Tsunami report indicates "worst case" scenario for Christchurch, Kaiapoi

Environment Canterbury today released a report indicating a possible “worst case” scenario for Christchurch and Kaiapoi in the unlikely event that a tsunami arrives on our shores following a large earthquake in a distant location.

The National Institute of Water and Atmospheric Research has modelled the possible effects of “distant-source tsunami inundation” (flooding) in the areas most affected by the February 2011 earthquake.

Environment Canterbury Commissioner Donald Couch says NIWA had been asked to produce the report to make sure the communities and agencies concerned have the most up-to-date information possible for distant-source tsunami evacuation planning.

“It was important to get this information now because the earthquakes have caused the ground level to go down in some areas,” Mr Couch said.

The modelled inundation is based on a “worst case” scenario – a tsunami from the Peru (South American) coastline, with the largest tsunami surge arriving at high tide.

“Although this modelling is based on the 1868 tsunami, scientists believe that the earthquake that generated this tsunami was exceptional,” Mr Couch said. “At magnitude 9.0 to 9.1, it was the biggest earthquake ever recorded. In addition, the location of the epicentre and orientation of the fault on which the earthquake occurred meant a very large tsunami was generated, and that it was directed straight towards the South Island of New Zealand.

“The likelihood of all these factors occurring together in the same way is very low, so the level of tsunami inundation represented by the modelling only happens every few thousand years. In the vast majority of tsunamis that may affect us, the inundation will not reach the levels shown on the maps in the report.”

Mr Couch pointed out that the report does not cover possible inundation from a local source tsunami from Pegasus Bay. “Potential flooding from a tsunami just off the Christchurch or Waimakariri coast is likely to be much less than a worst case distant source tsunami from across the Pacific Ocean,” he said.

NIWA modelled other coastal areas of Canterbury for potential distant source inundation in 2010, including Waikuku and Woodend beaches and Lyttelton and Akaroa harbours.

Mr Couch said these areas were not remodelled after the February earthquake because the earthquake did not significantly change ground levels there. “Some parts of the Canterbury coast, such as the northern Hurunui, Kaikoura and Banks Peninsula bays have not been modelled at all because we do not yet have accurate enough topographic data.”

Evacuation planners in the areas covered by the report are considering its findings and possible responses. The models in the report are not designed to be used for land use or infrastructure planning.

For the NIWA report and related material, go to www.ecan.govt.nz/tsunami