Randolph, Utah, is a tiny town with just 467 residents. Early on February 24, 1979, it experienced a 3.8 magnitude earthquake. Surprisingly, no one in town noticed it enough to report it. Geologists at the University of Utah were puzzled. Earthquakes of similar size frequently make headlines in places like California.
Fast forward nearly fifty years, and researchers have made new discoveries. They believe this quake might belong to a newly identified category called “mantle earthquakes.” These quakes happen deep beneath Earth’s surface, between 43 to 55 miles down—much deeper than most typical tremors.
George Zandt, a geologist from the University of Arizona, took a special interest in this mystery. He noticed the Randolph quake while working in seismology years ago and decided to help with the latest research. Zandt explained that the depth of the quake is why it wasn’t felt by anyone. “I had data that suggested it was real, even if others found it hard to believe,” he said.
So, what makes an earthquake noticeable? According to the U.S. Geological Survey, it’s not just about the magnitude but also about the intensity. Intensity is a combination of shaking measurements and personal reports from people. Generally, people feel quakes that are above a magnitude of 3.0.
Keith Koper, Zandt’s former student and now director of the University of Utah Seismograph Stations, is investigating these deep quakes. He recently confirmed nine cases originating from deep underground for a paper published in Geophysical Research Letters. He also studied a new quake that occurred beneath Utah’s Uinta Basin. Koper describes these deep quakes as unique. “At that depth, materials behave like taffy. They flow slowly over millions of years,” he explained.
The recent quake on September 10, 2025, was found to be 42 miles deep. Despite being categorized as a magnitude 4.1 event, it likely went unnoticed by anyone above. Yet scientists can see the evidence of these deep events in the rocks that eventually rise to the surface. “We can tell how they were stretched,” Koper noted.
Interestingly, these mantle earthquakes seem to happen in singular bursts without aftershocks or preliminary tremors. They often occur near ancient rock formations, like the Wyoming Craton, which have remained stable for billions of years. Koper believes the deep earthquakes might be triggered by movements of molten rock beneath these stable areas.
Understanding these deep seismic events is crucial. Koper emphasizes that learning more could help assess their potential danger. “We need to figure out just how powerful they can be,” he said.
In summary, while the Randolph earthquake may have seemed insignificant at the time, it has opened up new dialogues in the geoscience community and raises questions about what other mysteries lie beneath our feet.
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Earthquakes,Geology,Plate tectonics,scientific mysteries
