Abstract. On 5 December 2018, a magnitude Mw 7.5 earthquake occurred southeast of Maré, an island of the Loyalty Islands archipelago, New Caledonia. This earthquake is located at the junction between the plunging Loyalty Ridge and the southern part of the Vanuatu Arc, in a tectonically complex and very active area regularly subjected to strong seismic crises and earthquakes higher than magnitude 7 and up to 8. Widely felt in New Caledonia, it was immediately followed by a tsunami warning, confirmed shortly after by a first wave arrival at the Loyalty Islands tide gauges (Maré and Lifou), and then along the east coast of Grande Terre of New Caledonia and in several islands of the Vanuatu Archipelago. Two solutions of the seafloor initial deformation are considered for tsunami generation modeling, one using a non-uniform finite-source model from USGS and the other being a uniform slip model built from the Global Centroid Moment Tensor (GCMT) solution, with the geological knowledge of the region and empirical laws establishing relationships between the moment magnitude and the fault plane geometry. Both tsunami generation and propagation are simulated using the Semi-implicit Cross-scale Hydroscience Integrated System Model (SCHISM), an open-source modeling code solving the shallow-water equations on an unstructured grid allowing refinement in many critical areas. The results of numerical simulations are compared to tide gauge records, field observations and testimonials from 2018. Careful inspection of wave amplitude and wave energy maps for the two simulated scenarios shows clearly that the heterogeneous deformation model is inappropriate, while it raises the importance of the fault plane geometry and azimuth for tsunami amplitude and directivity. The arrival times, wave amplitude and polarities obtained with the uniform slip model are globally coherent, especially in far-field locations (Hienghène, Poindimié and Port Vila). Due to interactions between the tsunami waves and the numerous bathymetric structures like the Loyalty and Norfolk ridges in the neighborhood of the source, the tsunami propagating toward the south of Grande Terre and the Isle of Pines is captured by these structures acting like waveguides, allowing it to propagate to the north-northwest, especially in the Loyalty Islands and along the east coast of Grande Terre. A similar observation results from the propagation in the Vanuatu islands, from Aneityum to Efate.

Abstract. On the 5th of December 2018, a magnitude Mw 7.5 earthquake occurred southeast of Maré, an island of the Loyalty Archipelago, New Caledonia. This earthquake is located at the junction between the plunging Loyalty ridge and the southernmost Vanuatu arc, in a tectonically very active area regularly subjected to strong seismic crises and events higher than magnitude 7 and up to 8. Widely felt in New Caledonia it has been immediately followed by a tsunami warning, confirmed shortly after by a first wave arrival at the Loyalty Islands tide gauges (Maré and Lifou), then along the east coast of Grande Terre of New Caledonia and in several islands of the Vanuatu Archipelago. Seafloor initial deformation linked to tsunami generation has been modeled with MOST numerical code using earthquake parameters available from seismic observatories. Then the wave propagation has been modeled using SCHISM, another modelling code solving the shallow water equations on an unstructured grid based on a new regional DEM of ~180 m resolution and allowing refinement in many critical areas. Finally, the results have been compared to tide gauge records, field observations and testimonials from 2018. The arrival times, wave amplitude and polarities present good similarities, especially in far-field locations (Hienghène, Port-Vila and Poindimié). Maximum wave heights and energy maps for two different scenarios highlight the fact that the orientation of the source (strike of the rupture) played an important role, focusing the maximum energy path of the tsunami south of Grande-Terre and the Isle of Pines. However, both scenarios indicate similar propagation toward Aneityum, Vanuatu southernmost island, the bathymetry acting like a waveguide. This study has a significant implication in tsunami hazard mitigation in New Caledonia as it helps to validate the modelling code and process used to prepare a scenarios database for warning and coastal evacuation.

