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Drilling deep into the Alpine Fault

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« on: February 10, 2011, 08:01:26 pm »


Alpine Fault dams water to east, study finds

By PAUL GORMAN - Science Reporter - The Press | 7:26AM - Wednesday, 09 February 2011

IMMENSE FORCES along the South Island's Alpine Fault have created a waterproof seal between two of the world's major crustal plates.

Scientists working on the Alpine Fault drilling project near Whataroa on the West Coast have found that a belt of intensely crushed, powdery clay around the immediate fault is keeping huge quantities of water underneath the Southern Alps — on the Pacific Plate - from escaping on to the Australian Plate on the western side of the fault.

Project co-leader Dr Rupert Sutherland, of GNS Science, said the fault was acting like a "large umbrella or dam".

Drilling down into and across the fault had effectively been like "punching a hole in a dam" and had created a sudden waterfall underground.

The speed of flow of water from one side to the other was equal to that of a 33m-high waterfall, he said.

"Right at the beginning we wondered if the fault itself would be a seal for water flow, which is quite important because the fluid processes on a fault make a difference to friction."

"I was quite surprised though at the magnitude of the pressure difference across the rocks."

"The rock has been absolutely crushed into a very fine powder that doesn't let water go through it because it is so fine- grained."

The discovery was one of the first of the international drilling project. More research was needed on the fluid characteristics of the fault zone to be able to draw definite conclusions, Sutherland said.

The project at Gaunt Creek has attracted more than 20 researchers and engineers from around the world.

The Alpine Fault runs more than 450km from Marlborough down the western edge of the Alps and offshore at the entrance to Milford Sound.

Researchers believe the fault ruptures every 300 or so years and last moved in 1717.

It is expected to generate a magnitude-8 earthquake when it next yields to pressure from the Australian and Pacific plates.

Sutherland said site work would be completed in the next day or two.

The project's underground fault observatory was now in place, with five fluid pressure sensors, 25 temperature sensors and two seismometers installed underground in the two bore holes.

The drilling crew had sealed the bottom 100m of the 151m hole with a cement and bentonite (clay) solution and also the bottom 82m of the 100m bore.

Three tonnes of rock cores — about 100m — had been carefully collected and were being sent to Otago University to be analysed over the next few months, he said.

"It's been a complete success, but twice as hard as I thought it would be."

"There's always issues — technically it is very difficult work; the first time it has ever been done."

Scientists might return to drill deeper into the fault next summer, but that still had to be finalised, Sutherland said.


http://www.stuff.co.nz/the-press/news/canterbury-earthquake/4634396/Alpine-Fault-dams-water-to-east-study-finds
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