Almost fifty years ago, an earthquake was recorded in the northern part of Utah that puzzled seismologists due to its unusual depth. This event occurred much deeper than is typical for earthquakes beneath continental crust. A recent study by the University of Utah has confirmed that this earthquake did indeed take place and belongs to a rare category of deep seismic events that occur within the Earth's mantle.

Characteristics of Deep Earthquakes

The earthquake happened early in the morning on February 24, 1979, near the city of Randolph, close to the borders of Utah, Idaho, and Wyoming. It had a magnitude of 3.8, but the tremors were not felt at the surface. Seismic records revealed unusual features, which drew the attention of researchers. Data analysis pinpointed the epicenter at a depth of about 90 kilometers, significantly below the Earth's crust and deep within the upper mantle—a zone where earthquakes are rarely observed.

At the time, additional studies were conducted that confirmed the reality of such depth, but convincing the scientific community of such an anomalous event proved difficult. The initial report about the earthquake did not attract much attention, but decades later, University of Utah researchers revisited the original seismic records. Under the guidance of specialists, a new assessment was made of the 1979 earthquake’s waveforms and those of eight other suspected deep earthquakes in northern Utah and southwestern Wyoming. The analysis confirmed that all nine events occurred well below the crust, providing evidence for the existence of so-called continental mantle earthquakes (CME).

New Data and Modern Research

The significance of this discovery increased after another deep earthquake occurred on September 10, 2025, near the city of Maeser in the Uinta Basin, Utah. This event had a magnitude of 4.1, with its epicenter located at a depth of about 68 kilometers—more than 20 kilometers below the Mohorovičić discontinuity (Moho), which separates the Earth's crust from the mantle. In a separate study published in The Seismic Record, this earthquake was described as an archetypal continental mantle event.

Unlike most earthquakes, deep events occur under extreme temperatures and pressures. At such depths, rocks typically deform slowly rather than breaking suddenly. However, traces of stretching can be found in rocks that have been uplifted to the surface.

To determine the origin of an earthquake, seismologists analyze the arrival times of various seismic waves recorded by instruments at the surface. Small differences in wave arrival times help pinpoint the event’s source. The University of Utah’s seismic stations have preserved decades of seismic records, creating a valuable archive for modern research. These data have made it possible to identify additional deep events that were previously classified as crustal earthquakes.

Features and Geological Context

Deep earthquakes differ from typical ones in that they occur in isolation, without the foreshocks and aftershocks that are common for shallow events. They are concentrated near the western edge of the Wyoming craton block and arise in regions where temperatures often exceed 700 degrees Celsius.

The Wyoming craton is an ancient and stable block of lithosphere that extends beneath parts of Wyoming and neighboring states. Its structure varies across the region due to erosion, and the lithosphere becomes thinner toward Idaho and Utah. The newly discovered deep earthquakes occur precisely in this transitional zone.

Over millions of years, the mantle has interacted with the craton, flowing around it, which leads to increased deformation rates and additional stresses. According to researchers, this interaction is what triggers deep earthquakes in this region.