Report: TR 2006/49

Author: James Goff, Roy Walters, Fraser Callaghan (NIWA, on behalf of Environment Waikato, Auckland Regional Council, Environment Bay of Plenty, Northland Regional Council)

Abstract

New Zealand sits in a precarious position astride the boundary between the Pacific and Australian Plates. There is a wide range of potential tsunamigenic sources in this area including fault movements, submarine landslides, volcanic activity, and other mechanisms. In addition, considerable palaeotsunami information indicates that large tsunamis have inundated the coastline several times in the past.

NIWA was engaged by the combined councils of Northland Regional Council, Auckland Regional Council, Environment Waikato and Environment Bay of Plenty to undertake a tsunami source investigation. Part of this study has been directed toward using palaeotsunami and some historic data to evaluate possible tsunami sources. Dislocation and submarine landslide models were used to simulate the displacement of the sources, and a finite element numerical model was used to simulate generation and propagation of the resultant tsunami.

The initial aim of the project was to identify the three most significant tsunami sources for the combined region. During the investigation process it was decided to expand this to four. Based upon available data, the identified sources included a distant one from South American, distant/regional tsunamis from the Solomon Sea and New Hebrides areas, a subduction zone event along the Tonga-Kermadec Trench, and selected local sources. The local sources investigated included those from the Bay of Plenty which have been previously published and some poorly-defined ones north and east of the North Island that may explain traces in the palaeotsunami record. The report discusses the general settings for each source and presents results for the areas encompassed by the four regional councils: Northland, Auckland, Environment Waikato, and Environment Bay of Plenty.

Using a combination of an inverse solution (palaeotsunami data guiding source selection) and source modelling, we determined that a subduction zone event along the Tonga-Kermadec Trench represented the most significant tsunami source for the combined region. In some cases, surface water elevations were in excess of 10m. The distant South American, Solomon Sea and New Hebrides source areas generated maximum water surface elevations around 3 m, although there was considerable variability. The eastern coast of the Northland region records the highest elevations from all sources, with a marked signal also noted along the western coastline.

Some of the focus of the project was determined by unusually high runups recorded from events such as the Krakatoa tsunami in 1883. Such northern sources were inferred as being potentially problematic particularly for the north-facing Bay of Plenty and Hauraki Gulf. However, this does not appear to be the case.

High elevations and distinctive depositional records from some sites noted in palaeotsunami evidence, particularly in Northland and eastern Coromandel Peninsula, indicated that these could not have been laid down by any of the scenarios modelled for regional and distant sources. Unknown or poorly studied local sources were probably the cause. We modelled three potential local source scenarios. These scenarios were based upon an examination of all available data including bathymetric data and palaeotsunami records.

Tsunami Source Study [PDF, 971 KB]