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 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. These earthquake sources are primarily based on the largest Pacific earthquakes of M w ≥ 8.1 since 1950 and other large credible sources of tsunami that may impact Tokelau. Earthquake-source location and moment magnitude are related to tsunami-wave amplitudes and tsunami flood depths simulated for each of the three atolls of Tokelau. This approach yields instructive results for a community advisory but is not intended to be fully deterministic. Rather, the underlying aim is to identify credible sources that present the greatest potential to trigger an emergency response.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. Our limited simulations suggest that trans-Pacific tsunami from distant earthquake sources to the north of Tokelau pose the most significant inundation threat. In particular, our assumed worst-case scenario for the Kuril Trench generated maximum modelledwave amplitudes 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 distant earthquakes from Chile and Peru, and regional earthquake sources to the south. Flooding is dependent on the wave orientation and direct alignment to the incoming tsunami.Our "worst-case" tsunami simulations of the Tokelau Islands suggest that dry areas remain around the villages, which are typically built on a high islet. 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 around low-lying islets of the atoll. Where potential tsunami flooding is inferred from the modelling, recommended minimum evacuation heights above local sea level are compiled, with particular attention paid to variations in tsunami flood depth around the atolls, subdivided into directional quadrants around each atoll. However, complex wave behaviours around the atolls, islets, tidal channels and within the lagoons are also observed in our simulations. Wave amplitudes within the lagoons may exceed 50 cm, increasing any inundation and potential hazards on the inner shoreline of the atolls, which in turn may influence evacuation strategies. Our study shows that indicative simulation studies can be achieved even with only basic field information. In part, this ...

Nearshore tsunami amplitudes across the Maldives archipelago due to worst-case seismic scenarios in the Indian Ocean

“…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), who showed similar effects in Tokelau 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) and Xisha archipelago (also known as the Paracel Islands; Xie et al, 2019), 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 propaga-tion in a complex multi-atoll system and learn how acrossatoll 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 2. 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).…”

Sea-Level Rise Effects on Changing Hazard Exposure to Far-Field Tsunamis in a Volcanic Pacific Island

“…In the Pacific Islands region, far-field or distant-sourced tsunamis which take up to several hours from initiation to coastal impact are typically considered lower risk events compared with locally sourced tsunamis (e.g., [16]). However, there is a dearth in detailed hazard risk modelling studies which demonstrate the effects that SLR will have on exacerbating tsunami inundation risk to far-field events in Pacific Island environments.…”