Cyclone Cody made predicting Tonga eruption tsunami spread harder

Tuesday, 18 January 2022

Scientists were unable to predict the speed with which a tsunami would reach New Zealand after the Hunga Tonga-Hunga Ha’apai eruption.

Instead, Crown research agency GNS Science tracked the tsunami using the network of Deep-ocean Assessment and Report of Tsunamis (DART) buoys littered across the South Pacific.

But even then, tracking the tsunami was complicated by Cyclone Cody, which was altering swell height and making it harder to spot the tsunami.

Jonathan Hanson, GNS Science’s earthquake and tsunami duty officer, said it was much different from a tsunami generated by an earthquake.

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“Where our experts usually use tools like modelling and forecasting, for this volcano-source event they relied on real time observations from our DART buoy network,” Hanson said.

“The DART network was invaluable in our ability to make assessments about when and where the tsunami might arrive as well as its amplitude.”

Niwa hydrodynamics scientist Dr Emily Lane said the tsunami was far larger than what was expected.

“Prior to this, I would have said, yeah, a volcanic tsunami could cause damage maybe 200 or 300 kilometres away from the volcano,” Lane said. “But we're seeing boats being destroyed in New Zealand, in Japan, there were cars and boats being washed away and boats being destroyed in the US.

“These are all thousands of kilometres away from this volcano, so that's an incredible amount of energy that's been blown out to cause that.”

It is one of the reasons GNS Science struggled to predict its speed and size, and to offer advice to the National Emergency Management Agency which warns people about incoming tsunami threats.

Hanson said Cyclone Cody also made things tougher.

“The storm caused heightened swells around the east coast of the North Island and made our data more difficult to decipher,” he said.

“Storm energy shows up in recordings with a much shorter period than tsunami waves, and it can make picking out the signal of the tsunami very difficult. You can see this at the Great Barrier Island gauge from the weekend – the sea level is clearly going up and down, but it is very hard to detect the longer period waves of the tsunami from within all of the wave noise created by the storm.”

Scientists are now trying to understand how such a powerful tsunami could come from a relatively small source, such as the Hunga Tonga-Hunga Ha’apai volcano.

There are four possible causes, according to Lane: the explosive eruption itself, a caldera and sea floor collapse, pyroclastic flows, and the shock wave created by the eruption.

She said more than one mechanism could be at play.

Hanson has his theory for what triggered the tsunami.

“It is likely that the earlier 14 January eruption blew away part of the volcano above water, so water flowed into the extremely hot vent,” Hanson said. “This meant that the Saturday evening eruption initially occurred underwater and exploded through the ocean, causing a widespread tsunami.

“There are also a few other potential mechanisms that could have contributed to the tsunami. We won’t know for sure until scientists can investigate what happened at the volcano – and right now both ash clouds and limited communications with Tonga mean that we can’t be certain.”

The first sign a tsunami was headed for New Zealand came at 5.48pm Saturday, when the DART buoy closest to Tonga was triggered by the wave.

As further buoys were triggered in the following hours, GNS was able to get an idea of what the threat might be to New Zealand.