Scientists Confirm a Deep Earthquake That Shouldn't Exist— A Discovery That Challenges Everything We Know About Earth's Interior

For decades, scientists believed they understood where earthquakes could and could not happen. Most earthquakes occur within Earth's crust, the thin outer layer of our planet. Deep below that layer, temperatures and pressures become so extreme that rocks are expected to bend and flow slowly rather than suddenly break.

But a remarkable new discovery has forced researchers to rethink this long-held assumption.

Scientists have now confirmed that a mysterious earthquake that occurred beneath Utah in 1979 originated nearly 90 kilometers underground, deep inside Earth's upper mantle. This finding has revealed the existence of a rare and unusual type of earthquake that many experts once thought was impossible beneath a continent.

The discovery not only solves a geological mystery that puzzled scientists for nearly 50 years but also opens new questions about how Earth's interior behaves.

A Strange Earthquake from 1979

The story begins in the early morning hours of February 24, 1979, near the small town of Randolph in northern Utah, close to the borders of Idaho and Wyoming.

The earthquake measured a modest magnitude of 3.8. Interestingly, no one on the surface reported feeling it. However, when scientists examined the seismic recordings, they noticed something unusual.

The earthquake appeared to have originated far deeper than normal earthquakes in continental regions.

At the time, University of Utah researcher George Zandt carefully analyzed the data. His calculations suggested that the earthquake had started approximately 90 kilometers below sea level.

This depth placed the event well beneath Earth's crust and deep inside the upper mantle.

That conclusion was surprising because scientists generally believed earthquakes could not occur under such conditions.

"The deep depth explained why it wasn't felt by people at the surface," Zandt later explained. However, convincing other researchers was difficult because the idea of a mantle earthquake beneath a continent seemed highly unlikely.

As a result, the finding received little attention and remained largely forgotten for decades.

Revisiting a Forgotten Mystery

Years later, researchers at the University of Utah decided to revisit the old seismic records.

Led by geology professor Keith Koper, the team reexamined the original data from the 1979 earthquake. They also studied eight other suspicious seismic events that had occurred in northern Utah and southwestern Wyoming.

Using modern analysis techniques, the researchers discovered something extraordinary.

All nine earthquakes had originated deep below Earth's crust.

This provided strong evidence for the existence of a rare class of earthquakes known as continental mantle earthquakes (CMEs).

Unlike ordinary earthquakes, these events occur deep within Earth's mantle, a region where rocks are expected to behave more like a slowly flowing material than a brittle solid.

The results confirmed that the 1979 earthquake was not an error or a miscalculation. It was a genuine deep-mantle earthquake.

Another Deep Earthquake Strengthens the Evidence

The case became even stronger when another unusual earthquake struck Utah on September 10, 2025.

This earthquake occurred near Maeser in Utah's Uinta Basin and reached a magnitude of 4.1.

Scientists determined that it originated approximately 68 kilometers below the surface.

More importantly, its source was located over 20 kilometers beneath the Mohorovičić discontinuity, often called the "Moho." This boundary separates Earth's crust from the mantle beneath it.

The earthquake showed many of the same characteristics as the earlier events, making it one of the clearest examples of a continental mantle earthquake ever observed.

For researchers, this was powerful confirmation that these mysterious deep earthquakes are real.

Why These Earthquakes Shouldn't Happen

The most fascinating aspect of the discovery is that these earthquakes occur in an environment where scientists generally do not expect rocks to break.

At depths of 70 to 90 kilometers, temperatures can exceed 700 degrees Celsius.

Under such intense heat and pressure, rocks usually deform slowly over long periods rather than fracture suddenly.

Keith Koper compares the mantle's behavior to taffy.

If stretched over millions of years, mantle rocks flow and deform much like soft candy. Yet somehow, under certain conditions, these rocks can still generate earthquakes.

This presents a major scientific puzzle.

Researchers understand fairly well how shallow earthquakes occur along faults in Earth's crust. However, the physical processes responsible for these deep mantle earthquakes remain unclear.

As Koper put it, "How in the world can these things happen?"

That question is now driving a new area of research in earthquake science.

A Different Kind of Earthquake

The newly identified continental mantle earthquakes differ from ordinary earthquakes in several important ways.

First, they tend to occur alone.

Most shallow earthquakes are accompanied by foreshocks and aftershocks. Deep mantle earthquakes, however, usually appear as isolated events.

Second, they occur in regions with unusually high temperatures.

Third, scientists currently have very little information about how large these earthquakes can become.

For shallow earthquakes, researchers can estimate potential maximum magnitudes by studying fault lengths visible at the surface.

For mantle earthquakes, no such surface faults exist.

This means scientists still do not know the full hazard potential of these rare events.

The Key Role of the Wyoming Craton

The answer may lie in an ancient geological structure known as the Wyoming Craton.

A craton is an extremely old and stable section of Earth's lithosphere that has survived for billions of years.

The Wyoming Craton stretches beneath parts of Wyoming and neighboring states.

Koper compares it to an iceberg.

Just as an iceberg extends far below the ocean surface, the craton extends deep into Earth's mantle like a massive underground keel.

Over millions of years, flowing mantle material encounters this rigid structure and is forced to move around it.

Researchers believe this interaction creates additional stresses and deformation within the surrounding mantle.

The newly confirmed deep earthquakes occur precisely in this transition zone near the western edge of the Wyoming Craton.

Scientists think the mantle's movement around the craton may be generating the conditions necessary for these unusual earthquakes.

A Discovery with Big Implications

The confirmation of continental mantle earthquakes represents a major advance in Earth science.

It challenges long-standing assumptions about how rocks behave deep underground and reveals that Earth's interior may be far more dynamic than previously thought.

The discovery also demonstrates the value of preserving scientific data. Without decades-old seismic records stored by the University of Utah Seismograph Stations, researchers might never have solved this mystery.

Perhaps most importantly, the findings highlight how much remains unknown about our planet.

Even after centuries of geological research, Earth continues to surprise scientists with phenomena that defy expectations.

A small earthquake that went unnoticed in 1979 has now become the key to uncovering an entirely new category of seismic activity. As researchers continue investigating these deep mantle earthquakes, they hope to understand the forces driving them and determine whether similar hidden events may be occurring beneath other continents around the world.

The discovery is a reminder that some of Earth's greatest mysteries are still buried deep beneath our feet.

References:

1. Keith D. Koper, Sean J. Hutchings, Relu Burlacu, Katherine Whidden, Valerie Springer, Rigobert Tibi, Guanning Pang. The 10 September 2025 Mw 4.1 Earthquake in Northeastern Utah, United States: An Archetypal Continental Mantle Event. The Seismic Record, 2026; 6 (2): 167 DOI: 10.1785/0320260006
2. Sean J. Hutchings, Keith D. Koper, Relu Burlacu, Qicheng Zeng, Fan‐Chi Lin, George Zandt. Upper Mantle Earthquakes Along the Edge of the Wyoming Craton. Geophysical Research Letters, 2025; 52 (9) DOI: 10.1029/2024GL114073