Potential tsunami hazard of the southern Vanuatu subduction zone: tectonics, case study of the Matthew Island tsunami of 10 February 2021 and implication in regional hazard assessment

Abstract

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.

Forearc crustal faulting and estimated worst-case tsunami scenario in the upper plate of subduction zones. Case study of the Morne Piton Fault system (Lesser Antilles, Guadeloupe Archipelago)

“…COMCOT is widely used by the research community and constantly tested notably through various real tsunami cases (e.g. Prasetya et al, 2011;Gusman et al, 2019;Paris et al, 2021;Gusman et al, 2022;Roger et al, 2023). COMCOT uses a modified staggered finite-difference scheme to solve linear and non-linear shallow water equations in either spherical or Cartesian coordinate systems throughout a set of nested grids allowing refinement of the bathymetric resolution in coastal areas.…”

A brief history of tsunamis in the Vanuatu Arc

“…On 10 February 2021, a powerful Mw 7.7 thrust-type earthquake occurred at the southeasternmost part of the VSZ, south of Matthew and Hunter Islands, and generated a tsunami which was observed on most coastal gauges and DART stations in the southwest Pacific region as far as Tasmania to the south and Tuvalu to the north at distances of 3,000 and 1,800 km from the epicentre, respectively (detailed in Gusman et al, 2022;Roger et al, 2023a). The maximum amplitude of about 1.4 m was recorded on the closest tide gauge at Lenakel on Tanna Island, southern Vanuatu.…”

Sedimentary evidence of tsunamis in New Caledonia, southwest Pacific

“…The 28 March 1875 tsunami caused by a large earthquake with an estimated magnitude Mw 8.1 in southern Vanuatu was the oldest and the largest, and also the only deadly event reported, with 25 deaths at Lifou, Loyalty Islands (Louat and Baldassari, 1989;Sahal et al, 2010;Le Duff et al, 2016;Ioualalen et al, 2017). The two most recent tsunamis triggered by local earthquakes on the southern part of the Vanuatu subduction zone (the 5 December 2018 Mw 7.5 Tadine earthquake, and the 11 February 2021 Mw 7.7 Matthew earthquake) were particularly well recorded by the tide gauge network, and thus used to validate and/or improve local and regional tsunami hazard models and warning systems (Gusman et al, 2022;Roger et al, 2021Roger et al, , 2023.…”