An international team of scientists and engineers preparing highly specialized instruments to be deployed on the seafloor offshore the North Island’s east coast. (from left), Pete LIljegren (Columbia University), Laura Wallace (GNS Science), Neville Palmer (GNS Science), Carlos Becerril (Columbia University), Spahr Webb (Columbia University), Yusuke Yamashita (Kyoto University), and Ted Koczynski (Columbia University). PHOTO: SUPPLIED/ Margaret Low, GNS Science.

PAM GRAHAM
pam.graham@age.co.nz

Scientists around the world are watching the information three instruments now on the Wairarapa seabed will provide about New Zealand’s largest fault, the Hikurangi subduction zone.

They’re the latest effort in an ongoing well-resourced study of the area where the Pacific Plate dives, or subducts, beneath the Australian Plate. The boundary between the two is the actual fault.

“There are probably close to 100 international scientists involved in research of this subduction zone, and over the last few years, and the next couple of years the level of international investment in this is pretty, to my knowledge, unprecedented,” said Dr Laura Wallace, one of the project leaders.

“We are looking at $50 to $60 million of international investment.”

Scientists on board NIWA’s research ship Tangaroa on Sunday placed three precision transponders in a tight array on the seabed offshore from Castlepoint near the Hikurangi trench where it comes up to the seafloor.

A wave glider, or boat, will go out to the instruments in January and record their exact position to within a few centimetres and the process will be repeated each year for at least five years, possibly longer.

Scientists already know from GPS measurements the subduction zone on land is locked up and is building up pressure and stress that will probably be released in a future large earthquake.

The measurement of horizontal movement in the seafloor will help scientists work out if the plate boundary is locked up offshore as well.

That’s important because it will give insights into the possibility of fault ruptures that could cause a tsunami.

The array off the Wairarapa coast has just been purchased by GNS Science. Dr Wallace would not disclose the cost, beyond saying the instruments “weren’t cheap”.

A similar array owned by Scripps Institution of Oceanography in the US is being placed off the Hawke’s Bay coast.

On this trip the Tangaroa is putting a total of 40 instruments of different types off the coast of Gisborne, Hawke’s Bay, and Wairarapa.

The expedition includes scientists from GNS Science, Victoria University of Wellington, Lamont Doherty Earth Observatory (US), Scripps Institution of Oceanography (US), as well as Tohoku, Kyoto and Tokyo Universities in Japan.

“This is the first time we have had a long-term deployment of this type of instrument on the seafloor,” Dr Wallace said about the instruments on the Wairarapa seafloor.

Potentially they could become part of a tsunami warning system if they were set up to provide real-time information.

“What we can learn about this fault and how it moves will help us understand and prepare for the next great earthquake,” said GNS Science expedition leader Dr Daniel Barker.

There has previously been drilling off Gisborne where “slow slips” have been occurring, and also seismic surveys to image the plate boundary.

Slow slips are movements of tectonic plates over weeks and months, rather than in an earthquake, and they’ve only been known about for about 15 years.

The world’s shallowest slow slip occurs just offshore near Gisborne.
Dr Wallace said slow slips are believed to happen as far south as Castlepoint but not much further than that.

“It is possible they are happening out there, we just can’t see them with the GPS,” she said.

The effort that has just got under way is trying to measure horizontal movement of the seabed near the trench.

“There is a huge amount of interest in this,” Dr Wallace said.

It is also hoped different instruments currently trying to measure slow slip movements off Gisborne will be moved south to the northern Wairarapa coast to learn more about what is happening off there.

Subduction zones are where most of the world’s deadliest earthquakes, volcanoes and tsunamis occur, such as the Sumatra [2004] and northern Japan [2011] Magnitude 9 earthquakes and tsunami.