The M_w 7.5 Tadine (Maré, Loyalty Islands) earthquake and related tsunami of 5 December 2018: seismotectonic context and numerical modeling

Abstract

Abstract. On 5 December 2018, a magnitude Mw 7.5 earthquake occurred southeast of Maré, an island of the Loyalty Islands archipelago, New Caledonia. This earthquake is located at the junction between the plunging Loyalty Ridge and the southern part of the Vanuatu Arc, in a tectonically complex and very active area regularly subjected to strong seismic crises and earthquakes higher than magnitude 7 and up to 8. Widely felt in New Caledonia, it was immediately followed by a tsunami warning, confirmed shortly after by a first wave arrival at the Loyalty Islands tide gauges (Maré and Lifou), and then along the east coast of Grande Terre of New Caledonia and in several islands of the Vanuatu Archipelago. Two solutions of the seafloor initial deformation are considered for tsunami generation modeling, one using a non-uniform finite-source model from USGS and the other being a uniform slip model built from the Global Centroid Moment Tensor (GCMT) solution, with the geological knowledge of the region and empirical laws establishing relationships between the moment magnitude and the fault plane geometry. Both tsunami generation and propagation are simulated using the Semi-implicit Cross-scale Hydroscience Integrated System Model (SCHISM), an open-source modeling code solving the shallow-water equations on an unstructured grid allowing refinement in many critical areas. The results of numerical simulations are compared to tide gauge records, field observations and testimonials from 2018. Careful inspection of wave amplitude and wave energy maps for the two simulated scenarios shows clearly that the heterogeneous deformation model is inappropriate, while it raises the importance of the fault plane geometry and azimuth for tsunami amplitude and directivity. The arrival times, wave amplitude and polarities obtained with the uniform slip model are globally coherent, especially in far-field locations (Hienghène, Poindimié and Port Vila). Due to interactions between the tsunami waves and the numerous bathymetric structures like the Loyalty and Norfolk ridges in the neighborhood of the source, the tsunami propagating toward the south of Grande Terre and the Isle of Pines is captured by these structures acting like waveguides, allowing it to propagate to the north-northwest, especially in the Loyalty Islands and along the east coast of Grande Terre. A similar observation results from the propagation in the Vanuatu islands, from Aneityum to Efate.

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

“…The Loyalty Islands Ridge and the Vanuatu subduction arc act as waveguides as well, focusing the tsunami waves on the Loyalty Islands (Maré, Tiga, Lifou and Ouvéa) and the Vanuatu Islands (Aneityum, Tanna, Erromango, Efate mainly). This has already been highlighted by Roger et al (2021) for the 5 December 2018 tsunami propagation. They are also two tsunami pathways clearly focusing the tsunami waves on Tasmania and along the Gold Coast (Australia).…”

“…LENA is particularly inclined to such phenomena and a dedicated study would provide keys to the understanding of Lenakel Bay's reaction to long waves. Higher amplitudes can be expected in nearby exposed areas showing particular geometries like Vshape bays, harbours and river mouths, or specific submarine features like submarine canyons and seamounts able to trigger amplification and/or resonance effects of the incoming waves as was highlighted in the 5 December 2018 tsunami (Roger et al, 2021). At the regional scale, the tsunami amplitude is higher close to the source region (New Caledonia, Vanuatu) and in the south-western quadrant (New Zealand, Australia).…”

“…The area of the southern part of the VSZ between the latitudes 21.5 and 22.5 • S and the longitudes 169 and 170 • E is very active seismically and has produced several seismic crises with earthquakes of magnitude M w 7.0+ during recent decades (1980, 1995, 2003-2004, 2017, 2018). These events are felt by the population in New Caledonia and Vanuatu, as discussed by Roger et al (2021). From a geological point of view, this region is characterized by the progressive subduction/collision of the NW-SE trending Loyalty Ridge located on the Australian plate under the southern Vanuatu microplates.…”

“…From a geological point of view, this region is characterized by the progressive subduction/collision of the NW-SE trending Loyalty Ridge located on the Australian plate under the southern Vanuatu microplates. This leads to strain accumulation that is regularly partially released through moderate to strong earthquakes during remarkable sequences (1980, 2003-2004, 2017, 2018), which include both interplate thrust faulting earthquakes and outer rise normal faulting earthquakes west and south-west of the trench, in which events of one mechanism appear to trigger events of the other (Roger et al, 2021).…”

“…The ratio 5/17 is to be considered with caution: most of the small tsunamis have been recorded by coastal gauges (but not reported by witnesses) during the last decade, and thus the real number of tsunamis having reached New Caledonia, at least from the VSZ, is probably considerably bigger than 17. The latest earthquake-generated tsunami triggered by the VSZ occurred on 5 December 2018, following an M w 7.5 normal faulting earthquake (Roger et al, 2019a(Roger et al, , b, 2021: its amplitude reached more than 2 m in some locations in the south of New Caledonia and Vanuatu (note: at the time of the article submission, there are at least two new tsunamigenic earthquakes of magnitude M w 6.9 and 7.0 having occurred on the VSZ on 30 and 31 March 2022).…”