Tsunami could strike New Zealand before public officially alerted - scientists

Experts warn that if a tsunami strikes New Zealand, there might not be enough time to issue an official warning.

The Ministry of Civil Defence and Emergency Management sends out warnings via the media and through its Emergency Mobile Alert service - while some coastal communities have sirens to alert the public to head to higher ground.

Last weekend, Auckland tested its tsunami sirens as part of its disaster management preparations, blaring a series of tones across the Rodney and Waitakere ward areas.

But if a local tsunami struck, GNS Science senior tsunami scientist Dr William Power says it could hit so quickly there wouldn't be time to activate these emergency measures.

"In the case of locally-generated tsunamis, it is unlikely that official warnings will be issued before damaging waves arrive. So people in coastal areas should take immediate action," he told Newshub.

"If you feel an earthquake that makes it hard to stand, or lasts more than a minute - move immediately to higher ground or as far inland as possible. Remember: LONG OR STRONG, GET GONE."

And the threat of a locally-generated tsunami is real.

Last week there was a swarm of earthquakes in the Hikurangi subduction zone - the massive fault which lies off the East Coast of the North Island.

While these swarms are common, this one was associated with one of the larger "slow-slips" ever observed in New Zealand.

And experts warn if the zone ruptures, it could potentially cause a magnitude 8 or greater earthquake and tsunami.

"We know a large earthquake and tsunami is something we will face in our lifetime, or that of our children and grandchildren," according to the East Coast LAB's (Life at the Boundary) Hikurangi Response Plan Project.

"The reality is it isn't a matter of if, but when."

Dr Power says every part of the New Zealand coastline could potentially be affected by a tsunami.

"Tsunami hazard for the Pacific is higher than for other oceans because of the 'Ring of Fire' - the zone of earthquakes associated with the tectonic plate boundary that bounds the Pacific," he told Newshub.

"In general terms, the east-facing coasts of the North Island and the southwest corner of the South Island have higher tsunami hazard than the rest of New Zealand as these areas are more exposed to local subduction zones."

Modelling by GNS Science indicates there's a 1 in 300 chance per year of a tsunami 3m or more affecting the Auckland region, most likely to be caused by an undersea earthquake or an undersea or coastal landslide.

Regardless of the level of risk, scientists say people in the region should be aware of the risks and know how to respond to a tsunami threat.

"Around the world, when a tsunami has hit a coastal city, we have seen a range of impacts," Dr Power says.

"This includes damage to ports and structures on and close to the coast. Beaches, bays, estuaries, and river mouths can be severely affected.

"All of these impacts might be expected in Auckland if a tsunami hits there."

The last significant tsunami to hit New Zealand was in 1960, caused by a magnitude 9.5 quake off the coast of Chile.

It caused severe fluctuations in water levels along much of the country's east coast for several days, and damaged harbours, bridges, and coastal buildings.

To find out more about how to prepare for a tsunami or other emergencies, visit happens.nz.

Explainer - for the 1 in 300 figure for the Auckland region referred to above:

Each section of coastline has a different level of tsunami hazard based on a number of factors including the direction it faces and the likelihood of an offshore earthquake.

Scientists include all these factors when they model the probability of a tsunami of a given height reaching a particular stretch of coastline.