Tsunami hazard in Wallis and Futuna
Could Wallis and Futuna be affected by tsunamis? And if so, what characteristics would such tsunamis have?
Awareness of the potentially disastrous consequences of tsunami for island and coastal regions has increased in the Pacific region over the last 10 years due to damaging tsunamis such as the Sumatra (2004), Tonga-Samoa (2009), and Tohoku, Japan (2011), events.
In response to these events, the scientific community has increased efforts to improve understanding of how tsunamis are generated, how they travel across the ocean, flood land, and affect coastal communities and infrastructure. The French Territory of Wallis and Futuna (Fig. 1), in the South-West Pacific, has experienced at least two tsunamis in the recent past - in 1993 and 2009. Both these caused flooding on Futuna. Studies of pre-historic tsunami deposits have also shown that at least two other tsunamis are likely to have flooded parts of the island prior to the arrival of Europeans. However, the history of tsunami in Wallis and Futuna is not well documented.
The Tonga-Samoa tsunami on 29 September 2009 hit the islands of Futuna and Alofi early in the morning. Waves at the beach reached up to 3 metres in height and in places the tsunami penetrated up to 100 metres inland. Fortunately there was minimal damage to community infrastructure located close to the coastline (Fig. 2).
Our work on understanding and characterising tsunami hazard in Wallis and Futuna started immediately after the 29 September 2009 event. The project involved NIWA, the Institut de Recherche pour le Développement (IRD) in New Caledonia, and the University of New South Wales, in Sydney.
The research was divided into four phases:
- A post-tsunami field survey collecting physical evidence of flooding and wave levels around the two islands of Futuna and Alofi. This evidence was subsequently used to calibrate numerical models of the tsunami.
- Identification of pre-historical evidence of tsunami flooding in both Wallis and Futuna.
- Identification and characterisation of earthquake sources around the Pacific Rim capable of generating tsunamis large enough to travel to Wallis and Futuna. Such sources are large earthquakes, typically of magnitude 8 or larger, with large seafloor displacement and an orientation that focuses the tsunami's energy toward Wallis and Futuna.
- Application of computer models to simulate the potential for different tsunami sources to cause flooding on Wallis and Futuna and to better understand how long a tsunami from these various sources would take to arrive on both islands.
14 tsunami sources with the potential to impact Wallis and Futuna were identified:
- Pacific-wide tsunamis, specifically from the Kuril region, Chile and south Peru. Tsunamis from Japan and the Aleutian and Cascade regions are unlikely to significantly affect Wallis and Futuna
- regional tsunamis from the Tonga Trench and central Vanuatu back-arc
- local tsunamis generated along the active fault zone immediately south of Futuna. This fault zone is responsible for the 1993 earthquake and subsequent tsunami.
The modelling indicated that tsunamis generated by a full rupture of the Tonga Trench would have the strongest impact on both islands. In Wallis, the second-strongest impact is likely to be from a tsunami originating in the Kuril Trench. In Futuna, the second strongest impact is likely to be from a locally generated tsunami.
Generally, the risk of significant damage from tsunami flooding is greater for Futuna than Wallis, as the barrier reef around Wallis provides more protection than the narrow fringing reef around Futuna. On Futuna there is the potential for significant flooding from tsunamis. The modelling suggests that flood levels could be up to 2 metres above land in certain populated areas close to the coastline.
The coastline between Futuna and the island of Alofi (Fig. 3) is particularly at risk, as the strait between the two islands tends to focus tsunami waves. This affected region includes the airport runway. However, areas along the south, north and east coasts of Futuna and the southeast coast of Alofi may also be prone to significant flooding.
Tsunami flooding in Wallis is most significant on the outer reef islands along the edge of the barrier reef. Uvéa, the main village, is less prone to flooding, although the modelling indicates that the meeting place and harbour facilities in the administrative centre of Mata'Utu, and taro fields along the SE coast in Tepa and Falaleu, could experience flooding (Fig. 4).
There would be little or no warning to either of the islands for a regionally or locally generated tsunami event, other than possibly earthquake shaking or sea withdrawal. Fig. 5 shows the estimated arrival times and durations of tsunamis from the various earthquake sources.
Much of the Wallis and Futuna population lives on low-lying coastal margins a few metres above sea level. It should be assumed that all coastal areas below 5 metres above sea level could experience tsunami flooding.
The essential message is: if an earthquake is felt strongly or if the sea is behaving abnormally or quickly withdrawing, move to higher ground immediately.
About the study
This project has been carried out to provide initial information to help Wallis and Futuna develop emergency responses in the form of community preparedness and evacuation plans. This could include tsunami warning procedures and community-developed evacuation routes and shelters.
The results could also be used to help inform future land-use planning - for example, the relocation of critical community facilities, such as the hospital, away from areas of potential flooding or other natural hazard risk. The study was funded by the EU EDF 9-C Envelope project to support Disaster Risk Reduction in Pacific EU Overseas Countries and Territories and the Pacific Fund of the French Ministry of Foreign and European Affairs.