Drilling Planned as Part of Alpine Fault Research

Drilling Planned as Part of Alpine Fault Research

New Zealand scientists are taking the first steps in an ambitious long-term research project focussed on understanding how the Alpine Fault in the South Island has evolved and produces earthquakes.

Subject to consent applications being successful, the research team intends to drill two boreholes about 150m-deep and 50m apart in Gaunt Creek, near Whataroa in early 2011.

The boreholes will enable the scientists to examine unweathered rock samples from the shallow part of the Alpine Fault, install earthquake recorders and weather gauges, and measure temperature and water pressure.

The borehole and rock samples will be analysed using a variety of geophysical logging tools, including a camera-like device that takes photographs of the interior of each borehole and an X-ray scanner to examine each rock's mineralogy and structure in fine detail.

Researchers at GNS Science, the University of Otago, and Victoria University of Wellington are coordinating this work, in collaboration with colleagues at The University of Auckland, the University of Canterbury, and organisations in Germany, the United Kingdom, and the United States.

In preparation for the summer's study, the research team has submitted consent applications to the Department of Conservation and the West Coast Regional Council. The outcomes of these applications are expected to be known in late 2010.

Project co-leader John Townend, of Victoria University of Wellington, said in tandem with the drilling planned for Gaunt Creek, a number of more conventional surface surveys will be conducted using seismic imaging equipment and geological and geochemical mapping.

“Separate consent applications for those surveys are also in preparation. A separate site in the Whataroa River valley is also being considered for a later stage of the project,” Dr Townend said.

A tendering process would be conducted later this year to identify contractors for the Waitangitaona River valley drilling and borehole logging.

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

The Alpine Fault, which is visible from space, extends for more than 650km from south of Fiordland along the spine of the Southern Alps and into Marlborough. Geological evidence suggests it ruptures every 200 to 400 years producing earthquakes of about magnitude 8 that cause strong ground shaking throughout much of the South Island.

Scientists believe it last ruptured in 1717 in an earthquake that produced about 8m of horizontal movement along the fault. In between major ruptures, the fault is locked and produces few significant earthquakes.

"A key motivation for this project is to obtain new understanding of how large faults evolve and generate earthquakes in what is an amazing natural laboratory,” Dr Townend said.

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.

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. Eventually, by drilling into the crust, researchers hope to sample these rocks which are young (less than 1 million years old) and from the zone where earthquakes occur.

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 also hope to learn about chemical and physical changes occurring at various depths.

ENDS

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