Scientists for the first time have detected a slow slip earthquake in motion during the act of releasing tectonic pressure on a major fault zone at the bottom of the ocean.

The slow earthquake was recorded spreading along the tsunami-generating portion of the fault off the coast of Japan, behaving like a tectonic shock absorber. Researchers from The University of Texas at Austin described the event as the slow unzipping of the fault line between two of the Earth's tectonic plates.

Their results were published in Science.

"It's like a ripple moving across the plate interface," said Josh Edgington, who conducted the work as a doctoral student at the University of Texas Institute for Geophysics (UTIG) at UT Austin's Jackson School of Geosciences.

Slow slip earthquakes are a type of slow-motion seismic event that takes days or weeks to unfold. They are relatively new to science and are thought to be an important process for accumulating and releasing stress as part of the earthquake cycle. The new measurements, made along Japan's Nankai Fault, appear to confirm that.

This breakthrough research was made possible by borehole sensors that were placed in the critical region far offshore, where the fault lies closest to the seafloor at the ocean trench.

Sensors installed in boreholes can detect even the slightest motions—as small as a few millimeters, said UTIG Director Demian Saffer, who led the study. Such movement on the shallow fault is all but invisible to land-based monitoring systems such as GPS networks.

The slow slip earthquake, captured by the team's sensors in the fall of 2015, traveled along the tail of the fault—the region close to the seafloor where shallow earthquakes can generate tsunamis—easing tectonic pressure at a potentially hazardous location. A second slow tremor in 2020 followed the same path.

Although the Nankai Fault is known to generate large earthquakes and tsunamis, the discovery suggests that this part of the fault does not contribute energy to these events—acting more like a shock absorber.

The results will help researchers home in on the behavior of subduction zone faults across the Pacific Ring of Fire, the tectonic belt that spawns the planet's largest earthquakes and tsunamis.

The two events, which have only now successfully been analyzed in detail, appear as ripples of deformation traveling through Earth's crust. Originating about 30 miles off the coast of Japan, borehole sensors tracked this unzipping motion along the fault as it moved out to sea before dissipating at the edge of the continental margin.

Each event took several weeks to travel 20 miles along the fault, and each one happened in places where geologic fluid pressures were higher than normal. The finding is important because it is strong evidence that fluids are a key ingredient for slow earthquakes. This is an idea widely circulated in the scientific community, but finding a direct connection has been elusive until now.

The last time Japan's Nankai Fault produced a significant earthquake was in 1946. The magnitude 8 earthquake destroyed 36,000 homes and killed over 1,300 people.

Although another large earthquake is expected in the future, the observations suggest the fault releases at least some of its pent-up energy harmlessly in regular, re-occurring slow slip earthquakes. The location is also important, because it shows that the part of the fault nearest the surface releases tectonic pressure independently of the rest of the fault.

Armed with that knowledge, scientists can begin to probe other regions of the fault to better understand the overall hazard it poses. The knowledge is also vital for understanding other faults, Saffer said.

For instance, Cascadia, a massive earthquake fault facing the Pacific Northwest, appears to lack Nankai's natural shock absorber. Although some slow slip has been detected at Cascadia, none has been detected at the tsunami-generating, tail end of the fault, which suggests that it may be strongly locked to the trench, Saffer said.

"This is a place that we know has hosted magnitude 9 earthquakes and can spawn deadly tsunamis," Saffer said. "Are there creaks and groans that indicate the release of accumulated strain, or is the 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."

The borehole observatories used in the Japan study were installed by the Integrated Ocean Drilling Program. Other data were supplied by ocean floor cable observatories operated by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC).