Nearly 50 years ago, a puzzling earthquake beneath northern Utah jolted scientists' understanding of how Earth works. Now, research from the University of Utah confirms that the mysterious event was real, and part of a rare class of earthquakes occurring far deeper beneath the continental crust than scientists once believed possible.
In the early morning hours of o Feb. 24, 1979, the University of Utah Seismograph Stations recorded the quake under Randolph, a Utah town near the Idaho and Wyoming borders. No one reported feeling the quake, despite its magnitude 3.8 heft, and the accompanying seismic data didn't make sense.
George Zandt, then a postdoctoral seismology researcher at the U, took a closer look at the seismic readings and pinpointed this quake's focal depth at a jaw-dropping 90 kilometers below sea level. Such a depth was not thought possible, placing its hypocenter far below Earth's crust, well into the upper mantle.
"The deep depth explained why it wasn't felt by people at the surface," said Zandt, who went on to a long career on the University of Arizona's geology faculty. "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."
Fresh look at old data
He wrote an abstract about the Randolph quake for Earthquake Notes, but Zandt's findings remained largely unnoticed until last year. That's when a new generation of U seismologists re-evaluated waveform data from the 1979 quake and eight other suspected deep earthquakes that have since occurred in northern Utah and southwest Wyoming.
This study, led by U geology professor Keith Koper, confirmed locations for all nine well below Earth's crust, proving the existence of what are called "continental mantle earthquakes," or CMEs. Then on Sept. 10, 2025, another one struck at around 6 p.m. outside Maeser in Utah's Uinta Basin, clocking in at magnitude 4.1 with a focal depth of 68 kilometers.
That's more than 20 kilometers below the Mohorovičić discontinuity, the boundary separating Earth's crust from the underlying mantle, better known as the Moho. Koper's team characterized the Maeser quake as an "archetypal continental mantle event" in a subsequent study published last month 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," said Koper, a one-time protege of Zandt's and now the director of the U of U Seismograph Stations. "Nevertheless, you can still see it in rocks that have made their way back up to the surface; you can see how they were stretched."
Zandt came out of retirement to work on this study, which lists him as a co-author.
A completely different kind of earthquake
To locate the spots where earthquakes originate, seismologists analyze how long it takes different types of seismic waves to reach seismographs on the surface. The differences in arrival times provide vital clues. The Seismograph Stations have long preserved such data, creating a valuable archive that Koper's graduate student Sean Hutchings tapped to analyze known deep quakes and find several others that had been mischaracterized as crustal quakes.
"It's sort of a mystery in terms of fundamental physics. How in the world can these things happen?" Koper said. "Another reason why it's a big deal is that we have no idea how big they can be. With crustal earthquakes, we can measure what we think their maximum size is going to be. We measure the faults that we can map out near the surface. We can measure the length of a fault segment and that clues us into how big it can be, which helps us estimate seismic hazard."
The two papers found several traits in common that distinguish these deep quakes from the familiar ones commonly occurring along faults near the surface. Principally, they occur in isolation-no aftershocks or foreshocks-near the edge of the Wyoming Craton, accompanied by extremely high temperatures, often exceeding 700 degrees Celsius. At those temperatures, rock is soft and ductile.
Cratons are ancient cohesive blocks of Earth's lithosphere, which Koper likens to icebergs. Instead of bobbing in the ocean, however, cratons rest in Earth's mantle like the keel of a boat in water. Located at the boundary of the tectonically active Western U.S. and the stable interior of the North American plate, the Wyoming Craton has been heavily eroded, resulting in a heterogeneous structure and an overall thinning of the lithosphere westward across Idaho and Utah. This is where the deep quakes have occurred.
"On the scale of millions of years, the mantle is hitting the craton and then flowing around it," Koper said. "It's that interaction where that mantle flow is being diverted around this hard cratonic root that's causing the increased strain rate, the increased deformation and it's also creating extra stresses. We think it's that interaction between the keel of the iceberg and the medium around it that's leading to these earthquakes."
The research described here appeared April 10 in The Seismic Record, under the title "The 10 September 2025 4.1 Earthquake in Northeastern Utah, United States: An Archetypal Continental Mantle Event," and on May 5, 2025, in Geophysical Research Letters under the title, "Upper Mantle Earthquakes Along the Edge of the Wyoming Craton." Co-authors include Sean J. Hutchings, Fan-Chi Lin, Qicheng Zeng, Relu Burlacu, Katherine Whidden and Valerie Springer of the University of Utah's Geology & Geophysics Department. This work is supported by the State of Utah, the U.S. Department of Energy and the U.S. Geological Survey.
