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NZ’s major fault lines not connected through central Cook Strait

Geologists and other scientists have long believed that the major fault lines which run through Wellington and the Wairarapa run uninterrupted under the Cook Strait and reappear in the South Island around Clifford and Cloudy Bays. This was before they had the technology and the resources to take a really good look under the sea.

Scientists have recently discovered there is no connection between major fault lines in the North and South Islands. This is a very important finding as it means that if an earthquake occurred any fault rupture may be contained to one island. The discovery, which was announced on 26 August, is part of research by the National Institute of Water & Atmospheric Research (NIWA), GNS Science and Victoria University. The project dubbed It’s Our Fault is unearthing new information to assess the potential hazard of earthquakes in New Zealand. The Wellington and Cook Strait region is New Zealand’s most active seismic risk area.

The project involved scientists from NIWA mapping about 20 active faults in the Wellington and Cook Strait region capable of generating earthquakes of magnitude M7 or greater. They used NIWA’s deepwater research vessel Tangaroa to study the seafloor and the sediment layers beneath the seafloor. The scientists used this information, combined with new and archived seismic profiles from other sources, to create the first clear picture of all active faults in the offshore region. They discovered many new submarine faults, and that while some of the faults are the offshore extensions of large faults on land, others are entirely submarine. They found that none of the surveyed faults connected up between the North and South Islands.

NIWA’s Ocean Geology Principal Scientist, Dr Philip Barnes says the new findings are a breakthrough for earthquake research in New Zealand.

“We finally know what the big faults between Wellington and Blenheim are doing. By combining the seafloor imagery with other data of the faulted sediments and rocks beneath the seabed we have been able to get a much more accurate idea of the location, size and history of faults in the area.”

For the first time NIWA scientists have also been able to use high resolution images of the sedimentary layers below the seabed to assess the prehistoric earthquake activity of some of the faults – dating back as far as 18,000 years ago.

Dr Barnes says they can now estimate the interval of time between earthquakes on each fault, providing new estimates of the likelihood of earthquakes occurring.

GNS Science Geologist, and Project Leader for the It’s Our Fault project, Russ Van Dissen says “Knowing that there is no connection between the North and South island major fault lines will have a critical impact on the accuracy of modelling to assess earthquake risk.”

“We thought the faults were connected, and the thing we were most concerned about was whether a big South Island earthquake would rip right through Wellington or the Wairarapa. And the same was true for an earthquake starting up in the Wellington region, whether it would just go through the Cook Strait and wind up stopping somewhere in Marlborough. But with this new mapping that NIWA has done the picture is pretty clear that that is not the case. Earthquakes might start or stop in the Cook Strait, but probably not propagate through.”

The It’s Our Fault project is jointly funded by the Earthquake Commission, Wellington City Council and ACC.

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