An earthquake researcher says new technology deployed last year has enhanced New Zealand's tsunami alert system, but there would remain uncertainty over the arrival of ocean waves after an offshore seismic event.
Watching, waiting and macramé at Whangārei Heads Photo: RNZ / Nita Blake-Persen
On Friday a severe earthquake near the Kermadec Islands sparked tsunami alerts around the Pacific and left thousands of New Zealanders fleeing to higher ground. The 8.1 quake was the biggest of three major events in the oceans off New Zealand.
It took five hours for the evacuation notice to be lifted - in part because the earthquake happened in such a remote part of the ocean.
Researcher at the University of Auckland, Jennifer Eccles, told Morning Report the motion of the waves after a quake were complex as these travelled across the ocean and therefore there was inherent difficulty in giving precise predictions for arrival on shore.
She said there was a degree of 'better-safe-than-sorry' approach taken by authorities when an offshore seismic disturbance took place.
"They are relatively slow and they do take different paths across the ocean, some of which will be faster than others. So in terms of the expected time of a tsunami in New Zealand, there's about a two-hour spread, even across the North Island," she said.
"Scientifically, although we know when the earthquake occurs ... there is new technology that is coming online that the government is investing in DART (Deep-ocean Assessment and Reporting of Tsunami) buoys - and they will see the location of the tsunami wave, confirm that something has happened out there."
DART buoys detect tsunami threats by measuring associated changes in water pressure via sea floor sensors. The deep-sea instruments are capable of measuring sea-level changes of less than a millimetre in the deep ocean.
Two-way communication between a DART buoy and a 24/7 monitoring centre allows rapid assessment and subsequent warning advice to be provided to the public.
"What these do allow us to do compared to what we were doing 10 years ago, when basically all we had was the knowledge an earthquake out to sea, they do allow us to see essentially the change in the sea surface height as the tsunami does cross the ocean, so that allows us to confirm there was a tsunami.
"If there wasn't one, the advisory could be changed earlier, but if there is one there's still a lot of uncertainty about how that is going to interact in terms of a small tsunami."
Determining when small tsunami waves were due onshore however was more difficult and there is a degree of uncertainty, she said.
Friday's tsunami was expected to arrive at 10.20am and the threat alert was lifted at 1.20pm.
"In terms of Civil Defence, often, particularly because there is a lot of uncertainty over sampling out there in the ocean they do somewhat assume the worse-case scenario and with some of these warnings, for example, just a marine or coastal beach disturbance, there's a lot of uncertainty when we're talking about something quiet small...
"They were complex waves, they can bounce off coastlines in different ways and sometimes you do end up with delay arrivals in some places that could be relatively large.
She said communicating messages to the public could be improved in light of Friday's events.
Meanwhile, researchers from Victoria University of Wellington have brought us one step closer to more accurately forecasting when large earthquakes could occur.
Using data from the 2016 Kaikōura earthquake, they have been able to prove that sediment in under water landslides - called turbidites - can be used to find out how many large earthquakes have happened in a particular area in the past.
By assessing this pattern, they can then estimate when the next high-magnitude earthquake could occur.
It's the first time this method has been proven to work.
Jamie Howarth, leader author of the study and a senior lecturer in Victoria University of Wellington's School of Geography, Environment and Earth Sciences, told Morning Report their findings path to way to potentially determining quake threats.
"Seismologists were able to tell us that three large earthquakes that had happened on Friday occurred because we have this network of seismometers around New Zealand and in the world that record ground motions. So on that we can figure out where the earthquake happened and how big it was. That network of seisometrers has only really been around for 50 years or more," he said.
However, scientists also worked on the theory that sediments that accumulate on the seafloor in response to seismic shaking recording detailed information on the ground motion produced by prehistoric earthquakes. It's this information that adds to the tools used by scientists and potentially offers a way to see patterns and predict events.
New Zealand researchers proved that hypothesis after the Kaikōura earthquake. They went out on Niwa research vessel and studied the "sediment seafloor interface" at the time of the quake, using radiometric dating techniques.
He said it showed unequivocally that the turbidites had accumulated in response to the Kaikōura earthquake.
"We were able to demonstrate that those turbidites record information on the strength of shaking from the earthquake and also the spacial extent of shaking, the area over which those ground motions were felt."
This technique has the potential to foresee quakes by looking at historical long-term data and identifying patterns, he said.
"Watch this space, because New Zealand has the Hikurangi margin, our largest and most poorly-constrained seismic hazard and we're actually working at the moment on long calls, from along the margin, looking at these turbidites and reconstructing their spacial patterns... of many thousands of years and because you have that long-term perspective it gives a much more robust basis for forecasting the likelihood that we're going to see a similar event at some point in the future."
