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The Amplification of Tsunamis by Mercury Bay, New Zealand

The east coast of New Zealand is exposed to tsunami hazards, which are generated by both distant and near tsunamigenic sources. The impact of tsunami varies along the coast depending on the source region and the amount of local attenuation or amplification. Some regions have consistently amplified historic tsunami, including Mercury Bay on the east coast of the Coromandel Peninsula. Whitianga, located in the Mercury Bay, is now the fastest growing population centre in the Coromandel Region; hence, tsunami hazard is of particular concern. The natural resonance periods (eigen periods) of Mercury Bay are determined by its geometry and depth, and tsunami waves that enter the Bay will be amplified when their frequencies match the resonant frequencies of the Bay. The combination of tide levels with the amplified tsunami waves may lead to a destruction of the moored vessels and many coastal facilities. To date, amplification within the bay has mostly increased the trough depth, while having little effect on the crest height. This study assesses the tsunami hazard in Mercury Bay in response to a tsunami generated along the Kermadec subduction margin (Kermadec Trench). Even though historically, the Kermadec Trench has never produced a hazardous tsunami affecting the eastern coast of New Zealand, it is still important to develop an assessment of the worst scenarios of earthquake generated tsunamis from this source. In particular, the Sumatra 2004 and Tohuku 2011 tsunami events have suggested that a magnitude Mw 9 to 9.5 subduction megathrust earthquake is a plausible scenario. Merian’s formula was used to obtain the natural resonant period of Mercury Bay and 17 scenarios of tsunamigenic earthquakes were simulated using the tsunami model COMCOT version 1.7. Those scenarios included the recent various combination of the Kermadec earthquake, and hypothetical earthquakes that rupture the northern, middle, southern parts of Kermadec Trench. The results demonstrate that most of the initial tsunami waves generated by Kermadec Trench earthquakes are negative waves that arrive in the Bay within 56 to 158 minutes of the earthquake. This may explain the observed historical pattern of enhanced amplification of the troughs. However, significant amplification of the crests, producing waves that would threaten Whitianga Township, are generated by Kermadec earthquakes with magnitudes greater than Mw 8.5. From the spectral analysis of each tsunami model, Mercury Bay showed a consistent response of about 52 minutes dominant period. This period is also shown as the period of the Mercury Bay when tsunami is absent. In response to distant tsunamis, it seems that the geometry of the Mercury Bay control their periods as they enter the bay. Both the 2011 Tohoku and 2010 Chilean Events have the dominant periods close to the Mercury Bay period at 47 and 51 minutes respectively.
Type of thesis
Ramli, A. Y. (2012). The Amplification of Tsunamis by Mercury Bay, New Zealand (Thesis, Master of Philosophy (MPhil)). University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/6634
University of Waikato
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