Rare triple tsunami recorded off the coast of New Zealand

(ORDO NEWS) — The new DART buoy complex in the South Pacific recorded waves driven by three earthquakes that occurred within hours of each other.

In tsunami-prone regions, an earthquake is a signal to move to higher ground. But deadly waves may still be on the way if the earthquake is felt faintly or not at all. Now researchers working in the Southwest Pacific have deployed a new set of ocean sensors to detect tsunamis directly. At the beginning of this year, instruments recorded a rare triple tsunami, caused by three tsunamigenic earthquakes that occurred within hours of each other.

Measure the wave

The rupture of the fault, in particular, the vertical thrust that occurs at the border of converging subduction plates, could be a recipe for a tsunami. But judging the strength of a tsunami from ground-based seismometers – before the waves hit the coast – is fraught with uncertainty.

That’s why instruments that measure tsunamis directly – tsunameters – are key, says Bill Frye, a seismologist at GNS Science in Wellington, New Zealand and a member of the New Zealand Tsunami Expert Group.

“Instead of using an earthquake as a proxy, we are actually measuring the tsunami wave.”

Tsunameters come in many forms: Coastal sea level sensors measure changes in water level near the coastline. Cables stretched across the seabed register pressure changes caused by the passing tsunami. Deep sea tsunami assessment and reporting (DART) buoys record changes in sea surface height.

This video presents a computer model of a possible M8.9 plate boundary earthquake and its associated tsunami along the Hikurangi Fault for civil defense and emergency planning purposes.

DART buoys in particular have many benefits, says Fry. They are cheaper to deploy than cables on the seabed and are not as affected by coastal waves as coastal sea level sensors. If placed near the source of an earthquake – for example, over an ocean trench – they can also give early warning of an impending tsunami.

The first DART buoy was deployed in 2000 and five more were added by the end of 2001, all moored in the Pacific Ocean. Three years later, an official tsunameter network was created in response to the deadly tsunami that followed the December 26, 2004 earthquakes in Sumatra and Andaman. Today more than 60 DART buoys stand guard over the world’s oceans.

“These are amazing things,” says Jose Borrero, a coastal and tsunami scientist with eCoast Marine Consulting and Research in Raglan, New Zealand and a member of the Tsunami Expert Group. The DART buoy consists of a bottom seabed pressure recorder and a surface buoy for by transmitting data via satellites, and it can detect minuscule changes in sea surface elevation. “It can distinguish millimeters of water level fluctuations,” says Borrero.

Filling in the gaps

Despite the tsunami hazard, some subduction plate boundaries are still relatively poorly understood. One example is the Kermadec Basin north of New Zealand. According to Stephen P. Hicks, a seismologist at Imperial College London, this subduction zone is particularly prone to earthquakes as tectonic plates converge rapidly, at about 7 centimeters per year. “This is one of the fastest plate boundaries on Earth,” says Hicks.

Kermadec Trench and other nearby troughs could trigger tsunamis that hit New Zealand’s north coast. ”Armed with this knowledge, the New Zealand government decided to add 12 new DART buoys to the Southwest Pacific.

In December 2019, researchers aboard the Tangaroa installed the first four buoys off the east coast of New Zealand. Four more joined the bright yellow buoys labeled “New Zealand Tsunami Detection” during the year. Four more will be installed by the end of 2021. In total, they will control the active Hikurangi, Kermadec, Tonga and New Hebrides troughs.

Three earthquakes, three tsunamis

The buoys currently in the water are already collecting data, and on March 5 they noticed something rare: a triple tsunami.

On that day, three separate earthquakes occurred near New Zealand, all over 7 points. They occurred in rapid succession – at 2:27 am, 6:41 am and 8:28 am local time – within 1000 kilometers of each other. Each of them caused its own tsunami, and new DART buoys recorded these waves.

These three overlapping events were an excellent test for the new tsunameter network, Borrero said. “Any response system will have a hard time coping with this.”

New data is making it possible to more quickly assess the potential impact of a tsunami in the Southwest Pacific. Observations on March 5 of the waves set in motion by the third earthquake, the strongest – M8.1, allowed researchers to more accurately measure the maximum wave amplitude and better estimate the duration of the tsunami threat to the coast of New Zealand. In particular, officials used the measurements to conclude that dangerous waves were no longer likely to occur more than 4 hours earlier than previously possible.

“Prior to DART, disaster cancellation decisions were based on coastal tide gauges,” Frye told Eos. “Many of New Zealand’s tidal sensors are located in harbors that can trap energy for hours or days after the arrival of tsunami waves.”

These new DART buoys will protect more than just the Southwest Pacific, Borrero said. The data they collect will be transmitted to countries such as Chile and the United States, which also border the Pacific Ocean and therefore could experience a tsunami from a remote source. “They give us the warning we need, but then they also go to the global network,” Borrero said

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