Tonga eruption: Scientists still studying tsunami impact in Northland

Friday, 21 January 2022

Tsunami activity from Tonga left many boats underwater at Tūtūkākā Marina.

Scientists are interested in what tsunami impacts were experienced in New Zealand from the Tongan eruption, after acknowledging they do not know a lot about tsunamis caused by volcanic eruptions.

The eruption of the Hunga-Tonga-Hunga-Ha’apai volcano on Saturday sent literal shock waves around the world and was the most explosive eruption in 30 years.

Within minutes, it resulted in a devastating tsunami in Tonga, followed by tsunami surges in New Zealand, and as far away as Japan, South America – where two women reportedly died in Peru – and even in the Caribbean.

The boats damaged at Tutukaka Marina by the tsunami include Tim Alexander’s boat Justice, which will be rebuilt with insurance money, he says.

Niwa tsunami scientist Dr Emily Lane said it was surprising to see the waves travel so far from the volcanic eruption.

**READ MORE:

This satellite image, taken by Japanese weather satellite Himawari-8, shows the undersea volcano eruption in Tonga on Saturday.

* Tongan eruption tsunami felt in unusual waves at Port Taranaki

* Cyclone Cody made predicting Tonga eruption tsunami spread harder

* Helicopter used to rescue child from remote Northland harbour

As 95 per cent of tsunamis were caused by earthquakes, scientists were set up with seismic monitors around the world, but tsunamis caused by volcanoes were a lot less predictable, she told Stuff.

There are a range of things that could have happened to cause the tsunami, and Lane said it was likely the shock waves, which were heard around the world, created atmospheric pressure changes that reinforced the waves with extra energy.

The volcanic eruption in Tonga was one of the most explosive in 30 years and sent literal shock waves around the world.

In New Zealand, the tsunami caused millions of dollars in damage at Tutukaka Marina, where eight to 10 boats sunk and structures were damaged.

Some structures, including oyster farms, were also damaged at Whangaroa Harbour.

But the waves were also experienced on the west coast of Northland, where a group fishing for flounder in Hokianga Harbour had to run for their lives as water surged up around them.

The red sailing catamaran Amphitreete, left, was picked up by the tsunami surge at Tutukaka Marina and thrown into Justice, Tim Alexander says.

Lili Mullane told Stuff the water suddenly went out 50m then came rushing around them to about 1m high, inundating the shore.

The group managed to escape by running, then driving, through the water, driving onto private farms to get away.

Niwa's climate stations recorded a brief spike in air pressure as the atmospheric shock wave from Tonga's volcanic eruption pulsed across New Zealand.

Lane said information about such experiences will add to scientists’ knowledge about tsunamis caused by volcanoes, and she encouraged people to share what they saw.

“It’s really tricky because we didn’t know what to expect from this.”

A colleague visited Tutukaka, where no inundation was observed, she said.

The marina was impacted because of the shape of the bay which created a resonance in the tsunami waves, and the impact of Cyclone Cody, which also caused surges, Lane said.

More warning should have been given – Tutukaka resident says

Tutukaka resident Tim Alexander, whose boat Justice was badly damaged in the tsunami, said he was extremely disappointed in the lack of warning from Civil Defence.

A national advisory was issued at 8.15pm on Saturday to warn of strong and unusual currents and unpredictable surges, but there was no activation of the tsunami sirens nor phone notification.

The tsunami sirens, which warn people to seek more information, were most recently activated on March 5, 2021, in response to earthquakes in the Kermadec Islands.

But the sirens are used to warn of land evacuations, not threats to the water, and there was no need for a widespread evacuation for people on land, Northland Civil Defence Emergency Management spokesman Murray Soljak earlier told Stuff.

But Alexander said it was “extremely lucky” no one died, with many people who live aboard their boats not being aware of the advisory.

“The reality of the situation is that we had a 1.5–2m wave come into Tutukaka Harbour and then the marina. This was what I saw on the CCTV, and what many eyewitnesses saw when it occurred.”

The force of the wave was shown by the damage to boats, with red sailing catamaran Amphitreete being picked up by the wave and thrown onto Alexander's boat’s flybridge.

Lane reinforced the idea that the sirens are for warning people of a land evacuation, but admitted the volcanic tsunami was difficult to predict.

She encouraged anyone with information about the tsunami surges in New Zealand, especially land inundation, to email her on Emily.Lane@niwa.co.nz

Meanwhile, local business Dive! Tutukaka will be operating again from Saturday, after its boats were relatively unscathed from the tsunami.

Owner Jeroen Jongejans said the marina was closed, but the marina trust was being very accommodating by giving the business a few timeslots to get in and out, so it can operate its “chocka block” sailings.