Ocean-flank collapse on the south of Taʾu, Manuʾa Group, Samoa Islands: implications for risk management
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
Ocean-island flank collapses are amongst the most dangerous of all landslide related hazards in the world, as they have the potential to trigger ocean-wide tsunamis that can cause damage and loss of life to communities thousands of kilometres from their source of origin. The implications for landslide-induced tsunami originating from high volcanic islands in the Pacific are serious; and consequent hazards to life, infrastructure, and emergency management need to be constantly reviewed, monitored, and investigated. Ta’u, the easternmost inhabited island in the Samoa Islands volcanic chain, exhibits a series of down-faulted benches on its southern flank; believed to be the remnant of catastrophic collapse involving ~30km³. An historical map of Ta’u, charted during the first United States exploring expedition into the Pacific Ocean (Charles Wilkes Expedition), suggests that the event was recent; having occurred less than 170 years ago. A collapse event of this magnitude would have generated a locally devastating tsunami, with possible impacts experienced at the regional level. However, there exists no written or oral record of such an event. It appears that half the island, involving an estimated 30km³, disappeared off the map less than 170 years ago without anybody noticing it. A number of key questions thus emerged. Did this event actually happen within the last 170 years, and if so, how and why could it have gone unnoticed? Is the event much older than the impression obtained from the literature? More importantly, what is the likelihood of a future collapse and subsequent tsunami, and what would the hazard impacts be at the local and regional levels? These questions formed the research basis for this thesis. Specific aims were developed to address the issues identified, and a range of inter-disciplinary scientific techniques using innovative methods and new datasets were implemented to achieve them. The results demonstrate that the collapse most likely occurred more than 170 years ago, raising serious debate on the accuracy of observations made during the Charles Wilkes Expedition. The results also show that the eruptive-hazard at the site exists. Given that the nature and frequency of active volcanism in the area is uncertain, the risk of a future collapse and subsequent tsunami in the medium-term is considered high. The inter-disciplinary approach to landslide-tsunami hazard investigation on an oceanic island presented in this thesis, can be developed and applied by disaster managers to similar hazard investigations on other oceanic islands. Ultimately, the increase in knowledge-base can be used as a tool for developing safer and more resilient coastal communities.