20 Jan 2022

Tonga eruption: Waves up to 15 metres possible - Scientist

8:59 am on 20 January 2022

An expert in natural disasters says it's absolutely possible Tonga's volcanic eruption could have sparked tsunami waves reaching 15-metres high at their peak.

Press conference in the bunker at Parliament. Bill Fry from GNS

GNS principal scientist Bill Fry. Photo: RNZ / Rebekah Parsons-King

Five days on from one of the largest volcanic eruptions in the past 30 years, more details are emerging about the destructive tsunami triggered by Tonga's Hunga-Tonga-Hunga-Ha'apai volcano, and the damage it caused.

In the first official update from the island kingdom, the Tongan government said the eruption generated waves of up to 15m, that hit the west coasts of a series of islands.

GNS seismologist Bill Fry said while 15 metres is an enormous height to imagine, it's likely the waves splashed up to that height in some places, rather than rushing in as a wall of water 15m high everywhere.

"It's quite possible that the 15m is a maximum height that the water reached inland when it was coming in and sloshing around. It had a lot of force. If you think about throwing a bucket of water out onto a deck, if it hits a wall or it hits your fence, that could splash up a bit.

"The same type of thing can happen with a tsunami, and I expect that 15m is probably the maximum extent at which it splashed up."

Fry said it would not have reached 15m everywhere.

"When we look at point measurements of the wave amplitude or the wave height, with a coastal run up, we're always seeing something that's both the effect of the incoming tsunami - the tsunami that was generated, and also the local effects of the coastline, the shape of the coastline.

"Does the coastline look like a funnel that sends all of the energy into a particular place, and of course makes that energy more focused and has higher run ups? Or does it look like a shape that spreads the energy out and has much lower height waves - lower amplitude waves?

"So I think that 15m measurement was probably the former case, where there is some sort of local topography that funnelled the energy to a particular place and made it slosh up to that 15m."

Foreign Minister Nanaia Mahuta yesterday said the outer islands of Tonga were extensively damaged.

Evacuations began Wednesday morning for residents on some of the smaller islands of Ha'apai, with about 150 people evacuated to other islands, Mahuta said.

The Tongan government said Mango, Atatā, and Fonoifua islands were being evacuated, and multiple people were injured.

People were also being evacuated from the western side of Tongatapu, including Kanokupolu, where dozens of homes have been damaged.

Structures on two islands have been completely wiped out: on the island of Mango, all of the homes have been destroyed - aerial images from a New Zealand Defence Force flight show just a few temporary tarpaulin shelters still standing.

And it appears almost every structure on the island of Atatā has been destroyed. The New Zealand Defence Force described the damage there as "catastrophic" after surveillance flights.

The United Nations Institute for Training and Research (UNITAR) also released a preliminary assessment that 72 structures had been damaged on Atatā and the entire island covered in ash.

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