When a magnitude 8.8 earthquake ripped through the Kuril-Kamchatka subduction zone on July 29, 2025, it launched a Pacific-wide tsunami – and a rare natural experiment.
NASA and the French space agency’s SWOT satellite happened to pass overhead. The satellite captured the first high-resolution, spaceborne swath of a great subduction-zone tsunami.
Instead of a single neat crest racing across the basin, the image revealed a complicated, braided pattern of energy dispersing and scattering over hundreds of miles. These are details that traditional instruments almost never resolve.
The results go well beyond a pretty picture. They suggest the physics we use to forecast tsunami hazards – especially the assumption that the largest ocean-crossing waves travel as largely “non-dispersive” packets – need a revision.
Satellites transform tsunami mapping
Until now, deep-ocean DART buoys have been our best open-ocean sentinels: exquisitely sensitive, but sparse, each giving a time series at a single point.
SWOT maps a 75-mile-wide swath of sea surface height in one pass. This lets scientists see the tsunami’s geometry evolve in both space and time.
“I think of SWOT data as a new pair of glasses,” said study lead author Angel Ruiz-Angulo of the University of Iceland. “Before, with DARTs we could only see the tsunami at specific points in the vastness of the ocean.”
“There have been other satellites before, but they only see a thin line across a tsunami in the best-case scenario. Now, with SWOT, we can capture a swath up to about 120 kilometers (75 miles) wide, with unprecedented high-resolution data of the sea surface.”
From eddies to a tsunami
NASA and the French space agency CNES launched SWOT in December 2022 to survey surface water around the world.
Ruiz-Angulo and co-author Charly de Marez had been poring over its data for ocean eddies when the Kamchatka event hit.
“We had been analyzing SWOT data for over two years understanding different processes in the ocean like small eddies, never imagining that we would be fortunate enough to capture a tsunami,” noted the researchers.
Tsunami behavior breaks rules
Classic teaching holds that big, basin-spanning tsunamis behave as shallow-water waves. Their wavelength dwarfs ocean depth, so they march along without breaking into separate components.
SWOT’s snapshot argues otherwise for this event. “The SWOT data for this event has challenged the idea of big tsunamis being non-dispersive,” said Ruiz-Angulo.
When the team ran numerical models that included dispersive effects, the simulated wave field matched the satellite pattern far better than “non-dispersive” runs.
That matters because dispersion repackages the wave train’s energy as it approaches land. “The main impact that this observation has for tsunami modelers is that we are missing something in the models we used to run,” said Ruiz-Angulo.
“This ‘extra’ variability could represent that the main wave could be modulated by the trailing waves as it approaches some coast. We would need to quantify this excess of dispersive energy and evaluate if it has an impact that was not considered before.”
Blending every clue available
SWOT’s swath told scientists what the wave looked like mid-ocean. DART buoys anchored the timing and amplitude at key points.
Two gauges didn’t line up with tsunami predictions from earlier seismic and geodetic source models – one recorded the waves earlier than expected, and the other recorded them later.
Using an inversion that assimilated the DART records, the researchers revised the rupture. It extended farther south and spanned roughly 249 miles (400 kilometers), not the 186 miles (300 kilometers) that many initial models assumed.
“Ever since the 2011 magnitude 9.0 Tohoku-oki earthquake in Japan, we realized that the tsunami data had really valuable information for constraining shallow slip,” said study co-author Diego Melgar. Folding that information in isn’t yet routine.
As Melgar argued, this is because the hydrodynamic models needed to model DARTs are very different from the seismic wave propagation ones for modeling the solid Earth data.
“But, as shown here again, it is really important we mix as many types of data as possible.”
Old quakes guide new warnings
The Kuril–Kamchatka margin has a history of producing ocean-wide tsunamis. A magnitude 9.0 quake in 1952 helped motivate the Pacific’s international alert system, which issued basin-scale warnings during the 2025 event.
SWOT’s pass adds a new kind of evidence to that warning toolbox. With enough luck and coordination, scientists could use similar swaths to validate and improve real-time models.
This will be especially important if dispersion turns out to shape near-coast impacts more than we thought.
“With some luck, maybe one day results like ours can be used to justify why these satellite observations are needed for real or near-real time forecasting,” Ruiz-Angulo said.
A turning point for tsunami forecasts
Three takeaways emerge. First, high-resolution satellite altimetry can see the internal structure of a tsunami in mid-ocean, not just its presence.
Second, researchers now argue that dispersion – often downplayed for great events – may shape how energy spreads into leading and trailing waves, which could alter run-up timing and the force on harbor structures.
Third, combining satellite swaths, DART time series, seismic records, and geodetic deformation gives a more faithful picture of the source and its evolution along strike.
For tsunami modelers and hazard planners, the message is equal parts caution and opportunity.
The physics now has to catch up with the complexity that SWOT has revealed, and planners need forecasting systems that can merge every available data stream. The waves won’t get any simpler – but our predictions can get a lot sharper.
The study is published in the journal The Seismic Record.
Image Credit: NOAA
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