Linear propagation of tsunami and acoustic–gravity waves on a sphere: Geometrical focusing and defocusing

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This study investigates the propagation of tsunami and acoustic–gravity waves at oceanic scales, accounting for the Earth’s curvature within a linear, potential flow framework. While local, near-field analyses often neglect Earth’s curvature and employ Cartesian or cylindrical coordinate systems, this work utilises spherical coordinates to examine wave behaviour over large distances. The analysis reveals that wave amplitudes experience a defocusing effect as they travel from the source (e.g., the Pole) toward the equator, followed by a focusing effect as they approach the antipodal point beyond the equator. A qualitative comparison is made with the 2022 Hunga Tonga–Hunga Ha’apai volcanic eruption in the South Pacific. The study models surface-gravity (tsunami) waves propagating through a compressible water layer, as well as atmospheric acoustic–gravity waves propagating through the air. The entire analysis is carried out within the framework of linear theory.