Two sub-seafloor earthquake observatories are now in action at New Zealand's biggest danger area for tsunami risk - the Hikurangi Subduction Zone.

The observatories, just installed off the coast of Gisborne as part of a huge international science effort, are specifically designed to closely monitor long-lasting, silent "slow-slip" earthquakes that have been implicated in catastrophic disasters elsewhere on the planet.

The Hikurangi Subduction Zone, the boundary where the Pacific tectonic plate dives beneath the North Island, poses our country's largest geological hazard and is thought capable of unleashing massive "mega-thrust" earthquakes and quick-fire tsunamis.

Combined with a number of other international efforts to investigate the subduction zone over the next few years, the new observatories will help make Hikurangi one of the best-studied subduction zones anywhere in the world.

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One of them, named "Te Matakite" which means "to see into the future", will sense and measure changes in the fault zone that human eyes cannot see or record in any other way.

It would provide crucial new insights into the subduction zone behaviour, and help scientists better understand its threat.

Te Matakite wasn't the only observatory in the world at a subduction plate boundary, but was still a first for New Zealand.

The two sub-seafloor observatories will remain in place for up to 10 years, recording valuable information about the physical and chemical conditions inside the Hikurangi Subduction Zone. Graphic / GNS Science
The two sub-seafloor observatories will remain in place for up to 10 years, recording valuable information about the physical and chemical conditions inside the Hikurangi Subduction Zone. Graphic / GNS Science

And because of its nested design, with three packages of instruments lowered inside each other, it was one of the most complex that has ever been built.

During the expedition, aboard the Integrated Ocean Drilling Program (IODP) scientific drilling ship JOIDES Resolution, the observatory was installed 450m below the seafloor into a major active fault that splits off from the main Hikurangi megathrust fault.

Research into slow-slip events had pinpointed this thrust fault site as one where these silent earthquakes might be propagating up to the seafloor, so the scientists chose to position the observatory instruments below, within and above the fault.

READ MORE: Our sleeping taniwha: Hikurangi's tsunami threat

"Observatories are notoriously difficult to install, due to the deep-water depths and tools required, but we are thrilled both were deployed successfully over the two-month voyage," said the expedition's co-leader, Dr Demian Saffer of Pennsylvania State University.

"Te Matakite" which means "to see into the future", will sense and measure changes in the fault zone that human eyes cannot see or record in any other way. Photo / Supplied

"The observatories will be able to detect tiny changes in the rock and sediment surrounding the subduction fault and provide clues about how and where stress is building or being relieved."

Both observatories would listen to the creaks and groans of the subduction zone and were the primary method scientists had of being able to continuously collect this data over many years, directly from the source of the slow-slip events.

Saffer said the purpose of the observatories was to "test a broad range of questions we have about slow-slip events, and their relationship to large, destructive earthquakes and tsunamis".

Expedition co-leader Dr Laura Wallace, of GNS Science, hoped the observatories would pave the way to more long-term measuring equipment being installed in the Hikurangi subduction zone - something which might also contribute to improved tsunami warning in the future.

As slow-slip events occurred at least once a year, the observatories were expected to sense and record multiple cycles of these enigmatic, slow earthquakes over the next several years.

Within five years, scientists would return to both observatory sites with another scientific research ship to retrieve the data using a robotic submarine.

This would link up to instruments at the wellhead with a cable to download the pressure data and send it back to a computer on the ship.

Scientists would also use a winch to retrieve the delicate temperature and fluid sampling packages inside Te Matakite.

"The puzzle we are putting together on the JOIDES Resolution will help subduction zone scientists all around the world understand earthquakes and the slow-slip phenomenon better," Wallace said.

"And for people living near subduction zones, it will be used to improve hazard and risk models of earthquakes and tsunamis.

"Better understanding of what could happen will lead to improved preparedness and fewer lives lost."

The project, funded by the US National Science Foundation, was further expected to offer insights for scientists studying subduction zones elsewhere.