The ocean is nearly transparent to low frequency sound permitting the observation of distant events such as earthquakes or explosions at fully basin scales. For very low frequency the ocean acts as a shallow-water waveguide and lateral variability in bathymetry can lead to out-of-plane effects. In this paper, data from the International Monitoring System of the Comprehensive Test Ban Treaty Organization (CTBTO) is used to present two cases where robustly localized seismic events in locations clearly within the two-dimensional (2-D) shadow of a continent or large island generate T-phase signals that are received on a hydro-acoustic station. A fully three- dimensional parabolic equation model is used to demonstrate that lateral variability of the bathymetry can lead to diffraction, explaining both observations. The implications of this are that the CTBTO network has greater coverage than predicted by 2-D models and that inclusion of diffraction in future processing can improve the automatic global association of hydroacoustic events.
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