Nearly half a century ago, a small earthquake beneath northern Utah left seismologists puzzled. The event seemed to originate far deeper than earthquakes were thought capable of occurring beneath a continent. Now, new research from the University of Utah has confirmed that the unusual quake was real and part of a rare category of seismic events happening deep within Earth's mantle.
The earthquake struck in the early morning of February 24, 1979, beneath the town of Randolph near Utah's borders with Idaho and Wyoming. Although it registered a magnitude of 3.8, nobody reported feeling it. The seismic recordings also appeared unusual, prompting closer examination.
At the time, University of Utah postdoctoral researcher George Zandt analyzed the data and calculated that the earthquake originated about 90 kilometers below sea level. That depth placed it well below Earth's crust and deep within the upper mantle, a location where scientists generally did not expect earthquakes to occur.
"The deep depth explained why it wasn't felt by people at the surface," said Zandt, who later spent many years on the geology faculty at the University of Arizona. "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."
Decades Later, Old Data Reveals a Pattern
Zandt published a brief abstract about the Randolph earthquake in Earthquake Notes, but the finding attracted little attention. Interest was revived decades later when University of Utah researchers revisited the original seismic records.
Led by geology professor Keith Koper, the team reexamined waveform data from the 1979 earthquake along with eight other suspected deep earthquakes that had occurred in northern Utah and southwestern Wyoming.
Their analysis confirmed that all nine events originated well below the crust, providing strong evidence for the existence of what scientists call continental mantle earthquakes (CMEs).
The findings gained additional significance when another deep earthquake struck on September 10, 2025, near Maeser in Utah's Uinta Basin. That event reached magnitude 4.1 and originated about 68 kilometers below the surface.
Its source was located more than 20 kilometers beneath the Mohorovičić discontinuity, commonly called the Moho, which marks the boundary between Earth's crust and the underlying mantle. In a separate study published in The Seismic Record, researchers described the Maeser earthquake as an "archetypal continental mantle event."
Earthquakes in an Unusual Environment
Unlike most earthquakes, these deep events occur in an environment characterized by extreme heat and pressure. At such depths, rocks are generally expected to deform slowly rather than fracture suddenly.
"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, who directs the University of Utah Seismograph Stations and once studied under Zandt. "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 collaborate on the new research and is listed as a coauthor.
A Different Kind of Earthquake
To determine where earthquakes begin, seismologists examine the travel times of different seismic waves recorded by instruments at the surface. Small differences in arrival times help researchers pinpoint the earthquake's origin.
The University of Utah Seismograph Stations has preserved decades of seismic records, creating a valuable archive for modern analysis. Graduate student Sean Hutchings used this archive to study known deep earthquakes and identify additional events that had previously been classified as crustal earthquakes.
"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 researchers found several characteristics that set these deep earthquakes apart from more familiar seismic events. They occur alone, without the foreshocks and aftershocks commonly associated with shallow earthquakes. They are also concentrated near the western edge of the Wyoming Craton and occur in regions where temperatures often exceed 700 degrees Celsius.
The Role of the Wyoming Craton
The Wyoming Craton is an ancient, stable block of Earth's lithosphere that extends beneath parts of Wyoming and neighboring states. Koper compares cratons to icebergs. Rather than floating in the ocean, they extend downward into Earth's mantle like the keel of a ship.
Situated between the tectonically active western United States and the more stable interior of the North American plate, the Wyoming Craton has experienced significant erosion over geologic time. As a result, its structure varies across the region, and the lithosphere becomes progressively thinner toward Idaho and Utah.
The newly confirmed deep earthquakes occur in this transition zone.
"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 was published 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 May 5, 2025, in Geophysical Research Letters under the title "Upper Mantle Earthquakes Along the Edge of the Wyoming Craton."
Additional coauthors include Sean J. Hutchings, Fan-Chi Lin, Qicheng Zeng, Relu Burlacu, Katherine Whidden, and Valerie Springer from the University of Utah Department of Geology & Geophysics. Funding for the work was provided by the State of Utah, the U.S. Department of Energy, and the U.S. Geological Survey.