Natural Hazards Update - No.7 February 2006

Auckland tsunami hazard assessed

 

Modelling of a tsunami generated by a subduction earthquake along the Tonga–Kermadec Trench–Hikurangi Trough system.

 

 

The arrows on this image of Village 1, Ranong Province, show the direction of the Indian Ocean tsunami.

 

The Auckland region may experience a large tsunami about once every 900 years on average. A large tsunami is one greater than 5 metres.

NIWA scientists have been assessing the risk posed by tsunamis for the Auckland Regional Council.

The report identifies both local and distant tsunami sources. Locally, we don’t believe sources within the Hauraki Gulf would produce sizeable tsunamis. Outside the gulf there are numerous sources, but there’s no historical record of any producing a large tsunami. Preliminary modelling suggests that a subduction earthquake along the Tonga–Kermadec Trench–Hikurangi Trough system, east and north-east of New Zealand, could be one of the most damaging sources. A tsunami generated by a subduction earthquake could produce waves up to 4 metres and affect 30–100 km of the coast, with the first waves hitting Great Barrier Island about 70 minutes after the quake.

Chile has long been recognised as the most common distant source of tsunamis affecting Auckland, and waves from there would take about 15 hours or so to arrive. There’s considerable evidence that during the 15th century a tsunami reached as far as 14 metres above sea level as it ran up and over the land. This probably originated closer to home, or perhaps in the Solomon Sea or Fiji Basin.

Helping Thai people build 'sustainable livelihoods'

NIWA scientist Dr James Goff is part of a two-year USAID project on ‘Post-tsunami Sustainable Coastal Livelihoods’ in southern Thailand.

The programme aims to ensure the future livelihoods of rural communities affected by the Indian Ocean tsunami. Dr Goff is working on tsunami preparation, education, and mitigation for communities in the Ranong Province of Thailand. This involves reviewing recent, historical, and potential future disaster impacts and vulnerability in order to assess hazards, to provide warning, evacuation and mitigation guidance, to develop model ‘risk awareness’ educational and training products, and to help embed disaster planning and management to cover all hazards. This work is conducted in close cooperation and consultation with the local government and communities. Other agencies involved in the project include the Asian Disaster Prevention Center, Asian Institute of Technology, and the University of Rhode Island.

High-tech search for tsunami evidence in Otago

 

Evidence of past tsunami inundation from the North Island of New Zealand. (Photo: Dr Scott Nichol, School of Geography and Environmental Science, University of Auckland).

 

 

LIDAR image of the coast south-east of the Avon-Heathcote Estuary (Ihutai) from Taylors Mistake in the east to Moncks Bay in the west. (Image provided by the Christchurch City Council).

 

  

NIWA is trialling the use of LIDAR images to identify evidence of past tsunami along the entire Otago coast.

LIDAR (‘light detection and ranging’) is also known as aerial laser scanning. A laser is mounted under an aircraft and scans several hundred metres across the flight path, allowing large tracts of land, riverbeds, or beaches to be mapped to high accuracy. Large tsunamis leave a marked, yet subtle, imprint on the landscape, and LIDAR data provide the kind of high resolution data necessary to ‘read’ the evidence. The data are in digital form, making them suitable for use in computer modelling and for detailed investigations of the earth’s surface.

Evidence of pre-historic tsunamis found in Christchurch

In a study for the Christchurch City Council, NIWA has uncovered evidence of pre-historic tsunamis in the Avon-Heathcote area. So far we have identified four tsunamis larger than 5 metres within the estuary over the past 4000 years. Preliminary geological and archaeological evidence indicates that the last large tsunami, about 500 years ago, disrupted coastal settlement patterns. The city council has now commissioned a more detailed tsunami investigation.

Regional councils join forces on tsunami research

Environment Waikato, Environment Bay of Plenty, Northland Regional Council, and the Auckland Regional Council are jointly funding NIWA to identify the sources which are most likely to produce damaging tsunami for their combined coastlines. The main aim of the study is to produce tsunami source data for input into inundation models that can be run for the individual regional councils concerned. The work is due to be completed before mid 2006.

The three key sources which we believe could be the most significant for the combined regions are:

• Regional Eastern source:
  Tonga–Kermadec Trench.
• Regional/Distant Northern source:
  Fiji/New Hebrides Trench.
• Distant Eastern source:
  South America (Chile).