Satellite Captures Pacific Tsunami's Complex Waves, Challenging Forecast Models

A Pacific Tsunami Seen in Unprecedented Detail

A magnitude 8.8 earthquake in the Kuril-Kamchatka subduction zone on July 29, 2025, sent a massive tsunami across the Pacific Ocean. A fortunate satellite pass gave scientists something they had never recorded before: a high resolution view from space across a broad strip of a tsunami generated by a great subduction earthquake.

The Surface Water and Ocean Topography satellite, known as SWOT, captured a complicated, braided pattern of waves spreading and scattering over hundreds of miles. The image showed much more than the single, orderly crest often used to explain tsunamis. It revealed smaller features within the moving wave field that conventional ocean instruments almost never resolve.

SWOT is a joint mission between NASA and the French space agency CNES. Its observation has prompted researchers to reconsider how the largest tsunamis travel across an ocean, particularly whether their different wave components separate more than standard forecasting models allow.

The findings also helped researchers revise the earthquake's estimated rupture length from about 186 miles, or 300 kilometers, in many initial models to roughly 249 miles, or 400 kilometers. Combining the satellite view with measurements from ocean buoys produced a more detailed picture of both the waves and the fault movement that created them.

What the Satellite Could See That Buoys Could Not

The Deep-ocean Assessment and Reporting of Tsunamis system, known as DART, has long provided essential measurements of tsunamis in the open ocean. Its instruments are sensitive enough to detect changes in water pressure at the scale of millimeters.

Each DART buoy records how conditions change over time at one location. Those measurements are valuable, but the buoys are spread far apart. Large stretches of ocean lie between them, leaving scientists to calculate how the waves behave in those gaps.

SWOT can map sea surface height across a strip about 75 miles, or 120 kilometers, wide in a single pass. Earlier satellites could measure along a narrow line; SWOT provides a much broader view of the arrangement of waves.

Angel Ruiz-Angulo, the study's lead author from the University of Iceland, explained how that wider view changes what researchers can observe:

I think of SWOT data as a new pair of glasses," Ruiz-Angulo said. "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.

The two systems provide different pieces of the same event. DART follows the passing waves at fixed points, while SWOT shows their shape across a large area. Together, they help researchers understand where wave energy travels and how its arrangement changes.

A Chance Observation During Ocean Research

NASA and CNES launched SWOT in December 2022 to survey surface water around the world. Its main goals included improving understanding of global ocean dynamics and climate change.

Ruiz-Angulo and study co-author Charly de Marez had spent more than two years examining SWOT measurements, including the behavior of small ocean eddies. Eddies are swirling movements of water. Capturing a tsunami was an unexpected opportunity within that work.

Ruiz-Angulo and de Marez described their surprise:

"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," the researchers noted.

The timing mattered. SWOT passed directly over the affected area as the tsunami crossed the Pacific, allowing the researchers to use the event as a rare natural experiment. Its broad measurements recorded details that isolated buoy readings and narrow satellite tracks would struggle to show.

Why the Waves Challenged a Familiar Assumption

Large tsunamis that cross ocean basins are commonly described as shallow water waves. That term can seem surprising when the waves are traveling across a deep ocean. It refers to the relationship between wavelength and water depth: the distance between successive wave crests is much greater than the depth beneath them.

Under the classic explanation, those long waves travel as largely non-dispersive packets. In simple terms, their components move together, keeping much of their original shape instead of separating into different groups.

SWOT's view of the July tsunami showed a more complicated arrangement. Energy appeared in a braided pattern, with waves dispersing and scattering across a broad area. The researchers found that this event challenged the usual assumption about the behavior of very large tsunamis.

Ruiz-Angulo described the finding directly:

"The SWOT data for this event has challenged the idea of big tsunamis being non-dispersive," said Ruiz-Angulo.

The research team tested numerical models, which use mathematical calculations to simulate the moving water. Models that included dispersion reproduced the satellite's wave pattern much better than the non-dispersive versions typically used in forecasting.

The observation therefore gave modelers a concrete pattern against which to check their calculations. The more successful simulations allowed the tsunami's components to separate as they traveled, capturing complexity that the simpler runs missed.

How Dispersion Could Change Coastal Predictions

Dispersion happens when different wave components travel at different speeds. As they separate, the energy within a group of waves is rearranged. For a tsunami, that can affect the relationship between its leading crest and the waves following behind.

The July observation raised the possibility that trailing waves could change the main wave as it approaches a coastline. Their timing and energy could differ from what a model without dispersion predicts.

Ruiz-Angulo explained both the missing physics and the work needed to measure its effect:

The main impact that this observation has for tsunami modelers is that we are missing something in the models we used to run," Ruiz-Angulo explained. "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.

The size of that additional dispersive energy, and its effect near shore, still needed to be quantified. Those are practical questions because forecasts help authorities decide when to issue evacuation orders and help engineers plan coastal defenses.

Changes in how energy reaches land could affect the timing of run-up, meaning how far and high tsunami water advances onto the coast, as well as the force on harbor structures. The researchers' task is to work out how much the newly observed complexity changes those coastal outcomes.

