For most people, earthquakes occur relatively close to the Earth’s surface. They break through the Earth’s crust, shaking towns and cities and leaving faults that geologists can map. It is widely believed that the deeper parts of the Earth behave differently. At enormous pressures and temperatures far below those in the Earth’s crust, rocks are expected to slowly deform rather than burst suddenly.This assumption is why a series of earthquakes beneath northern Utah and southwestern Wyoming has attracted so much attention. Some of these events originate dozens of miles below the Earth’s crust, and conventional earthquake theory suggests that earthquake ruptures are extremely difficult to rupture in this part of the Earth. A series of recent studies have confirmed that these unusual tremors are not measurement errors but actual mantle earthquakes occurring deep in the western United States.
Earthquakes deep beneath Utah and Wyoming trace back to 1979 mystery
The story begins with a magnitude 3.8 earthquake recorded near Randolph, Utah, on February 24, 1979. At the time, its estimated depth was immediately noticeable. The data showed that the event originated about 94 kilometers below sea level, deep beneath the Earth’s crust and upper mantle.Such a position is difficult to accept. Continental earthquakes almost always occur within the Earth’s crust, where lower temperatures cause rocks to fracture in a brittle manner. Mantle rocks at these depths are generally expected to flow and deform rather than break suddenly.This debate raged on for decades. According to a study published in Geophysical Research Letters titled “Upper mantle earthquakes at the edge of the Wyoming Craton” A new examination of seismic records confirms that the 1979 quake was not an isolated anomaly. After reviewing regional earthquake catalogs and reassessing depth estimates, the researchers identified nine confirmed mantle earthquakes occurring along the edge of the Wyoming Craton, including the 1979 event.
Scientists confirm nine deep mantle earthquakes beneath Utah and Wyoming
The study, led by Sean J. Hutchings, examined earthquakes recorded in Utah and Wyoming from 1979 to 2023. By comparing the depth of the earthquakes to fourteen independent models of crustal thickness, the team concluded that all nine events occurred beneath the Moho, the boundary that separates the crust and mantle.Eight of the earthquakes occurred more than 15 kilometers below this boundary. The deepest is the 1979 Randolph earthquake, which is estimated to have occurred about 60 kilometers below the Moho. According to the study, these events represent the clearest evidence of continental mantle earthquakes outside of areas such as the Himalayas and Tibet, where deep continental seismic activity has been previously recorded.In 2013, these earthquakes ranged from small to magnitude 4.8 beneath the Wind River Mountains in Wyoming. Although the magnitudes vary, they all share one notable feature: they all appear to originate from mantle material rather than the overlying crust.
Scientists discover rare mantle earthquake to occur beneath Utah in 2025
Interest in this phenomenon increased again after another deep earthquake occurred in northeastern Utah on September 10, 2025.In a paper published in Earthquake Records, titled “Magnitude 4.1 earthquake in northeastern Utah, USA, on September 10, 2025: a typical continental mantle eventA 4.1-magnitude earthquake was reported near Mercer, Utah. Various seismic techniques located the event at a depth of approximately 65 to 70 kilometers below the surface. Previous studies estimated the crustal thickness in the region to be only about 40 to 45 kilometers.Earthquakes occur approximately 20 to 25 kilometers within the continental mantle. The researchers described it as a “classic continental mantle event” and noted that it shared some characteristics with early deep earthquakes found in Utah and Wyoming.Unlike many shallow earthquakes, the Messer event produced no significant foreshocks or aftershocks. Its seismic waves also contain unusually strong high-frequency energy, another feature previously observed in mantle earthquakes in the region.
Why deep earthquakes are concentrated near the Wyoming Craton
One clue may lie in the location of the earthquake. Both studies found that these events were concentrated near the western edge of the Wyoming Craton, an ancient continental lithosphere block that forms part of the geological core of North America.According to the study, all nine mantle earthquakes occurred within about 100 kilometers of the cratonic lithosphere boundary. This region marks the transition between thick, stable cratonic rocks and the surrounding mantle, which appears to be more dynamic and structurally complex.Researchers suspect that mantle currents may play a role. As mantle material slowly moves around the solid craton roots over millions of years, stress builds up along the boundaries. According to the study, enhanced strain rates associated with mantle convection near craton margins may help create conditions for earthquakes to occur at depths where rocks would otherwise be expected to gradually deform. Earthquake in unusually hot environment
How do mantle rocks with temperatures exceeding 700°C generate earthquakes?
According to the study, an estimated eight out of nine mantle earthquakes occur in areas with temperatures exceeding 700°C. Some are associated with temperatures approaching or exceeding 1,000°C. In this case, traditional brittle failure becomes increasingly difficult.To explain this, the researchers suggested that there may be more than one mechanism at work. Some earthquakes may involve brittle failure near the crust-mantle boundary, while deeper events may be related to thermal runaway processes. In this case, the deformation is concentrated in a narrow region, releasing energy rapidly despite the ductility of the surrounding mantle. These studies also raise the possibility that fluids within the Earth’s mantle may be involved in this process, although the exact role remains uncertain.
