Scientists Confirm an Earthquake That 'Shouldn't Exist' Struck 56 Miles Beneath Utah, Deep Inside the Earth's Mantle

Scientists have confirmed that a mysterious earthquake first detected nearly half a century ago really did rupture some 90 kilometers — about 56 miles — beneath the surface of Utah, far deeper than researchers long believed earthquakes could occur beneath a continent.

The tremor struck in the predawn hours of Feb. 24, 1979, beneath the small town of Randolph, near where Utah meets Idaho and Wyoming. It registered a modest magnitude of 3.8, and no one reported feeling it. For decades the event lingered as a seismological curiosity, its depth so improbable that many assumed the original measurement was simply an error. At those depths, deep within Earth's upper mantle, rock is hot enough that it is expected to slowly flow and deform rather than snap suddenly and release seismic energy.

A new study from the University of Utah has now put that skepticism to rest. By reanalyzing decades of accumulated seismic data with modern techniques, researchers led by geology professor Keith Koper identified a rare class of events they call "continental mantle earthquakes," confirming nine such quakes that originated well below the Earth's crust. The 1979 Randolph event, whose roughly 90-kilometer depth was first calculated by University of Utah researcher George Zandt, anchors the group.

The findings overturn a basic assumption about where earthquakes can happen. Conventional understanding holds that beneath continents, brittle faulting — the sudden slip that generates earthquakes — is confined to the relatively cool, rigid crust, typically within the upper few tens of kilometers. Below that, in the ductile mantle, rock was thought to yield gradually, incapable of the abrupt failure that produces a seismic jolt. The Utah quakes show that, under the right conditions, the mantle can break too.

Exactly what allows the deep rock to rupture remains an open scientific question. Researchers suspect that unusual stresses, the local thermal structure, or fluids and mineral changes at depth could make a normally pliable region brittle enough to fail. Pinning down the mechanism could reshape how geophysicists understand the strength and behavior of the lithosphere beneath stable continental interiors.

The work carries practical stakes as well. Understanding the full range of depths at which earthquakes can nucleate feeds directly into seismic hazard assessments, which guide building codes and risk maps for communities across the Intermountain West. While a deep, magnitude-3.8 tremor poses little direct danger, documenting that the continental mantle is seismically active forces scientists to reconsider models of how and where the ground can shake — a reminder that even decades-old data can still upend the textbooks.