Earthquakes deep inside Earth often follow strict physical limits. Heat, pressure, and rock softness usually stop large ruptures from spreading far. In July 2024, a powerful earthquake in northern Chile challenged long held ideas about deep earthquakes.
A hidden heat driven process allowed rupture to travel deeper and faster than expected, releasing much more energy than standard models allow.
On July 19, 2024, a magnitude 7.4 earthquake struck near Calama in northern Chile. Shaking damaged buildings and disrupted electricity. Chile experiences frequent earthquakes, yet most destructive events occur close to the surface.
Calama differed because rupture began far below ground, inside a sinking tectonic plate.
Chile has frequent strong earthquakes
Chile sits along a subduction zone where the Nazca Plate slides beneath South America. Movement along plate boundaries creates frequent earthquakes. In 1960, central Chile experienced a magnitude 9.5 megathrust earthquake, strongest ever recorded.
Most damaging Chilean earthquakes form near plate boundaries at shallow depths. Calama occurred inside the oceanic plate at about 125 kilometers (about 78 miles) depth.
Earthquakes at such depths usually cause weaker shaking at ground level. Calama broke that pattern.
Scientists from The University of Texas at Austin studied why such strong shaking occurred. Results appeared in Nature Communications and focused on rupture physics rather than surface damage alone.
Why deep earthquakes happen
Earthquakes between 70 and 300 kilometers (about 43 to 186 miles) depth fall into an intermediate category.
At such depths, heat and pressure usually prevent sudden rock failure. For decades, researchers linked most intermediate depth earthquakes to dehydration embrittlement.
As a cold oceanic plate sinks, minerals like serpentine hold water inside crystal structures. Rising temperature and pressure release water from minerals. Water increases pressure inside rocks, weakens mineral bonds, and allows sudden cracking.
Laboratory studies show dehydration works well below about 650 degrees Celsius (about 1,200 degrees Fahrenheit).
Above that temperature, rocks behave more like soft solids and resist sudden breakage. For that reason, scientists believed rupture should stop near that thermal boundary.
Faster slip means more heat
Calama challenged those limits. Seismic analysis revealed rupture traveled roughly 50 kilometers (about 31 miles) deeper than expected, reaching zones hotter than 650 degrees Celsius (about 1,200 degrees Fahrenheit).
The researchers identified a second mechanism called shear thermal runaway. During rupture, intense friction generates extreme heat along fault surfaces. Heat weakens surrounding rock even more, creating a feedback loop.
Faster slip creates more heat, and more heat allows faster slip. Rupture accelerates instead of stopping.
“These Chilean events are causing more shaking than is normally expected from intermediate-depth earthquakes, and can be quite destructive,” said Zhe Jia, research assistant professor at the UT Jackson School of Geosciences.
“It’s the first time we saw an intermediate-depth earthquake break assumptions, rupturing from a cold zone into a really hot one, and traveling at much faster speeds.”
“That indicates the mechanism changed from dehydration embrittlement to thermal runaway.”
Rupture spread in a series of events
Seismic data showed rupture did not occur in one smooth motion. Instead, multiple subevents activated one after another.
Early rupture began near 125 kilometers (about 78 miles) depth inside a colder slab core. Later rupture segments reached depths near 170 kilometers (about 106 miles), far into hotter regions.
Early segments released only a small fraction of total energy but produced many aftershocks. Later segments released most energy and produced fewer aftershocks.
Such behavior fits thermal runaway theory, since intense heating removes leftover stress that would otherwise trigger aftershocks.
Rupture moved mainly downward along a steep fault plane rather than spreading sideways.
Average rupture speed reached about 4.2 kilometers per second (about 2.6 miles per second), close to shear wave speed. Such rapid motion remains rare for intermediate depth earthquakes.
How temperature shaped the earthquake
The researchers used thermal models of the Chilean subduction zone to estimate temperature at rupture depths.
The models showed cold slab cores remain relatively thin. Rupture length exceeded cold core thickness, forcing propagation into warmer regions.
Minerals other than serpentine, such as chlorite and talc, also release water, yet those minerals occur in smaller amounts and at lower temperatures. Once rupture entered hotter regions, dehydration alone could not explain continued failure.
Thermal runaway provided a logical explanation for sustained rupture at high temperature. Heat from friction weakened rock enough to allow slip even in normally stable zones.
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Why risk estimates change
“The fact that another large earthquake is overdue in Chile has motivated earthquake research and the deployment of multiple seismometers and geodetic stations to monitor earthquakes and how the crust is deforming in the region,” said Professor Thorsten W. Becker.
Calama shows that deep earthquakes can activate regions once considered too hot for rupture. Hazard models must account for combined effects of dehydration and thermal runaway.
Allowing rupture transitions increases possible earthquake size and shaking intensity.
Understanding hidden heat driven processes improves earthquake forecasting, infrastructure design, and emergency planning.
Calama offers a rare window into Earth’s deep interior and highlights how extreme conditions can still produce powerful earthquakes.
The study is published in the journal Nature Communications.
NOTE – This article was originally published in Earth and can be viewed here
