Modal scattering: A key to understanding oceanic T‐waves

The excitation mechanism of oceanic T‐waves has been a puzzle for almost fifty years, with refraction from a sloping seafloor and seafloor scattering as two of the most commonly invoked mechanisms. By representing the earthquake source field as a normal mode sum, it can be seen that both mechanisms are very closely related. Strict modal orthogonality prohibits the existence of T‐waves in a laterally homogeneous semi‐infinite half‐space or radially symmetric sphere, as energy cannot be transferred from one mode to another in an homogeneous medium. Deterministic non‐planar bathymetry, random boundary roughness, upper crustal heterogeneity, or a combination of these provides a physical mechanism to break the strict orthogonality. We show that modal scattering from the rough seabottom in the epicentral region converts energy from the directly excited ocean crustal/water column modes to the propagating acoustic modes comprising the oceanic T‐wave. Submarine earthquake fault orientation also appears to be reflected in the T‐wave excitation.