Nearly half a century ago, a small earthquake beneath northern Utah left seismologists puzzled. The event seemed to originate far deeper than earthquakes were thought capable of occurring beneath a continent. Now, new research from the University of Utah has confirmed that the unusual quake was real and part of a rare category of seismic events happening deep within Earth's mantle.
The earthquake struck in the early morning of February 24, 1979, beneath the town of Randolph near Utah's borders with Idaho and Wyoming. Although it registered a magnitude of 3.8, nobody reported feeling it. The seismic recordings also appeared unusual, prompting closer examination.
At the time, University of Utah postdoctoral researcher George Zandt analyzed the data and calculated that the earthquake originated about 90 kilometers below sea level. That depth placed it well below Earth's crust and deep within the upper mantle, a location where scientists generally did not expect earthquakes to occur.
"The deep depth explained why it wasn't felt by people at the surface," said Zandt, who later spent many years on the geology faculty at the University of Arizona. "I did some other analysis that convinced me of the reality of the deep depth but it was hard to convince others of the highly anomalous mantle earthquake occurring in a region where none should exist."
