In the quiet town of Randolph, Utah, a mysterious event occurred in 1979 that left geologists scratching their heads. A 3.8-magnitude earthquake, though seemingly modest, went unnoticed by the town's residents, leaving scientists at the University of Utah Seismograph Stations puzzled. Fast forward to the present, and researchers, in collaboration with geologists from Sandia National Labs, have made a groundbreaking discovery, shedding light on this enigmatic phenomenon.
The Enigma of Mantle Earthquakes
The Randolph earthquake, once an enigma, is now understood as a unique category of seismic activity known as "mantle earthquakes." These quakes occur deep beneath the Earth's tectonic plates, at depths ranging from 43 to 55 miles (70 to 90 kilometers). George Zandt, a retired geologist who first noticed this anomaly while working as a postdoc, has rejoined the research team to unravel this mystery.
"The deep depth of these quakes explains why they often go unnoticed by people on the surface," Zandt explains. "It's a subjective science when it comes to earthquakes that are 'felt' by people." The U.S. Geological Survey defines this as a combination of the quake's magnitude and intensity, with people typically reporting feeling earthquakes of magnitude 3.0 or higher.
Unraveling the Mystery
Keith Koper, a geophysicist and former protégé of Zandt, has taken the lead in cataloging and studying these deep mantle quakes. In a paper published in Geophysical Research Letters last May, Koper confirmed nine cases of quakes originating below the Earth's crust. He also contributed to a follow-up study published in The Seismic Record this April, investigating a similar quake that occurred in Utah's Uinta Basin.
"These earthquakes are like taffy on long time scales, millions of years," Koper describes. "The material at that depth is under high temperature and pressure, and it flows like taffy." The September 10, 2025, earthquake, with a magnitude of 4.1, was traced back to a depth of 42 miles (68 km) below ground, revealing the power and uniqueness of these mantle events.
The Significance and Implications
Despite their deep origins and low-intensity surface impact, Koper emphasizes that evidence of these quakes can still be seen in rocks that have made their way back to the surface. "You can see how they were stretched," he says. These quakes remain a "mystery in terms of fundamental physics," but Koper and his team have identified some common traits.
The mantle quakes appear to occur in isolated bursts, without aftershocks or early tremors. Interestingly, they all seem to originate near the Wyoming Craton, a geologically ancient and stable formation. Cratons, extending deep below the Earth's surface, are like the keel of a ship, gliding through molten rock. Koper's team suspects that these deep mantle quakes are triggered when this molten material causes a disturbance.
The Importance of Understanding
While these deep mantle quakes may not pose an immediate threat to the surface, Koper believes it is crucial to study them to fully comprehend the "seismic hazard." "We have no idea how big they can be," he cautions. Understanding the potential magnitude of these quakes is essential for assessing the true risk they pose.
In conclusion, the discovery of mantle earthquakes highlights the fascinating and often mysterious nature of our planet. It serves as a reminder that there is still much to learn and uncover about the Earth's seismic activity, and the potential implications for human safety and understanding of our planet's fundamental physics.
