Research from the University of Utah, announced on May 27, confirms that a mysterious earthquake recorded nearly 50 years ago beneath northern Utah was real and part of a rare class of deep earthquakes. These events occur far deeper beneath the continental crust than previously believed possible.
In the early morning hours of Feb. 24, 1979, the University of Utah Seismograph Stations detected an earthquake under Randolph, near the Idaho and Wyoming borders. Despite its magnitude 3.8 strength, no one reported feeling it due to its unusual depth—90 kilometers below sea level—placing it well into Earth’s upper mantle. George Zandt, then a postdoctoral seismology researcher at the university, said, “The deep depth explained why it wasn’t felt by people at the surface. 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’s findings remained largely unnoticed until last year when Keith Koper, professor in geology at the University of Utah and director of its Seismograph Stations, led a study re-examining waveform data from this event and eight other suspected deep earthquakes in northern Utah and southwest Wyoming. The team confirmed all nine quakes occurred well below Earth’s crust as “continental mantle earthquakes” (CMEs). Another CME struck outside Maeser in Utah’s Uinta Basin on Sept. 10, 2025, with a magnitude 4.1 quake at a focal depth of 68 kilometers.
Koper described these events as originating under very unusual conditions: “This is an example of an earthquake that’s nucleating in very unusual conditions, the high temperature, the high pressure… It’s more like taffy on long time scales.” He added, “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 studies found these quakes typically occur near edges of ancient geological formations known as cratons—in this case near Wyoming Craton—where rock temperatures often exceed 700 degrees Celsius and are soft and ductile. These quakes tend to happen in isolation without aftershocks or foreshocks.
Koper said, “It’s sort of a mystery in terms of fundamental physics. How in the world can these things happen? Another reason why it’s a big deal is that we have no idea how big they can be.” The research appeared April 10 in The Seismic Record and May 5 in Geophysical Research Letters.
