[ad_1]
GNS Science staff landed on Raoul Island on Wednesday and restored a data communications link that was lost when major earthquakes struck the area on Friday.
The greatest seismic activity began with a magnitude 7.3 event off the East Cape around 2:30 a.m. M. That was followed by a 7.4 magnitude earthquake under the seafloor near Raoul Island around 6.40 am.
Then around 8.30am there was a magnitude 8.1 earthquake, also near Raoul Island, which is the largest and northernmost of the main Kermadec Islands.
The link was lost before it could send information about the tsunami caused by the second major earthquake in the area. No one was on the island at the time.
READ MORE:
* Navy ship heads to remote Raoul Island to check tsunami warning systems
* New Zealand’s Trifecta Tsunami Rare, Expert Says
* Minimal damage after earthquake and tsunami warnings, initial reports suggest.
“Last Friday, the sensors in Raoul provided information on the first tsunami, but due to the failure of the communications links, the data from the second tsunami could not be accessed. This data has now been lost, ”said Sam Taylor-Offord, GNS seismic service officer.
The understanding of the impact of earthquakes and tsunamis on the island was evolving.
Christel Yardley / Stuff
Whakatāne and Ōhope residents move to higher ground and park near the Ōhope Scenic Reserve on Friday.
The ground crew found that a frame, which was bolted to the ground and was supporting the GNS equipment, had been pulled out of the ground and tipped over. “Basically all the power cables came out. That’s quite unexpected, ”Taylor-Offord said.
“That would have been the earthquake. The question is how strong the earthquake was on the island, because when the power went out we lost the data.
“We don’t know how strong the earthquake was that caused the damage, but we can see the consequences,” he said.
NZDF / Supplied
Royal New Zealand Navy ship HMNZS Canterbury with Raoul Island in the background, taken during the 2018 refueling mission.
There was also a gap in the data during the first earthquake. “We are not sure what caused that yet, but it would have been the second earthquake that wiped out communications.”
When the power went out and communications were lost, the tsunami data was also lost. It may 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.
In addition to restoring existing communications links, work was underway to add an additional layer of robustness to systems that support data communications.
NZDF / SUPPLIED
An aerial view of the Raoul Island Station.
GeoNet said the three earthquakes produced tsunamis that overlapped and occurred in New Zealand.
The largest tsunami, which occurred after the East Cape earthquake, was about 30-35 cm at Lottin Pt in the East Cape, and about 10-20 cm at Great Barrier Island.
After the earthquakes near Raoul Island, the waves overlapped and produced a 35-40 cm tsunami. That was measured with a tsunami gauge on Great Barrier Island, and it was also recorded on many other gauges across the country.
Tsunami heights could vary significantly along the coast, so there could have been larger tsunamis in places where there were no tide gauges to measure them, GeoNet said.
GNS, MetService and the Department of Conservation have facilities on Raoul Island. After the earthquakes, the Canterbury Navy ship headed to the island, which is about 1000 km northeast of New Zealand, with small GNS and MetService teams on board.
GNS has two underwater tsunami gauges off the coast of Raoul Island.
They measure the difference between atmospheric pressure and the pressure at the sensor itself, which indicates the height of the water above the sensor, Taylor-Offord said.
GOOGLE MAPS / Supplied
The subtropical islands of Kermadec, with Raoul Island highlighted here, are roughly midway between New Zealand and Tonga at the boundary of the Pacific Plate and the Australian Plate.
When a tsunami passes over the sensor, we observe it by the change in water pressure it causes as it travels towards the coast. How much the water pressure has changed from the tsunami gives us an indication of its height at the sensor location.
The two tsunami gauges at Raoul provided useful information on 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 cause tsunamis, they gave an early and clear indication of a tsunami threat, often hours before the arrival time of the tsunami on the New Zealand mainland.
While tsunami gauges and other geophysical sensors on Raoul Island were important, GNS and GeoNet could monitor earthquakes and tsunamis in the Tonga-Kermadec subduction zone without them.
SUPPLIED
The view from Prospect Ridge on Raoul Island, overlooking Green Lake and Blue Lake (file photo).
There was a regional sensor network in the South Pacific with many seismometers and tsunami gauges like the ones on Raoul Island, as well as several tsunami sensors in the deep ocean.
A DOC spokesperson said the department had received a report from Raoul that there was no significant damage to DOC buildings on the island.
The kitchen had been a mess with loose items on the floor and it was neat. The holding tank of the fire depot was severely damaged.
Until a year ago, the DOC had a team of about 15 people on the island, but they were pulled when New Zealand entered Covid-19 alert level four.
With the island uninhabited for now, and with the deadly Whakaari / White Island eruption in December 2019 in mind, a health and safety review of the DOC’s work on the island is underway.
