On 15 January 2022, Tonga was hit by a tsunami caused by the eruption of the HUNGA TONGA –
HUNGA HA'APAI (HT-HH) volcano. This study aims to detect the vulnerable structural systems from
this disaster, using fragility curves for future disaster mitigation planning. I gathered data on the
inundation depth and structural damages through the government ministries' post-disaster field surveys
and completed the required dataset on an independent field survey. This study focused on Hihifo and
Nuku'alofa on Tongatapu island. The method used to determine fragility functions was the Grid Search
method. Structural damages have two classifications: destroyed and survived. Fragility curves for
structural damages were developed for all building types of Hihifo and Nuku'alofa, timber buildings of
Hihifo and Nuku'alofa, and Reinforced Concrete (RC) buildings with masonry infill of Hihifo. The
result showed that the Hihifo area has a stronger structural system than Nuku'alofa. The fragility curves
of Hihifo and Nuku'alofa were compared with those developed in other countries. It showed that the
timber buildings and RC structures with masonry infill of Hihifo are stronger than in other countries.
However, American Samoa's RC structures are more resilient than Hihifo at higher inundation depths.
Therefore, the timber buildings of Nuku'alofa need to be assessed in detail. The RC with masonry infill
buildings of Hihifo must follow the building code so that all RC buildings will be equally resilient to a
tsunami. Lastly, areas exposed to tsunami disasters should have RC structures instead of timber.
Fragility function, tsunami, Hunga Tonga-Hunga Ha'apai, structural damages
Keywords: Fragility function, tsunami, Hunga Tonga-Hunga Ha'apai, structural damages