Image Courtesy: ScienceDaily
A mysterious earthquake that struck beneath northern Utah nearly 50 years ago has been confirmed as a genuine deep-mantle seismic event, challenging long-held assumptions about where earthquakes can occur inside Earth.
The magnitude 3.8 quake occurred near the town of Randolph in February 1979 and appeared to originate roughly 90 kilometers below the surface, far deeper than continental earthquakes were believed capable of forming. After decades of skepticism, researchers at the University of Utah have reanalyzed the original seismic records and concluded that the event was real, according to a newly published study.
The findings place the Randolph earthquake within a rare class of events known as continental mantle earthquakes, or CMEs. Unlike conventional earthquakes that occur within Earth’s brittle crust, these quakes originate deep inside the upper mantle, where temperatures and pressures are so extreme that rocks are generally expected to bend and flow rather than fracture suddenly.
Researchers revisited the 1979 event alongside eight other suspected deep earthquakes recorded in northern Utah and southwestern Wyoming. Their analysis confirmed that all nine originated well below the crust, providing some of the strongest evidence yet that continental mantle earthquakes are a real and recurring phenomenon.
The discovery gained additional importance after another deep earthquake struck Utah’s Uinta Basin in September 2025. That magnitude 4.1 event originated around 68 kilometers below the surface, more than 20 kilometers beneath the boundary separating Earth’s crust from the mantle, further reinforcing the existence of these unusual seismic events.
What makes the earthquakes especially significant is that scientists still do not fully understand how they happen. At depths where temperatures can exceed 700 degrees Celsius, rocks behave more like extremely slow-moving taffy over geological timescales. Conventional earthquake theory suggests sudden fault ruptures should be far less likely under such conditions.
The events also differ from typical earthquakes in other ways. Researchers found they tend to occur in isolation, without the foreshocks and aftershocks commonly associated with shallow seismic activity. That makes them more difficult to study and potentially harder to anticipate.
Scientists believe the answer may lie beneath the ancient Wyoming Craton, a massive block of stable continental crust that extends deep into Earth’s mantle. As mantle material slowly flows around this dense underground structure over millions of years, stresses may build along its edges, eventually triggering deep earthquakes in regions where Earth’s lithosphere becomes thinner.
Beyond solving a geological mystery, the findings could have implications for seismic hazard assessments. While scientists can estimate the maximum size of many crustal earthquakes by mapping known fault systems, deep mantle earthquakes offer far fewer clues about how large they could ultimately become.
For now, the newly confirmed Utah events highlight how much remains unknown about Earth’s interior and suggest that some of the planet’s most surprising earthquakes may be occurring in places where scientists once thought they were impossible.