Scientists probe Alpine Fault


An international science team plans to drill a 1.3km-deep hole into the Alpine Fault, near Whataroa, later this year to find out more about the ‘inner workings’ of New Zealand’s main faultline, and the earthquakes it produces.
The team, led by New Zealand scientists, will start drilling in early October.
Project co-leader Rupert Sutherland, of GNS Science, said drilling technology had matured to a point where a lot of crucial information about the fault’s inner workings could be gleaned from a deep borehole.
“The Alpine Fault saves up all its energy for one big showdown every few hundred years. In between its big ruptures, it stays locked and produces minor earthquakes and tremor,” Dr Sutherland said.
An international science panel has reviewed the project plans in detail and concluded that every precaution will be taken to ensure the operation is safe.
The Whataroa location is regarded by scientists as one of the best sites in the world. There, scientists will be able to examine rock samples extracted from the fault zone and install sensitive monitoring equipment to record small earthquakes and measure temperature, pressure and a range of chemical conditions.
The project will be one of the first attempts to probe the inside of a major fault before it ruptures. As part of a lead-up to this year’s project, a smaller group of scientists drilled two boreholes to about 150m in early 2011.
It found the existence of a finely-ground, impermeable layer of rock in the centre of the fault zone, holding back large amounts of fluid on the upper east side of the fault. This was a surprise as it had not been anticipated from the many surface studies of the fault dating back to the 1970s.
Scientists believe the large difference in fluid pressures on either side of the fault zone could play a role in initiating the first slipping movements as an earthquake begins.
Their aim is to intersect the fault at about 1km depth and drill a further 300m into the underlying Australian tectonic plate. They will take rock samples from the borehole for analysis.
The inside walls of the 10cm-diameter borehole will be studied with a camera-like device. They will also lower other scanning equipment to examine rock structures.
Near Whataroa, the fault cuts into the Earth’s crust at about 45 degrees, which means it can be investigated with a vertical borehole without the need for expensive angle drilling. Scientists believe it last ruptured in 1717 in an earthquake that produced about 8m of horizontal movement and vertical movement of 1m to 2m along the fault.
The project involves scientists — and funding — from more than a dozen organisations in New Zealand, Canada, France, Germany, Japan, the United Kingdom, and the United States. It is being led by scientists from GNS Science, Victoria University of Wellington, and the University of Otago.