In a scientific first, researchers have tracked the elusive motion of a variety of “silent” earthquake during a seismic event along a major fault zone on the ocean floor.
The phenomenon, known as a slow-slip earthquake, was caught in the act of exerting tectonic pressure as it moved, all while researchers tracked its spread along a fault region off Japan’s coast known to have generated tsunamis in the past.
The recent observations of this phenomenon, which scientists at the University of Texas at Austin have described as behaving similar to a shock absorber, were detailed in a new research study that appeared in Science.
A Fault Line Slowly Unzips
Josh Edgington, whose doctoral work was undertaken at the University of Texas Institute for Geophysics (UTIG) at UT Austin’s Jackson School of Geosciences, likened the phenomenon to “a ripple moving across the plate interface.”
Slow slip earthquakes, which are sometimes also referred to as “silent earthquakes,” involve seismic phenomena that release energy over long periods, often lasting anywhere from a few hours, to even several months. Although they are not truly “silent” events, slow earthquakes are very quiet, and distinct from the kinds of slow rupture velocity events that give rise to tsunami earthquakes.
The detailed measurements obtained by the UT Austin team along Japan’s Nankai Fault represent a breakthrough that could potentially help reshape the way scientists address seismic hazards. Of key significance, the new research helps show how these events could facilitate the release of tectonic pressures without triggering devastating tsunamis.
Tracking “silent” Earthquakes
The findings were obtained using precision borehole sensors that were installed near the fault’s most shallow reaches beneath the ocean trench in the fall of 2015.
In contrast with land-based monitoring technologies, which can often encounter problems while attempting to detect subtle fault movement occurring offshore, the sensors employed by the UT Austin team were able to record seismic events occurring at scales as small as just a few millimeters.
During the 2015 observations, the team’s sensors captured a slow slip earthquake creeping along the shallowest part of the fault closest to the seafloor, which is the same area where tsunamis are generated. Fortunately, the event in question served in helping to ease tectonic stress around this potentially hazardous location, and no tsunami occurred.
Five years later, a second slow slip tremor occurred, following a similar path to the one in 2015. By analyzing the two events, the researchers were able to demonstrate that each seismic event lasted several weeks, during which the slow slip earthquakes moved along the fault before dissipating at the edge of the continental margin.
Another finding had been that both seismic events occurred in tandem with higher-than-normal fluid pressures in the Earth’s crust. This finding strongly supports the idea that fluids play a significant role in driving slow earthquakes, an idea that has long been suspected by seismologists, but previously remained unconfirmed.
A History of Major Seismic Events
In 1946, a major seismic event occurred along the Nankai Fault, killing 1,300 individuals and destroying close to 36,000 homes.
The 1946 earthquake registered as a magnitude 8 event, and while similar major seismic events are expected in the future, the team’s recent observations suggest that, fortunately, the occurrence of slow-slip earthquakes helps to dissipate some of the pent-up energy in such areas, especially those at the shallow end of the Nankai Fault closest to the surface.
“This discovery points to a part of the fault that releases tectonic pressure independently of the deeper, more hazardous segments,” said UTIG Director Demian Saffer, who led the team’s research.
Seismic Concerns Along the Ring of Fire
Additionally, their results could help scientists to refine existing earthquake and tsunami hazard models, which may not only apply to undersea regions off Japan’s coast, but also for other regions along the Pacific Ring of Fire.
Another potential region that could spell problems resulting from seismic phenomena occurring there is the Cascadia subduction zone off the U.S. Pacific Northwest. There, similar slow slip phenomena has been detected, although not at the tsunami-generating end of the fault.
“Cascadia is a place that we know has hosted magnitude 9 earthquakes and can spawn deadly tsunamis,” Saffer says. “Are there creaks and groans that indicate the release of accumulated strain, or is fault near the trench deadly silent?”
“Cascadia is a clear top-priority area for the kind of high-precision monitoring approach that we’ve demonstrated is so valuable at Nankai,” Saffer added.
The team’s findings were detailed in a new paper, “Migrating shallow slow slip on the Nankai Trough megathrust captured by borehole observatories,” by Josh. Edgington, Demian Saffer, et al, was published in Science on June 26, 2025.
Micah Hanks is the Editor-in-Chief and Co-Founder of The Debrief. He can be reached by email at micah@thedebrief.org. Follow his work at micahhanks.com and on X: @MicahHanks.