Potential tsunami hazard of the southern Vanuatu Subduction Zone: 1 tectonics, case study of the Matthew Island tsunami of 10 February 2021 2 and implication in regional hazard assessment.

The Vanuatu subduction zone (VSZ) is known to be seismically very active, releasing a significant 17 energy resulting of the quick convergence rate between the Australian and Pacific tectonic plates. That 18 is not the case on its southernmost part south of latitude 22.5°S and east of longitude 170°E which is 19 neither known as being highly tectonically active nor having produced large tsunamis over the past 150 20 years, and by the way, has not been much studied. On the 11 th of February 2021 (10 February UTC), a 21 magnitude M w 7.7 earthquake triggered a tsunami warning in New Caledonia and Vanuatu twenty 22 minutes after midnight (local time). With an epicentre located close to the volcanic islands of Matthew 23 and Hunter, this shallow reverse-faulting rupture (< 30 km depth) was able to disturb the seabed and 24 produce a tsunami. In fact, it was confirmed 45 min later by the coastal gauges of the Loyalty and the 25 south Vanuatu islands which recorded the first tsunami waves. Showing an overall recorded amplitude 26 of less than 1 m with a maximum of ~1.5 m in Lenakel, (Tanna, Vanuatu), it has been recorded on most 27 coastal gauges and DART stations of the southwest Pacific Region as far as Tasmania in the South and 28 Tuvalu in the North respectively at distances of ~3000 and ~1800 km from the epicentre. In this study, 29 the tsunamigenic potential of the southernmost part of the VSZ and the implications in terms of regional 30 hazard assessment are discussed through (1) the presentation of the complex tectonic settings of this 31 “transition zone” between the Solomon-Vanuatu and the Tonga-Kermadec Trenches; (2) the case study 32 of the 10 February 2021 tsunami at a southwest Pacific regional scale using three different tsunami 33 generation scenarios computed with COMCOT modelling code on a set of 48 nested bathymetric grids; 34 and (3) the simulation of a plausible M w 8.2 scenario encompassing the active part of this “transition 35 zone”. In fact, the validation of the M w 7.7 parameters for tsunami modelling provides keys to further 36 assess the hazard from potential tsunami triggered by higher magnitude earthquakes in this region. 37 Finally, it helps to highlight the significant role played by the numerous submarine features in the 38 region, the Norfolk Ridge being the most important acting like a waveguide toward the north and the 39 south. 40 directly toward Lord Howe Island. Part of the energy is still propagating toward New Zealand, using the ridges like the Three Kings Ridge toward North Cape. To the north, the tsunami propagates within the North Fiji Basin, (between Vanuatu and Fiji) and is able to go through the Vityaz Trench region, reaching Tuvalu islands. Just a little part of energy is propagating toward the east and seems to disappear when crossing the Kermadec-Tonga Trench. In details, the tsunami seems to be caught within the different lagoons or trapped by shelf surrounding oceanic islands: Norfolk Island’s shelf, for which a high-resolution DEM has been specifically built using nautical charts, is the best example of waves being caught around an island in this study, leading to consequent amplitudes of 1.5 m and more. High amplitudes are also shown in Vanuatu, especially on the southern coast of Aneityum Island, its southernmost island, but also in Tanna or Erromango, at the same locations already highlighted with the M w 7.7 scenario herein, but also for the 5 December 2018 tsunami study (Roger et al., 2021). In the nearby islands of New Caledonia, the amplitudes are less important as would have been expected, especially in the Loyalty Islands, but Ouvéa and Grande-Terre respective lagoons catch tsunami waves leading to amplitude records of around 0.5 m. Similarly, the tsunami waves are caught within the islands in Fiji, Tonga and in Tuvalu’s Te Namo atoll. It is interesting to see that the tsunami can particularly affect the west coast of New Zealand much more than its northern shore: locations as Jackson Bay (southwest coast of the South Island), already identified as reacting very easily to tsunami coming from a wide range of azimuths, still shows amplitudes of more than 1 m.