Researchers Identify Rare Deep-Mantle Earthquakes Beneath Utah

A team of geologists from the University of Utah has confirmed the existence of a rare class of seismic events known as continental mantle earthquakes (CMEs). By re-evaluating seismic data from a mysterious 1979 event near Randolph, Utah, researchers validated that the quake originated approximately 90 kilometers below the surface. This depth places the event well within the Earth's upper mantle, a region previously thought to be too hot and pressurized for the brittle fracturing required to generate earthquakes.

For decades, the 1979 Randolph earthquake was dismissed as an anomaly or a potential error in measurement. However, by applying modern analytical techniques to historical waveform data and comparing it with a more recent 2025 event in the Uinta Basin, the research team identified a pattern of nine distinct seismic events originating beneath the Mohorovičić discontinuity. These findings suggest that the mantle beneath the western United States is capable of brittle failure under conditions that were once considered impossible for such activity.

This discovery carries significant implications for our understanding of geodynamics and seismic risk. Because these earthquakes occur deep within the mantle, they often go unnoticed by surface populations, yet they challenge fundamental models of how tectonic plates behave and how stress is distributed deep beneath continental crust. By confirming that these events are not mere outliers, scientists are now tasked with re-evaluating the rheology of the upper mantle and the physical mechanisms that allow rock to fracture at such extreme depths. This shift in perspective may lead to more accurate seismic hazard modeling as researchers continue to investigate the unique conditions that permit these deep-seated tremors.