Sirens for 'distant' tsunami

20:11, May 14 2012

Christchurch's tsunami warning system will only be used to alert residents of a "distant" tsunami triggered by a major offshore earthquake.

The Christchurch City Council is installing 22 tsunami sirens on the coastline between Waimairi Beach and Sumner this month at a cost of $550,000.

Council, Environment Canterbury and GNS Science are holding community meetings in coastline suburbs to answer questions about the system, due to be operational by July 1.

CCC civil defence emergency manager Murray Sinclair said the system would only be used for distant tsunami – those that would take more than three hours to reach our shores . "If those sirens go off it means get out of the area. For the regional source, where we've got maybe an hour and a half to two hours, we can't guarantee we can get them activated."

Tsunami fell into three categories: "local" tsunami which would reach the coast within an hour of an offshore earthquake, "regional" which would take between one and three hours, and "distant" .

Tsunami triggered by quakes on major faultlines in central or South America, where the tsunami alert system would be most effective, could take 12 to 15 hours to reach New Zealand coastline.


The most likely source of a regional tsunami for Christchurch was the Hikurangi subduction zone, off the east coast of the North Island, which would take 1.5 to two hours to strike.

GNS Science natural hazards scientist Graham Leonard said the chances of a tsunami inundating Christchurch were remote.

"This is a very low risk. If [an earthquake] was offshore it would be very unlikely to generate any tsunami. If it did generate any tsunami it would be very unlikely that that tsunami was big enough to reach any houses.

Worst-case scenario modelling recommended evacuation to four metres above sea level or within 300-400m of the coast.

The second storey of a modern building would also suffice, Leonard said.

ECan hazard analyst Helen Grant said the earthquakes themselves would be the warnings for local and regional tsunami.

"Two to three hours is not a very long time to be able to give an official warning. For this kind of scenario, do not wait for the sirens."

The Press