The 2023 South Vanuatu doublet of earthquakes and tsunamis: observations, numerical simulations and gray areas

Over a period of ~5 years from August 2018 to December 2024, the Vanuatu Subduction Zone (VSZ) has demonstrated its potential to trigger large earthquakes and threatening tsunamis, particularly in its southern section, southeast of the Loyalty Islands archipelago. Of particular interest is the region where the Loyalty Ridge collides with the Vanuatu arc, where the subduction zone’s predominant strike direction changes sharply from roughly N-S to E-W. It is within this region that three earthquakes with a moment magnitude (Mw) >= 7.5 have occurred: Mw 7.5 on 5 December 2018, Mw 7.7 on 11 February 2021 and Mw 7.7 on 19 May 2023.

The latter is of major interest for three reasons: (1) it is an outer-rise event having occurred very close to the epicentre of the 1995 Mw 7.7 earthquake, which was the largest outer-rise normal faulting event globally at that time; (2) it was followed by a large set of aftershocks including a Mw 7.1 event ~1 hour after the main shock; (3) both the main shock and the larger aftershock triggered a tsunami; and (4) diverse records of the tsunamis exists, including data from the New Zealand DART network and the recently deployed SWOT satellite.

The first tsunami had sufficiently large wave amplitude to be recorded on most gauges of the southwestern Pacific Ocean, as far as Tasmania in the southwest (~3000 km) and Fongafale to the north-east (~1900 km), although the second tsunami was barely noticeable on the deep-ocean monitoring systems (i.e., DART) and sea-level coastal gauges.

Numerical simulations of tsunami generation and propagation using COMCOT modelling code were performed with different source models to try to deduce the source characteristics, however despite the array of available finite fault sources, none were able to fit the tsunami observations fully, including deep-ocean DART locations. In addition, SWOT 2D measurements, if generally showing a good correlation with the simulation outputs, still reveal elevations quite different from the simulation, notably in terms of amplitude of the main tsunami wavefront propagating towards the northeast. Investigations of ionospheric response to the event using GNSS records highlights the existence of a secondary source associated with the main Mw 7.7 shock, which may be linked to later release of seismic energy and/or the breaking of a second rupture patch.

This presentation aims to show what is known so far, and what are the key pieces of information still missing which may help us to explain the tsunami observations induced by the 2023 earthquake.