A small earthquake that rattled northern Utah in 1979 was dismissed for decades as a mistake in the data. Now scientists have confirmed it really happened nearly 90 kilometers below the surface, a depth where earthquakes should not exist beneath a continent.
A quake no one felt, but the ground moved anyway
At 4:23 a.m. on February 24, 1979, the ground shook near the town of Randolph, close to the borders of Idaho and Wyoming. The earthquake registered a magnitude of 3.8, but no one reported feeling it. Seismologists at the University of Utah noticed something strange in the recordings. The waves did not look like a typical shallow crustal quake.
George Zandt, then a postdoctoral researcher at the university, calculated that the earthquake originated about 90 kilometers below sea level. That placed it deep in the upper mantle, far below the crust. At the time, the scientific consensus held that earthquakes under continents could not happen at such depths. The rock in the mantle is hot and under high pressure, expected to flow slowly rather than snap and break. Zandt published a brief abstract in Earthquake Notes, but the finding was largely ignored.
Old seismic records reveal a hidden pattern
Decades later, researchers at the University of Utah took another look at the original waveform data. Led by geology professor Keith Koper, the team reexamined the 1979 event along with eight other suspected deep earthquakes recorded in northern Utah and southwestern Wyoming. Their analysis confirmed that all nine earthquakes originated well below the crust, providing strong evidence for a rare category of events now called continental mantle earthquakes, or CMEs.
The findings gained new urgency when another deep earthquake struck on September 10, 2025, near Maeser in Utah's Uinta Basin. That quake reached magnitude 4.1 and originated about 68 kilometers below the surface. Its source was more than 20 kilometers beneath the Mohorovičić discontinuity, the boundary between Earth's crust and the mantle. The 2025 event matched the pattern of the 1979 quake and the other historical events, reinforcing the conclusion that these deep mantle earthquakes are real and not anomalies in the data.
Why this matters for understanding how the planet works
For local residents, the confirmation of these deep earthquakes changes little about daily life. The quakes are small and occur so far underground that people on the surface do not feel them. But for geologists, the discovery overturns a long held assumption about where and how earthquakes can happen beneath continents. The mantle was thought to be too hot and ductile to host brittle failure. These events suggest that under certain conditions, rock in the upper mantle can break suddenly, producing seismic waves that travel to the surface.
The research, conducted at the University of Utah and published in The Seismic Record, opens a new line of inquiry into the behavior of Earth's interior. The nine confirmed events cluster in a region where the North American plate has a complex tectonic history. Scientists do not yet know what conditions allow these deep mantle earthquakes to occur. The answer may lie in the composition of the rock, the presence of fluids, or the stress state of the mantle beneath the western United States.
