Scientists have just confirmed the existence of a type of earthquake that shouldn’t exist beneath North America

A decades-long seismic mystery has finally been solved. Researchers have confirmed that the 1979 earthquake beneath the Utah-Wyoming region originated deep in the Earth’s upper mantle, well below the crust where earthquakes are typically expected to occur. This finding, published in The Seismic Record, validates a controversial observation that has puzzled geologists for nearly half a century, and establishes a new class of seismic events known as continental mantle earthquakes.

A forgotten earthquake that refused to abide by the rules

On February 24, 1979, seismometers operated by the University of Utah recorded a magnitude 3.8 earthquake near Randolph, a small community close to the Idaho-Wyoming border. Under normal circumstances, an earthquake of this size would have been felt across the region. However, no reports from people have surfaced.

The reason soon became clear to postdoctoral researcher George Zandt, who analyzed the seismic records and discovered something unusual. The quake appears to have originated approximately 90 kilometers below the surface, deep in the Earth’s upper mantle rather than the crust. Such a site challenges conventional geological understanding because rocks at those depths are exposed to temperatures and pressures that would prevent the brittle fracture associated with earthquakes.

“The deep depth explained why people didn’t feel it on the surface,” Zandt said.

At the time, this discovery was met with skepticism. The scientific community had little reason to expect earthquakes to occur in that environment, and this observation was largely ignored for decades.

“I’ve done some other analyzes that have convinced me of the reality of the deep abyss, but it’s been difficult to convince others of a very anomalous mantle earthquake occurring in an area where there shouldn’t be anything like it.”

This event remained an unresolved anomaly until modern researchers revisited the original data using improved analytical techniques and a much larger archive of seismic observations.

Researchers confirm the existence of a new class of earthquakes

The new investigation, led by seismologist Keith Cooper of the University of Utah, examined the original 1979 quake along with eight additional seismic events suspected to have originated at an unusual depth underground. The study, published in The Seismic Record, confirmed that all nine earthquakes occurred beneath the Earth’s crust in the Earth’s upper mantle.

This confirmation has led scientists to officially classify the phenomenon as continental mantle earthquakes (CMEs). This designation represents a major development in seismology because these events occur in an environment that behaves very differently from the shallow crust where most earthquakes begin.

Map of the craton area of ​​southwestern Wyoming, modified from Hutchings et al. (2025). The background color is the shear velocity at a depth of 60 km (Schmandt et al., 2015). The yellow stars are continental mantle earthquakes (CMEs) during the period 1979–2023 that have been verified in previous studies (Hutchings et al., 2025, and references therein). The orange stars are six recently identified CMEs that occurred during the period 2007-2010 (Woo and Chen, 2025). The white stars are four suspected CMEs identified by the University of Utah Seismic Stations (UUSS) in 2025, and the red star is the location of the 2025 Macer earthquake. The thick black line indicates the approximate boundaries of the lithospheric keel in the Wyoming craton established by Bezada et al. (2024) using the mid-depth (60-125 km) high-resistivity ocean from Bedrosian and Frost (2023). The black arrow indicates east-northeast asthenosphere (ENE) motion of ∼4 cm/yr relative to the stationary North American plate (Smith and Sbar, 1974). In focal mechanisms, black circles represent compressive P axes, and white circles represent tensile T axes. Despite the difference in mechanisms, the P axes are nearly horizontal and aligned from north-south (NS) to northeast-southwest (NE-SW). Credit: Seismic Record

Unlike traditional seismic events, newly confirmed mantle earthquakes tend to occur alone. The researchers found no evidence of the sequence of aftershocks and aftershocks typically associated with crustal earthquakes. Their distribution also revealed a geographic pattern, clustering near the western edge of the ancient Wyoming Craton, a huge mass of stable continental lithosphere underlying much of Wyoming and adjacent areas.

The study also identified a more recent example. On September 10, 2025, a magnitude 4.1 earthquake struck near Macer in Utah’s Uinta Basin, with a depth of about 68 kilometers. This location placed the event more than 20 kilometers below the Mohorovičić discontinuity, or Moho, the boundary that separates the Earth’s crust from the mantle.

