NZ And Japan Combine in Wellington Quake Study

NZ And Japan Combine in Wellington Quake Study

New Zealand and Japanese scientists are combining forces to investigate the boundary between the Pacific and Australian tectonic plates under Wellington to better understand its threat to New Zealand.

The collision zone between the two plates is about 25km beneath Wellington and the two-year project is designed to produce precise information on the structure and processes occurring at this boundary in the Earth’s crust.

The Pacific and Australian tectonic plates are locked beneath Wellington and, based on the behaviour of similar locked plates in other parts of the world, scientists expect this plate boundary will eventually rupture and produce a large, damaging earthquake.

Evidence for large earthquakes from this source in the geological past is incomplete as it is hard to distinguish this type of earthquake from one on a fault above the plate boundary.

The project is being coordinated by GNS Science and Victoria University, in conjunction with the Ministry of Economic Development’s Crown Minerals group, and with support from scientists of the Earthquake Research Institute at Tokyo University.

In the first phase, 50 portable seismometers on loan from the United States have been deployed around the Wellington region, supplementing permanent seismographs operated by the GeoNet Project. Scientists will use this network of instruments, known as a seismic array, to get an accurate picture of earthquake activity under the lower North Island.

The instruments were deployed in November 2009 and will remain in place until March 2010 recording earthquakes in the lower North Island. As well as recording many hundreds of small earthquakes, they will also record thousands of sound sources made by a seismic survey ship off the North Island coast.

A second part of the project, starting next month, will see the 50 portable seismometers redeployed in a straight line between the Kapiti and Wairarapa coasts. The line of instruments will be extended off both coasts with 20 ‘ocean-bottom seismometers’ from Japan being placed on the seafloor.

The onshore and offshore instruments will remain in place for one month recording sound sources from a seismic survey ship undertaking an oil and gas prospectivity study of the nearby Pegasus and Wanganui basins for the Ministry of Economic Development.

In the final fieldwork phase of the project in early 2011, 900 portable seismometers from Japan and the US will be placed in a straight line across the lower North Island from coast to coast, again between Kapiti and Wairarapa. They will record seismic waves from borehole blasts set off by scientists.

The project will be one of the biggest seismic deployments in New Zealand’s history. It will also be the first time the plate subduction zone under the Wellington region has been studied in such detail.

Scientists will combine all the seismic recordings like a cat-scan to build a three-dimensional image of the subduction zone under the lower North Island.

They will then be able to compare this image with a similar 3D image of the subduction zone beneath Tokyo, which has a similar geological setting to Wellington.

Project co-ordinator Stuart Henrys, of GNS Science, said it was important to understand the physical processes that led to large subduction zone earthquakes.

“Together with our Japanese collaborators, we anticipate this research will lead to a better understanding of what controls seismic behaviour on the plate interface beneath the lower North Island.”

Dr Henrys said the project would produce many terabytes of data that would take several years to fully analyse. The data will be shared among the collaborating scientists for a combined research effort.

ENDS

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