Multinational probe of Hikurangi subduction zone gets underway - 16/10/2017

Scientists are preparing for New Zealand’s largest ever deployment of seafloor earthquake recording instruments in a bid to learn more about the earthquake behaviour of the tectonic plates beneath the east coast of the North Island.

The seismometers will record earthquakes and slow-slip events that occur in an area known as the Hikurangi subduction zone which marks the boundary of the Pacific and Australian tectonic plates.

It is arguably New Zealand’s most significant active fault and is capable of generating a magnitude 8.5 earthquake that, in addition to widespread ground shaking, is also likely to produce a tsunami, coastal uplift and subsidence, landslides and liquefaction.

Involving scientists from New Zealand, Japan, the US, and the UK, the large-scale collaborative project is aimed at better understanding the potential threat to New Zealand from the Hikurangi subduction zone.

Scientists Dan Bassett, Dan Barker, and Katie Jacobs, all of GNS Science, with some of the recording instruments that will be placed  on the seafloor off the East Coast as part of the project to study the Hikurangi subduction zone. The instruments are on loan from Japan for the duration of the project. Photo – Margaret Low, GNS Science.

Scientists Dan Bassett, Dan Barker, and Katie Jacobs, all of GNS Science, with some of the recording instruments that will be placed on the seafloor off the East Coast as part of the project to study the Hikurangi subduction zone. The instruments are on loan from Japan for the duration of the project. Photo – Margaret Low, GNS Science.

It will kick off in late October when NIWA’s research ship Tangaroa will lower 100 specially made seismometers, known as Ocean Bottom Seismometers and on loan from Japan, onto the seafloor in a grid pattern running from offshore Wairarapa north to east of Ruatoria.

In addition to the seafloor instruments, scientists will deploy more than 200 land-based seismic instruments across the Raukumara Peninsula.

The seafloor and land-based instruments will record echoes from within the Earth from both naturally occurring earthquakes and from acoustic signals generated by a US research ship, Marcus Langseth, which will bepositioned off the East Coast. This will enable scientists to create images of the plate boundary fault zone both underneath the sea and beneath the land.    

The cat scan-like images will show the structure of the Earth’s crust down to a depth of about 30km.

In most places over the study area the images will be two-dimensional, or like slices through the Earth. However, a more intensive survey area northeast of Gisborne will produce three-dimensional images of the plate boundary collision zone.

The images will show the position of the two tectonic plates and also help scientists determine the physical properties of the various rock layers that make up the subduction zone.

The nature of the rock material on the grinding surface of each plate affects how the two plates move past each other. 

Using this information, scientists can learn more about how the plates are locked together at numerous points along the subduction zone.

Scientists from four countries are working together to produce the most detailed subsurface images yet of the Hikurangi Subduction Zone. The graphic shows planned deployment lines for marine and land seismometers that are a key part of the project. Spacings of the Ocean Bottom Seismometers will be between 10km and 15km. In the more intensive survey area east of Gisborne, spacings will be 2km between instruments. In the on-land part of the project, instruments will be placed on the ground at 2km intervals. All the instruments will record marine seismic pulses from a ship and this will enable scientists to develop cat scan-like images of the Earth’s crust to a depth of about 30km. The survey area is over the region where megathrust earthquakes and slow-slip earthquakes are known to occur. Scientists regard this as the best place in the world to study and understand these types of earthquakes.

Scientists from four countries are working together to produce the most detailed subsurface images yet of the Hikurangi Subduction Zone. The graphic shows planned deployment lines for marine and land seismometers that are a key part of the project. Spacings of the Ocean Bottom Seismometers will be between 10km and 15km. In the more intensive survey area east of Gisborne, spacings will be 2km between instruments. In the on-land part of the project, instruments will be placed on the ground at 2km intervals. All the instruments will record marine seismic pulses from a ship and this will enable scientists to develop cat scan-like images of the Earth’s crust to a depth of about 30km. The survey area is over the region where megathrust earthquakes and slow-slip earthquakes are known to occur. Scientists regard this as the best place in the world to study and understand these types of earthquakes.

New Zealand project leader Dr Stuart Henrys, of GNS Science, said the data will help the understanding of why different areas of the plate boundary are behaving so differently.

Some parts of the plate interface slide past each other every year or so in events called slow earthquakes, while other parts appear to be stuck fast and are storing energy for a future large earthquake.

Lead US investigator from the University of Texas, Harm Van Avendonk, said the project would produce detailed images of the entire fault system across this part of the North Island.

“A better understanding of what causes the marked differences in tectonic behaviour on this plate boundary will help New Zealand government agencies in their efforts to reduce the danger posed by earthquakes and tsunami in this area,” Dr Van Avendonk said.

“What makes subduction zones rupture in huge, tsunami-generating earthquakes is one of the most pressing questions facing Earth scientists today.”

Dr Henrys said the more intensive study zone northeast of Gisborne was an exciting prospect as it should provide the best images yet, anywhere in the world, of the zone where slow-motion earthquakes are known to occur repeatedly.

Scientists regard the area off the East Coast as the best place in the world to study and understand the phenomenon of slow-motion earthquakes. This is because it is more accessible and shallower than other subduction zones around the world, which are typically further from shore.

Dr Henrys added that the marine seismic reflection system being used in this project involved releasing compressed air quickly into the water with sensors towed behind the ship picking up ‘echoes’ from different rock types in the upper layers of the Earth’s crust.

Note: The project is called Seismogenesis at Hikurangi Integrated Research Experiment or SHIRE for short. It is one of several large multi-agency research projects working to better understand the Hikurangi subduction zone. Over the next four years international research organisations are planning to invest between $30M and $40M in science projects to investigate the Hikurangi subduction zone.  

SHIRE is funded by the US National Science Foundation and the Ministry of Business, Innovation and Employment. Participating organisations are GNS Science, University of Texas, JAMSTEC, Imperial College London, Penn State University, University of Southern Mississippi, University of Southern California, California State Polytechnic University, Victoria University of Wellington, and the University of Tokyo. The Marcus Langseth is owned by the US National Science Foundation and is operated by the Lamont-Doherty Earth Observatory of Columbia University in New York. 

https://www.tvnz.co.nz/one-news/new-zealand/team-international-scientists-studying-active-zone-off-north-islands-east-coast-capable-triggering-megathrust-quake