Tsunami Hazard in the Firth of Thames

A review of active faults in the Auckland region identified the Kerepehi Fault as a potential tsunami source. A tsunamigenic event along the Kerepehi Fault has not occurred during historical times making this hazard hard 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. 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. Results of modelling indicate that 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. The largest shoreline wave heights (~4 m) were generated by displacement in deep water and had the most severe impact upon Pakotoa, Ponui, and Rotorua Islands. The maximum mainland wave impacts at Deadmans Point and has a height of the order of 2.8 m. The overall tsunami hazard associated with this fault is low. Modelling was also undertaken of a distantly generated tsunami proagating into the Hauraki Gulf. The confined nature of the Firth of Themes acts to focus wave energy resulting in elevated wave heights within this embayment. For townships adjacent to the Firth of Themes, the maximum rise above mean sea level casued by teletsunamis of similar magnitude to the 1969 Chilean tsunami was between 0.36 and 0.49 m, with the largest height observed at Tapu. The probable extent of tsunami inundation occurring in the Thames region was investigated using the non-linear finite difference model, 'TSUNAMI N2'. The results indicate that should a tsunami of at least moderate amplitude (3 m) be generated in the Firth of Thames, 7 m of vertical run up is likely to occur between Tararu and Moanataiari, and land adjacent the the Thames aerodrome will be horizontally inundated by up to 450 m.

The study included a review of the regional tectonic setting, and Holocene sea level fluctuations. Analysis of shallow seismic sub-bottom profiles to locate the submarine extension of the Kerepehi Fault. modelling of tsunami generation and propagation from fault displacement using TSUNAMI, 3DD and empirical equations. Modelling of teletsunami propagation using 3DD. Modelling of tsunami inundation in the Thames Region using TSUNAMI N2.

Firth of Thames and Hauraki Gulf

Seismic data collected between 1973 and 1995 Modelling undertaken in 1998 and 1999.

Details of seismic data collection given in thesis. Details of models given in thesis, also see data content and abstract above. Frequency of collection: Irregular. One off modelling exercise.

Gaps in collection: n/a Attribute accuracy: Detailed in thesis.

Raw data available on CD in back of thesis. MS Word format tables and guide to data. Modelling files also included in respective formats.

Assessment of tsunami risk in the Thames area for Auckland Regional Council and Civil Defence planning purposes.

Freely available.

Study completed.