In the predawn hours of February 24, 1979, a small earthquake rumbled beneath the tiny town of Randolph in northern Utah, near where the state meets Idaho and Wyoming. It registered a modest magnitude 3.8, and no one reported feeling it. Nearly half a century later, that forgotten tremor has become the cornerstone of a discovery that rewrites part of the rulebook on where earthquakes can happen.<\/p>\n
Researchers at the University of Utah have confirmed that the 1979 quake struck about 90 kilometers, roughly 56 miles, below the surface. That places it deep within Earth’s upper mantle, far beneath the brittle crust where virtually all continental earthquakes are supposed to occur. According to the team, it is one of nine such events they have now verified beneath northern Utah and southwestern Wyoming.<\/p>\n
The depth is what makes the finding so startling. At those levels, temperatures climb past 700 degrees Celsius, and conventional wisdom holds that rock under such heat and pressure should deform slowly, flowing like taffy over geological time rather than snapping suddenly. A sudden snap, the brittle fracture that radiates seismic waves, was thought to be off the table at mantle depths beneath a continent.<\/p>\n
Yet the evidence held up. The team, led by geology professor Keith Koper, director of the University of Utah Seismograph Stations, reexamined the waveform data from the 1979 event along with eight other suspected deep quakes. The original 90-kilometer estimate had been calculated decades earlier by a postdoctoral researcher poring over the same station records. Modern reanalysis confirmed that all nine originated well below the crust, in what scientists now call continental mantle earthquakes.<\/p>\n
The phenomenon is not purely historical. A magnitude 4.1 event in September 2025 near Maeser, Utah, struck at a depth of about 68 kilometers, adding a fresh example to the catalog and underscoring that these deep ruptures are an ongoing process, not a one-time fluke.<\/p>\n
Exactly why the mantle breaks here remains an open question. The leading idea points to the cold, dense root of rock that hangs beneath this part of the continent like the submerged keel of an iceberg. Where that frigid keel grinds against the warmer, more pliable material surrounding it, stresses may build to the point of sudden failure, producing quakes at depths the textbooks ruled out.<\/p>\n
The implications reach beyond academic curiosity. Understanding the full range of depths at which earthquakes can occur feeds directly into how scientists assess seismic hazard for a region. Utah sits along the Wasatch Front, one of the most seismically watched corridors in the American West, and any revision to the picture of how and where the ground can shake carries weight for the millions who live there.<\/p>\n
The confirmed events were detailed in recent peer-reviewed papers, including work appearing in the journal The Seismic Record. For now, the 1979 Randolph quake stands as a reminder that even decades-old data can hold surprises, and that the boundary between rock that flows and rock that fractures is less settled than geologists long believed.<\/p>\n","protected":false},"excerpt":{"rendered":"
In the predawn hours of February 24, 1979, a small earthquake rumbled beneath the tiny town of Randolph in northern Utah, near where the state…<\/p>\n","protected":false},"author":0,"featured_media":0,"comment_status":"","ping_status":"","sticky":false,"template":"","format":"standard","meta":[],"categories":[3],"tags":[],"yoast_head":"\n