Almost half a century ago, a small earthquake struck northern Utah that baffled seismologists. The phenomenon appears to have occurred much deeper beneath the continent than is thought earthquakes can occur. Now, new research from the University of Utah confirms that this unusual quake is real and part of a rare category of seismic events that occur deep within the Earth’s mantle.
The earthquake occurred in the early morning hours of February 24, 1979, beneath the town of Randolph, near the Utah-Idaho-Wyoming border. It recorded a magnitude of 3.8, but no one reported feeling it. Earthquake records also appeared to be abnormal, necessitating a detailed investigation.
At the time, George Zandt, a postdoctoral researcher at the University of Utah, analyzed the data and calculated that the quake’s origin was about 90 kilometers below sea level. Its depth placed it well below the Earth’s crust, deep in the upper mantle, a place where scientists generally don’t expect earthquakes to occur.
“The depth explained why people on the surface didn’t feel it,” said Zandt, who later worked for many years in the geology department at the University of Arizona. “I did other analyzes that convinced me of the deep reality, but it was difficult to convince others that highly unusual mantle earthquakes were occurring in areas where they shouldn’t exist.”
Decades later, old data reveals patterns
Zandt published a brief summary of the Randolph earthquake in 2016. earthquake memoHowever, this discovery received little attention. Decades later, interest was revived when researchers at the University of Utah reexamined the original earthquake records.
A research team led by geology professor Keith Copper reviewed waveform data from the 1979 earthquake and eight other suspected deep earthquakes in northern Utah and southwestern Wyoming.
Their analysis confirmed that all nine events occurred well below the Earth’s crust, providing strong evidence for the existence of what scientists call continental mantle earthquakes (CMEs).
This discovery became even more important when another deep earthquake occurred near the maser in Utah’s Uinta Basin on September 10, 2025. The earthquake had a magnitude of 4.1 and its origin was approximately 68 kilometers above the ground.
The source was located more than 20 kilometers underground in the Mohorovicić discontinuity, commonly referred to as the Moho, which marks the boundary between the Earth’s crust and the underlying mantle. In another study published in Earthquake recordresearchers described maser earthquakes as a “typical continental mantle phenomenon.”
Earthquakes in unusual environments
Unlike most earthquakes, these deep earthquakes occur in environments characterized by extreme heat and pressure. At such depths, rocks are typically expected to deform slowly rather than suddenly rupture.
“This is an example of an earthquake where a core occurs under very unusual conditions of high temperature and pressure, and almost all the material at that depth is ejected. It’s more like candy, but it’s candy on long time scales, like millions of years,” said Professor Kopel, director of the University of Utah Seismograph Station and a former student of Zandt. “Yet you can still see it in the rocks that have come back to the surface, and you can see how they were stretched.”
Zandt came out of retirement to collaborate on the new study and is listed as a co-author.
different types of earthquakes
To determine where an earthquake begins, seismologists look at the propagation times of various seismic waves recorded by equipment on the ground. Small differences in arrival times can help researchers pinpoint the origin of an earthquake.
The University of Utah Seismograph Station preserves decades of earthquake records, creating a valuable archive for modern analysis. Graduate student Sean Hutchings used this archive to study known deep earthquakes and identify additional events previously classified as tectonic earthquakes.
“From a fundamental physics perspective, it’s kind of a puzzle. How could something like this happen?” Koper said. “Another reason this is important is that we don’t know how big an earthquake will be. With tectonic earthquakes, we can measure what the maximum size is likely to be. We measure faults that can be mapped near the surface. We can measure the length of the fault section, which gives us a clue as to how big it will be, which helps us estimate the risk of an earthquake.”
Researchers have discovered several characteristics that distinguish these deep earthquakes from more familiar seismic phenomena. These occur on their own, without the foreshocks and aftershocks that often accompany shallow earthquakes. It is also concentrated near the western edge of Craton, Wyoming, where temperatures often exceed 700 degrees Fahrenheit.
The role of Wyoming’s cratons
The Wyoming Craton is an ancient, stable block of Earth’s lithosphere that extends beneath parts of Wyoming and neighboring states. Koper compares the craton to an iceberg. Rather than floating in the ocean, they extend downward into the Earth’s mantle like the keel of a ship.
Located between the tectonically active western United States and the more stable interior of the North American plate, the Wyoming craton has undergone significant erosion over geological time. As a result, its structure varies from region to region, and the lithosphere gradually thins toward Idaho and Utah.
The newly confirmed deep earthquakes occur in this transition zone.
“Over millions of years, the mantle collides with and flows around the craton,” Kopel said. “The interaction of mantle flow being diverted around this hard cratonic root causes increased strain rates and increased deformation, creating additional stresses. We believe that interactions between the iceberg’s keel and its surrounding medium are causing these earthquakes.”
The study was published on April 10th. Earthquake record On May 5, 2025, with the title “4.1 Earthquake that occurred in northeastern Utah, USA on September 10, 2025: a typical continental mantle phenomenon”, Geophysical Research Letters with the title “Upper Mantle Earthquakes Along the Edge of the Wyoming Craton.”
Other co-authors include Sean J. Hutchings, Fan-Chi Lin, Qicheng Zeng, Relu Burlacu, Katherine Whidden, and Valerie Springer of the University of Utah’s Department of Geology and Geophysics. Funding for this research was provided by the state of Utah, the U.S. Department of Energy, and the U.S. Geological Survey.
