New Zealand fault hole yields insights into earthquake processes

An international team of scientists have halted a bid to drill a 1.3-km-deep hole into the Alpine Fault in New Zealand's South Island because of equipment problems -- but they said Wednesday the hole had already yielded a lot of information.

Drilling had stopped at 893 meters -- the deepest ever borehole drilled into a fault in New Zealand -- and was probably 100 to 200 meters short of the fault itself, they said in a joint statement.

The Deep Fault Drilling Project at Whataroa, north of Franz Josef, was jointly led by New Zealand's Victoria and Otago universities and the government's Institute of Geological and Nuclear Sciences (GNS Science) and included scientists from more than a dozen countries.

They discovered that temperatures rose by more than 140 degrees centigrade for every kilometer below ground at the fault, which had important implications for the way rocks behaved and for understanding earthquake processes along major faults worldwide.

At 6 km below the Southern Alps mountain range, earthquakes stopped occurring because the rocks were so hot they become ductile and bent and flowed rather than fractured.

The scientists were installing equipment inside the borehole to measure a range of physical and chemical conditions.

"The decision to stop drilling was made after steel casing inside the borehole was damaged part-way down and could not be repaired," Dr. Rupert Sutherland, of GNS Science and Victoria University, said in the statement.

"It is definitely feasible to reach all the way to the fault and we are keen to attempt this in the future. The very high rate of temperature increase with depth means that deeper drilling may enable us to sample rocks and fluids and document processes that usually happen at totally inaccessible depths."

Project co-leader Dr. John Townend, of Victoria University, said they had installed a fiber-optic cable along the full length of the borehole that could be used to make seismic recordings as if they had dozens or even hundreds of seismometers strung out along the borehole.

"A special seismometer has also been installed in the borehole 400 meters below the surface. We will incorporate it into the existing seismic monitoring network to record earthquakes deep within the Alpine Fault zone," Townend said in the statement.

The Alpine Fault, the on-land boundary between the Pacific and Australian tectonic plates, moved about 27 meters horizontally every 1,000 years, in three or four separate large ruptures.

Scientists had evidence that it had ruptured 24 times in the past 8,000 years at an average interval of 330 years, although individual intervals ranged from 140 to 500 years.

The fault last ruptured 297 years ago in 1717 and scientists estimated it had a 28 percent probability of rupturing in the next 50 years, which was high by global standards. Endi