A tsunami is created when a large volume of water is disturbed suddenly. This can be caused by undersea earthquakes, large seafloor landslides, volcanic eruptions, large coastal or lakeside landslides and very occasionally by meteorites. The disturbance causes waves to radiate out from the event source. These waves can travel at speeds of around 500 kph and have significant wave lengths (the distance from crest to crest).
In deep water the height of a tsunami can be less than 50 cm but as it reaches shallow water (such as a continental shelf or near the coast) it is lifted up by the seafloor causing it to slow down. The tsunami can rapidly become a large wave as the back of the wave continues to press forward - building up the wave on the coast.
The damage from tsunamis can occur as a result of inundation (flooding roads, buildings and land), the impact of the moving water (erosion, structural damage) and debris impacts (debris carried by the wave moving inland and receding).
Nature of the Tsunami Threat
Tsunamis have not generally been perceived as a particular threat to the Bay of Plenty. Eleven tsunamis are recorded in the historical record (since 1840) - all of less than three metres. However, recent work indicates that tsunami may be more of a threat than previously thought. In the past 4000 years two major regional and four localised paleo-tsunami events have been recorded - all equal to or greater than the five metre resolution level for detection in the paleo record.
The distance from the coast is clearly a key attribute in defining responses and determines whether a warning system will provide assistance. Sources for tsunamis can be classified by their distance from the areas that may be impacted:
- Local - for example, eruption of Mayor Island or White Island, or fault movement within the offshore Taupo Volcanic Zone
- Regional - for example, volcanism in the Tonga/Kermadec system or landslide in the Hikurangi Trough
- Distant - for example, South American earthquake.
Within the Bay of Plenty Civil Defence Emergency Management Group, Bay of Plenty Regional Council will be working with other local authorities and emergency services to:
- Further understand the threat from tsunamis so that the risk to communities and infrastructure is minimised
- Prepare communities for tsunami events so that there is the ability to respond effectively in an emergency.
Tsunami Inundation Maps
The Bay of Plenty CDEM Group has produced indicative tsunami inundation maps for tsunami of specific heights at the mean high water mark. One of the primary aims of the maps is to inform communities on the tsunami hazard and to generate interest in developing Community Response Plans. Contact your local council Civil Defence Emergency Management Officer to enquire further.
Tsunami Research Project
In June 2002 a project was initiated under Bay of Plenty Regional Council's regional civil defence function to look at the threat posed by the tsunami hazard to the Bay of Plenty. The project was undertaken on a shared-cost basis with Environment Waikato who wished to investigate the tsunami threat to the eastern Coromandel, and was divided into three stages.
Stage one involved the identification of prehistoric tsunami activity. This stage was designed to identify whether there was a significant threat to the Bay of Plenty and Coromandel Peninsula beyond what is historically known. It involved original research into the paleo record to look for tsunami signatures in sediment cores.
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Stage two involvd an analysis of the nature of the threat from tsunamis. This involved linking the historical record and findings from the paleo-tsunami research from Stage One. This was designed to provide an understanding of the threat and tsunamigenic origins faced in the Bay of Plenty and Coromandel Peninsula from tsunami.
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For stage three, Bay of Plenty Regional Council contracted NIWA to undertake a tsunami study for the Wairakei/Te Tumu development area. The focus of this study was to determine the potential tsunami inundation hazard for this greenfield development site.
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