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

Abstract

Abstract. Devastating tsunami over the last decade have significantly heightened awareness of the potential consequences and vulnerability to tsunami for low-lying Pacific islands and coastal regions. Our tsunami risk assessment for the atolls of the Tokelau Islands was 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 series of fourteen earthquake-source experiments that apply a combination of well-established fault parameters to represent plausible "high-risk scenarios" for each of the tsunamigenic sources. Earthquake source location and moment magnitude were related to tsunami wave heights and tsunami flood depths simulated for each of the three atolls of Tokelau. This approach was adopted to yield indicative and instructive results for a community advisory, rather than being fully deterministic. Results from our modelling show that wave fields are channelled by the bathymetry of the Pacific basin in such a way that the swathes of the highest waves sweep immediately northeast of the Tokelau Islands. From our series of limited simulations a great earthquake from the Kuril Trench poses the most significant inundation threat to Tokelau, with maximum modelled-wave heights in excess of 1 m, which may last a few hours and include several wave trains. Other sources can impact specific sectors of the atolls, particularly from regional sources to the south, and northern and eastern distant sources that generate trans-Pacific tsunami. In many cases impacts are dependent on the wave orientation and direct exposure to the oncoming tsunami. This study shows that dry areas remain around the villages in nearly all our "worst-case" tsunami simulations of the Tokelau Islands. Consistent with the oral history of little or no perceived tsunami threat, simulations from the recent Tohoku and Chile earthquake sources suggest only limited flooding. Where potential tsunami flooding was inferred from the modelling, recommended minimum evacuation heights above local sea level were compiled, with particular attention paid to variations in tsunami flood depth, subdivided into directional quadrants around each atoll. But complex wave behaviours around the atolls, islets, tidal channels and lagoons were also observed in our simulations. Wave amplitudes within the lagoons may exceed 50 cm, increasing any inundation and risks on the inner shoreline of the atolls, which may influence evacuation strategies. Our study shows that indicative, but instructive, simulation studies can be achieved even with only basic field information, due in part to the relative simplicity of the atoll topography and bathymetry.

Reply on RC1

“…Even though the bathymetry dataset used (Rasheed et al, 2020) is the highest resolution currently available for the Maldives, it is comprised of data from a variety of sources and the defects of these datasets will manifest as defects in this study. We find that the bathymetry exerts a major influence in directing tsunami waves across the complex geometry of the Atolls of the Maldives, in agreement with Orpin et al (2016), which showed similar effects in Toekalu in the Pacific. Availability of better bathymetry data, particularly near the coastline, can be used to further improve the model and thus the collection of such data should be a priority for the Maldives as it will have clear and direct benefits for risk management across the archipelago.…”

“…Even where hydrodynamic simulations of such domains have been carried out, e.g. for Tokelau in the South Pacific (Orpin et al, 2016), complex multi-Atoll interactions have not been considered. The Maldives archipelago, consisting of a string of multiple Atolls, each characterized by many islands and bathymetric features, provides an excellent case study to assess tsunami propagation in a complex multi-Atoll system and learn how across-Atoll and within-Atoll effects can influence the local impact of tsunamis.…”

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

“…that portion of the coast that is likely to be inundated (say less than 10 m above mean sea level) is most often lacking accuracy for the Pacific Island. The issue is of dramatic importance for low lying islands such as a number of Pacific atolls (Orpin et al, 2015) where the highest elevation is often more than 5 m above mean sea level. The elevation data we acquired for the coastal fringe in Wallis enabled us to calibrate the DTM and strengthen our models.…”

“…This is critical for the numerous Published by Copernicus Publications on behalf of the European Geosciences Union. low-lying islands of the Pacific which have not been the recipient of many tsunami hazard studies such as in Tokelau (Orpin et al, 2015) or in the Marquesas (Hébert et al, 2001).…”

Comment on nhess-2022-95

“…Even where hydrodynamic simulations of such domains have been carried out, e.g. for Tokelau in the South Pacific (Orpin et al, 2016), complex multi-Atoll interactions have not been considered. The Maldives archipelago, consisting of a string of multiple Atolls, each characterized by many islands and bathymetric features, provides an excellent case study to assess tsunami propagation in a complex multi-Atoll system and learn how across-Atoll and within-Atoll effects can influence the local impact of tsunamis.…”