Report: TR04/32

Author: R.G. Bell (NIWA), J. Goff (GeoEnvironmental Consultants Ltd), G. Downes (GNS), K. Berryman (GNS), R.A. Walters (NIWA), C. Chagué-Goff (NIWA), P. Barnes (NIWA), I. Wright (NIWA)

Abstract

Tsunami is a series of ocean waves generated by the sudden displacement of large volumes of water and is caused by the vertical movement of the sea as a result of earthquakes or landslides (land-based or submarine), submarine volcanic eruptions or meteor impacts. During the initial development of the Civil Defence and Emergency Management Group (CDEM) Plan, the risk associated with tsunami was considered high enough to allocate a ranking within the regional hazardscape of 1st place for a locally generated tsunami and 6th for a far-field generated tsunami . Prior to this both Environment Waikato and Environment Bay of Plenty recognised that the threat of tsunami may have been greater than traditionally thought, asking NIWA and associates to undertake research on the eastern coastline of the North Island.

The research has been split into three stages:

• Stage 1 included identifying tsunami activity in the previous 5000 years along the eastern coastline of the Bay of Plenty and the Coromandel Peninsula, developing a tsunami magnitude to suit this research, identifying geographical areas where further site study could be undertaken and providing recommendations for additional research. This was undertaken by field investigations of paleo-tsunami deposits from sediment cores and partial laboratory analysis.
• Stage 2 included more detailed research of several key core samples to determine a historical catalogue of tsunami events for the Bay of Plenty and Coromandel eastern coastline, identify hot-spots and providing an overview of the potential for geo-sourced tsunami, both local and remote, and
• Stage 3 will use the information provided during research and collation for Stages 1 and 2 to develop appropriate response, planning and management actions. This may include the development of a management/action plan, inclusion and input for District Plan change submissions, communications and implementation strategies and further investigations on inundation mapping.

The Stage 1 report will be released in early 2005 as part of Environment Waikato’s Technical Report series. This report details research from Stage 2.

A credible tsunami hazard profile for the East coast of the Bay of Plenty and Coromandel has been obtained, using core samples, eye witness accounts, sea level and tsunami run-up data, geophysical marine surveys and modelling. The findings from this research indicate that the eastern coastline can expect a 1m wave height tsunami every 80 years and a 2.5m tsunami wave every 320 years.

For the eastern coastline of the Bay of Plenty and Coromandel there have been 7 tsunami recorded in the past 85 years that have generated a wave height of up to 1m, with 5 events since 1700 generating wave heights of 1 to 3m. The paleo-tsunami record also shows 6 events back as far as 3000 years before present that generated a wave height greater than 5m.

As well as distal tsunami from across the eastern edge of the Pacific Plate, there are local and regional sources of tsunami. These include:

• Earthquakes along the Tonga-Kermadec-Hikurangi plate boundary,
• Undersea volcanism along the Tonga-Kermadec sections of the plate boundary,
• Landslide sources along the Hikurangi section of the plate boundary,
• Regional fault activity through the Taupo Volcanic Zone (TVZ),
• Offshore volcanic sources such as Tuhua (Mayor) Island and Whakaari (White) Island, and
• Pressure waves generated onshore by by volcanic eruptions from within the Taupo Volcanic Zone or Mt. Taranaki.

The areas identified (within the Waikato Region) as having greater risk from the impacts of a local or regional tsunami are:

• The open coast from Otama Beach to Port Charles and out to Great Mercury Island (especially Whangapoua and Port Charles),
• Mercury Bay (especially Whitianga).

Tsunami Hazard for the Bay of Plenty and Eastern Coromandel Peninsula : Stage 2 [PDF, 3.7 MB]