10 December 2014

MEDIA RELEASE

Updated modelling confirms previous thoughts on Tsunamis

Tsunami surges over 2.5 metres high in places could inundate low-lying areas along the Canterbury coastline, particularly river mouths and estuaries, in the event of a large earthquake off the South American coast, latest modelling shows. But the tsunami would take at least 14 hours to arrive, allowing time to evacuate.

Environment Canterbury has today published a report[1], prepared by NIWA, on the likely effects of a magnitude 9.485 earthquake off the coast of Peru – a one-in-2500 year “worst-case” event. The report updates previous reports that were based on a one-in-500 year event of a magnitude 9.1 earthquake in a similar location.. City and district councils along the coast and the Canterbury Civil Defence Emergency Management Group have been involved in the study.

“The likely areas of flooding are very similar to the earlier reports,” says Environment Canterbury Commissioner Donald Couch. “The updated modelling confirms, rather than changes, evacuation zones along the coast.”

“The worst tsunami flooding would be along the Pegasus Bay coast, including Waikuku Beach, Kairaki, the Pines, Brooklands, Christchurch’s beach suburbs, the Avon Heathcote Estuary and Lyttelton Harbour. There would also be some flooding in the Kaikoura district, Motunau, Akaroa Harbour, Washdyke, Timaru Port and at the major river mouths in the region.

Mr Couch says the South American tsunami would take between 14 and 15 hours to arrive, but the second surge would be the largest and would take 17 to 20 hours to arrive. Significant surges would still be arriving 21 to 25 hours later, gradually decreasing in size. The waves will arrive at the rate of five metres a second in the worst-inundated places such as the Estuary and other river mouths, and the deepest inundation will be 2.5 metres along low-lying parts of the Pegasus Bay coastline and South Canterbury coast.

The modelling assumes, like the previous reports, the largest tsunami surge arrives at high tide, which is a worst-case scenario. It is likely that at least one surge would arrive around high tide, since the largest surges would be spread over several hours.

“This information is very useful for our civil defence emergency preparedness and planning, where we use worst-case scenarios,” Mr Couch says. “We know we’ve got a few hours to evacuate the low-lying coast areas in this situation, we know how big the waves are likely to be and how deep the flooding could be, and we can now prepare for that with more certainty.”

All affected city and district councils will be supplied with the new information, and it will be publicly available at www.ecan.govt.nz/tsunamiand www.canterburymaps.co.nz.

The Canterbury coast, particularly North Canterbury, is also vulnerable to tsunamis created closer to our shores for which we have less warning time, and Environment Canterbury is continuing its studies into these local tsunamis. “This new information doesn’t change our usual message,” Mr Couch says. “If you are at the coast and feel an earthquake that is so strong you can’t stand up, or that goes on for more than a minute, move to higher ground or as far inland as you can straight away.”

[1] “Updated inundation modelling in Canterbury from a South American tsunami” report prepared by NIWA for Environment Canterbury.

© Scoop Media