Scientists set to probe Alpine Fault

Scientists are preparing to drill deep boreholes into the South Island's Alpine Fault, in a bid to uncover clues about what happens before major earthquakes up to 10 times the size of the Canterbury quake.

Though the Alpine Fault, running the length of the South Island, has not produced a large earthquake in three centuries, it is regarded as one of New Zealand's most hazardous faults.

The Government has been told that there is a 20 per cent chance of a big quake on the fault in the next 20 years, compared with a 15 per cent chance of a major one affecting Wellington over the next 50 years.

The Alpine Fault last ruptured 290 years ago and moved the ground 27m horizontally along 300km.

Geophysicist Rupert Sutherland, of GNS Science, is part of a group of 65 international researchers which wants to drill boreholes directly into the faultline - eventually to a depth of 1.5km - to investigate its structure, mechanics and evolution.

Dr Sutherland was last year granted $920,000 over three years to discover what physical conditions in the Earth's mid-crust allow active continental faults to evolve and generate earthquakes, while Jennifer Eccles, of Auckland University, was awarded $300,000 to analyse data from seismometers to be installed at various depths in one of the boreholes.

Millions more dollars are expected to be contributed to the project by overseas researcher so that measurements, analysis of core samples and establishing long-term observatories within the boreholes can provide a basis for future deeper drilling experiments.

The project will be kicked off with two boreholes only 150m deep, but geologists have previously talked about the potential to eventually drill a borehole up to 5km deep, which could give new clues about what happens in the lead-up to a major quake about 10 times as powerful as the Canterbury's 7.1-magnitude quake on September 4 last year.

Preparations for drilling are expected to start on January 24 at Gaunt Creek, near Whataroa, on the West Coast.

Professor Kevin Furlong, from Pennsylvania State University in the United States, told NZPA that the initial boreholes would be shallow, but the project would provide a focal point for a broad range of research into the faultline.

The Alpine Fault runs for more than 650km, from south of Fiordland and along the spine of the South Island into Marlborough. It has moved an average of 27mm a year over the past 50,000 years - but this has included catastrophic quakes in which it has moved 8m in seconds.

It ruptures every 200 to 400 years - producing earthquakes of about magnitude 8 - most recently in 1717 along nearly 400km of the southern two thirds of the fault.

A similar project at Parkfield on California's San Andreas Fault cost $44 million and has equipment inserted 3km deep.

- NZPA