Researchers have verified a rare deep earthquake that struck beneath Utah in 1979 at a depth once thought impossible for such events under continents. New analysis of old data and a 2025 quake support the existence of continental mantle earthquakes.
The February 24, 1979, magnitude 3.8 earthquake occurred near Randolph, Utah, at about 90 kilometers below the surface. University of Utah researchers reexamined seismic records and confirmed its mantle origin, along with eight similar events in the region. A magnitude 4.1 quake on September 10, 2025, near Maeser originated 68 kilometers deep. It was described in published studies as an archetypal continental mantle event. Geology professor Keith Koper noted the unusual conditions, stating the rocks at such depths behave more like taffy yet can still fracture. The events cluster near the western edge of the Wyoming Craton. The findings appeared in Geophysical Research Letters in 2025 and The Seismic Record in 2026. Coauthors included Sean Hutchings and George Zandt.
