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Potential tsunami hazard associated with the Kerepehi Fault, Hauraki Gulf, New Zealand

A review of 'active' faults in the Auckland region identified the Kerepehi Fault as a potential tsunami source (de Lange and Hull, 1994). The Kerepehi Fault trends NNW, transecting the central region of the inner Hauraki Gulf. A tsunamigenic event along the Kerepehi Fault has not occurred during historical times making this hazard difficult to quantify. Consequently, this study assessed the hazard represented by the Kerepehi Fault using numerical simulation, which required that the locations of offshore segments and behaviour of each segment of the Kerepehi Fault be determined. Shallow seismic sub-bottom profiles totalling 135 km in length were analysed and used to locate the submarine extension of the Kerepehi Fault. Without core data, only the relative timing of movement along the fault could be determined. The seismic data clearly showed that movement has occurred since the last observable reflector/unconformity. Onland evidence indicates that this unconformity corresponds to a surface flooding event which occurred at 6.5 ka; suggesting that the submarine extension of the Kerepehi Fault is still active and potentially tsunamigenic. Seismic data indicates the offshore Kerepehi Fault trends NNW up the central Firth of Thames and is divided into four segments by three WSW-ENE trending transverse faults. The displacement that is most likely to occur for any given event ranges from 2.1-7.35 m. Tsunami generation and propagation was modelled using a linear, finite element model, 'TSUNAMI'; a finite difference hydrodynamic circulation model, '3DD'; and an empirical set of parametric equations (Abe, 1995). The results indicate: (i) The greatest risk to Thames township is associated with displacement along an adjacent fault segment, which produces wave heights of the order of 1.8 m. (ii) The largest shoreline wave heights (~4 m) were generated by displacement in deep water and had the most severe impact upon Pakatoa, Ponui, and Rotorua Islands. The maximum mainland wave resulting from displacement along the Kerepehi Fault impacts at Deadmans Point and has a height of the order of 2.8 m. (iii) The overall tsunami hazard associated with this fault is low. Comparison of wave heights generated by the three methods indicated that for shallow water source regions, 'TSUNAMI' generally under predicts maximum wave height. This is suggested to occur because 'TSUNAMI' is unable to cope with the highly nonlinear wave processes that occur in shallow water. The behaviour of tsunami waves within the Firth of Thames was further investigated through the simulation of teletsunami (distantly generated tsunami) propagation into the Hauraki Gulf, which was undertaken using the model '3DD'. Wave height time histories for selected regions and amplitude attenuation graphs produced from '3DD' output indicate: (i) Generally, for wave periods between 10 and 30 minutes, the maximum predicted sea level rise increases as teletsunami wave period becames longer. (ii) The confined nature of the Firth of Thames acts to focus wave energy resulting in elevated wave heights within this embayment. The maximum amplitude reinforcement occurring in the Firth of Thames is equivalent to 50 % of the amplitude at the open ocean boundary and is considered significant. (iii) For townships adjacent to the Firth of Thames, the maximum rise above mean sea level caused by teletsunamis of similar magnitude to the 1960 Chilean tsunami was between 0.36 and 0.49 m. The largest of these, 0.49 m, is observed at Tapu. (iv) A teletsunami of this magnitude, represents a less significant hazard than tsunamis locally generated along the Kerepehi Fault. Wave heights observed during the 1960 Chilean tsunami and those predicted by model '3DD' are in strong agreement. However, the coarse grid employed by '3DD' (1500 m) limits the number of grid cells per wavelength. Consequently wave propagation in shallow water may not be fully represented, particularly for short period waves. Hence, the above results should be regarded with caution. The probable extent of tsunami inundation occurring in the Thames region was investigated using the non-linear finite difference model, 'TUNAMI N2'. The results indicate that should a tsunami of at least moderate amplitude (3 m) be generated in the Firth of Thames, 7 metres of vertical run-up is likely to occur between Tararu and Moanataiari, and land adjacent to the Thames aerodrome will be horizontally inundated by up to 450 m.
Type of thesis
Chick, L. M. (1999). Potential tsunami hazard associated with the Kerepehi Fault, Hauraki Gulf, New Zealand (Thesis, Master of Science (MSc)). University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/8511
University of Waikato
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