Study to unravel faultline secrets

A large research project about to get under way should help explain why parts of the major fault off the North Island east coast appear to be stuck fast, storing energy for a massive earthquake.

Scientists from New Zealand, Japan, the US and UK hope to get a better understanding of the potential threat posed by the Hikurangi zone. It is capable of generating an

8.5-magnitude earthquake, causing widespread ground shaking, and likely to produce a tsunami, coastal uplift and subsidence, landslides and liquefaction.

The research involves lowering

100 specially-made seismometers on loan from Japan onto the sea floor in a grid pattern running from offshore Wairarapa north to east of Ruatoria. More than 200 land-based seismic instruments will also be placed across the Raukumara Peninsula.

The instruments will record echoes from within the Earth from both naturally occurring earthquakes and from acoustic signals generated by US research ship Marcus Langseth, which will be positioned off the East Coast, GNS Science said.

Cat scan-like images created as a result will show the structure of Earth’s crust down to a depth of about 30km. The project will produce detailed images of the entire fault system across this part of the North Island.

‘‘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.’’

Scientists hope to better understand why the plates are locked together at numerous points along the subduction zone, while in others they slide past each other in the slow-slip events.

Lead US investigator Dr Harm Van Avendonk said: ‘‘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.’’

Learning why subduction zones ruptured in huge, tsunami-generating earthquakes was one of the most pressing questions facing Earth scientists.

Scientists regard the area off the East Coast as the best place in the world to study slow-motion earthquakes, because it is more accessible and shallower than other subduction zones around the world, which are typically further from shore. The ocean bottom seismometers will be lowered into place from Niwa ship Tangaroa, which will be leaving Wellington after Labour Weekend.

New Zealand project leader Dr Stuart Henrys, of GNS Science, said the more intensive study zone should provide the best images yet, anywhere in the world, of the zone where slow-motion earthquakes were known to occur repeatedly.

During the next four years international research organisations will spend $30-40 million investigating the Hikurangi zone.