GNS Science staff went ashore on Raoul Island on Wednesday and restored a data communications link that was lost when large earthquakes rocked the area on Friday.
The major earthquake activity started with a 7.3 magnitude event off East Cape about 2.30am. That was followed by a 7.4 magnitude quake under the seabed near Raoul Island about 6.40am.
Then about 8.30am there was an 8.1 magnitude earthquake, also near Raoul Island, which is the largest and most northernmost of the main Kermadec Islands.
The link was lost before it was able to send information about the tsunami caused by the second large earthquake in the area. No-one was on the island at the time.
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“Last Friday, the sensors on Raoul provided information about the first tsunami, but due to the failure of communications links, data was not able to be accessed for the second tsunami. This data has now been lost,” GNS seismic duty officer Sam Taylor-Offord said.
Understanding of the impact of the earthquakes and tsunamis on the island was evolving.
The staff who went ashore found that a rack, which was bolted to the floor and held GNS equipment, had been pulled out of the ground and tipped over. “All the power cables essentially came out. That’s quite unexpected,” Taylor-Offord said.
“That would have been the earthquake. The question is how strong was the shaking on the island, because when the power went out we lost the data.
“We don’t know how strong the shaking that caused the damage was, but we can see the aftermath,” he said.
There was also a gap in the data during the first earthquake. “We’re not sure what caused that yet, but it would have been the second earthquake that took out the communications.”
When the power went out and communications were lost, data from the tsunami was also lost. It might be possible to get an idea of the size of the tsunamis and how far inland they went by looking at the impact the events had on the island, Taylor-Offord said.
As well as restoring the existing communications links, work was being done to add an additional layer of robustness to the systems supporting data communications.
GeoNet said all three quakes produced tsunami that overlapped and were recorded around New Zealand.
The largest tsunami, which occurred after the East Cape earthquake, was about 30-35cm at Lottin Pt on East Cape, and about 10-20cm at Great Barrier Island.
After the earthquakes near Raoul Island, waves overlapped and produced a tsunami of 35-40cm. That was measured by a tsunami gauge at Great Barrier Island, and also recorded on many other gauges around the country.
Tsunami heights could vary significantly along the coast, so there could have been larger tsunamis at places where there were no tidal gauges to measure them, GeoNet said.
GNS, MetService and the Department of Conservation all have facilities on Raoul Island. After the earthquakes, the Navy ship Canterbury headed to the island, which is about 1000km north-northeast of New Zealand, with small teams from GNS and MetService on board.
GNS has two tsunami gauges underwater near the Raoul Island coastline.
They measure the difference between atmospheric pressure and the pressure at the sensor itself, which indicates the height of water above the sensor, Taylor-Offord said.
When a tsunami passes over the sensor, we observe it by the change in water pressure it causes as it travels onshore. How much the water pressure is changed by the tsunami gives us an indication of its height at the sensor’s location.
The two tsunami gauges at Raoul provided useful information about the tsunami threat posed by earthquakes in the region, Taylor-Offord said.
Because the sensors were close to the source of many earthquakes that could potentially cause tsunamis, they gave an early and clear indication of tsunami threat, often hours ahead of tsunami arrival time on mainland New Zealand.
While the tsunami gauges and other geophysical sensors on Raoul Island were important, GNS and GeoNet could monitor earthquakes and tsunami in the Tonga-Kermadec subduction zone without them.
There was a regional sensor network in the South Pacific with many seismometers and tsunami gauges like those on Raoul Island, as well as several deep ocean tsunami sensors.
A DOC spokesperson said the department had received a report from Raoul that there was no significant damage to DOC’s buildings on the island.
The kitchen had been a mess with loose items on the floor and was tidied up. The fire reservoir holding tank was significantly damaged.
Until a year ago, DOC had a team of about 15 people on the island, but they were taken off when New Zealand went into Covid-19 alert level four.
With the island uninhabited for now, and with the deadly December 2019 Whakaari/White Island eruption in mind, a health and safety review of DOC’s work on the island is under way.
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