Jamie Morton is the NZ Herald's science reporter.

New Zealand's tsunami risk from East Coast offshore area to be revealed in new study

The 7.1 magnitude quake that shook Te Araroa on the North Island's East Coast this month showed why more information is needed on quake-making offshore areas threatening the country. Photo / Supplied
The 7.1 magnitude quake that shook Te Araroa on the North Island's East Coast this month showed why more information is needed on quake-making offshore areas threatening the country. Photo / Supplied

A multi-million dollar study will reveal the true threat New Zealand faces from an offshore area thought to be capable of triggering massive earthquakes and devastating tsunamis.

It focuses on what's called the Hikurangi subduction zone, where the Pacific Plate dives beneath the North Island off the East Coast.

It has potential to be the country's single biggest geological hazard, yet little is known about the danger it poses.

"Megathrust" quakes in subduction zones have been the cause of major disasters, including the 2004 Boxing Day tsunami that killed 230,000 people around the Indian Ocean, and the 2011 Tohuku tsunami in Japan that left nearly 16,000 people dead.

While geoscientists have unearthed evidence showing that the East Coast had been hit by large tsunami and megathrust earthquakes in the past, much about the size and frequency of those events remained unclear.

Dr Laura Wallace of GNS Science, which is leading a major collaborative project that has just received $6.5 million in Government funding over the next five years, said there was an urgent need to better understand it.

The project will be leveraging off of more than $25 million in international investment planned over the next several years to further probe offshore records of past and present movements along the zone, and gain more insights into the physical processes involved.

The project links in with plans by the 26-nation International Ocean Discovery Programme (IODP) to create an underwater earthquake observatory, with an expedition in 2018 set to drill three holes of up to 1.5km deep in the seafloor in different parts of the plate boundary, about 40 to 80km east of Gisborne.

"Because New Zealand's historical record is so short, we have no idea what the Hikurangi subduction zone is capable of - it is arguably the largest, unknown, geohazard facing the country," Wallace said.

If a magnitude 9.0 earthquake occurred on the subduction zone, it could cause strong shaking and damage throughout much of the North Island.

"We can see from GPS measurements that large parts of the subduction zone is basically locked up and probably building up stress that will eventually be relieved in a very large earthquake - such an event would rupture not just beneath the offshore area, but also continue beneath the North Island, with very strong shaking associated with it."

If the subduction zone ruptures in a magnitude 9 earthquake, it could threaten tens of thousands of lives, cause billions of dollars in damage and impact large population centres including Wellington, Auckland, and Christchurch.

As part of the project, Dr Philip Barnes of the National Institute of Water and Atmosphere will lead a voyage in November to investigate submarine sediment deposits and search for offshore clues of past megathrust quakes.

The new scientific information provided will eventually help to inform improved tsunami evacuation procedures, help guide engineering practices to lower risks and slash the costs of economic recovery, and allow asset and insurance managers to make better decisions.

Wallace said seafloor monitoring capabilities developed as part of the project would give New Zealand the technology needed to eventually progress to an earthquake and tsunami early warning system, similar to those being developed in Japan and the US.

"There's a real need for more instrumentation to respond and to understand these offshore quakes more quickly, such as the recent earthquake and small tsunami generated offshore of Te Araroa on September 2," she said.

"We will be developing New Zealand-based capability in seafloor monitoring instrumentation, so that potentially in the future, we could develop a permanent system that's cabled to land, giving us data on earthquakes and approaching tsunamis in real time."

The project is supported by the Ministry of Business, Innovation and Employment's Endeavour Fund, which this year awarded researchers $209 million over five years.

- NZ Herald

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