An international scientific expedition aimed at improving knowledge of possible east coast earthquake risks, has produced the most detailed 3D images ever recorded of the Hikurangi Subduction Zone.

The US research ship Marcus Langseth arrived in Napier Port on Thursday night, following a 35-day ocean-based expedition involving 11 scientists from around the world.

Subduction zones such as the Hikurangi subduction zone, where the Pacific Plate descends (subducts) beneath the North Island, are known to produce the planet's biggest and deadliest earthquakes, also referred to as megathrust earthquakes, as well as devastating tsunami.

Subduction zones are also know for submarine "slow-slip" events, which act like normal earthquakes but over a longer period of time.

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The reasons why slow-slip events happened, and the potential relationship to earthquakes, were still unknown.

The current study involving the 71m-long Marcus Langseth focused on obtaining detailed 3D images of where slow-slip events (or slow-slip earthquakes) occur off the coast of Gisborne to help better understand those reasons.

Following a public tour of the vessels yesterday, University of Wisconsin-Madison geophysicist Dr Harold Tobin told Hawke's Bay Today he was "incredibly pleased" with the smooth running of the operation.

"The 3D seismic operation is complicated but we're here to really understand the science much better and these images will help with that.

"The thing that's been really surprising is that there is a really incredible complexity to the subsurface geology. We previously did not have too much data but now we really know what the structure of the two plates looks like within the geology itself.

"There are difference along the fault, and we can now map those out - there are sticky spots and weak spots - and that will help us understand what that means."

Dr Tobin said the project, which involved five separate US universities, costs "several million dollars".

The study had also employed five specially trained observers to ensure no mammal or protected seabirds came within an exclusion zone during seismic testing.

Marcus Langseth senior science officer Robert Steinhaus said the voyage had all gone very smoothly.

"We saw a lot of marine life but it rarely came near the exclusion zone. That allowed us to acquire much more data because previous surveys around the world we've had 200 shutdowns. On this one we had one shutdown."

Mr Steinhaus added that it cost between $75,000 and $100,000 a day to pay for the 71m ship and 48 crew during the voyage.

The ship, which is owned by the US National Science Foundation and is operated by the Lamont-Doherty Earth Observatory of Columbia University in New York, has been working off the East Coast to produce CAT scan-like images that will show the structure of the Earth's crust down to a depth of about 20km.

The US$29m ship has also been towing four 6km-long streamers, and using an airgun array that acts acoustic source generators, which use sensors towed behind the ship to pick up 'echoes' from different rock types in the upper layers of the Earth's crust.

Alongside the ship's acoustic sources, which act by rapidly expanding a bubble in the water then exploding them, naturally occurring earthquakes also recorded information on specially installed seafloor and land-based instruments.

Lead US investigator from the University of Texas, Dr Nathan Bangs, said the project would use all the land and marine data gathered to produce detailed images of the source of slow-slip events offshore of Gisborne.

"The intensive survey being undertaken northeast of Gisborne will produce three-dimensional images of the subduction zone both underneath the sea and beneath the land, and will help to reveal why slow-slip events happen there," he said.

The images will show the position of the two tectonic plates and help scientists determine the physical properties of the various rock layers that make up the subduction zone and how this affects how the two plates move past each other.

"Already the data acquired are revealing never-before-seen details of the shape and location of the subduction zone faults."

GNS Science marine geophysicist Dr Stuart Henrys said it would take about two years to fully interpret the data collected during the voyage, which had taken five years to plan.

"At GNS we have people here, people on shore, so we are part of the collaborative team effort. We also have instruments on shore that will feed into the interpretation of that CAT scan."

The three-dimensional imaging project is an international collaboration between University of Texas and other US academic Institutions, Imperial College London, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), and GNS Science in New Zealand.

Funding is being provided by US National Science Foundation, the Natural Environment Research Council in the UK, JAMSTEC IN Japan, and MBIE in New Zealand.