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Otago scientists part of planned deep drill probing of Alpine Fault

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Wednesday, 25 June 2014 11:08am

B Carpenter and Dr V Toy
Brett Carpenter (Penn State University) and Dr Virginia Toy (Otago) with an Alpine Fault drill core extracted during the 2011 lead-up project.

A New Zealand-led international science team that includes University of Otago geologists is planning to drill a 1.3km-deep hole into the Alpine Fault in the South Island later this year to find out more about the nature of the fault and the earthquakes it produces.

The scientists will drill a single deep borehole near Whataroa, north of Franz Josef on the West Coast, starting in early October. The location is regarded by scientists as one of the best sites in the world to study the inner workings of a major plate boundary fault.

The borehole will enable the scientists 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 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.

There have been a number of projects to drill into plate boundary faults after large earthquakes. The Alpine Fault 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 into the fault, also near Whataroa, in early 2011. That was the first substantial drilling investigation on the Alpine Fault.

One of the main findings of the 2011 project was 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 hope is that the proposed 1.3km-deep borehole will shed more light on the relationship between fluid pressure, the internal structure of the fault zone, and the mechanics of earthquakes.

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 using a variety of techniques. The inside walls of the 10cm-diametre borehole will be studied with a camera-like device that measures the shape and geometry of the borehole. They will also lower other scanning equipment into the borehole to examine rock structures in detail.

Project co-leader Rupert Sutherland, of GNS Science, says the Alpine Fault represented a major hazard to the South Island and drilling technology had matured to a point where a lot of crucial information about the fault’s inner workings could be gleaned from a borehole such as this.

“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 says.

"The borehole will be drilled using low-impact techniques used routinely in environmentally sensitive groundwater and geotechnical applications.

“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 Alpine Fault, which is visible from space, extends for about 650km from south of Fiordland along the spine of the Southern Alps and into Marlborough. It ruptures on average every 330 years, plus or minus 90 years, producing earthquakes of about magnitude 8 that cause strong ground-shaking throughout much of the South Island.

It is among the more active plate boundary faults in the world and is one of the most attractive large faults in the world to study because of its size, rate of movement, and accessibility. 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. In between major ruptures, the fault appears to be locked and produces mostly small earthquakes and deep-seated vibrations known as seismic tremor.

University of Otago Department of Geology members, guided by another project co-leader, Dr Virginia Toy, will be responsible for the rock samples recovered from the borehole. They will describe this core, and perform analyses of it that include feeding it through the CT scanner in the Oncology Department at Otago Hospital.

Dr Toy says “These images will reveal the internal structure of the core, which results from fracturing associated with the passage of seismic waves. Characterising this sort of fracturing is important if we are to make proper predictions of the type of seismic waves the Alpine Fault could produce during large earthquakes.”

These results will enable scientists to develop better models of local shaking hazard which can be used by engineers and planners.

See two short videos here:

Project overview – 4 min 40 seconds
Safety review video – 1 min 50 seconds

For more information, contact:

Dr Virginia Toy
Project Co-leader
Department of Geology
University of Otago
Tel 64 3 479 7506
Email virginia.toy@otago.ac.nz

Dr Rupert Sutherland
Project Co-leader
GNS Science
Mob 027 273 1164

Background

The Alpine Fault is the on-land boundary between the Pacific and Australian tectonic plates. It moves about 27m horizontally every 1000 years, in three or four separate large ruptures. In between major ruptures, it does not move at the surface. Scientists have evidence that it has ruptured 24 times in the past 8000 years. The average interval between ruptures was 330 years. However, individual intervals ranged from 140 years to 500 years. The fault has a 28 percent probability of rupturing in the next 50 years, which is high by global standards.

The two tectonic plates that meet at the Alpine Fault are moving past and pushing against each other and this forces the Southern Alps higher. It also uplifts rocks from deep within the Earth’s crust. So rocks found at the surface at the fault have been uplifted quite quickly in geological terms from inside the fault zone.

By comparing rocks retrieved by drilling with rocks exposed at the surface, the research team hopes to discover how the Earth’s crust deforms during earthquakes. They will also learn about chemical and physical changes occurring at various depths inside the fault zone.

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