Deep Earthquakes Rattle Conventional Wisdom in Northern Utah
A series of unusual seismic events beneath northern Utah is forcing scientists to reconsider long-held beliefs about where earthquakes can occur in continental regions.
A 1979 Anomaly Confirmed
In 1979, a magnitude 3.8 earthquake recorded in northern Utah appeared to originate at a depth of over 55 miles, well below the Earth’s crust. This depth was anomalous because conventional geology holds that earthquakes in continental regions do not occur in the mantle.
“The depth was anomalous because conventional geology holds that earthquakes in continental regions do not occur in the mantle.”
Recent analysis by University of Utah seismologists Keith Koper and George Zandt, published in The Seismic Record, has confirmed that eight subsequent earthquakes in the same region also occurred in the upper mantle. The researchers describe these as "archetypal continental mantle events."
Location and Cause
The quakes are associated with the western edge of the Wyoming Craton, a stable block of lithosphere extending across northern Utah and southwest Wyoming. Temperatures there can exceed 1,300°F.
The team proposes that the earthquakes are caused by mantle material flowing around and squeezing the craton over millions of years, creating strain and stress.
Unknowns and Implications
Keith Koper noted that the maximum possible magnitude of such deep earthquakes is unknown, unlike crustal earthquakes whose fault sizes can be measured. The deep quakes also lack foreshocks and aftershocks.
“The maximum possible magnitude of such deep earthquakes is unknown, unlike crustal earthquakes whose fault sizes can be measured.”
The research highlights ongoing unknowns about tectonic dynamics at mantle depths.