Assessing the tsunami hazard potential for the equatorial Southwest Pacific atolls of Tokelau using scenario-based modelling

Abstract

Devastating tsunamis over the last decade have significantly heightened an awareness of the potential consequences and vulnerability to tsunami for low-lying Pacific islands and coastal regions. This tsunami inundation assessment was based on applying a tsunami source-propagation-inundation model to assess the potential for tsunami flooding from fourteen large, high-risk earthquake sources in terms of magnitude, orientation, and distance from the three atolls of the Tokelau Islands. For each earthquake source, potential tsunami were simulated using a well-established but relatively recent computational method, Gerris Flow Solver. For the purposes of informing a civil evacuation strategy, a relationship was established between an earthquake warning (source location and moment magnitude) and potential tsunami risk (wave height, tsunami-flood height). Wave fields are channelled by the bathymetry of the Pacific basin such that swathes of the highest waves sweep immediately northeast of the Tokelau Islands. From our limited series of simulations, great earthquakes from the Kuril Trench pose the most significant inundation threat with maximum wave heights in excess of 1 m, and 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. Dry areas remain around the villages in nearly all our tsunami simulations. Consistent with the oral history of little or no perceived tsunami threat, simulations from the recent Tohoku and Chile earthquake sources infer only limited flooding. Where potential tsunami flooding was identified, recommended evacuation heights above local sea level were compiled, with particular attention paid to variations in tsunami flood depth around the atolls, subdivided into directional quadrants around each atoll. But complex wave behaviours around the atolls, islets, lagoons and tidal channels 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, and may influence evacuation strategies. Our study shows that provisional, but informative, tsunami simulations can be achieved even with only basic field information, due in part to the relative simplicity of the atoll topography and bathymetry.