Tsunami Resonance and Wave Amplification in Semi-enclosed Basins: A case study of the Messiniakos Gulf, Greece

Tsunami wave amplification in semi-enclosed coastal basins is fundamentally influenced by resonance effects, as the local geometry and bathymetry determine the hydrodynamic response to an incoming tsunami wave. Previous research studies emphasize that the shape of the basin and its bathymetric features often exert a more decisive influence on the resulting coastal impact than the characteristics of the seismic source itself.  This study investigates the natural oscillation modes of the Messiniakos Gulf, a deep semi-enclosed basin on the peninsula Peloponnese, Greece, to characterise the spatial distribution of tsunami amplitudes from near-by tectonic sources and its implications for coastal hazard assessment.

We employed a Delft3D Flexible Mesh model of the Messiniakos Gulf to determine the natural oscillation modes of the gulf. For this purpose, we analysed a set of tsunami events using the Okada approach, with different source locations and fault parameterisations within the subduction zone of the Western Hellenic Arc. The numerical outputs were validated against background spectra derived from long-term tidal gauge records at Kalamata harbour, located at the north coast of the gulf.

Our results show a high correlation between the observed and simulated spectral peaks, indicating that the resonance periods in the Messiniakos Gulf remain stable across all tested scenarios. This suggests that the local bathymetry and the resulting natural modes have a greater influence on the propagation patterns and spectral distribution of the tsunami energy at the coast than the source mechanism itself.
The results further demonstrate that the impact of a tsunami shows significant spatial variability across the gulf. While the oscillation period remains consistent throughout the basin, energy concentrates at specific coastal areas, and can lead to extreme local wave heights that may even persist for longer time spans than the original wave itself. In contrast, other areas remain relatively unaffected. Identifying these high-amplification zones is essential for hazard assessment, as it provides a basis for local evacuation planning and effective early warning strategies.