Tsunami Modelling Study 3

Executive Summary

Northland Regional Council contracted NIWA to undertake an initial study on the risk of tsunami inundation facing communities in the Northland Region. The following credible sources were identified:

• Remote source: South American origin. Return period 50-100 years. This represents the most
  probable tsunami risk in the next 100 years.
• Local/Regional source: Tonga Kermadec. Two events were modelled, Mw 8.5 and Mw 9.0. The return period of these events is much longer (500-2000 years) but these represent a worst-case scenario for a tsunami striking the Northland coast.

Tsunami propagation to the Northland coastline including inundation at 15 specific communities for each of the above scenarios was simulated using a computer model. The simulations were performed for current sea levels and for a mean sea level elevated by 50 cm, representing the 100 year projection by the IPCC Fourth Assessment Report.

The remote South American tsunami did not cause significant inundation for most of the settlements studied; however, some relatively minor flooding was predicted to affect properties in the settlements of Hihi and Dargaville/Ruawai. Modelled inundation from the Mw8.5 Tonga-Kermadec subduction zone event was largely similar in extent and degree to the South American tsunami, but was greater in
depth and extent at Bland Bay, Helena Bay and Taupo Bay. The Mw9.0 Tonga-Kermadec event caused major inundation at Bland Bay, Helena Bay, Tauranga Bay, Taupo Bay, Te Ngaire and Hihi. In other locations, the extent of inundation was similar to the Mw8.5 event, though inundation depth was inevitably deeper.

The simulation results presented here cover the likely range of tsunami that might be expected in Northland, increasing from the relatively common but smaller South American tsunami to the largest likely event emanating from the Tonga-Kermadec subduction zone, and therefore provide information on the likely range of impacts on which to base local contingency planning.

In this final part of the three-stage study, the results from the entire project are summarised and recommendations for improving the modelling and reducing the uncertainty in the results are made. Overall, in all three studies, the relatively likely, but smaller South American event caused the least inundation, the Mw8.5 event slightly more, and the Mw9.0 event had much more severe impacts. The
modelling study is dependent on, and therefore limited by, the initialisation of the tsunami for each earthquake source, the quality of the LiDAR topographic data and also the quality of bathymetric data in inshore waters. A lack of knowledge of the effects of buildings and land features on wave drag also adds uncertainty to the simulations. Despite these limitations, we believe that the current modelling
exercise provides the best possible estimate of inundation in Northland from remote and regionally sourced tsunamis available to date.

Download the report

File size: Please note that this report is very large due to the number of maps and graphs. It has been separated into sections to make it easier to download. The Table of Contents .pdf outlines what information is in each section. 

Find out what a tsunami is and what we are doing to prepare for managing one if it does happen: www.nrc.govt.nz/tsunami