For most people, earthquakes are something that happens relatively close to the Earth’s surface. They break through the crust, shake towns and cities, and leave behind faults that geologists can map. The deeper parts of the planet are generally thought to behave differently. Under the intense pressures and temperatures found far below the crust, rocks are expected to deform slowly rather than fracture suddenly.That assumption is why a cluster of earthquakes beneath northern Utah and southwestern Wyoming has attracted such attention. Some of these events originated tens of miles beneath the crust, in a part of the Earth where conventional earthquake theory suggests seismic ruptures should be extremely difficult. A series of recent studies has strengthened the case that these unusual tremors are not measurement errors but genuine mantle earthquakes occurring deep beneath the western United States.
Deep earthquakes beneath Utah and Wyoming trace back to a 1979 mystery
The story began with a magnitude 3.8 earthquake recorded near Randolph, Utah, on 24 February 1979. At the time, its estimated depth stood out immediately. Data suggested the event originated about 94 kilometres beneath sea level, placing it far below the crust and deep within the upper mantle.Such a location was difficult to accept. Continental earthquakes almost always occur within the crust, where cooler temperatures allow rocks to break in a brittle manner. Mantle rocks at those depths are generally expected to flow and deform rather than rupture suddenly.That debate continued for decades. According to a study published in Geophysical Research Letters, titled “Upper Mantle Earthquakes Along the Edge of the Wyoming Craton”, a fresh examination of seismic records confirmed that the 1979 earthquake was not an isolated anomaly. After reviewing regional earthquake catalogues and re-evaluating depth estimates, researchers identified nine confirmed mantle earthquakes along the edge of the Wyoming Craton, including the 1979 event.
Scientists confirm nine deep mantle earthquakes beneath Utah and Wyoming
The research, led by Sean J. Hutchings, examined earthquakes recorded between 1979 and 2023 in Utah and Wyoming. By comparing earthquake depths with fourteen independent models of crustal thickness, the team concluded that all nine events occurred beneath the Moho, the boundary separating Earth’s crust from the mantle.Eight of those earthquakes were located more than 15 kilometres below that boundary. The deepest, the 1979 Randolph earthquake, was estimated to have occurred roughly 60 kilometres beneath the Moho itself. According to the study, these events represent some of the clearest evidence for continental mantle earthquakes outside regions such as the Himalayas and Tibet, where deep continental seismicity has been documented before.The earthquakes ranged from small tremors to a magnitude 4.8 event beneath Wyoming’s Wind River Range in 2013. Despite their differences in size, they shared one striking characteristic: they all appeared to originate within mantle material rather than the overlying crust.
Scientists identify a rare mantle earthquake beneath Utah in 2025
Interest in the phenomenon increased again after another deep earthquake struck northeastern Utah on 10 September 2025.In a paper published in The Seismic Record, titled “The 10 September 2025 Mw 4.1 Earthquake in Northeastern Utah, United States: An Archetypal Continental Mantle Event”, a magnitude 4.1 earthquake near Maeser, Utah, was reported. Multiple seismic techniques placed the event at a depth of roughly 65 to 70 kilometres beneath the surface. Previous studies had estimated crustal thickness in the area at only about 40 to 45 kilometres.That left the earthquake approximately 20 to 25 kilometres inside the continental mantle. The researchers described it as an “archetypal continental mantle event” and noted that it shared several features with earlier deep earthquakes identified in Utah and Wyoming.Unlike many shallow earthquakes, the Maeser event produced no obvious foreshocks or aftershocks. Its seismic waves also contained unusually strong high-frequency energy, another feature previously observed in mantle earthquakes from the region.
Why do deep earthquakes cluster near the Wyoming Craton
One clue may lie in the location of the earthquakes. Both studies found that the events cluster near the western edge of the Wyoming Craton, an ancient block of continental lithosphere that forms part of North America’s geological core.As per the study, all nine mantle earthquakes occurred within roughly 100 kilometres of the craton’s lithospheric boundary. The region marks a transition between thick, stable cratonic rock and the surrounding mantle that appears more dynamic and structurally complex.Researchers suspect that mantle flow may play a role. As mantle material slowly moves around the rigid cratonic root over millions of years, stresses can accumulate along the boundary. According to the study, enhanced strain rates linked to mantle convection near the craton edge may help create conditions that allow earthquakes to occur at depths where rocks would otherwise be expected to deform gradually. Earthquakes in a surprisingly hot environment
How earthquakes occur in the mantle rock hotter than 700°C
As per the study, estimated that eight of the nine mantle earthquakes occurred in regions where temperatures exceeded 700°C. Some were associated with temperatures approaching or exceeding 1,000°C. Under such conditions, conventional brittle failure becomes increasingly difficult.To explain this, researchers proposed that more than one mechanism could be operating. Some earthquakes may involve brittle failure near the crust-mantle boundary, while deeper events could be linked to a process known as thermal runaway. In this scenario, deformation becomes concentrated within a narrow zone, generating a rapid release of energy despite the surrounding mantle behaving in a ductile manner. The studies also leave open the possibility that fluids within the mantle could contribute to the process, although the exact role remains uncertain.
