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What Lies Beneath? Earthquake Expert Talks Rocks

20 October 2003

What Lies Beneath? Earthquake Expert Talks Rocks
Geology Professor Rick Sibson to give Distinguished Research Medal Lecture

“What unknown affinity
Lies between mountain and sea
In country crumpled like an unmade bed?”

The answer to that Denis Glover poem* is, quite simply, “Plate Tectonics”.

Plate Tectonics? Indeed, says Rick Sibson. He should know -- he’s been studying the fault zones crumpling this nation’s bed for over 25 years. A University of Otago Professor of Geology, and one of only seven living New Zealanders made a Fellow of the Royal Society of London, Sibson will be presenting his Distinguished Research Medal Lecture at the University this week: Inside an Earthquake: the Rock Evidence.

His audience will accompany Sibson on his life-long journey in search of rocks from ancient and active fault zones. Not just any old rocks, mind you, but ones that help us map the movement of past earthquakes, and which provide us some understanding of the forces that drive that action.

The Outer Hebrides, the hills of Juneau Alaska, the Makran region of Iran, the deep mines of Val D’Or, Quebec, the Mt Isa mining district of Australia, California’s San Andreas Fault, and the trails of Westland, have all been Sibson’s “laboratory”. Earthquakes, both ancient and modern, have left a distinctive geological imprint on each of these regions, convincing Sibson that earthquakes are just “too important” to be left to seismologists alone.

People often associate the study of earthquakes with the jagged lines scratched on a seismograph. But Sibson argues that earthquakes are more properly the domain of geology, touching as they do every crease and wrinkle and fold on the surface of the earth. They are also quite astonishingly “useful”, he says.

Wellington Harbour, for example, which provides safe sanctuary to moored vessels, is the result of continuing earthquake activity along a series of major faults. And every miner knows that you find gold and other valuable minerals along old fault zones. The legendary Californian, Australian, and Canadian Gold Rushes, as well as smaller deposits in Otago such as Macraes Mine, occurred because ancient earthquake ruptures allowed massive amounts of very hot, pressurised water to flow intermittently through narrow fault zones, creating the mineralisation we now prize.

“Wellington Harbour contains about one cubic kilometre of water” Sibson explains. “If you heated the water up to about 300 degrees, and it flowed from the right source rocks through the Wellington fault, it might result in a rather puny gold deposit of about 10 tonnes.”

By contrast, the gold now being mined at Macraes Mine probably resulted from fluid flowing in many thousands of pulses following successive earthquakes, the total fluid volume being equivalent to about 10 Wellington Harbours.

“The significance of these studies,” he says, “is that in at least some settings earthquakes are triggered not only by stress accumulating, but also by (water) pressure build up”.

Earthquakes have other important uses as well. Without them, the South Island would simply not exist. “We exist only because we are being actively shortened east-west along the plate boundary. If that didn’t happen, we wouldn’t be above sea level.”

But such activity has its risks, he concedes. Lurking beneath the stunning valleys and mountains of Westland and Fiordland is enough potential force to release about 30 times as much energy as the earthquake Southerners experienced last August. “And the shaking will go on perhaps 10 times as long.” The last earthquake of such magnitude occurred around 1717.

Hold on to your beds…

WHAT: Distinguished Research Medal Lecture – “Inside an Earthquake: The Rock Evidence”
WHO: Professor Richard Sibson, Department of Geology, University of Otago
WHEN: Wednesday 22 October, 5.00 pm
WHERE: Castle Lecture Theatre 1, Albany St.

* From Arawata Bill – The Search in Denis Glover, Selected Poems edited by Bill Manhire, (Victoria University Press)

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