In the early morning hours of Feb. 24, 1979, the University of Utah Seismograph Stations recorded the quake under Randolph, a Utah town near the Idaho and Wyoming borders. No one reported feeling the quake, despite its magnitude 3.8 heft, and the accompanying seismic data didn’t make sense.
George Zandt, then a postdoctoral seismology researcher at the U, took a closer look at the seismic readings and pinpointed this quake’s focal depth at a jaw-dropping 90 kilometers below sea level. Such a depth was not thought possible, placing its hypocenter far below Earth’s crust, well into the upper mantle.
“The deep depth explained why it wasn’t felt by people at the surface,” said Zandt, who went on to a long career on the University of Arizona’s geology faculty. “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.”
Fresh look at old data
He wrote an abstract about the Randolph quake for Earthquake Notes, but Zandt’s findings remained largely unnoticed until last year. That’s when a new generation of U seismologists re-evaluated waveform data from the 1979 quake and eight other suspected deep earthquakes that have since occurred in northern Utah and southwest Wyoming.
This study, led by U geology professor Keith Koper, confirmed locations for all nine well below Earth’s crust, proving the existence of what are called “continental mantle earthquakes,” or CMEs. Then on Sept. 10, 2025, another one struck at around 6 p.m. outside Maeser in Utah’s Uinta Basin, clocking in at magnitude 4.1 with a focal depth of 68 kilometers.
That’s more than 20 kilometers below the Mohorovičić discontinuity, the boundary separating Earth’s crust from the underlying mantle, better known as the Moho. Koper’s team characterized the Maeser quake as an “archetypal continental mantle event” in a subsequent study published last month in The Seismic Record.
“This is an example of an earthquake that’s nucleating in very unusual conditions, the high temperature, the high pressure, and almost all the material at that depth is going to flow. It’s more like taffy, it’s taffy on long time scales, like millions of years,” said Koper, a one-time protege of Zandt’s and now the director of the U of U Seismograph Stations. “Nevertheless, you can still see it in rocks that have made their way back up to the surface; you can see how they were stretched.”
Zandt came out of retirement to work on this study, which lists him as a co-author.