On the morning of February 24, 1979, the ground beneath Randolph, Utah trembled with a magnitude 3.8 earthquake that no one felt—and, for nearly half a century, almost no one believed was real. Now, researchers from the University of Utah have solved a 47-year scientific mystery, confirming that this deep tremor belonged to an entirely new category of seismic events: continental mantle earthquakes, a phenomenon that shouldn't exist according to conventional geology.
When postdoctoral researcher George Zandt first analyzed the Randolph quake's seismic recordings, he calculated that it originated about 90 kilometers below sea level—far deeper than the Earth's crust, deep within the upper mantle itself. At depths where rocks are expected to flow like taffy under extreme heat and pressure, earthquakes simply weren't supposed to happen. Zandt published his findings in a brief abstract, but the geological establishment was skeptical. "It was hard to convince others of the highly anomalous mantle earthquake occurring in a region where none should exist," Zandt recalled decades later.
The mystery remained dormant until geology professor Keith Koper and his team at the University of Utah seismic stations revisited the original waveform data. Using the institution's archive of decades-old seismic records, graduate student Sean Hutchings methodically studied known deep earthquakes and reclassified events previously labeled as crustal earthquakes. Their reanalysis confirmed what Zandt had suspected: nine separate earthquakes in northern Utah and southwestern Wyoming all originated well below the crust, providing irrefutable evidence of a genuine class of continental mantle earthquakes.
The breakthrough gained fresh urgency when another deep earthquake struck on September 10, 2025, near Maeser in Utah's Uinta Basin. This magnitude 4.1 event originated approximately 68 kilometers below the surface—more than 20 kilometers beneath the Mohorovičić discontinuity, the boundary separating Earth's crust from the mantle. Researchers described it as an "archetypal continental mantle event," a living confirmation of the pattern Zandt had first glimpsed decades earlier.
What makes these earthquakes so unusual extends beyond their depth. They occur in isolation, without the foreshocks and aftershocks that typically accompany shallow earthquakes. They cluster near the western edge of the Wyoming Craton, an ancient, stable block of lithosphere, and emerge only in regions where temperatures exceed 700 degrees Celsius—conditions where rock should bend rather than break. "It's sort of a mystery in terms of fundamental physics," Koper said. "How in the world can these things happen?"
The discovery carries weighty implications. With conventional earthquakes, scientists can measure fault segments visible at the surface to estimate maximum possible magnitude and assess seismic hazard. Continental mantle earthquakes offer no such guideposts. "We have no idea how big they can be," Koper emphasized. Zandt, now retired from the University of Arizona's geology faculty, collaborated on the new research, lending his decades of expertise to finally resolve the puzzle he first encountered in 1979.
The confirmation represents more than academic vindication for an overlooked researcher—it expands humanity's understanding of the dynamic forces moving beneath our feet, in realms we are only beginning to comprehend.