Numerical simulation of the tsunami triggered by the 17 December 2024 Port Vila, Vanuatu, Mw 7.3 earthquake

On 17 December 2024, a strong, damaging earthquake (Mw7.3) occurred 30 km WNW of the capital of Vanuatu, Port Vila, on Efate Island. The epicentral depth of the event is at ~57 km depth in the central region of the Vanuatu Subduction Zone (VSZ). Within minutes following the main shock, a small tsunami was recorded on Port Vila coastal gauge (VANU), located at the end of Mélé Bay, along a wharf of the commercial port, with a  maximum recorded amplitude of ~29 cm, below the threshold (30cm) for the Beach and Marine Threat warning category. However, it should be noted that it may have overtopped this value in other local locations; for example, nearby Erakor Lagoon, which often showed larger tsunami impact during past events. Later, the tsunami was recorded on the deep-ocean Bottom Pressure Recorders of the NZ DART regional network (DART NZL and NZK), and other Vanuatu gauges and in New Caledonia, including the Loyalty Islands. Apart from VANU and LENA (Lenakel, Tanna Island, ~17 cm), the records do not show maximum tsunami amplitude greater than 10 cm. There is also no evidence that the tsunami waves reached other gauge locations in the southwest Pacific region, such as Fiji, Tonga, New Zealand, and Australia. At present, we are not aware of reports of damage related to the tsunami.

In order to simulate the tsunami generation and propagation in the region to support rapid response, simple seismic rupture models were quickly designed based on available moment tensor solutions from different seismological agencies. It also used empirical laws, past events, and geologic knowledge of the region. Several models were tested, as the moment tensor solutions did not elucidate which structure within the over-arching active process of the VSZ was responsible. Interestingly, the azimuths and dip angles of the nodal planes do not fit well with the subduction interface as it is known. The simulation of the tsunami was initially performed on a limited number of nested grids to reduce the computational costs in an event response framework, focused on providing refined solutions for the Efate and Port Vila region only. Further simulations encompassed more nested grids in New Caledonia, Fiji, New Zealand, Tonga and Australia (including Norfolk Island). Comparison of the simulation results with the recorded waveforms in Vanuatu and New Caledonia show a good agreement in terms of arrival time, phase and amplitude. Modelled maximum wave amplitude maps confirmed that the tsunami did not exhibit amplitudes larger than ~30 cm and did not propagate out of the Vanuatu-New Caledonia region. More detailed simulations demonstrate that tsunami arrivals may have exceeded 40 cm locally, for example in Fatumaru Bay (northeast of Mélé Bay).

In addition to the validation of a quickly-designed rupture model for tsunami assessment within minutes of an earthquake occurrence, the strong alignment between the simulations and observations suggests that the source of this tsunami was sea-floor displacement related to oblique-normal faulting during the Mw7.3 earthquake. As of time of writing, no aftershocks have generated tsunami.