Together, these discoveries provide compelling evidence that mantle earthquakes are not isolated anomalies, but part of a broader geological process occurring beneath the western United States.

Why deep mantle earthquakes should be impossible

The confirmation of continental mantle earthquakes raises a larger question: How can earthquakes occur in rocks that should not break?

At depths of approximately 70 to 90 kilometres, temperatures can exceed 700 degrees Celsius. Under these extreme conditions, mantle rocks would be expected to deform gradually rather than suddenly break apart. Geologists often describe this behavior as elastic flow, meaning that the rock behaves more like a slowly moving material than a solid solid.

“It’s like candy,” Cooper said. “It’s sweet on long time scales, like millions of years.”

(a) Nearby stations (triangles) that recorded the 2025 Messer earthquake (black star). Red triangles indicate stations with seismograms plotted in the following panels: (b) UU.VNL2, (c) UU.LIUT, (d) UU.HDUT, (e) C0.SPVA, and (f) US.BW06. For each trace, the direction was removed, and the instrument response was deconvolved using a trapezoidal taper with transition frequencies between 0.01-0.02 Hz and 40-50 Hz. The two horizontal components were rotated in the radial and transverse directions. The vertical lines show the arrival times of the P wave that arrived first (p, red) and the S wave that arrived first (s, blue). Note that the timelines differ between panels (b)-(f). Credit: Seismic Record

This comparison highlights the scientific mystery at the heart of this discovery. Conventional earthquake theory is based on the sudden release of stress through brittle failure. However, mantle rocks at these temperatures must absorb and redistribute pressure over time, rather than storing it until rupture occurs.

The existence of continental mantle earthquakes suggests that unknown mechanisms may allow stress to build up and release even under these seemingly unfavorable conditions. The researchers have proposed several possibilities, including localized areas of weakness, mineral transitions, or unusually concentrated strain, although none of them fully explain the observations.

As a result, these earthquakes have become an important focus for geophysicists seeking to understand the deep workings of the Earth’s interior.

The Wyoming Craton may hold the key

Scientists believe the answer may lie in the unique geology of western North America. Confirmed mantle earthquakes occur near the boundaries of the Wyoming Craton, one of the oldest and most stable parts of the continental lithosphere on the planet.

Cooper compares the craton to an iceberg embedded within the Earth’s mantle. While the visible crust only makes up the top, the structure extends deep underground, creating a solid root surrounded by slowly moving mantle material.

Results of modeling S spectra at UU.PNSU station, 128 km away. (a) Ground speed recorded on the transverse component. Vertical gray lines define the noise window, and blue lines define the S window. (b) Amplitude spectra of the noise window (gray) and the S window (blue). The red dashed line shows the best-fitting Brun model. The dashed gray lines indicate the frequency range used in the fitting, which is defined by the region where the signal-to-noise ratio is greater than three. Details of the methodology are provided elsewhere (Edwards et al., 2010; Holt et al., 2021). Spectral fits at additional stations are shown in Figure S8. Specifications. Spectral velocity. Credit: Seismic Record

Over millions of years, mantle rocks have flowed around this ancient formation. This interaction appears to generate concentrated stress along the craton margins, which may create favorable conditions for deep seismic activity.

“On a scale of millions of years, the mantle collides with the craton and then flows around it,” Cooper explained.

“It’s this interaction where the mantle flow is diverted around this solid cratonian root that causes the strain rate to increase, the deformation to increase, and it also creates additional stresses. We think it’s this interaction between the keel of the glacier and the surrounding medium that leads to these earthquakes.”

The researchers note that the western edge of the craton has undergone extensive geological modification over hundreds of millions of years. The lithosphere becomes progressively thinner toward Utah and Idaho, creating sharp contrasts in structure and mechanical behavior. These variations may focus stress on specific regions deep within the mantle, generating the unusual earthquakes that are now being documented.

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