The Kermadecs - a sub-tropical paradise with earthquakes, volcanoes and tsunami

Monday, 17 June 2019

A GNS senior scientist talks about tsunami warnings after Civil Defence briefly issued a tsunami threat following a magnitude 7 earthquake in the Kermadec Islands.

Sunday's tsunami warning was a good reminder New Zealand is surrounded by trouble.

That includes the Kermadec Islands, which stretch northeast off the Bay of Plenty towards Tonga.

The magnitude 7.0 quake at 10.54am on Sunday, centred 174km south of Raoul Island - the biggest and northernmost main island of the arc about 1000km northeast of New Zealand - has been followed by a handful of magnitude 5 and 6 aftershocks.

Could you handle the remoteness? Dave Whitelaw on North Meyer Island with Raoul Island in the background.

Department of Conservation (DOC) senior ranger and supervisor on the Island, Matiu Mataira, said the quake had not damaged buildings or equipment.

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Artist Robin White on sub-tropical Raoul Island in May 2011.

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A picture from the GNS webcam on Raoul Island in the Kermadecs. The Department of Conservation camp can be seen at extreme right.

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'The seven staff have experienced some small aftershocks and are coping well. DOC are staying in regular contact with staff on the island and have been in touch with their families.

Looking south, the sub-tropical Kermadec Islands, with Raoul Island highlighted here, are about halfway from New Zealand to Tonga on the boundary of the Pacific Plate and the Australian Plate.

'Island staff remain vigilant and are following post-earthquake protocol, which includes staying away from landslide-prone areas until the ground is deemed stable.'

This quake is par for the course for the region. There's more going on around these warm, sub-tropical South Pacific islands than just conventional heat.

The Niwa research vessel Tangaroa mapped the Kermadec volcano that erupted 800 kilometres northeast of Tauranga in July 2012, producing a pumice raft the size of Canterbury.

The 2500-kilometre long island chain consists of about 80 submarine volcanoes rising some 8000m from the Kermadec ridge, with White Island in the Bay of Plenty the southernmost. Parallel to the boundary between the Pacific and Australian crustal plates, this arc forms a highly active section of the 'Pacific Ring of Fire'.

Sometimes it gets too hot for comfort. There have been evacuations of DOC and MetService staff from Raoul Island when its volcano erupts.

The location of the July 2011 earthquake.

Significant eruptions of that volcano, classified by GNS Science as one of the Kermadecs' 'most explosive and potentially destructive', have occurred in 1814, 1870, 1964 and 2006.

A DOC worker, Mark Kearney, was killed in the sudden eruption on the morning of March 17, 2006, while taking water measurements at the crater lake. The eruption destroyed a large area of bush and scattered rocks and ash over half the island.

An area of floating pumice, from an undersea eruption, 250 nautical miles in length and 30 nautical miles wide was spotted southwest of Raoul Island in August 2012 by the crew of an RNZAF Orion on maritime patrol from Samoa to New Zealand.

GNS Science has a webcam on the crater rim of the Raoul Island volcano.

There have also been major earthquakes on the plate boundary, with a magnitude 8.1 in late 1986, magnitude 7.4 and 7.6 quakes in 2006 and 2007 respectively, and two others of the same sizes in 2011.

Helen Bostock holds some of the pumice discovered after an eruption southwest of Raoul Island in August 2012.

Mainland New Zealand is just down the line and a big enough earthquake on the plate boundary near the Kermadecs could easily generate a tsunami which will affect Kiwis living along the coast.

Following Sunday's quake, the Ministry of Civil Defence and Emergency Management issued a tsunami warning for New Zealand, prompting an urgent call for further details from people on social media. Eight minutes later it revoked the warning, based on advice from GNS Science's national geohazards monitoring centre and its tsunami experts panel.

GNS Science duty seismologist Dr John Ristau told Stuff a tsunami from a major quake at the southern end of the island chain could reach New Zealand in 45 minutes. If it was near Raoul Island it would take about two hours.

'The most vulnerable parts would be along the northern coast nearest to the epicentre - Northland, Auckland, Bay of Plenty, East Cape. If the earthquake is large enough, the entire coast of New Zealand would be threatened to varying degrees.'

Sunday's quake generated a tsunami of about 10cm at Raoul Island. To affect New Zealand it would probably need to be of magnitude 7.8 or higher and be from a reverse-faulting quake, in which one plate pushed over the top of the other, with a depth of 30-40km or less, he said.

The Australian and Pacific plates near Raoul Island were converging about 30 per cent faster than along the Hikurangi subduction zone east of the North Island, with the Pacific Plate pushing below the Australian plate at about 55cm each year.

'US Geological Survey figures show that over the past 10 years there were 70 quakes of magnitude 6 and above in the Kermadec-Tonga Trench.

'This is more active than the New Zealand mainland, but less active than regions such as Japan and Papua New Guinea,' Ristau said.

MetService employees Kevin Alder and Steve Knowles are two of dozens who spent year-long tours of duty on Raoul Island before the weather-observation programme became largely automated.

Alder was there from late 1986 to late 1987 and experienced 'quite a bit of earthquake activity', including a magnitude 8.1 quake.

'It was early evening, I'd had a shower and was just getting dressed in my room when an earthquake started. I rushed out to tell the others and it was still going. It lasted three minutes.'

Another staff member reported seeing an explosion of water north of the island, later confirmed as an eruption from an undersea vent.

Knowles, who was on Raoul from late 1992 to the end of 1993, remembers a cluster of earthquakes in March 1993 getting larger and larger, with up to three a minute. He was in the island's volcanic caldera when a very shallow magnitude 4.7 quake struck directly below.

'I'd just walked out into the crater. Rocks were falling off the hills around the crater and all the shrubs were shaking.

'There were volcanologists in the area and they decided to take us off the island. We stayed off overnight - there was real concern that the island was going to erupt.'

Alder and Knowles have both been back to Raoul and recall their years there fondly. They say one major difference now is the island is predator-free and the sub-tropical bird life is amazing.

The March 2006 eruption only lasted about half an hour, but forced the helicopter evacuation of five DOC workers.

In the November 21, 1964 event, a nine-man expedition team and several visiting scientists were evacuated by a Royal New Zealand Navy ship.

In 2006, Christchurch man and former MetService member of the team, Brian Phipps, told Stuff he was at the Raoul met office at 5.50am when he got a call from the hostel to say the island was erupting.

'I said, 'oh yeah, pull the other one' because I hadn't seen or heard anything. I went over to one of the windows and looked back and there were these huge plumes of smoke heading up into the sky.'

A mayday call was sent. Those on the island then got as close to the eruption as they could.

'We could see the smaller eruptions going off. It was quite exciting,' Phipps said in 2006.