Nearly 50 years after George Zandt recorded a puzzling 3.8 magnitude earthquake beneath Randolph, Utah on February 24, 1979, researchers have finally confirmed what he long suspected: earthquakes can rupture Earth's mantle far deeper than scientists once believed possible. The 1979 Randolph quake, with its focal depth of 90 kilometers below sea level, was virtually ignored by the scientific community at the time. But last year, a new generation of seismologists at the University of Utah re-evaluated decades of archived seismic data and proved that this anomalous event was real—and part of a rare class of continental mantle earthquakes, or CMEs.
The existence of these deep earthquakes fundamentally changes how scientists understand Earth's interior. Most earthquakes occur within the crust, relatively close to the surface. But CMEs happen far below the Mohorovičić discontinuity, the boundary that separates Earth's crust from the mantle. On September 10, 2025, researchers witnessed this phenomenon firsthand when a magnitude 4.1 earthquake struck near Maeser in Utah's Uinta Basin at a depth of 68 kilometers—more than 20 kilometers below the Moho. Keith Koper, director of the University of Utah Seismograph Stations and a former student of Zandt's, characterized it as an "archetypal continental mantle event."
The discovery emerged from painstaking detective work. Koper's graduate student, Sean Hutchings, combed through the Seismograph Stations' historical archive, analyzing the arrival times of different seismic waves at the surface. By comparing these timing patterns, the team re-evaluated nine suspected deep earthquakes across northern Utah and southwest Wyoming that had previously been misidentified as shallow crustal quakes. A study published in Geophysical Research Letters last May confirmed the deep locations of all nine. The September 2025 Maeser earthquake has now been documented in a subsequent study published in The Seismic Record, adding another confirmed case to an ever-growing catalog.
What makes these earthquakes so extraordinary is not just their depth, but the conditions under which they occur. At depths where CMEs rupture, temperatures often exceed 700 degrees Celsius—hot enough that rock behaves less like solid stone and more like taffy, flowing imperceptibly over millions of years. Despite these extreme conditions, the rocks can still rupture suddenly, releasing seismic energy. Unlike typical crustal earthquakes, which often trigger foreshocks and aftershocks along mapped fault lines, CMEs occur in isolation with no warning or follow-up events. They cluster near the edge of the Wyoming Craton, an ancient, thickened block of continental lithosphere.
This mystery presents both scientific fascination and practical challenge. Seismologists have long estimated the maximum size of crustal earthquakes by measuring the length of surface fault segments—a method rooted in decades of observation. But with CMEs, researchers face a fundamental uncertainty. "We have no idea how big they can be," Koper said. "Another reason why it's a big deal is that we have no idea how big they can be." Understanding these rare deep earthquakes may force a complete recalibration of seismic hazard assessments across regions where they occur. For the first time in nearly half a century, Zandt emerged from retirement to co-author the latest research, witnessing the vindication of his initial discovery.