makes news in states like California all the time.

Now, nearly half a century later, researchers at the university, in partnership with geologists at Sandia National Laboratories and elsewhere, believe they have finally discovered why. The previously unexplained quake represents what the team now describes as an emerging new class of seismic activity, “mantle earthquakes,” which have been documented to occur beneath the Earth’s tectonic plates 43 to 55 miles (70 to 90 kilometers) underground.

University of Arizona geologist George Zandt, who first noticed the mysterious Randolph earthquake decades ago while working as a postdoctoral researcher in seismology, has come out of retirement to participate in ongoing new research on these deep quakes.

“I have done some other analyzes that have convinced me of the reality of the deep abyss, but it has been difficult to convince others of a highly anomalous mantle earthquake occurring in a region where none should exist,” Zandt recounted his history with the phenomenon in a statement. But he noted, “The deep depth explains why people don’t feel it on the surface.”

Earthy dessert

Whether an earthquake people “feel” enough to report the event is a bit of a subjective science, according to the USGS. It includes not only the magnitude of the earthquake, which scientists determine by the oscillating energy waves emanating from the source, but also by a value known as the “intensity” of the earthquake.

This last measure combines measurements of actual ground shaking at the surface along with incident reports from those who experienced the earthquake. But, as a general rule, “people reported feeling tremors larger than about magnitude 3.0,” the study notes.

Geophysicist Keith Cooper, who was one of Zandt’s students, led the project to better catalog and understand these deep mantle earthquakes, confirming nine cases that emanated from somewhere beneath the Earth’s crust in a paper in Geophysical Research Letters last May. Cooper, who is currently director of the UUSS, also helped investigate a new, similar earthquake that occurred beneath the Uinta Basin in Utah, a follow-up study published last April in The Seismic Record.

“This is an example of an earthquake that forms under very unusual conditions, such as high temperature and high pressure, and almost all the material at that depth will flow out. It’s like candy. It’s candy on long time scales, like millions of years,” Cooper said in the statement.

The September 10, 2025, earthquake was traced to a focal depth of 42 miles (68 km) below the Earth’s surface and 12 miles (20 km) below the boundary between the Earth’s crust and mantle, known as the Mohorovicić discontinuity.

Although the citizens above in the town of Macer probably wouldn’t have guessed it, this was a “typical continental mantle event” with a magnitude of 4.1. But, despite the deep, low-intensity quality of most of these mantle earthquakes at the surface, Cooper said strong evidence of this candy-like molten shaking is still making its way to the surface.

“You can still see it in the rocks that have come back to the surface,” Cooper said. “You can see how it’s stretched out.”

Molten oceans

However, Cooper describes these earthquakes as “kind of a mystery in terms of basic physics,” but he and his colleagues were able to identify some common features between them. To begin with, mantle earthquakes appear to occur in one-off bursts, without aftershocks or early aftershocks. All seismic events appear to originate near a very ancient geological formation called the Wyoming Craton near the edge of Utah.

Cratons are known for their ancient stability, often remaining intact for billions of years, despite extending from near the Earth’s surface to 155 miles (250 km) below, with segments sliding across the molten rock like the keel of a ship. Cooper’s team suspects that deep mantle earthquakes arise when this molten material figuratively rocks the boat.

Despite the apparent lack of evidence of a risk from either surface or deep quakes, Cooper believes it will be necessary to understand them first to truly define “seismic risk.”

“We have no idea how big it is,” he said.

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