Coast-to-Coast Seismic Study to Yield Deep Secrets

4 MAY 2011

Scientists from New Zealand, the United States, and Japan will spend the next three weeks using a 100km-long array of earthquake recorders to find out more about the geological stucture deep under the lower North Island.

They will place portable seismographs at 900 locations right across the lower North Island between the Wairarapa and Kapiti coasts for the 21-day project. The instruments will be only 100m apart in some places.

By detonating explosive charges in 50m-deep boreholes near some of the seismographs and recording the echo on their portable equipment, scientists will be able to build a three-dimensional image of rock structures and the tectonic plate boundary beneath the Lower North Island. The technique can be likened to producing a CAT scan of the earth’s crust.

The project will be one of the biggest temporary deployments of seismic equipment in New Zealand’s history. It will also be the first time the boundary zone of the two tectonic plates under the Wellington region has been studied in such detail.

The Pacific plate, to the east of the North Island, is being forced under the Australian plate but the two giant plates are not sliding past each other uniformly. In places, they are locked and significant amounts of energy are being stored in this locked part of the interface.

The boundary between the two plates is about 15km beneath the Wairapapa coast and gets progressively deeper to the west. It is 25km below Wellington and about 35km below the Kapiti coast.

“We have a limited knowledge of how the plates lock together deep under the North Island,” said project co-ordinator, Stuart Henrys, of GNS Science.

“This project will tell us about the properties of the locked interface and how this interface might behave when it ruptures in an earthquake,” Dr Henrys said.

“Knowing more about the earth’s crust beneath the Wellington region will lead to improved design of buildings and infrastructure. It will also help communities become better prepared for future earthquake and tsunami events.”

The small portable seismographs, which have been brought to New Zealand from the US and Japan, will be placed in 30cm-deep holes so they make firm contact with the ground. Some will be placed near roads, but are unlikely to be seen by motorists. Much of the project is taking place on remote private land and the scientists have spent several months approaching landowners seeking permission to carry out their work.

Scientists and technicians will start deploying the earthquake recording equipment on May 7 and about a dozen detonations will take place over several nights starting on May 10. By the middle of May, when the project is scheduled to finish, scientists will retrieve the instruments.

The large amount of data collected during the project will be combined with data from a similar offshore experiment completed in 2010. Scientists will spend some months analysing the data and initial findings will be available at the end of this year.

The organisations involved in the project are GNS Science, Victoria University of Wellington, the Earthquake Research Institute at Tokyo University, and the University of Southern California.

ENDS

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