Forced and free waves of simulated volcanic meteo-tsunamis in the South China Sea

Volcanic meteo-tsunamis (VMTs) are rare, devastating phenomena first recognised after the 1883 Krakatau eruption in Indonesia and recently quantified following the 2022 Hunga Tonga-Hunga Ha’apai volcanic eruption in the Kingdom of Tonga. While most studies have focused on their fast-moving leading waves, little attention has been given to their trailing free waves, which have been shown in the past to represent a major hazard. We conducted high-resolution simulations of VMTs in the South China Sea, examining their characteristics in Singapore, Manila, and Hong Kong. Our results show a clear separation between forced leading and free trailing waves in large shallow-water areas, confirmed by spectral analysis, which revealed characteristic periods for both types of waves. Temporal gaps between the arrivals of forced and free waves correlate with the continental shelf extent and local bathymetry. These gaps have significant implications for designing VMT early warning systems, and advancements in tsunami hazard assessment.