Utah earthquakes explained: researchers pinpoint cause after decades

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New analysis of decades-old seismograms has cast fresh light on a baffling class of earthquakes beneath Utah and southwest Wyoming — events that originate far below the crust where rock should behave like warm putty, not snap. The findings, published this week by researchers at the University of Utah in The Seismic Record, suggest scientists are beginning to untangle how brittle failure can occur in the deep continental mantle and what that means for our understanding of continental interiors.

What began as a puzzling, nearly imperceptible tremor in 1979 has grown into a pattern researchers can no longer ignore. Instruments at the University of Utah first recorded a magnitude 3.8 event near Randolph on February 24, 1979. Early analysis placed the source around 90 kilometers down — well into the mantle — a depth many seismologists at the time deemed implausible for traditional earthquake mechanics.

Revisiting old records reveals a new class of quakes

A recent team reanalyzed that 1979 event alongside several similar, previously unexplained earthquakes across the region. Their comparison identified at least nine episodes that share the same deep origin and uncommon behavior. The authors label these occurrences **continental mantle earthquakes** — a term that signals their location and how they differ from shallow crustal tremors.

These deep events lack the usual seismic signatures: they don’t show clear foreshock sequences and are followed by few, if any, aftershocks. They arrive with the main shock and then fade into silence, even though they originate where temperatures exceed roughly 700°C and rocks generally deform rather than fracture.

How could rock that hot break?

Researchers propose a structural explanation tied to ancient pieces of rigid lithosphere embedded in the mantle. The team points to the **Wyoming Craton** — a stable, old block of continental material — and to what they call a lithospheric “keel,” an extended, dense root of cold rock that penetrates downward from continental plates.

The current hypothesis holds that flowing mantle material moving past a stationary keel builds stress where the two meet. Over long time scales, that interface could accumulate sufficient stress to produce sudden brittle failure well below the crust’s base. In effect, the keel behaves like a stubborn obstacle, concentrating forces that then release as these unusual deep quakes.

That explanation aligns with the spatial clustering of the events, but it does not yet connect all the dots. Scientists still lack a detailed physical model that explains how such high-temperature rocks rupture, or how large these deep mantle quakes might become.

Why this matters now

Understanding these deep earthquakes reshapes parts of seismology and continental tectonics. While the immediate hazard to communities remains small — the known events have been modest in size and often went unfelt — the discovery affects seismic models, mantle flow studies, and how geologists interpret the long-term evolution of stable continental interiors.

Scientific significance: Confirms brittle-style rupture can occur far below the crust, forcing revisions to models of lithosphere–mantle interaction.

Seismic hazard: Present evidence points to low short-term risk for large damaging quakes from these events, but magnitude limits are still unknown.

Research needs: Better seismic networks, higher-resolution imaging, laboratory experiments at high temperature and pressure, and refined numerical models are needed to test proposed mechanisms.

Broader implications: Offers insight into how ancient continental roots influence present-day tectonic stresses and mantle convection patterns.

Open questions and next steps

The study marks an important step but raises clear follow-ups. Can high-temperature rock rupture mechanics be replicated in experiments or simulated with realistic mantle rheology? How widespread are continental mantle earthquakes beyond the Utah–Wyoming area? And could some deep events escape detection because existing seismic networks are optimized for shallower events?

Researchers will likely expand monitoring in the region, reexamine archived data elsewhere, and push lab and computational work to bridge the gap between observed seismic signals and plausible physical mechanisms. For now, the finding is a reminder that Earth still holds surprises beneath its surface, and that even familiar landscapes can conceal complex, active processes operating on geological time scales.