Abstract. The Vanuatu subduction zone (VSZ) is known to be seismically very active, due to the high convergence rate between the Australian and Pacific tectonic plates for the majority of the margin. However, this is not the case on its southernmost part south of latitude 22.5∘ S and east of longitude 170∘ E, which is neither highly tectonically active nor has it produced large tsunamis over the past 150 years. It has also not been widely studied. On the 11 February 2021 (10 February UTC), a magnitude Mw 7.7 earthquake triggered a tsunami warning in New Caledonia and Vanuatu 20 min after midnight (local time). With an epicentre located close to the volcanic islands of Matthew and Hunter, this shallow reverse-faulting rupture (< 30 km depth) was able to deform the seabed and produce a tsunami. This was confirmed 45 min later by the coastal gauges of the Loyalty and the south Vanuatu islands, which recorded the first tsunami waves. Showing a typical recorded amplitude of less than 1 m, with a maximum of ∼ 1.5 m in Lenakel (Tanna, Vanuatu), it was observed on most coastal gauges and DART stations in the south-west Pacific region as far as Tasmania to the south and Tuvalu to the north at distances of ∼ 3000 and ∼ 1800 km from the epicentre. In this study, the tsunamigenic potential of the southernmost part of the VSZ and the implications in terms of regional hazard assessment are discussed through (1) the presentation of the complex tectonic settings of this “transition zone” between the Solomon–Vanuatu and the Tonga–Kermadec trenches, (2) the case study of the 10 February 2021 tsunami at a south-west Pacific regional scale using three different tsunami generation scenarios computed with the COMCOT modelling code on a set of 48 nested bathymetric grids, and (3) the simulation of a plausible Mw 8.2 scenario encompassing the active part of this “transition zone”. The validation of the Mw 7.7 parameters for tsunami modelling provides the means to further assess the hazard from potential tsunamis triggered by higher magnitude earthquakes in this region. Tsunami records highlight that > 28 cm wave amplitudes were recorded at eight different coastal gauges, including one with an amplitude of more than 1 m (Lenakel, Tanna, Vanuatu). The tsunami threat at that location would be large enough to warrant an onshore evacuation. Finally, it helps to highlight the significant role played by the numerous submarine features in the region, the Norfolk Ridge being the most important, which acts like a waveguide from the north to the south.

Abstract. The Vanuatu subduction zone (VSZ) is known to be seismically very active, releasing a significant energy resulting of the quick convergence rate between the Australian and Pacific tectonic plates. That is not the case on its southernmost part south of latitude 22.5° S and east of longitude 170° E which is neither known as being highly tectonically active nor having produced large tsunamis over the past 150 years, and by the way, has not been much studied. On the 11th of February 2021 (10 February UTC), a magnitude Mw 7.7 earthquake triggered a tsunami warning in New Caledonia and Vanuatu twenty minutes after midnight (local time). With an epicentre located close to the volcanic islands of Matthew and Hunter, this shallow reverse-faulting rupture (< 30 km depth) was able to disturb the seabed and produce a tsunami. In fact, it was confirmed 45 min later by the coastal gauges of the Loyalty and the south Vanuatu islands which recorded the first tsunami waves. Showing an overall recorded amplitude of less than 1 m with a maximum of ~1.5 m in Lenakel, (Tanna, Vanuatu), it has been recorded on most coastal gauges and DART stations of the southwest Pacific Region as far as Tasmania in the South and Tuvalu in the North respectively at distances of ~3000 and ~1800 km from the epicentre. In this study, the tsunamigenic potential of the southernmost part of the VSZ and the implications in terms of regional hazard assessment are discussed through (1) the presentation of the complex tectonic settings of this “transition zone” between the Solomon-Vanuatu and the Tonga-Kermadec Trenches; (2) the case study of the 10 February 2021 tsunami at a southwest Pacific regional scale using three different tsunami generation scenarios computed with COMCOT modelling code on a set of 48 nested bathymetric grids; and (3) the simulation of a plausible Mw 8.2 scenario encompassing the active part of this “transition zone”. In fact, the validation of the Mw 7.7 parameters for tsunami modelling provides keys to further assess the hazard from potential tsunami triggered by higher magnitude earthquakes in this region. Finally, it helps to highlight the significant role played by the numerous submarine features in the region, the Norfolk Ridge being the most important acting like a waveguide toward the north and the south.