Regional Impact of the 29 September 2009 North Tonga Tsunami on the Futuna and Alofi Islands (Wallis & Futuna)
Abstract
The north Tonga earthquake occurred at 5:48am on 30 September local time in Futuna, ~650 km west of the epicentre. The PTWC issued a warning at 6:04am for tsunami arrival in Wallis (Wallis & Futuna) at 6.35am. No warning was issued by the territorial authorities for Wallis nor for Futuna, located 230 km to the south-west. There was no reported tsunami on Wallis. However a tsunami hit the archipelago of Futuna (islands of Futuna and Alofi) between 7.00 and 7.20am on 30 September. The tide was approximately 3/4 out. We took advantage of an 8 days survey funded by the French Ministry of Foreign Affairs, previously planned for investigating palaeotsunamis on Futuna and Alofi. We measured run-up and inundation from the mid- to low-tide mark, as well as flow depths, and sediments associated with the 30 September tsunami at 41 sites around the islands. Run-ups were estimated based on visual evidence of recent coastal impact - burnt grasses and plants, sand and other displaced debris (e.g., on the road). We interviewed the population on multiple occasions. The maximum run-up of 4.5 m was observed on the eastern beach of Alofitai in Alofi, associated with an inundation of 85 m and a flow depth of 3m at the coast. On Futuna, we measured maximum run-ups of 4.4 m on the eastern tip and 4.3 m on the NW tip of the island, with maximum inundations of 95 and 72m, respectively. A flow depth of 2 m was inferred on the NE tip. Overall, the tsunami impact was more severe on the northern coast of Futuna, with run-ups ranging from 2.1 to 4.3 m. Very small run-ups and inundations were observed along the southern coast, with a 1.0 m run-up and 10 m inundation measured in Léava, the capital of Futuna. Most witnesses report two main waves equivalent in amplitude, the second one being sometimes described as the largest. All witnesses indicate that the sea withdrew first. A video suggests only a few minutes between the successive waves (likely not the first) in Léava. The video shows the reef exposed well below the lowest tides. There were no casualties. One inhabitant was warned by LCI television at 06:30am and was able to witness the tsunami. There were unconfirmed reports of two women taken by surprise by the arrival of the tsunami on the reef near the eastern end of Futuna, but who managed to hold on to trees to avoid being taken out to sea by the backwash. A significant disaster was avoided essentially because it was early and the tide was low when the tsunami hit. Such an event at high tide would have added about 0.8-1m in height to the wave and have undoubtedly resulted in severe damage, injuries and possibly deaths. This event, together with a small tsunami triggered by a Mw 6.4 local earthquake in March 1993 and an oral legend about a deadly and destructive wave indicate that the tsunami risk for Futuna is high for the >4000 inhabitants who live almost exclusively on a 50-400 m-wide coastal strip, between a narrow reef and landward coastal cliffs. However, the hour and 10 minutes that the 30 September tsunami took to reach the island provided sufficient time to issue a warning to the population who can rapidly reach safety on this mountainous landscape.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.U23F..05L
- Keywords:
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- 4564 OCEANOGRAPHY: PHYSICAL / Tsunamis and storm surges