Roger, J.H.M.; Wang, X. 2023 Impact of tsunami in Porirua Harbour : focus on Mana Marina. GNS Science report 2023/31. 62 p.; doi: 10.21420/WETZ-0X92
Abstract
New Zealand is naturally exposed to tsunami due to its location sitting over a subduction zone of the Pacific Ring of Fire. Many tsunami have been reported around the country and, more recently, recorded by a dense array of coastal gauges and DART stations. In this study, the objective was to analyse in detail the effects of tsunami waves in Mana Marina, Porirua Harbour using an up-to-date digital elevation model (DEM) and a set of historical tsunami events and maximum plausible crustal fault and subduction zone scenarios. The first phase of this modelling study consists of validating the numerical model through the comparison of simulated waveforms with records at Mana Marina tide gauge in past events. For this purpose, six past events have been selected, including the tsunami associated with, respectively, the 2011 Mw 9.1 Tohoku-oki earthquake (Japan), the 2015 Mw 8.3–8.4 Illapel earthquake (Chile), the 2016 Mw7.8 Kaikoura earthquake (New Zealand), the 2021 Mw7.7 Matthew Island earthquake (Vanuatu), the 2021 Mw8.1 Raoul Island earthquake (Kermadec, New Zealand), and the 2022 Hunga Tonga–Hunga Ha‘apai (HTHH) volcanic eruption (Tonga). The second phase consists of detailed simulations of tsunami effects triggered by potential credible earthquake sources (local and regional) and analyses of the resulting wave amplitudes, current speeds, and tsunami directionalities. Results show that Mana Marina is vulnerable to tsunami, even though it is apparently geographically protected due to its location on the southwest coast of the New Zealand North Island. According to the simulation results, a Mw 9.3 Kermadec earthquake could cause a tsunami with maximum amplitude waves of 2–3 m, as well as current speed of up to 10 m/s in the marina. Even for very small tsunami in the marina, strong tsunami currents of more than 1.5 m/s could still be of concern for the marina assets and navigation. During the Kaikoura (2016) and the HTHH (2022) tsunami, the maximum tsunami amplitudes were only about 20 cm and 50 cm, respectively; tsunami currents could reach up to 4 m/s, as shown by the modelling. Interestingly, all the modelled scenarios show persistent, unidirectional tsunami currents in the middle of the marina, causing distinct circulations in both the northern half and the southern half of the marina.
