Full Breakdown
Continental Mantle Earthquakes Confirmed Beneath Utah and Wyoming
6/13/2026, 10:49:13 PM
Confirmation of Deep Mantle Earthquakes in Utah and Wyoming
In May 2025 a study in *Geophysical Research Letters* confirmed nine earthquakes that originated 68–90 km beneath the Utah-Wyoming border, depths previously thought impossible for continental interiors. The series begins with a magnitude 3.8 tremor near Randolph, Utah, recorded in February 1979, and includes a magnitude 4.1 event near Maeser on 10 September 2025. All events lie along the eastern edge of the ancient Wyoming Craton.
Historical Context and Early Skepticism
At the time of the 1979 recording, seismologists assumed continental quakes ceased within the brittle crust, roughly the top 40 km. Postdoctoral seismologist George Zandt calculated a 90-km depth for the Randolph event, a result dismissed for 46 years as physically implausible.
Key Researchers and Official Statements
George Zandt’s original estimate was later validated by a University of Utah graduate student who re-examined archived waveforms using modern timing-gap methods. Keith Koper, director of the University of Utah Seismograph Stations, oversaw the dense sensor network that captured the recent events and announced the need for a systematic review of historic catalogs.
Timeline of Confirmed Events
1979 – Randolph, UT, M 3.8, 90 km; 2007-2010 – six additional deep quakes (originally misfiled); 10 Sept 2025 – Maeser, UT, M 4.1, 68 km; 2025 – four more candidates pending confirmation.
Depth, Magnitude, and Frequency
The nine events cluster along the Wyoming Craton’s eastern rim, with depths of 68-90 km (20-30 km below the Moho) and magnitudes from 3.8 to 4.1; each occurred in isolation, without aftershocks.
Scientific Implications and Hazard Assessment
The mantle beneath the western U.S. flows slowly, diverting around the craton and concentrating strain at its edge, where the hot, ductile rock can fail abruptly. Koper likens the behavior to taffy and notes that the absence of a mapped surface fault leaves the maximum possible magnitude of these deep events uncertain.
Criticism and Knowledge Gaps
Some seismologists argue that mantle-depth ruptures lack the cold, brittle slab typical of deep earthquakes, questioning the mechanism behind isolated slips. Zandt emphasized that the event occurred in a region where such earthquakes were thought impossible, underscoring the skepticism. The catalog remains incomplete, the frequency uncertain, and the maximum possible magnitude unknown.
Verbatim Quotes
- “occurring in a region where none should exist,” — George Zandt, postdoctoral seismologist, University of Utah
- “more like taffy” — Keith Koper, director, University of Utah Seismograph Stations
- “no idea how big they can be.” — Keith Koper, director, University of Utah Seismograph Stations
- “The next step is a full pass through the historical catalogue to count how many more have already happened unnoticed beneath northern Utah and southwest Wyoming.” — K. D. Koper, lead author, University of Utah (Geophysical Research Letters, 2025)
Future Monitoring and Research
The team will re-examine the full seismic archive, refine mantle-quake detection algorithms, and incorporate deep-event scenarios into Utah’s hazard models, while the University of Utah’s sensor network continues real-time monitoring.
