New techniques used to investigate Port Hills rock falls

New scientific techniques used to investigate Port Hills rock falls

Researchers are using cutting-edge techniques never used before in New Zealand to establish the history of ancient earthquakes around the Port Hills.

University of Canterbury Rutherford Foundation post-doctoral fellowship recipient Dr Ben Mackey is investigating the fault that was previously unknown before the 22 February quake. The work is being done in collaboration with UC geologists Dr Mark Quigley and Dr Ben Kennedy.

“The idea behind the research is to try and find evidence of ancient earthquakes of similar intensity to the series we had last year,” he says.

“At present there is little constraint on how frequently the faults near Banks Peninsula rupture. I am going to try and find out how regularly they have ruptured in past major earthquakes, so that we may be more informed of when it could happen again.”

Dr Mackey says that because the Port Hills fault is hidden beneath Banks Peninsula and the Canterbury Plains, rather than exposed on the surface like the Greendale fault, he will need non-traditional techniques to ascertain the timing of previous earthquakes.

“Normally when a geologist tries to determine the timing of old earthquakes there is a fault that ruptures the ground where we can see the physical fault structure, trench across it, and work out when past events happened. But the fault that caused the 22 February quake is blind, meaning it did not propagate to the surface. This means we have to look for secondary evidence of ancient earthquakes, such as rock falls, rather than study the actual fault plane.”

Dr Mackey, who was awarded $190,000 to conduct the research, will painstakingly date boulders around the Port Hills to discover the timing of ancient rock falls, similar to those that occurred over the past year.

“The technique we are using is called cosmogenic nuclide exposure dating, and it’s never been applied in New Zealand on basalt, the rock type that forms Banks Peninsula. We collect surface samples from rock fall boulders, crush them up and extract small olivine crystals from the basalt. We can measure the concentration of the isotope helium-3 on a mass spectrometer,” he says. “The amount of helium in the crystals reflects how long the boulder has been sitting on the surface.”

“When a rock is exposed to the sky, it gets bombarded with cosmic rays, which cause small nuclear reactions in atoms within the rock. The by-products if these reactions (such as helium-3) accumulate in olivine crystals at a predictable rate, enabling calculation of the rock exposure age. Put simply, we are basically working out the sun tan of a rock, in order to figure out how long it has been lying in its position for.”

Dr Mackey, also a UC science and law graduate, used the technique while dating Ice Age floods in Idaho and analysing waterfall retreat in Hawaii.

“I saw images of Shag Rock before and after the quakes and thought this would be a good technique to apply in Banks Peninsula. Hopefully the information we collect can then be built into hazard modules in the future.”

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