Indonesia tsunami: New Zealand vulnerable to similar event, scientist says

New Zealand could face a similar event to the tsunami that struck Indonesia on Saturday night (local time), a scientist says. 

The general consensus is that the tsunami in Indonesia was triggered by an underwater landslide from volcanic eruptions. It came without warning, killing at least 222 people as of Monday morning (NZ time). 

While there is still some uncertainty about the cause of the tsunami, GNS seismologist Sam Taylor-Offord says it was likely caused by Anak Krakatau, an active volcano roughly halfway between Java and Sumatra, which has been spewing ash and lava for months. 

It's possible the two are unrelated but the consensus coming from scientists around the world, he said, is the most likely source being an underwater landslide caused by the volcano.  

"Part of that volcano has collapsed under the ocean and caused that tsunami to occur."

When a volcano erupts underwater, material moves down a slope and pushes water down which moves sideways into more water and generates a wave that propagates outward in all directions, Mr Taylor-Offord explained. 

"The energy builds up and collects together and it forms that damaging tsunami."

New Zealand is at risk, he says, because of similar circumstances around the 2016 Kaikōura earthquake. The magnitude 7.8 quake caused a massive underwater landslide. It moved down the deep canyon system that lies just offshore. 

There are about 100 ocean canyons around New Zealand, and although the Kaikōura Canyon is special, scientists say it's just part of a much larger, very active canyon complex on the north east coast of the South Island.

"There is potential anywhere you have a body of water with slopes on either side underwater for this kind of thing to happen," Mr Taylor-Offord said, adding that it's "not necessarily a volcanic phenomenon". 

"It's more the idea that you have soft material that's less stable which could slide. Yes, this could happen with a volcano, but it could also happen on the side of a lake, or a continental shelf off the coast of New Zealand.

"New Zealand is an incredibly active environment. We really have almost everything going on in terms of geological hazards."

Mr Taylor-Offord said GNS has been working since the Kaikōura quake to build a 24/7 monitoring centre which went live a few days ago. He said the centre is staffed around clock, monitoring data feeds from New Zealand and globally for hazards. 

He said the challenge in monitoring an event like the Indonesia tsunami is that it didn't have a clear precursor. 

"In a lot of cases tsunamis are formed by large earthquakes and in that kind of case we can observe the earthquake using seismic instruments with generally enough time to know whether or not a tsunami will be happening."

Coastal residents reported not seeing or feeling any warning signs, such as receding water or an earthquake, before waves of 2-3 metres washed ashore, according to media.

"In this case, the event has happened quite close to populated areas and hasn't given us any precursor signals so the only way to monitor this ahead of it making contact is through an in-ocean instrument looking for a tsunami."

The timing of the Indonesia tsunami, over the Christmas holiday season, has evoked memories of the Indian Ocean tsunami triggered by an earthquake on December 26 in 2004, which killed 226,000 people in 14 countries.