47 years after the first of a series of mystery quakes shook Utah area residents, scientists have determined that these Earth-shaking events originate deep in the mantle rather than in the crust, where typical earthquakes originate.
Although the research team behind the published study outlining their mystery quake origin theory suggests a potential cause of these continental mantle earthquakes (CMEs), they said that there remain inherent challenges in studying these transient events, which occur in isolation without foreshocks or aftershocks, helping explain the decades-long mystery surrounding them.
“It’s sort of a mystery in terms of fundamental physics,” said the study’s leader, geology professor Keith Koper, “How in the world can these things happen?”
How a 1979 Mystery Quake Shook up Science for Decades
According to a statement announcing the new mystery quake findings, the enigma began on February 24th, 1979, when the University of Utah’s seismographic instruments detected an earthquake beneath the town of Randolph, near the Montana-Wyoming border. Although the relatively sophisticated instruments indicated it was a respectable 3.8 on the Richter scale, there was a surprising lack of public reports of shaking and rumbling that typically accompany such events.
When the young researcher decided to try to pinpoint the origin of the mystery quake that appeared suddenly without a foreshock, civilian reports, or a local fault line, his calculations didn’t make sense. According to Zandt, his data revealed that the mystery quake had originated 90 kilometers (about 56 miles) beneath the surface.
Because earthquakes originating below the so-called ‘Moho’ (Mohorovičić) region separating the Earth’s upper crust and lower mantle were considered impossible, the researcher was initially perplexed at the stubbornly consistent data. Still, he noted, the depth would help explain why people hadn’t felt the event, despite its relatively robust energy signature.
After some further analysis, Zandt, who has enjoyed a long career on the University of Arizona’s geology faculty since the initial mystery quakes investigation that he performed as a postdoctoral candidate and came out of retirement to co-author this new study, said the results “convinced me of the reality of the deep depth.” However, he added, “It was hard to convince others of the highly anomalous mantle earthquake occurring in a region where none should exist.”
New Analysis Finds Eight Additional CME’s and a Possible Cause
After submitting an abstract about the mystery quake for the journal Earthquake Notes, the young researcher’s findings of a mantle-originating event remained largely unnoticed. Then, in 2025, a new generation of university geologists took a fresh look at the data. According to the team’s published study, this included reexamining the waveform data from the original mystery quake and from eight other events that had occurred since then in the same general region.
After a thorough analysis and some input from Zandt, Professor Koper’s team confirmed that all nine events originated below the crust, resulting in the creation of the new CME category. Before the team published their findings, another CME was detected on September 10th, 2025. Measured at a magnitude of 4.1, the event originated approximately 68 kilometers beneath the surface, or over 20 km below the Moho line.
According to Koper, the ‘archetypical continental mantle event’ was an example of an earthquake “nucleating in very unusual conditions.”
“The high temperature, the high pressure, and almost all the material at that depth is going to flow,” the professor explained, adding that the stretched deep Earth material is more like “taffy on long time scales, 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,” he added.
‘Little Icebergs’ of Earth’s Lithosphere Direct the Mantle’s Flow, Like a Ship’s Rudder
Although the University of Utah team is confident in their identification of a new type of earthquake that originates beneath the crust in the mantle, they note that the newly identified CME’s still present a few mysteries. For example, the nine events characterized in their study occurred without any foreshocks or aftershocks. Koper said that another point that makes the study’s findings “a big deal” is that researchers have no idea how big a CME can be.
“With crustal earthquakes, we can measure what we think their maximum size is going to be,” the professor explained. “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.”
More research will be needed to further understand the mystery quakes that randomly shake the relatively isolated region, but the study authors said they already have a working theory to explain these little-understood, transient events.
Resting within the Earth’s mantle are ancient blocks of the planet’s lithosphere, structures that the team compared to icebergs. According to the team’s theory, the area where the quakes are occurring has a geological composition and history that make it susceptible to the events leading up to a CME.
“On the scale of millions of years, the mantle is hitting the craton and then flowing around it,” Koper explained. “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 professor added.
The study “The 10 September 2025 4.1 Earthquake in Northeastern Utah, United States: An Archetypal Continental Mantle Event” was published in The Seismic Record.
Christopher Plain is a Science Fiction and Fantasy novelist and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at plainfiction.com, or email him directly at christopher@thedebrief.org.