Quick Dips

Explosive Investigation
of Earthquake Mystery

More than $5 million is being invested by the United States in an explosive "CAT scan" of the South Island that should help evaluate earthquake risks in the region.

A team of scientists from Victoria University, the Institute of Geological and Nuclear Sciences and US institutions will work its way across the South Island, starting along the Rangitata River on the Canterbury side, crossing the Alps and continuing through Whataroa to the West Coast. Along most of the way they will be drilling holes every kilometre, setting off small explosions in them and recording the reflections sent back from rock layers below the surface.

Computer analysis of these reflections will produce a cross-section view of the rock layers beneath the central South Island to depths of about 40 kilometres, indicating the types of rock there and how the rock is being deformed. The imaging process works in much the same way as a CAT scan in a hospital, but uses explosions instead of X-rays. The study will begin this month with preliminary work in the Lake Pukaki region, and the main part of the project will get under way next year.

Victoria University geophysicist Dr Tim Stern, a principle investigator for the project, says the central South Island holds mysteries for geologists. The region is important because the Pacific and Indian-Australian plates on either side of the Alpine Fault are moving past each other, causing a zone of deformation of the Earth's crust about 100km wide.

Such a situation could be expected to produce large earthquakes, but this does not seem to be happening.

"We see much earthquake activity along the eastern North Island, and in Fiordland, but comparatively little activity along the most conspicuous section of the Alpine Fault -- between Arthur's Pass and Fiordland," Stern says. "Some argue that the lack of activity is a consequence of the short time we have reliably recorded earthquakes -- 40 years or so -- compared to the earthquake cycle' that may have a time span of some 200 years. This school of thought proposes that the central portion of the Alpine Fault is a zone of potentially very large earthquakes but we just have not waited long enough to witness them."

However, there are two ways that deformation can occur at moving plate boundaries. As well as the sharp movement of earthquakes, the deformation can happen gradually as rocks flow "like treacle".

"The opposing view is that deformation is distributed over a much wider area of the South Island and is occurring largely by a flow in the crustal rocks. If so, the seismic risk for the central South Island would be low," says Stern.

The planned research should show how, and where, deformation is occurring relative to the Alpine Fault. It may also reveal why crustal rocks "flow" in one place but fracture and cause earthquakes in another.

New Zealand is an ideal location for this sort of study. The data obtained in the traverse can be related to an existing high-quality base of information, while low population levels in the region make possible work that could not be carried out in, for example, southern California.

A broad range of other studies is planned to supplement the seismic investigations and give a detailed knowledge of the plate boundary and its deformation, according to Dr Fred Davey of the IGNS. The institute's contribution to the project is part of a larger study of the whole plate boundary zone, which extends along the Kermadec Trench toward Tonga, northeast of New Zealand, and southward along the Macquarie Trench.

"The history of deformation associated with this plate boundary is important not only for understanding earthquake risk, but also for resource exploration, climate change, the influence of mountain uplift, and other significant scientific questions," says Davey. The institute has developed computer software which can dramatically improve the resolution of seismic data.

Geologically, New Zealand is a narrow piece of continent surrounded by water. This will allow the team to use a ship on each side of the South Island to carry out a specialised procedure called "onshore-offshore seismics", which is proposed for the second year of the project. The ships will set up acoustic signals using devices called "air guns" and the resulting seismic waves will be recorded on land as part of the study.

"To our knowledge there is no other continental plate boundary zone in the world where the opportunity for this onshore-offshore experiment exists, on both sides of the boundary," says Stern.

The project is of great interest internationally and has been made possible by a grant of $5.4 million from the US National Science Foundation, approved earlier this month to supplement New Zealand funding. Scientists from several US institutions including the University of Southern California, Woods Hole Oceanographic Institute, San Diego State University, and the Massachusetts Institute of Technology will take part in the work over the next four years.