The Earth has its secrets, and some of them it whispers so softly that only the most sophisticated instruments can hear them. This is what happened in the depths of the Pacific, where sensors installed 500 meters below the seabed have captured something unprecedented: slow earthquakes that propagate like invisible waves along underwater faults. For the first time in the history of seismology, we have witnessed earthquakes that “unroll” like zippers, taking days or weeks to complete movements that normally take seconds. A discovery destined to rewrite the textbooks.
The Earth's Hidden Shock Absorbers
When Josh Edgeton analyzed data dating back to 2015, he did not imagine he was facing something so particular. The recordings showed an imperceptible movement along the Nankai fault, one of the most dangerous zones for tsunamis on the planet. "It's like a ripple moving across the interface between the plates," explained the researcher of theUniversity of Texas. But this ripple was different: slow, steady, inexorable.
Slow earthquakes are nothing like what we expect from a seismic event. There is no sudden jolt, no buildings shaking. Demian Saffer, director of the institute that conducted the research, describes them as tectonic shock absorbers. While normal faults build up stress and then violently release it, these events dissipate energy gradually, week after week.
The Japanese Underwater Laboratory
Japan has turned its seabed into a giant natural laboratory. After the devastating tsunami of 2011, which killed nearly 20.000 people, the country invested $120 million in underwater sensor networks. More than 200 monitoring stations now monitor subduction zones, the areas where one tectonic plate slides beneath another, generating the most powerful earthquakes.
Sensors installed in drilled wells can detect movements of a few millimeters, impossible to detect with terrestrial GPS. It is thanks to this technology that in 2015 and 2020, two slow-slip events along the same portion of the Nankai fault.
Slow Earthquakes, a Matter of Fluids Under Pressure
The key to everything may lie in the fluids trapped in the rocks. As I pointed out in this article, earthquake prediction remains one of science’s “holy grails.” But these slow-moving earthquakes offer valuable clues. Records show that the events coincide with zones of high fluid pressure, suggesting that water trapped in fractures in the rock acts as a lubricant, allowing gradual movements rather than catastrophic ruptures.
Comparison with Cascadia
Not all faults are created equal. The Cascadia fault, off the west coast of North America, appears to lack these natural “shock absorbers.” According to observations published on Science, this zone could be “deadly silent” in its shallowest part, the one that generates tsunamis. A crucial difference that could explain why some faults produce catastrophes while others release energy gradually.
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Slow Earthquakes, Implications for the Future
The last time the Nankai Fault produced a large earthquake was 1946: magnitude 8, over 1.300 dead and 36.000 homes destroyed. The new observations suggest that at least some of the fault regularly releases its energy through these slow events, potentially reducing the magnitude of future earthquakes.
But beware: this is not the time to let your guard down. Slow earthquakes only affect the most superficial portions of the faults. The deeper parts could still be “charged” and ready to be unleashed. Each slow event is like a photograph in a slow-motion movie of the seismic cycle, and scientists are still learning to read this film to understand when the next big “click” will arrive.
