Geologists have detected a series of puzzling seismic tremors beneath the US state of Utah, none of which were perceptible on the surface. Scientists have concluded that these represent a novel category of geophysical events: earthquakes occurring within the continental mantle at depths previously thought incapable of generating such phenomena.
In the early morning hours of February 24, 1979, seismologists recorded a magnitude 3.8 earthquake near Randolph, Utah. However, no local residents reported feeling it. The reason remained an enigma for quite some time, as the epicenter was situated approximately 90 kilometers deep – well below the Mohorovičić discontinuity, which demarcates the Earth’s solid crust from the more ductile mantle.
According to established geological understanding, rocks at such depths are too pliable to accumulate the stress necessary for a tectonic rupture. Instead of a sharp seismic jolt, they are expected to deform slowly, exhibiting a “flowing” behavior under pressure.
The explanation began to emerge on September 10, 2025, with another “imperceptible” earthquake, measuring 4.1, occurring in Utah’s Uinta Basin. Upon examining the data, researchers from the University of Utah determined its focal point was at a depth of 68 kilometers, again substantially beneath the Moho. Both of these events, along with a cluster of similar tremors beneath northern Utah and southwestern Wyoming, shared a common anomaly: they originated in locations where, by all geophysical principles, they should not have occurred.
In a recent study published in The Seismic Record, a team of scientists asserts they have encountered a relatively recently identified class of events – earthquakes within the continental mantle. Over the past few years, geologists have documented over 450 such occurrences globally. Their existence was long met with skepticism within the scientific community, but sufficient data has now been compiled to confirm the authenticity of this phenomenon.
“The significant depth explains why surface dwellers did not experience the earthquake,” explains George Sandt, a geophysicist at the University of Arizona who analyzed the 1979 data.
The question remains: why has this particular region of the United States become a “hotspot” for mantle tremors? Researchers attribute the answer to the Wyoming Craton, an ancient continental crustal plate that has remained largely undisturbed for billions of years. This colossal “rock iceberg” is situated at the boundary between the geologically active West and the stable interior of the North American plate. For millions of years, it has thinned and shifted westward, creating a zone of stress at its interface with the mantle.
Keith Koper, lead author of the study from the University of Utah, elaborates that mantle flows circumvent the rigid cratonic root, leading to an increase in deformation and stress. It is this interaction, he states, that generates the mysterious deep earthquakes. As more data is accumulated, such events may become invaluable tools for geologists in studying the processes concealed within the planet’s interior.
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