Buoy Measurements Redrew the Earthquake Rupture

SWOT showed the tsunami's shape in the open ocean. DART records supplied measurements of its arrival time and amplitude, or wave size, at particular locations. Bringing those observations together helped the researchers examine the earthquake that generated it.

Two gauges recorded timing that did not match tsunami predictions based on earlier seismic and geodetic source models. Seismic measurements describe earthquake waves traveling through the solid Earth, while geodetic measurements track ground deformation. One gauge detected the tsunami earlier than predicted; the other detected it later.

The researchers used an inversion technique to incorporate the DART records. Inversion works backward from observations to estimate the event that produced them. Here, the team used the passing tsunami to revise its understanding of where the earthquake fault had moved.

The resulting estimate placed the rupture farther south and gave it a length of about 400 kilometers, or 249 miles, rather than the approximately 300 kilometers, or 186 miles, assumed in many initial models. Using those kilometer figures, the revised length was about 100 kilometers greater, an increase of roughly one third.

That revision matters because the location and extent of fault movement help determine the water disturbance that starts a tsunami. A better description of the earthquake source gives simulations a better starting point for calculating the waves that follow.

Why Different Measurements Need to Work Together

Study co-author Diego Melgar connected the approach to lessons from Japan's magnitude 9.0 Tohoku-oki earthquake in 2011. Tsunami observations, he explained, can help researchers determine shallow slip, the movement of a fault in its shallower portions.

Melgar described when researchers recognized the value of those measurements:

"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," Melgar said.

Combining those observations with standard earthquake models is still not routine. The hydrodynamic models needed to interpret DART records describe moving water. Seismic wave propagation models describe waves traveling through solid Earth. Bringing the two together requires researchers to connect different kinds of calculations.

Melgar said the July event showed why that effort matters:

"But as shown here again, it is really important we mix as many types of data as possible," he argued.

The study brought together complementary evidence: satellite measurements of the wave field, buoy records of timing and amplitude, seismic records, and measurements of ground deformation. Each helped refine a different part of the earthquake and tsunami, producing a more faithful picture of the source and its evolution.

How the 2025 Earthquake Compared With 1952

The Kuril-Kamchatka margin has a history of earthquakes that generate tsunamis across the ocean. A magnitude 9.0 earthquake there in 1952 helped motivate the creation of the Pacific's international alert system. That system issued warnings across the basin during the 2025 event.

The study compared the two earthquakes and concluded that the 2025 rupture likely broke significant portions of the megathrust that had ruptured in 1952. A megathrust is the major fault where one tectonic plate descends beneath another.

The 2025 movement occurred farther down-dip, meaning deeper along the fault, and involved little to no slip near the trench. The researchers found that this difference in rupture depth was consistent with the smaller tsunami impact in 2025 compared with the catastrophic waves of 1952.

The key dates place the new observation within that history:

• 1952: A magnitude 9.0 earthquake in the region generated catastrophic tsunami waves and helped motivate the Pacific's international alert system.
• 2011: Japan's magnitude 9.0 Tohoku-oki earthquake showed researchers the value of tsunami measurements for estimating shallow fault slip.
• December 2022: NASA and CNES launched SWOT to survey surface water worldwide.
• July 29, 2025: A magnitude 8.8 earthquake in the Kuril-Kamchatka subduction zone generated the Pacific tsunami captured by SWOT.

What the Observation Could Bring to Forecasting

SWOT's pass added a new form of evidence to the tsunami warning toolbox: a detailed view of the internal structure of a major tsunami in the middle of the ocean. With favorable timing and coordination, similar observations could help scientists test and improve models used for forecasting.

Ruiz-Angulo described that prospect as a goal requiring both opportunity and further work:

"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.

The study points to three connected needs. Models must capture the wave structure revealed by high resolution satellite measurements. Researchers must measure how dispersion redistributes energy between leading and trailing waves. Forecasting systems must also combine observations of the ocean with measurements of the earthquake itself.

For hazard planners, the practical focus is on more accurate estimates of what arrives at the coast and when. The July tsunami gave researchers a detailed test of their calculations, while the revised rupture estimate showed how ocean measurements can improve their understanding of an earthquake beneath the sea.

The Essentials

• A magnitude 8.8 earthquake struck the Kuril-Kamchatka subduction zone on July 29, 2025, launching a tsunami across the Pacific.
• SWOT captured the first high resolution view from space across a broad strip of a tsunami generated by a great subduction earthquake.
• The NASA and CNES satellite can measure sea surface height across a swath about 120 kilometers, or 75 miles, wide.
• Braided wave patterns challenged the assumption that large tsunamis travel as largely non-dispersive packets.
• Simulations including dispersion matched the satellite observations better than non-dispersive runs.
• DART records helped researchers extend the estimated earthquake rupture from about 300 to 400 kilometers and place it farther south.
• The 2025 earthquake likely ruptured significant portions of the fault that broke in 1952, but deeper movement and little to no slip near the trench were consistent with its smaller tsunami impact.
• Researchers still need to quantify how the observed dispersive energy affects waves approaching coastlines.