How deep-sea buoys trigger tsummani warnings after major earthquakes

The Tsunami Warning System, spread around the depths of the ocean, helps keep New Zealanders safer from the tsunami waves.

The buoys, which have been deployed in the past 15 months, have been an opportunity to prove their worth during recent large undersea earthquakes in the South Pacific.

Dart buoys – ocean depth assessment and tsunami reporting – have improved the accuracy of tsunami impact forecasts.

In the wake of major earthquakes on March 5, the DART network allowed the National Emergency Management Agency (NEMA) to cancel the tsunami warning several hours earlier than would have been possible if officials had relied on coastal tide gauges alone.

Read more: * Tsunami science and what parts of New Zealand are most at risk * A 7.3-magnitude earthquake is actually two earthquakes that broke separate faults, as the GNS discovers * Coastal communities threatened by undersea earthquakes cannot feel them

The buoys also provide more accurate estimates of the size, location and timing of tsunamis off the New Zealand coast, said NEMA spokesman Anthony Frith.

No

The red triangles indicate where modern technology monitors marine earthquakes and tsunami danger. Dart buoys A, B, C, E, F, G, H and I were deployed during flights in December 2019 and September 2020. Final flight will deploy buoys J, K, and L near the New Hebrides Trench as well as Buoy D, near Plenty Bay .

How do floats work

Dr Bill Fry, a seismic and tsunami researcher at GNS Science, said the Dart network measured tsunamis in the open ocean, away from the complex impacts of coastlines and usually long before many coastal impacts.

Data from the buoys was sent via satellite communications in near real time to GeoNet and to the Pacific Tsunami Warning Center in Hawaii, Frey said.

Sarah Sarson / Niwa

A dart buoy being loaded onto the Newa Tangaroa on the way to sea.

The quality of the information captured in dart buoys depends on the suitability of their location. That was enough to give a good picture of the earthquake causing the wave, and a good indication of the size of the tsunami, but it wouldn’t be perfect because the wave wasn’t captured everywhere.

Given the limited information shortly after the earthquake, early predictions of a tsunami were very conservative. “Then as we get more information, we can become less conservative, and thus less uncertainty,” Frey said.

Small tsunamis were on the surface and moving rapidly across the open ocean. When they reached shallow water, they slowed down and grew in height.

With three major earthquakes on March 5 – the East Cape event, followed by earthquakes of magnitude 7.4 and 8.1 in the Kermadyk Islands – tsunami activity was particularly difficult to understand.

Kristel Yardley / Staff

Coast residents were evacuated to higher ground on March 5 after tsunami warnings. Residents of Whakatāne and Ōhope move to higher ground and park their cars near the Ōhope Scenic Reserve.

Fry said that waves from earthquakes tend to bounce around the coast and harbors and stay there for a long time.

An early understanding of the risk of the tsunami on 5 March was “very good”, with time the uncertainty diminished, and an “ever-evolving picture of increased comfort with expectations” emerged.

Frey said he knew more about the tsunami on March 5 than it would have been if the circling buoy network were not in place.

Within about 30 minutes of the largest earthquake on Kermadyk Island, information about the tsunami came from a buoy, Frey said. The tsunami took about an hour to reach the North Cape.

Tide gauges recorded some tsunami information in coastal waters, but did not capture everything, and work was underway to search for evidence of the tsunami waves and their reach inland and above sea level.

NOAA

This National Oceanic and Atmospheric Administration animation shows what happens when a large tsunami strikes.

How do buoys help in other earthquakes

Frey said the Dart network also measured the 7.7-magnitude tsunami generated in the New Caledonia region on February 11th.

Within five to 10 minutes of this event, the earthquake data allowed the National Geographic Hazard Monitoring Center (NGMC) to identify a potential threat to New Zealand.

Improved understanding of earthquake magnitude and type through links with international agencies in approximately 45 minutes.

A team of tsunami experts quickly used the available earthquake data to run the prototypes of the tsunami waves before their arrival in New Zealand.

When water waves were measured on Dart networks and tides from the February 11 tsunami, Frey said, they were larger than expected from analyzing the earthquake data.

The University of Waikato / Supplied

In deep water, tsunamis move very fast and have long wavelength and small amplitude. When it enters shallow water, it slows down and the wavelength decreases. This makes the wave much longer.

These observations allowed us to improve the forecast, by highlighting that the earthquake was either larger and / or more complex than we originally thought.

Where are these buoys?

Frey said the Dart network of buoys has provided improved safety for all countries with borders on the Pacific Ocean, with a focus on New Zealand and its closest neighbors.

Eight of the 12 planned buoys (Depth Ocean Assessment and Tsunami Reporting) were placed along the Tonga-Karmadiq-Hikorangi Trench system, which runs northeast below the sea from near the top of the South Island to near Samoa.

Three of these floats are located off the east of the North Island between Wellington and the East Cape, while the others extend further to the northeast.

The plan is to deploy the remaining four buoys this year, depending on favorable ocean and weather conditions and cargo availability.

Supplied

The locally produced tsunami near Gisborne on March 26, 1947 caused extensive damage along the coast. This is all that remains of a four-room house at Turihaua Point that filled its head with water. I left seaweed hanging from power lines in the background.

Three of the remaining buoys will be located in northwest New Zealand, around New Caledonia and Vanuatu, and the other will be further from the sea off the east of the Upper North Island.

Frith Nehme said that dart buoys can also provide quick confirmation that a major earthquake, or other potential event, did not trigger a tsunami.

“We can decide sooner that there is no threat and that people do not need to evacuate, or give the people who evacuated themselves when they felt a long or strong earthquake everything clear to go home.”

The science behind the floats

Each buoy has two main components – a bottom pressure recorder located on the sea floor, and a large yellow float on the surface above.

Susan Pepperl of Niwa, who is involved in the Dart project, said deploying both components required an advanced combination of precision positioning on the high seas, sophisticated marine mapping below, and a deep understanding of ocean currents.

Equipment on the sea floor recorded the height of the ocean above it. If the measurement is outside the normal parameters, it sends a special signal to the surface float.

That was when the system switched to “event mode” – a trigger that means it will update the GNS server every few minutes instead of the usual once every hour. Event Mode can also be triggered remotely if it is thought the tsunami may be heading towards a buoy.

When the warnings come too late

Frith said a tsunami of local earthquakes near New Zealand, such as the first major event on March 5 – a 7.3-magnitude event off the East Cape – could arrive within minutes, before there is time to issue an official warning.

Even with the improvement in prediction provided by Dart’s floats, Nima emphasized the importance of following Long or Strong’s advice: Get Gone.

People near the coast should move immediately to higher ground or inland if:

Feel a strong earthquake that makes it difficult to stand or a long earthquake that lasts more than a minute Watch a sudden rise or fall in sea level Hear loud or unusual noises from the sea.

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