Tsunami generation by island edifice failure at White Island and Motuhora Volcanos, New Zealand W. P. de Lange A. Hansford V.G. Moon Department of Earth Sciences, University of Waikato, Private Bag 3105, Hamilton. Abstract There are 28 active offshore subaerial and submarine volcanoes within 1000 km of the Bay of Plenty coast, and many of these volcanoes experience repeated cycles of cone growth and edifice failure. Edifice failure involving debris avalanches interacting with the ocean can result in the generation of tsunami. White Island has undergone cycles of cone growth and edifice failure (Moon et al. in this volume). Using geotechnical modelling, debris avalanche characteristics were estimated for a ~0.24 km 3 collapse forming the horseshoe-shaped crater. Side-scan sonar and multi-beam imagery has identified ~0.23 km 3 of submarine debris avalanche deposit downslope of the crater. Motuhora consists of 3 domes, one of which has collapsed catastrophically depositing ~0.05 km 3 of debris in shallow water near the coast. Tsunamis were simulated for both sources, and predict wave heights of 2-10 m at the coast, with maxima at Matata. White Island tsunami arrived at the coast 35±3 minutes after generation. The wave heights are sufficiently large to account for palaeotsunami deposits at Waihi Beach, Ohiwa Harbour and Waiotahi Estuary. It is suggested that an earthquake in 2200-2500 BP caused subsidence within Ohiwa Harbour, island edifice failure at White Island and a tsunami that affected the Bay of Plenty. Introduction Within New Zealand’s offshore region there are ~800 “seamounts” (Rowden et al., 2005), including 28 active subaerial and submarine volcanoes within 1000 km of the Bay of Plenty (Wright et al., in press). Subaerial offshore volcanoes have been long recognised as potential sources of catastrophic tsunami due to a variety of processes including (Latter, 1981): • Emplacement of pyroclastic materials, such as occurred during the eruption of Krakatau in 1883 (de Lange et al., 2001; Simkin and Fiske, 1983); • Earthquake activity and collapse in reponse to the withdrawal of magma, such as occurred during the 1975 Kalapana earthquake, Hawaii (Cox and Morgan, 1984; Day et al., 2005; Owen and Bürgmann, in press); • Large scale collapse of the volcanic edifice as occurred during the Mt St Helens Eruption in 1980 (Keating and McGuire, 2000; 2004); Within the Bay of Plenty there are three active or recently active subaerial volcanoes (Figure 1): White Island (Whakaari); Mayor Island (Tahua); and Motuhora (Whale Island). In the early 1980s it was suggested that White Island (Weir and White, 1982) and Mayor Island (Buck, 1985) were both capable of generating large tsunami similar to those associated with the Krakatau Eruption in 1883. Subsequent modelling indicated that the greatest hazard was associated with eruptions that resulted in pyroclastic flows (de Lange and Healy, 1986; de Lange et al., 2001). However, tsunami generation by island edifice failure was not considered, and since similar volumes to pyroclastic flows could be involved, it was necessary to evaluate the potential hazard posed by island edifice failure. This meant that Motuhora, the closest volcano to the coast, could also represent a hazard, even though activity is now limited to hydrothermal venting and the volcano could be considered inactive (Burt et al., 1996;