The tiny town of Randolph, Utah (population: 467), sat atop 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 Seismograph Stations (UUSS) were baffled. The episode’s modest rumbling was well 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 Labs and elsewhere, believe they have finally figured out why. The once inexplicable quake represents what the team now describes as an emerging new category of seismic activity, “mantle earthquakes,” which have been documented occurring beneath Earth’s tectonic plates as deep as 43 to 55 miles (70 to 90 kilometers) underground.
University of Arizona geologist George Zandt, who first noticed Randolph’s mystery quake decades ago while working as a seismology postdoc, came out of retirement to pitch in on the ongoing new research into these deep quakes.
“I did some other analysis that convinced me of the reality of the deep depth, but it was hard to convince others of the highly anomalous mantle earthquake occurring in a region where none should exist,” as Zandt recounted his history with the phenomena in a statement. But, he noted, “the deep depth explained why it wasn’t felt by people at the surface.”
Earthy taffy
An earthquake that is “felt” by people enough to report the event is something of a subjective science, according to the U.S. Geological Survey. It includes not just the magnitude of the quake, which scientists define via the oscillating waves of energy emitted from the source, but also via a value known as the quake’s “intensity.”
That latter metric pools together measurements of the actual shaking of the ground at the surface alongside incident reports from those who experienced the quake. But, as a general rule, the survey notes, “people report feeling earthquakes larger than about magnitude 3.0.”
Geophysicist Keith Koper, once Zandt’s protégé, has led the project to catalogue and better understand these deep mantle quakes, confirming nine cases that emanated from somewhere below Earth’s crust for a paper in Geophysical Research Letters last May. Koper, now director of the UUSS, also assisted in the investigation of a new and similar quake that rattled under Utah’s Uinta Basin, a follow-up study published this April in The Seismic Record.
“This is an example of an earthquake that’s nucleating in very unusual conditions, the high temperature, the high pressure, and almost all the material at that depth is going to flow. It’s more like taffy, it’s taffy on long time scales, like millions of years,” Koper said in the statement.
The September 10, 2025 earthquake was traced back to a focal depth of 42 miles (68 km) below ground and 12 miles (20 km) below a boundary between Earth’s crust and mantle, known as the Mohorovičić discontinuity.
Although the citizens above in the town of Maeser probably would not have guessed it, this “archetypal continental mantle event” had a magnitude 4.1. But, despite the deep and low-intensity quality of most of these mantle earthquakes at the surface, Koper said hard evidence of this molten, taffy-like shaking still makes its way to the surface.
“You can still see it in rocks that have made their way back up to the surface,” Koper said. “You can see how they were stretched.”
Molten oceans
Still, Koper describes these quakes as “sort of a mystery in terms of fundamental physics,” but he and his colleagues have managed to identify some common traits between them. For starters, the mantle quakes appear to occur in one-off bursts, without aftershocks or early tremors. The seismic events also all seem to originate near a geologically very old formation called the Wyoming Craton near the edge of Utah.
Cratons are defined by their ancient stability, often staying intact for billions of years, despite extending from near the surface of the Earth to 155 miles (250 km) below, where portions glide through molten rock like the keel of a ship. Koper’s team suspects that the deep mantle quakes arise when this molten material metaphorically rocks the boat.
Despite the seeming lack of evidence of danger from any surface-rattling, deep mantle quakes, Koper believes it will be critical to understand them first to truly determine the “seismic hazard.”
“We have no idea how big they can be,” he said.
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