Finding faults

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Just after midnight on November 14, 2016, more than 24 fault lines around Kaikōura ruptured in spectacular fashion. One of these rifts—the previously unmapped Papatea Fault—threw up a few extra surprises for scientists.

Normally, the rupture of a fault is caused by a build-up of stress. But Papatea was stress-free until its rupturing neighbours squeezed it, triggering a violent fracture. In a matter of seconds, the earth split open along 19 kilometres and sections of mountainous land were shifted upwards by eight metres. Of the 24 fault lines, Papatea produced the largest vertical movement.

At first, Papatea’s behaviour confused scientists, because the fault wasn’t under strain. In a study published in Science Advances in October, New Zealand and Canadian researchers used before-and-after images of the fault line to create a model and figure out what happened.

It’s another unusual aspect of the most complex earthquake ever studied, with implications for assessing seismic risk. Current earthquake forecasting is based on the strain model, where faults accumulate tension until they fail, but Papatea shows that displacement is also a risk, says Mark Stirling, chair of earthquake science at the University of Otago: “A ten-metre displacement in a built-up area or beneath a critical facility, such as a large dam, would have significant consequences.”

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