Modal scattering: a key to understanding oceanic T-waves

Abstract

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.