Comment on nhess-2022-157

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.