Into the subduction zone: Scientists drill into the Hikurangi

Monday, 7 May 2018

Why the seabed cores drilled into the Hikurangi Subduction Zone from research ship Joides Resolution in 2018 are so important to understanding New Zealand's geology.

Imagine being on a boat in the heaving ocean, trying to keep it steady while drilling a small hole on the sea floor more than a kilometre below you.

The equipment - and the trip - has cost millions of dollars and years to plan, and you've got limited time to tick off your goals.

The crew of the research vessel Joides Resolution have just returned to Auckland from doing just that during a two month expedition to the Hikurangi Subduction Zone off the East Coast of the North Island.

It's an area scientists have dubbed a 'sleeping geological giant', and they were there to gather information about slow slip earthquakes.

* Exploring the 'megathrust' risk of the Hikurangi

* Talking to scientists about the Hikurangi subduction zone

* Hikurangi subduction zone part of a 3500km-long system**

Research vessel Joides Resolution docked at the Port of Auckland Port after returning from the Hikurangi subduction zone.

GNS scientist Doctor Laura Wallace was co-chief scientist on the mission.

'What we were trying to understand … was, we've been studying these things called slow slip events off the East Coast of Gisborne,' she said.

'We also see them off the Hawkes Bay, off the Kapiti region just west of Wellington, but the reason the ones off Gisborne are really special is they are occurring really close to the sea floor - within one or two kilometres.

'Most other slow slip events in the world are happing really deep - 20, 30, 40km deep. This gives us an opportunity to get rock samples that are actually involved in the slow slip behaviour.'

Slow slips are similar to earthquakes but occur over weeks or months instead of seconds. They were only discovered about 15 - 20 years ago.

Dr Katerina Petronodis (right) talks about core samples taken from drill sites near the Hikurangi Trench, off the coast of Gisborne.

Sometimes, the movement in a slow slip can be equivalent to that during a Magnitude 7 quake.

They redistribute pressure to different parts of the Earth's crust, and can trigger other earthquakes that were ready to occur.

In the short term, they cause a slight change in the rate of quakes but in the long term they relieve a lot of stress.

But scientist didn't yet understand why they happen, Wallace said.

Some of the most popular theories were that properties of different rocks in the fault zone allow slow slips to occur, and that water building up in the fault zone may also enable them.

WHAT THEY WERE DOING

The Joides Resolution is a big ship - 143 metres long with a 63m tall drilling rig - equivalent to a 20-storey building.

On board it has laboratories, a helicopter landing pad, a gym and enough room to house up 130 people. It's also a class one icebreaker.

The scientists, engineers and crew drilled holes into the seabed, collecting more than 1km of rock and sediment core samples and installing monitoring stations.

'Coring at the Hikurangi subduction zone, we're actually getting our hands on rocks that are involved in the slow slip process and we can test some of the theories,' Wallace said.

The observatories - which are below the sea floor at the slow-slip fault region and a first in New Zealand - are rare, expensive, and an 'engineering feat', Wallace said.

'These things will be in place for 10 years or more and continuously monitoring a lot of different changes within the Earth's crust during the slow slips.'

They will take three types of measurement: Pressure, temperature and chemicals in the fault zone.

Expedition co-leader, Dr Demian Saffer of The Pennsylvania State University, said: 'These technically complex observatories are two of only a handful of such instruments at subduction zones in the world.

'The observatories allow detection of even the smallest movements on the subduction fault, and will yield unparalleled insight into what happens during slow-slip earthquakes.'

The 143 metre-long vessel has a 63m tall drilling rig.

LOCATION

Positioned about 40 and 80km offshore from Gisborne, each station can cost up to $1 million.

The closer site is in about one km of water and extends 300m below the sea floor, while the outer site is in about 2.5km deep and extends about 450m below the sea floor.

Two of the coring drill sites were on a section of the plate boundary that is not subducted yet, including the top of a seamount (an extinct underwater volcano).

Cores were also collected from inside one of the major faults accommodating the motion of the tectonic plates, and from the sediments directly above the area where the largest slow slip occurs.

The data will be later collected by a ship with a submersible remotely operated vehicle and studied.

DRILLING ON THE HIGH SEAS

Dr Katerina Petronotis was a staff scientist on the vessel and its expedition project manager.

She described the time-consuming drilling work which can operate 24 hours a day if the boat is in position.

The white patches in these core samples are most likely sedimentation from the Taupō volcano erupting.

Pulling up cores takes quite a while, for example - one 10m section can take a couple of hours.

'Overall, the whole process (to drill about one km of cores) probably took two weeks.'

That may seem deep, but the ship has the capability to drill down about seven km through the sea and sea floor.

To do that, about 8km of drilling pipes were stored on board, Petronotis said.

Keeping the boat in place can be hard work in itself.

Some of the 8km of drilling pipes stored on board the vessel.

The Resolution has 12 powerful thrusters on board. They're linked to GPS and fire up in order to keep it in position above the drilling hole on the sea floor.

There's a sonar dome under the hull for positioning, and it can stay in a radius of the drill hole on the sea floor by about one per cent of the depth of water it's in.

The drill string enters the sea thorough the 'moon pool' - an opening to the sea in the centre of the hull.

It also has a mechanism that helps keep the drilling system level.

But once the heave - the up and down motion of the boat - exceeded 4m, they had to stop work, Petronotis said.

Luckily, the vessel only abandoned work for two days due to rough sea during the whole trip.

They have to be careful with the expensive equipment.

'If the drill string gets stuck, in extreme cases, we have to send a stick of dynamite down to blow if off.

WHAT ELSE THEY WERE HOPING TO KNOW

Improved understanding of the relationship between slow slips and earthquakes may mean better earthquake forecasting, Wallace said.

'You can never say predict, you'll never be able to predict them but to get a better understanding of the likelihood in the near future of an earthquake.'

Scientists have seen 'dozens and dozens' of slow slips events in New Zealand.

Storage racks on board the Joides Resolution used for holding core samples from the sea bed.

So far, they haven't triggered a bigger earthquake on the subduction zones, but they're a factor to consider when looking at hazards in the future.

Wallace described the funding for the research as 'probably the biggest international investment in New Zealand-based science that has ever been made'.

The trip went well - all of the objectives were accomplished - a rare feat.

It was the culmination of years of hard work - work to write the proposal for the research had begun eight years ago.

The Joides Resolution will leave Auckland later this week on a trip to drill into an active submarine volcano northeast of White Island.

Both trips are part of six voyages in New Zealand waters by the boat under the 23-nation International Ocean Discovery Program.

The ship - run by Siem Offshore, a fleet of Norwegian vessels serving the oil and gas industry - only visits New Zealand once every 10-12 years.

The expedition involved approximately $NZ30 million of international investment, funded by the United States National Science Foundation and the International Ocean Discovery Program.

It involved 30 scientists from New Zealand, the United States, Europe, Japan, China, South Korea, and Brazil.