The tiny town of Randolph, Utah (population: 467), sat atop a deep 3.8 magnitude earthquake in the early morning hours of February 24, 1979—but, mysteriously, none of its residents noticed enough to bother filing a report. Geologists monitoring the quake at the University of Utah Seismograph Stations ( UUSS ) were baffled. The episode’s modest rumbling was well within the range that typically makes news in states like California   all the time . Now, nearly half a century later, researchers at the university, in partnership with geologists at Sandia National Labs and elsewhere, believe they have finally figured out why. The once inexplicable quake represents what the team now describes as an emerging new category of seismic activity, “mantle earthquakes,” which have been documented occurring beneath Earth’s tectonic plates as deep as 43 to 55 miles (70 to 90 kilometers) underground.…