Fault line's movement studied

PAUL GORMAN/The Press

REVEALED: earthquake scientist Rob Langridge inspects the fault line in the Inchbonnie trench across the Alpine Fault.

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A gash has been opened up across the Alpine Fault, but the split has been made by geologists and not nature.

Scientists from New Zealand, Switzerland and the United States have spent the past three days on the West Coast at Inchbonnie, a dairy farming settlement about 60km from Greymouth that straddles the Alpine Fault.

The fault is the boundary between the Australian and Pacific plates, which are moving past and pushing against each other, forcing the Southern Alps higher.

One of the world's major fault lines, and visible from space, it runs for about 650km from the Wairau River valley through the central upper South Island, down the western side of the Southern Alps and out to sea near the Milford Sound entrance.

On Saturday, a 26m long and nearly 2m deep trench was dug in a paddock across a complex section of the fault, allowing geologists to set up a grid and get a three-dimensional view of movement on the fault.

Where the fault crosses the trench, layers of gravel and sand laid down by the nearby Taramakau River have been twisted and squeezed almost vertically.

Any wood found in the cross-section can then be carbon-dated, allowing geologists to calculate when the last major earthquake caused by the Alpine Fault occurred.

GNS Science earthquake geologist Rob Langridge said evidence from this latest trench, and previous trenches dug in the area, appeared to support the theory that shaking from a major Alpine Fault earthquake could be worse around the central South Island than previously thought.

Inchbonnie was at the junction of several faults, including the Hope Fault, and movement on the Alpine Fault there could effectively shift earthquake shaking along those other faults.

"It's like a freight yard where rail wagons are being changed on to different lines. So we've shunted off movement on to the Hope Fault -- the hazard goes down another line," Langridge said.

At Inchbonnie, the average slip rate of the fault was about 10mm a year, but that increased to more than 25mm further south-west on the fault.

The last major rupture on the fault has generally been acknowledged as happening in 1717, which generated an earthquake of at least eight on the Richter scale. There is also evidence of a similar-sized quake about 1620, and other fault movements about 1450 and 1100.

The consensus is that an Alpine Fault earthquake occurs on average every 250 to 280 years and that New Zealand is overdue for a big one.

However, Langridge believed the Inchbonnie work gave room for re-examining whether the 1717 and 1620 events might have been the same earthquake.

Geophysicists from Swiss technical institution ETH Zurich have also been at Inchbonnie, researching the position and behaviour of the Alpine Fault up to 1km below the surface using ground-penetrating radar and seismic surveying methods.

- © Fairfax NZ News