University of Utah geologists have confirmed that a long-dismissed 1979 earthquake beneath the town of Randolph really did rupture about 90 kilometers underground — far deeper than scientists thought quakes could occur beneath a continent. Led by geology professor Keith Koper, the team reanalyzed decades of waveform data and identified nine of these rare "continental mantle earthquakes," all originating well below the Moho, the boundary between crust and mantle.
The magnitude-3.8 Randolph event, so faint nobody reported feeling it, was joined by a magnitude-4.1 quake near Maeser in September 2025 at roughly 68 kilometers depth. The findings were published in The Seismic Record and Geophysical Research Letters, drawing on early calculations by researcher George Zandt.
The results challenge a basic assumption in seismology: at such depths and temperatures, rock is expected to slowly flow rather than suddenly snap. "At that depth it is going to flow — it's more like taffy on long time scales, like millions of years," Koper said. Confirming that the mantle can still break opens new questions about seismic hazard across the region.