Scenario-based numerical modelling and the palaeo-historic record of tsunamis in Wallis and Futuna, Southwest Pacific

Abstract

International audienceWe investigated the tsunami hazard in the remote French territory of Wallis and Futuna, Southwest Pacific, using the Gerris flow solver to produce numerical models of tsunami generation, propagation and inundation. Wallis consists of the inhabited volcanic island of Uvéa that is surrounded by a lagoon delimited by a barrier reef. Futuna and the island of Alofi form the Horn Archipelago located ca. 240 km east of Wallis. They are surrounded by a narrow fringing reef. Futuna and Alofi emerge from the North Fiji Transform Fault that marks the seismically active Pacific-Australia plate boundary. We generated 15 tsunami scenarios. For each, we calculated maximum wave elevation (MWE), inundation distance and expected time of arrival (ETA). The tsunami sources were local, regional and distant earthquake faults located along the Pacific Rim. In Wallis, the outer reef may experience 6.8 m-high MWE. Uvéa is protected by the barrier reef and the lagoon, but inundation depths of 2–3 m occur in several coastal areas. In Futuna, flow depths exceeding 2 m are modelled in several populated areas, and have been confirmed by a post-September 2009 South Pacific tsunami survey. The channel between the islands of Futuna and Alofi amplified the 2009 tsunami, which resulted in inundation distance of almost 100 m and MWE of 4.4 m. This first ever tsunami hazard modelling study of Wallis and Futuna compares well with palaeotsunamis recognised on both islands and observation of the impact of the 2009 South Pacific tsunami. The study provides evidence for the mitigating effect of barrier and fringing reefs from tsunamis

Tsunami hazard potential for the equatorial southwestern Pacific atolls of Tokelau from scenario-based simulations

“…For each tsunamigenic-source area, simulate the maximum potential tsunami based on a series of the very large, plausible earthquakes from published fault displacement and historical data (cf. Lamarche et al, 2015).…”

“…Moment magnitude can be derived from an estimate of the fault rupture's physical parameters (length, width, depth, Table 2. Compilation of plausible worst-case earthquake scenarios based on a combination of the largest historical earthquakes and/or established earthquake fault models (after Lamarche et al, 2015). The rake angle is 90 • for all sources.…”

“…These uncertainties are not formally quantified herein and are beyond the scope afforded by the current study. Earthquake parameterization herein (Table 2) is adapted from that described in the companion paper by Lamarche et al (2015) from a scenario-based study of tsunami hazard for the South Pacific Islands of Wallis and Futuna. Parameters are obtained from the literature and knowledge of regional tectonics and geodynamics (e.g.…”

“…Our investigation closely follows a companion study by Lamarche et al (2015), which examines tsunami hazards in the remote French territory of Wallis and Futuna, Southwest Pacific, using the Gerris Flow Solver to numerically model tsunami generation, propagation, and inundation. We adapt Gerris to create a series of scenarios, which are not necessarily comprehensive, generated by earthquakes originating from the seismogenic Pacific Rim, the sources of which have been identified and characterized in the literature.…”

“…Devastating tsunami over the last decade have significantly heightened awareness of the potential consequences and vulnerability of low-lying Pacific islands and coastal regions. Our appraisal of the potential tsunami hazard for the atolls of the Tokelau Islands is based on a tsunami source-propagation-inundation model using Gerris Flow Solver, adapted from the companion study by Lamarche et al (2015) for the islands of Wallis and Futuna. We assess whether there is potential for tsunami flooding on any of the village islets from a selection of 14 earthquake-source experiments.…”