The M_w 7.5 Tadine (Maré, Loyalty Is.) earthquake and related tsunami of December 5, 2018: implications for tsunami hazard assessment in New Caledonia

Abstract

Abstract. On the 5th of December 2018, a magnitude Mw 7.5 earthquake occurred southeast of Maré, an island of the Loyalty Archipelago, New Caledonia. This earthquake is located at the junction between the plunging Loyalty ridge and the southernmost Vanuatu arc, in a tectonically very active area regularly subjected to strong seismic crises and events higher than magnitude 7 and up to 8. Widely felt in New Caledonia it has been immediately followed by a tsunami warning, confirmed shortly after by a first wave arrival at the Loyalty Islands tide gauges (Maré and Lifou), then along the east coast of Grande Terre of New Caledonia and in several islands of the Vanuatu Archipelago. Seafloor initial deformation linked to tsunami generation has been modeled with MOST numerical code using earthquake parameters available from seismic observatories. Then the wave propagation has been modeled using SCHISM, another modelling code solving the shallow water equations on an unstructured grid based on a new regional DEM of ~180 m resolution and allowing refinement in many critical areas. Finally, the results have been compared to tide gauge records, field observations and testimonials from 2018. The arrival times, wave amplitude and polarities present good similarities, especially in far-field locations (Hienghène, Port-Vila and Poindimié). Maximum wave heights and energy maps for two different scenarios highlight the fact that the orientation of the source (strike of the rupture) played an important role, focusing the maximum energy path of the tsunami south of Grande-Terre and the Isle of Pines. However, both scenarios indicate similar propagation toward Aneityum, Vanuatu southernmost island, the bathymetry acting like a waveguide. This study has a significant implication in tsunami hazard mitigation in New Caledonia as it helps to validate the modelling code and process used to prepare a scenarios database for warning and coastal evacuation.

The 2021 Loyalty Islands Earthquake (Mw 7.7): Tsunami Waveform Inversion and Implications for Tsunami Forecasting for New Zealand

“…(2021). Since 1995, four large normal faulting earthquakes had occurred in the Southern Vanuatu subduction zone in 1995 (Mw 7.7), 2004 (Mw 7.1), 2017 (Mw7.0), and 2018 (Mw 7.5) (Roger et al., 2021). Three of these four intraplate earthquakes in the outer‐rise generated small to moderate tsunamis in the area (Roger et al., 2019; Sahal et al., 2010).…”

“…Normal faulting tsunamigenic earthquakes as the 5 December 2018 Mw 7.5 earthquake must be considered, especially in this region of the Vanuatu subduction zone as detailed by Roger et al. (2021). Since 1995, four large normal faulting earthquakes had occurred in the Southern Vanuatu subduction zone in 1995 (Mw 7.7), 2004 (Mw 7.1), 2017 (Mw7.0), and 2018 (Mw 7.5) (Roger et al., 2021).…”

“…Some of the coastal gauges in Vanuatu, New Caledonia, and Fiji are located inside a bay or lagoon showing complex bathymetric features potentially leading to arrival time shifts, as shown for New Caledonia and Vanuatu by Roger et al. (2021). This problem of random arrival time shifts, which may be caused by instrumental clock problems, inaccurate station positions, bay effects, harbor effects, or unknown instrument responses, can be solved by applying optimum waveform time shifts in the inversion as demonstrated in previous studies (Ho et al., 2019; Romano et al., 2016).…”

“…Since the 17 May 1995 Mw 7.7 Walpole tsunamigenic earthquake, the Vanuatu subduction zone, which is amongst the most seismically active, has produced at least 12 small (amplitude < 50 cm) to moderate (0.5 m < amplitude < 5 m) tsunamis. These tsunamis were triggered by earthquakes, such as the 19 November 2017 Mw 7.0 earthquake or the 5 December 2018 Mw 7.5 Tadine earthquake (Figure 1) whose maximum amplitude reached more than 2 m in New Caledonia and 4 m in Aneityum Island, Vanuatu (Roger et al., 2019, 2021; Sahal et al., 2010). Although the central and northern part of the Vanuatu subduction zone is also known to have experienced tsunamis triggered by even larger earthquakes of Mw 8.0+ (Ioualalen et al., 2017), there is no clear evidence for Mw8.0+ earthquakes in the southeasternmost part of the subduction zone (170°E−175°E), where the 10 February 2021 earthquake occurred.…”

“…According to the United States Geological Survey (USGS), the earthquake occurred at 13:19:55 UTC with a hypocenter located at 23.051°S–171.657°E and 10 km depth southeastward of the Loyalty Islands archipelago, New Caledonia, and south of Aneityum, Vanuatu, in the southern part of the Vanuatu subduction zone. It is located to the east of the region where the Loyalty Ridge, part of the Australian Plate, is subducted under the overriding Pacific Plate at a convergence rate of ∼12 cm/yr and where Mw7.0+ tsunamigenic earthquakes occurred during the last century (Roger et al., 2021). Since the 17 May 1995 Mw 7.7 Walpole tsunamigenic earthquake, the Vanuatu subduction zone, which is amongst the most seismically active, has produced at least 12 small (amplitude < 50 cm) to moderate (0.5 m < amplitude < 5 m) tsunamis.…”