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“It’s like a ripple moving across the plate interface,” said Josh Edgington, who analyzed the data while completing his PhD at the University of Texas Institute for Geophysics (UTIG).
The event – technically a slow-slip earthquake – was first spotted in autumn 2015 and repeated in 2020. Each episode unzipped roughly 20 miles of the fault in slow motion, starting about 30 miles off Japan’s Kii Peninsula and migrating seaward toward the ocean trench.
Onshore seismometers and GPS receivers were oblivious. Only a new network of borehole observatories, drilled hundreds of feet into the seabed, was sensitive enough to detect motion so subtle.
Detecting earthquakes from below
Those boreholes are part of Japan’s ambitious scientific-drilling program, which set out to plug the blind spot in global earthquake monitoring.
Land-based arrays can pinpoint sudden jolts but cannot “listen” to the shallows of subduction zones – precisely the places where tsunami-spawning ruptures begin. Installed sensors measure fluid pressure, strain and tilt with exquisite precision, allowing scientists to see how strain accumulates and releases in real time.
For UTIG director Demian Saffer, who led the study, the advantage is obvious. Slow-slip signals, he said, give researchers a direct view of how the shallow plate boundary behaves between major quakes.
If this creeping zone regularly releases stress, it could limit the size of future tsunamis. If not, the locked portion of the fault farther down-dip might still be primed for a magnitude-8 or 9 shock, similar to 1946, when a great Nankai earthquake leveled towns and killed more than 1,300 people.
Water helps faults slip
Both slow-slip events tracked by the borehole array unfolded in regions where pore-fluid pressures are unusually high.
That correlation supports a popular but difficult-to-prove theory: overpressured fluids lubricate faults, allowing sections to move quietly rather than break catastrophically.
In the Nankai data, the link is as clear as it has ever been, offering a new metric for judging the tsunami potential of similar faults worldwide.
Tsunami signs from afar
While parts of Nankai appear to “creak and groan” in slow motion, the equivalent shallow segment off the Pacific Northwest known as Cascadia may be silent.
That worries scientists, because a silent, locked interface stores energy that can unleash one of Earth’s rare magnitude-9 megathrusts and the devastating tsunamis that follow.
“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 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.”
Installing similar borehole observatories along Cascadia, Chile, and Indonesia – other corners of the Pacific “Ring of Fire” – could reveal whether those margins harbor their own stealthy slow quakes or remain locked tight to the trench.
The answer would refine tsunami-hazard forecasts and perhaps buy coastal communities critical minutes of warning.
Creeping fault limits big earthquakes
Taken together, the 2015 and 2020 Nankai slow-slip episodes suggest the shallow fault functions more like a tectonic shock absorber than a ticking bomb. By periodically releasing energy, it might reduce how much strain transfers to deeper, more dangerous segments.
Yet the scientists caution against complacency. The deeper Nankai interface and neighboring segments could still fail suddenly, as history shows.
For now, geophysicists are analyzing the rich new dataset to model how fluids, temperature, and rock composition govern the transition from silent creep to violent rupture.
Each slow-slip event is another frame in an expanding time-lapse of the earthquake cycle – one that could eventually reveal when the next big snap is likely to occur.
Hearing earthquakes before they roar
Catch a fault in the middle of a slow-motion glide and you learn a simple truth: not every earthquake shouts. Some only whisper, rippling quietly through kilometres of rock.
By wiring the seabed for sound, scientists have begun to hear those whispers and, with them, the hidden conversations that decide when Earth decides to roar.
The study is published in the journal Science.
Image Credit: Japan Meteorological Agency
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