Chile is no stranger to major earthquakes. The country experienced the strongest earthquake ever recorded in 1960, when a magnitude 9.5 megathrust event struck central Chile, triggering a massive tsunami and killing between 1,000 and 6,000 people. While devastating earthquakes are often linked to these massive megathrust events, the Calama earthquake stood apart from that familiar pattern.

Why This Earthquake Was Different

Megathrust earthquakes typically occur relatively close to the Earth’s surface, where tectonic plates collide. In contrast, the Calama earthquake originated far deeper underground. It ruptured at a depth of about 125 kilometers beneath the surface, inside the subducting tectonic plate itself.

Earthquakes that occur at these depths usually produce weaker shaking at the surface. However, the Calama event broke that expectation. Researchers at The University of Texas at Austin discovered that a rare sequence of underground processes significantly boosted the earthquake’s strength. Their findings were recently published in Nature Communications.

Beyond explaining why this earthquake was unusually intense, the study may also improve how scientists assess earthquake hazards in the future.

“These Chilean events are causing more shaking than is normally expected from intermediate-depth earthquakes, and can be quite destructive,” said the study’s lead author Zhe Jia, a research assistant professor at the UT Jackson School of Geosciences. “Our goal is to learn more about how these earthquakes occur, so our research could support emergency response and long-term planning.”

How Scientists Thought Deep Earthquakes Worked

Earthquakes at intermediate depths, including the Calama event, were long believed to be triggered mainly by a process known as “dehydration embrittlement.” This occurs as an oceanic tectonic plate sinks deeper into the Earth’s interior. As temperatures and pressures rise, water trapped in minerals is released.

When the rock loses this water, it becomes weaker and more brittle. Cracks can form, allowing the rock to suddenly rupture and generate an earthquake within the slab.

Scientists have generally believed that this dehydration process stops once temperatures exceed about 650 degrees Celsius.

A Rare Heat Driven Process Takes Over

The Calama earthquake challenged that assumption. According to the research team, the rupture continued well beyond the expected temperature limit. It traveled roughly 50 kilometers deeper into much hotter rock due to a second process known as “thermal runway.”

During this process, intense friction from the initial rupture generates extreme heat at the front of the fault. That heat weakens the surrounding material, allowing the rupture to keep moving forward and grow stronger as it spreads.

“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,” said Jia, who is part of the University of Texas Institute for Geophysics (UTIG), a research unit of the Jackson School. “That indicates the mechanism changed from dehydration embrittlement to thermal runaway.”

Tracking the Rupture Deep Underground

To understand how the earthquake unfolded and how far the rupture traveled, the University of Texas team worked with scientists in Chile and across the United States. They combined several lines of evidence to build a detailed picture of the event.

The researchers examined seismic records from Chile to track how fast and how far the rupture spread. They also used data from the Global Navigation Satellite System to measure ground movement and fault slip. Computer models helped estimate the temperatures and rock properties at the depths where the earthquake occurred.

Improving Earthquake Risk Forecasts

“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 Thorsten Becker, a co-author of the study and a professor at the Jackson School’s Department of Earth and Planetary Sciences and a senior research scientist at UTIG.

Becker and Jia emphasized that understanding how earthquakes behave at different depths could improve predictions of future seismic events. Better models could help estimate how strong shaking might be, while also guiding infrastructure design, early warning systems, and rapid emergency response planning.

Research Support and Funding

The research was supported by the National Science Foundation, Agencia Nacional de Investigación y Desarrollo (ANID), Chile, UC Open Seed Fund, Fundamental Research Funds for the Central Universities, and the University of Texas Institute for Geophysics.