Contribution of microtopography off the Ryukyu Islands to coastal sea-level amplification during the 2022 Tonga meteotsunami

Abstract

The January 2022 Tonga volcanic eruption generated atmospheric pressure waves that propagated over the ocean's surface and triggered a meteotsunami. This meteotsunami caused significant amplitudes exceeding 100 cm along various Pacific coastlines far from the volcano. However, the factors driving such amplification remain unclear. This study presents numerical simulations of the meteotsunami, focusing on the Ryukyu Islands in Japan, where a maximum amplitude of 100 cm was recorded. Two models for simulating pressure waves from the eruption were utilized: one based on the superposition of waves tuned using the dispersion relation of atmospheric gravity waves (synthetic waves), and the other based on a detailed numerical model that assumes the release of a heat source from the eruption vent. The synthetic pressure wave simulations showed good agreement with the observations, accurately reproducing the 100 cm amplitude at Amami. To further analyze the factors contributing to the large amplitude at Amami, additional simulations were conducted by limiting the resolution to offshore areas deeper than 2000 m while maintaining a high resolution in coastal bathymetry. These simulations showed that reducing offshore resolution decreased the amplitude at Amami from approximately 100 to 60 cm, highlighting the significant role of offshore microtopography such as Daito Ridge and Oki-Daito Ridge in coastal amplification. The difference in amplitude was particularly notable in ridge areas with depths of 2000–5000 m. Moreover, the proportion of free waves to the total tsunami amplitude was estimated by terminating atmospheric pressure forcing during the computation. The results indicated that free waves alone could amplify from less than 5 cm offshore to 50 cm at the Amami coast, which is approximately half the amplitude when forced waves are also considered. These findings provide crucial insights into assessing the future predictability of meteotsunamis. Future research should investigate the necessary resolution and relationship between atmospheric wave properties and tsunami amplification. Understanding these factors is essential to improve the prediction and risk assessment of meteotsunamis.