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.…