Magnitude 7.4, a year and a half ago. The way the earthquake occurred surprised scientists – who searched for answers.
One earthquake of magnitude 7,4 reached, in July 2024, the vicinity of Calama, in the north of Chile, causing damage to buildings and interruptions in electricity supply in several areas of the region.
What most caught the researchers’ attention, however, was not just the scale of the event, but the way it happened: unlike large typical subduction earthquakes, which tend to rupture relatively close to the surface, this one had origin at around 125 kilometers deep, within the tectonic plate itself which is diving beneath the continent.
Earthquakes at these depths — known as “intermediate depth” events — tend, in general, to produce less surface agitationbecause the energy dissipates as it passes through more rock before reaching inhabited areas.
The Calama episode contradicted this expectation. According to one of researchers from The University of Texas at Austin, a rare sequence of underground processes will have “supercharged” the ruptureincreasing the intensity of the tremor felt at the surface.
For decades, the dominant explanation for these deep earthquakes was the so-called “weakening due to dehydration”. As the oceanic plate descends into the Earth, pressure and temperature increase and water trapped in minerals is released, describes the .
The loss of water makes the rock more fragile and prone to cracking, allowing a sudden rupture to occur within the plate. However, scientists believed that this mechanism stopped operating when temperatures exceeded around 650 ºC.
The Calama earthquake, the authors say, advanced beyond this thermal limit: the rupture will have continued for another 50 kilometers in depthcrossing much hotter zones thanks to a second mechanism, called “thermal runaway”.
In this process, the friction generated by the initial rupture produces intense heat at the front of the fault, weakening the surrounding material and allowing the rupture to propagate faster and gain energy.
To reconstruct the evolution of the earthquake, the team combined seismic records in Chile, ground deformation measurements by GNSS (Global Navigation Satellite System) and computer models to estimate temperatures and rock properties at the depths involved.
In addition to explaining the anomaly in the Calama case, the researchers argue that understanding how these earthquakes behave at different levels of the upper mantle could improve the assessment of seismic risk, helping to estimate the likely intensity of the shaking, guide the design of infrastructures and support emergency warning and response systems.
