DART Network Proves Invaluable In Assessing Tsunami Threat Following The Hunga Tonga-Hunga Ha’apai Eruption

Aotearoa New Zealand’s network of DART buoys proved vital over the weekend as tsunami experts raced to analyse the tsunami threat to our coastlines following a large, explosive eruption at Hunga Tonga-Hunga Ha’apai volcano in Tonga.

DART (Deep-ocean Assessment and Reporting of Tsunami) buoys are deep-ocean instruments that monitor changes in sea level. They detect tsunami by measuring changes in water pressure via sea floor sensors. Right now, they are the only accurate way we can rapidly confirm a tsunami has been generated before it reaches our coast.

Anything abnormal that is picked up on a DART buoy is immediately communicated to GNS Science’s National Geohazard’s Monitoring Centre / Te Puna Mōrearea i te Rū (NGMC). On Saturday, The NGMC was already watching the situation closely, following observations of an earlier eruption at Hunga Tonga-Hunga Ha’apai Volcano.

When the DART network picked up signs of tsunami activity, the NGMC immediately activated the GNS Science duty response team, including earthquake and tsunami duty officer Elizabeth Abbott. She worked alongside the Tsunami Experts Panel (a panel of scientists from GNS Science and partner research organisations) to build a picture of possible tsunami threat to New Zealand coastlines.

“The first sign of unusual activity came when the New Zealand DART buoy closest to Tonga triggered at 5:48pm. We could see that a significant signal had been picked up by the sensor. This was about 30 minutes after the eruption (the time we had previously estimated to be about the travel time of a tsunami from the volcano to DART G) so we were immediately pretty confident that this was a volcanic source tsunami.

“From there we triggered the second closest DART to the volcano, DART F, and when we saw a significant signal recorded there too, we proceeded to activate the entire network”.

As data was recorded at more of our DART buoys and coastal tsunami gauges, we were able to build a picture of what threat there might be to New Zealand and provide advice to the National Emergency Management Agency (NEMA) about what wave amplitudes might be expected at New Zealand shores.

“Assessing this tsunami was completely different from assessing earthquake-source tsunamis that we’re more used to seeing” says Dr Bill Fry, GNS Science Principal Scientist and Tsunami Experts Panel member.

“Our forecasting commonly involves relying on seismic data and precalculated models of earthquake generated tsunamis. In this event, we were fortunate to have real time observations from the DART buoys. Based on tsunami measurements from the buoys we were able to make assessments about when and where the tsunami might arrive as well as its amplitude, despite having very little information about the volcanic eruption itself”.

NIWA hydrodynamics scientist and Tsunami Experts Panel member Dr Emily Lane says the DART network was key to us being able to assess the threat of this tsunami.

“If this had happened three years ago, before we had the DART network, we would have had no idea that there was a tsunami heading our way until it arrived at our coasts and was picked up by our coastal tsunami gauges.

“The network not only helps protect us in Aotearoa New Zealand, but it also fills what had previously been a gap in the global DART buoy network and provides warning information for our neighbouring countries in the Southwest Pacific too”.

Gary Knowles, Director Civil Defence Emergency Management, National Emergency Management Agency says the network has once again proved to be invaluable.

“Over the past weekend we once again saw the critical importance of the network demonstrated. The rapid detection, assessment and reporting of the tsunami threat following the volcanic eruption in Tonga meant that we were able to issue a National Advisory alerting people to the threat of tsunami activity more quickly and accurately than we otherwise would have been able to for this type of unpredictable and rare event.

“The data from the DART network was also instrumental in allowing us to update and cancel the National Advisory in a timely manner.”

Coastal tsunami gauges are still picking up activity in areas like Wellington Harbour that are prone to lingering wave activity. This is mostly caused by the effects of Cyclone Cody but is likely to contain a small amount of tsunami energy input, residual from the event on 15 January.

Strong and unusual currents are expected to continue for at least another 24 hours. This particularly applies to the East Coast of the North and South Islands, and the south and west coasts of the South Island. Strong currents can injure and drown people. Take care and remain vigilant if you are in or near the water close to the shore (including swimming, surfing, fishing, small boats).

GNS Science’s National Geohazards Monitoring Centre (NGMC) continues to monitor around-the-clock, and our teams are ready to respond if we see further tsunami activity caused by the volcano.

As the picture becomes clearer of the impacts to Tonga from this eruption and tsunami, we hold all those affected in our thoughts, including our colleagues at Tonga Geological Services. Also, to the New Zealand Tongan community who are trying to get in touch with their loved ones back home, we hope your communication links are restored soon. We are thinking of you.

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