Tsunami wavefront backprojection to locate sources of dispersive waves imaged by SWOT during the 2025 M8.8 Kamchatka Earthquake

Diego Melgar , Angel Ruiz-Angulo, Aditya Gusman

The 29 July 2025 M8.8 Kamchatka earthquake generated a Pacific-wide tsunami that the Surface Water and Ocean Topography (SWOT) mission imaged about 70 minutes later, about 600 km from the source, as a leading crest trailed by a train of short-wavelength dispersive waves. Rather than matching an assumed source forward to the observations, as prior studies of this wavefield have done, we back-project the observed wavefronts. Using a dispersive ray-tracing engine and travel-time reciprocity, we trace each digitized, band-passed front to a source location and, from SWOT’s per-pixel acquisition times to an emission time, with no assumed source model parameters. The method recovers the known source region from the leading front, and per-front resolution is quantified by a bootstrap calibrated against blind re-digitization. Because a single front constrains an arc of source positions traded against emission time, many fronts are not localizable and are left uninterpreted. Ten trailing fronts are nonetheless robustly localized outside the coseismic source region and at emission times well after the origin. Their emission time increases with distance from the source, and each was emitted later than the earliest possible arrival of energy from the earthquake at its location. Independently of the back-projection, the whole trailing train is shown to arrive too late to have been radiated dispersively from the source. A substantial part of the short-wavelength tail imaged by SWOT therefore did not originate at the coseismic source but was generated along the propagation path, progressively delayed through interaction with the seafloor. Back-projection of wide-swath altimetry locates and times the origin of individual tsunami waves directly from the observed wavefield.