Scientists drill alpine fault for first time

Last updated 12:20 16/03/2011

 

A New Zealand-led team of international scientists has successfully drilled through the alpine fault in the western South Island, the first phase of a project to learn about earthquake mechanisms on the fault.

The scientists drilled adjacent boreholes to depths of 101m and 152m on river terraces next to Gaunt Creek, near Whataroa 140km south of Greymouth on the West Coast, early last month.

They collected rock cores and made geophysical scans of the borehole walls, project co-leader Rupert Sutherland of GNS Science said today.

"They installed permanent monitoring instruments to record temperatures, pressures, and seismic activity inside the boreholes before back-filling both holes," he said.

"We were astonished that we managed to collect such high-quality rock cores across a zone that has been smashed by literally thousands of magnitude 8 earthquake movements over millions of years."

The alpine fault defined the Australian-Pacific plate boundary. Visible from space, it extended for about 650km from south of Fiordland along the western spine of the Southern Alps and into Marlborough.

Geological evidence suggested it ruptured every 200 to 400 years, producing earthquakes of about magnitude 8 that caused strong ground shaking throughout much of the South Island.

It had never been drilled through before.

The rock cores have been taken to Otago University, where images of the core surfaces and scans of their physical properties, such as density, magnetic properties, and electrical resistivity would be studied, he said.

Samples would also be sent to Germany, the United Kingdom, United States and Australia for additional laboratory analysis.

Scientists would take some years to analyse the cores fully in combination with data from geophysical scanners that were lowered down both boreholes, Dr Sutherland said.

Sensitive seismometers near the base of both boreholes were now recording very small earthquakes associated with the fault, and an array of temperature and fluid pressure sensors was monitoring any changes.

Scientists found the alpine fault acted as a barrier to fluid flowing through the rocks on either side, Dr Sutherland said.

"Repeated earthquakes have shattered the mountains and created myriad fractures that water can filter through, but rock along the fault plane itself is so finely crushed that it behaves like clay and creates an underground dam."

The scientific analysis and results would eventually turn into engineering standards and other preparations that meant New Zealand would be better prepared for the big quake when it happened, Dr Sutherland said.

The scientists hoped to learn how large continental faults evolved and generated earthquakes.

- NZPA