SWOT detects dispersive tsunami tied to a near-trench source in the 2025 Kamchatka earthquake.

Tsunamis from large subduction earthquakes pose severe coastal hazards, yet their genesis near the trench remains poorly constrained by land-based seismic geodetic data and distant deep-water sensors. Following the 29 July 2025 magnitude 8.8 Kamchatka earthquake, the NASA/CNES Surface Water and Ocean Topography (SWOT) satellite captured a distinct train of short-wavelength tsunami waves, which we link to near-trench tsunamigenesis. Sensitivity analyses of earthquake slip indicated tsunamigenesis within 10 kilometers of the trench, an inference not attainable from land seismology and geodesy or sparse deep-water seafloor pressure records alone. These results provide the first high-resolution, two-dimensional spaceborne observation directly linking the measured dispersive tsunami wavefield to near-trench tsunamigenesis, extending earlier model- and gauge-based inferences. They establish SWOT as a constraint on source processes, with implications for tsunami hazard science and subduction-zone geodynamics. Editor's summary: On 29 July 2025, the Kamchatka moment magnitude 8.8 earthquake triggered a Pacific-wide tsunami that happened to be imaged during a flyover of the NASA/CNES Surface Water and Ocean Topography (SWOT) mission. Sepúlveda et al. used the centimeter-scale SWOT data to analyze the tsunami's trailing waves, which had the dispersion and short wavelengths typical of shallow subduction-zone earthquakes. Together with land-deformation data and seafloor pressure sensors, the SWOT data allowed a high-resolution estimate of the depth of slip during the Kamchatka event. This application shows how SWOT can integrate with other sensing methods to understand tsunami-prone earthquakes. —Angela Hessler [ABSTRACT FROM AUTHOR]