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 it out. The once unexplained earthquake represents what the team now describes as an emerging new category of seismic activity, “mantle earthquakes,” which have been documented to occur at depths of 43 to 55 miles (70 to 90 kilometers) beneath the Earth’s tectonic plates.
University of Arizona geologist George Zandt, who first observed Randolph’s mysterious earthquake while working as a seismology postdoc decades ago, came out of retirement to join the ongoing new research on these deep earthquakes.
“I did some other analysis that convinced me of the reality of the deep depth, but it was hard to convince others of a highly anomalous mantle earthquake occurring in a region where none should exist,” as Zandt explained his history with the event in a statement. But, he said, “the deeper depth explains why it was not felt by people on the surface.”
clay taffy
According to the US Geological Survey, what constitutes an earthquake is “felt” by people enough to report the event, is a subjective science. This includes not only the intensity of an earthquake, which scientists define through the oscillating waves of energy emitted from the source, but also through a value known as the “intensity” of the earthquake.
That latter metric aggregates together event reports from people who experienced an earthquake as well as measurements of actual ground shaking at the surface. But, as a general rule, the survey said, “people feel earthquakes larger than about magnitude 3.0.”
Geophysicist Keith Koper, once a disciple of Zandt, has led a project to catalog and better understand these deep mantle earthquakes, confirming nine cases that originated somewhere beneath the Earth’s crust for a paper in Geophysical Research Letters last May. Kopper, now director of the UUSS, also helped investigate a new and similar earthquake that occurred under Utah’s Uinta Basin, a follow-up study published in The Seismic Record this April.
“This is an example of an earthquake that occurred under very unusual conditions, high temperature, high pressure, and almost all the material at that depth is flowing. It’s like taffy, it’s taffy on a long time scale, like millions of years,” Kopper said in the statement.
The earthquake, which occurred on September 10, 2025, was detected 42 miles (68 km) below the ground and 12 miles (20 km) below the boundary between the Earth’s crust and mantle, known as the Mohorovicic Discontinuity.
Although the citizens above the town of Messer might not have anticipated it, this “ideal continental mantle event” had a magnitude of 4.1. But, despite the deep and low-magnitude quality of most of these mantle earthquakes at the surface, Kopper said there is still solid evidence of these molten, taffy-like tremors coming to the surface.
“You can still see it in the rocks that have come back up to the surface,” Kopper said. “You could see how stretched they were.”
molten ocean
Nevertheless, Kopper describes these earthquakes as “kind of a mystery in terms of fundamental physics”, but he and his colleagues have managed to identify some common traits between them. For starters, mantle earthquakes appear to occur in one-time bursts, without aftershocks or initial shocks. All of the seismic events appear to originate near a geologically very old structure called the Wyoming Craton near the coast of Utah.
Cratons are defined by their ancient stability, often remaining intact for billions of years despite extending from near the Earth’s surface to as much as 155 miles (250 km) below, with some parts sliding through the molten rock like a ship’s keel. Kopper’s team suspects that deep mantle earthquakes occur when this molten material metaphorically rocks the boat.
Despite the lack of evidence of danger from any surface-shaking, deep mantle earthquakes, Kopper believes it will be important to understand them first to truly determine “seismic danger.”
“We don’t know how big they can get,” he said.
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