A method for planning disaster risk management and calibrating disaster relief efforts: application to the 2009 and 2022 Tonga tsunamis

Abstract

Coastal communities are highly exposed to ocean- and climate-related hazards but often lack an accurate population and infrastructure database. On January 15, 2022 and for many days thereafter, the Kingdom of Tonga was cut off from the rest of the world by a destructive tsunami associated with the Hunga Tonga Hunga Ha’apai volcanic eruption. This situation was made worse by COVID-19-related lockdowns and no precise idea of the magnitude and pattern of destruction incurred. The occurrence of such events in remote island communities highlights the need for (1) precisely knowing the distribution of residential and public buildings, and (2) evaluating what proportion of those would be vulnerable to a tsunami. A GIS-based dasymetric mapping method, previously tested in New Caledonia for assessing and calibrating population distribution at high resolution, is improved and implemented in less than a day to jointly map population clusters and critical elevation contours based on runup scenarios, and is tested against destruction patterns independently recorded in Tonga after the two recent tsunamis of 2009 and 2022. Results show that 62% of the population of Tonga lives in well-defined clusters between sea level and the 15 m elevation contour. The patterns of vulnerability thus obtained for each island of the archipelago allow exposure and potential for cumulative damage to be ranked as a function of tsunami magnitude and source-area. By relying on low-cost tools and incomplete datasets for rapid implementation in the context of natural disasters, this approach works for all types of natural hazards, is easily transferable to other insular settings, can assist in guiding emergency rescue targets, and can help to elaborate future land-use planning priorities for disaster risk reduction purposes.

Greetings from HYDRO 2023 Organizing Committee

“…The Kingdom of Tonga is composed of 172 small islands, 45 of them inhabited distributed on a 650 km × 200 km frame along the Tonga-Kermadec subduction zone. The population is exposed to a diverse range of strong geological processes and hazards that regularly affects the archipelago (Thomas et al, 2023a). Tsunamis in Tonga may be generated from subduction megathrust earthquakes (past events: Okal et al, 2004;in 2006in : Tang et al, 2008; the doublet earthquake in 2009: Lay et al, 2010), subaerial (volcano fl ank collapse) or submarine landslides (Frohlich et al, 2009), volcanic eruptions (Terry et al, 2022), or multi-hazard events (Lynett et al, 2022) with even a possible meteorite impact (Lavigne et al, 2021).…”

“…Local observations from social media videos and interviews were available only a few weeks later, Tonga being under strict international lockdown during the COVID-19 pandemic. In an archipelago where more than 60 % of the population lives below 15 m elevation (Thomas et al, 2023a), communication was impossible due to deep-sea telecommunications cables being severed (Terry et al, 2022), creating a lack of real-time data. The tsunami was recorded on nearby tide gauges and DART sensors , as well as all across the Pacific Ocean, with impacts in Japan (Tsukanova & Medvedev, 2022), China and Mexico (Ramírez-Herrera et al, 2022).…”

“…This lack of possible observations points in one of the highest vulnerable tsunami hazard region echoes how poorly documented this region is in tsunami-related publications compared to other regions. A literature search show that Tonga totals around 100 tsunami-related publications, half of them published after the 2022 event and almost two orders of magnitude fewer than Japan (Thomas et al, 2023b). GNSS-based ship datasets would offer a unique cost-effective tool to fill gaps in ocean observations (Metcalfe et al 2018), contributing to more research studies and more real-time knowledge of tsunami hazard in this region.…”