Something hidden deep underground contributed to the Chilean earthquake

In July 2024, a 7.4 magnitude earthquake struck near the city of Calama in northern Chile. The tremors caused damage to buildings and caused power outages throughout the area.

Chile is no stranger to major earthquakes. The country experienced the strongest earthquake ever recorded in 1960, when a 9.5-magnitude quake struck central Chile, triggering a massive tsunami and killing 1,000 to 6,000 people. While devastating earthquakes are often associated with these massive driving events, the Kalama earthquake was different from this familiar pattern.

Why was this earthquake different?

Large earthquakes usually occur relatively close to the Earth's surface, where tectonic plates collide. In contrast, the Kalama earthquake originated much deeper underground. It exploded about 125 kilometers below the surface, within the subducting tectonic plates themselves.

Earthquakes that occur at these depths usually produce weaker vibrations at the surface. However, the Kalama event shattered this expectation. Researchers at the University of Texas at Austin discovered that a rare series of underground processes dramatically enhanced the strength of the earthquake. Their findings were recently published in the journal Nature Communications.

In addition to explaining why this earthquake was so unusually strong, the study may also improve how scientists assess earthquake risk in the future.

“These Chilean events cause more shaking than would normally be expected from medium-depth earthquakes, and can be very destructive,” said study 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 that our research can support emergency response and long-term planning.”

How scientists thought deep earthquakes worked

It has long been thought that earthquakes that occur at intermediate depths, including the Kalama event, are caused mainly by a process known as “drought embrittlement.” This happens when ocean tectonic plates sink deeper into the Earth's interior. As temperatures and pressures rise, water trapped in minerals is released.

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

Scientists generally believe that this dehydration process stops when temperatures exceed about 650 degrees Celsius.

A rare heat-based process takes over

The Kalama earthquake challenged this assumption. According to the research team, the rupture continued beyond the expected temperature. It traveled about 50 kilometers deeper into rocks that were hotter due to a second process known as “thermal gradient.”

During this process, the intense friction generated by the initial rupture generates intense heat at the crack front. This heat weakens the surrounding material, allowing the tear to continue moving forward and become stronger as it spreads.

“It's the first time we've seen a medium-depth earthquake break assumptions, go from a cold region to a very hot region, and move at much faster speeds,” said Jia, who is part of the University of Texas Geophysics Institute (UTIG), a research unit of Jackson College. “This indicates that the mechanism changed from drought-induced embrittlement to thermal escape.”

Tracking the rupture deep underground

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

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

Improving earthquake hazard predictions

“The fact that another large earthquake was delayed in Chile has stimulated seismic 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 in the Department of Earth and Planetary Sciences at Jackson College and a senior research scientist at UTIG.

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

Research support and funding

Ang and DeSarolo said the National Science Foundation's DANID served as a fund for Texistin.

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