SALT LAKE CITY — Nearly half a century ago, a minor tremor rumbling beneath the rugged landscape of northern Utah left seismologists deeply puzzled. The seismic event appeared to originate at a depth previously thought impossible for earthquakes occurring within the stable interior of a continent.
For decades, the anomaly languished as an unverified outlier in scientific literature. However, groundbreaking new research led by the University of Utah has now definitively confirmed that the 1979 event was entirely real. It is part of a newly illuminated, highly rare category of seismic phenomena taking place deep within the Earth’s mantle—challenging foundational assumptions about geophysics, rock mechanics, and seismic hazard assessment.
Main Facts: A Paradigm Shift in Seismology
The recent findings, published across multiple peer-reviewed journals, center on the discovery of continental mantle earthquakes (CMEs). Unlike standard earthquakes that rupture within the Earth’s brittle crust, these seismic events nucleate far below the crust-mantle boundary—known as the Mohorovičić discontinuity, or Moho—deep within the upper mantle.
Key takeaways from the research include:
- The 1979 Anomaly Confirmed: A 3.8-magnitude earthquake on February 24, 1979, near Randolph, Utah, originated approximately 90 kilometers below sea level, firmly within the mantle.
- A Growing Catalog: University of Utah researchers re-analyzed historical waveforms, identifying a total of nine suspected deep earthquakes in northern Utah and southwestern Wyoming.
- Modern Validation: A recent 4.1-magnitude earthquake on September 10, 2025, near Maeser, Utah, struck 68 kilometers deep—more than 20 kilometers beneath the Moho—providing an "archetypal" textbook example of a CME.
- Geological Drivers: These events are concentrated along the western edge of the ancient Wyoming Craton, where mantle flow collides with a rigid, deep-rooted continental "keel," generating extreme stress and strain.
- The Physics Puzzle: Occurring in environments with temperatures exceeding 700 degrees Celsius, these deep rocks theoretically should deform like ductile taffy over millions of years rather than snap and fracture suddenly. Yet, somehow, they break.
Chronology: From a Dismissed Postdoc Theory to Modern Vindication
The story of this scientific breakthrough spans nearly fifty years, bridging generations of geological research and technological evolution.
February 24, 1979: The Randolph Tremor
In the early morning hours, a 3.8-magnitude earthquake struck beneath the small town of Randolph near Utah’s borders with Idaho and Wyoming. Because of its extreme depth, nobody at the surface felt the shaking. However, the seismic waveforms recorded by local instruments looked distinctly unusual, triggering an intensive investigation.
At the time, University of Utah postdoctoral researcher George Zandt analyzed the data. He calculated that the earthquake originated roughly 90 kilometers below sea level.
"The deep depth explained why it wasn’t felt by people at the surface," recalled Zandt, who later became a professor 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."
Zandt published a brief abstract about the Randolph event in the journal Earthquake Notes, but without advanced computing power or a broader context of similar events, the finding was largely overlooked by the wider scientific community.
Decades Later: Mining the Archive
Interest in Zandt’s forgotten paper was eventually revived by a new generation of geoscientists at the University of Utah. Led by geology professor Keith Koper—who directs the University of Utah Seismograph Stations and remarkably once studied under Zandt—researchers began digging through decades of preserved analog and digital seismic records.
Graduate student Sean Hutchings utilized the extensive archive maintained by the University of Utah to cross-examine historical data. By applying modern waveform analysis techniques, Hutchings and the team reexamined the 1979 Randolph event alongside eight other historical tremors in northern Utah and southwestern Wyoming. Their collective analysis proved that all nine events originated deep within the continental mantle.
September 10, 2025: The Maeser Earthquake
The theoretical framework was put to the ultimate test in real time on September 10, 2025. A 4.1-magnitude earthquake struck near Maeser in Utah’s Uinta Basin, originating approximately 68 kilometers below the surface.
Caught by modern high-precision seismic networks, this event occurred more than 20 kilometers beneath the Moho. Researchers quickly documented it in a separate study published in The Seismic Record, classifying the Maeser tremor as an "archetypal continental mantle event."
To cement this collaborative triumph, George Zandt came out of retirement to coauthor the new research alongside his former academic descendants.
Supporting Data and Geological Mechanics
To understand why these mantle earthquakes baffle scientists, one must examine the extreme environments in which they occur.
The Taffy Paradox
Standard earthquakes happen in the Earth’s crust—the thin, brittle outer shell of the planet—where rocks are cold and prone to snapping under tectonic stress. In contrast, the upper mantle sits at depths where temperatures frequently exceed 700 degrees Celsius and pressures are immense.
"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," explains Professor Keith Koper. "It’s more like taffy, it’s taffy on long time scales, like millions of years. 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."
The Wyoming Craton "Keel"
The geographic distribution of these deep earthquakes holds the key to their existence. They are heavily concentrated along the western edge of the Wyoming Craton—an ancient, highly stable block of continental lithosphere.
Koper uses an iceberg analogy to describe cratons: rather than simply floating on top of the mantle, they extend deep downward like the massive keel of a ship.
Situated between the tectonically active, stretching landscape of the western United States and the stable interior of the North American plate, the Wyoming Craton acts as an immovable obstacle. Over millions of years, mantle material flowing beneath the Earth is forced to divert around this rigid cratonic root.
This violent hydrodynamic interaction creates localized zones of increased strain rates, severe deformation, and extraordinary stress concentrations—ultimately driving rocks past their breaking point even at mantle depths.
Official Responses and Scientific Publications
The findings have been formally documented in two prominent peer-reviewed publications:
- "Upper Mantle Earthquakes Along the Edge of the Wyoming Craton," published May 5, 2025, in Geophysical Research Letters.
- "The 10 September 2025 4.1 Earthquake in Northeastern Utah, United States: An Archetypal Continental Mantle Event," published April 10, 2025, in The Seismic Record.
The research team behind these publications represents a broad collaboration from the University of Utah Department of Geology & Geophysics, including:
- Keith Koper (Director, University of Utah Seismograph Stations)
- George Zandt (Professor Emeritus / Collaborator)
- Sean J. Hutchings (Graduate Student / Lead Data Analyst)
- Fan-Chi Lin, Qicheng Zeng, Relu Burlacu, Katherine Whidden, and Valerie Springer
Financial support for the multi-year investigation was provided by institutional and governmental backing, including the State of Utah, the U.S. Department of Energy, and the U.S. Geological Survey (USGS).
Implications: Rewriting Seismic Hazard Models
The discovery of continental mantle earthquakes is far more than an academic curiosity; it opens up profound new questions regarding fundamental physics and seismic hazard mitigation.
The Physics Mystery
Scientists still do not fully understand the exact failure mechanism that allows rock to snap elastically in an environment where it should behave like ductile plastic. Hypotheses range from localized dehydration reactions to ancient mineral phase changes under extreme stress, but definitive answers remain elusive.
Evaluating Seismic Hazards
For crustal earthquakes, seismologists can estimate maximum potential magnitudes by mapping surface fault segments, measuring their lengths, and calculating historical slip rates. However, CMEs operate entirely differently.
- No Surface Precursors: Unlike shallow earthquakes that are frequently preceded by foreshocks and followed by extensive aftershock sequences, these deep mantle events occur in complete isolation.
- Unknown Upper Limits: Because they do not map to surface faults, scientists currently have no reliable way of knowing how large continental mantle earthquakes can theoretically grow.
"It’s sort of a mystery in terms of fundamental physics. How in the world can these things happen?" Koper reflects. "Another reason why it’s a big deal is that we have no idea how big they can be… 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. With mantle events, we are starting from scratch."
As seismologists continue to mine historical archives and monitor modern networks like the one operated by the University of Utah, the mapping of continental mantle earthquakes will undoubtedly force a comprehensive reassessment of how deep-seated tectonic forces shape the American West—and what hidden threats may still lie waiting far beneath our feet.
