The risk of death from rockfall in some Port Hills areas was up to 100 times higher than the risk of dying in a car crash, a geologist says.

GNS Science engineering geologist Chris Massey said residents near some earthquake-hit cliffs could have died had they not been evacuated after the February 2011 quake.

Speaking at the University of Canterbury at the weekend, Massey gave a presentation on rockfall risk.

He said geologists had assessed the risk of death from quake-hit cliff faces, including those in Redcliffs, Whitewash Head and Wakefield Ave in Sumner.

"If people hadn't been evacuated after the February 22 earthquake, they likely would have gone down the slope in the June 13 earthquake," he said.

Massey said the average annual risk of death for some of the houses near the affected cliffs was one in 1000, with "very, very few" areas as high as one in 100. The average annual risk of death from a car crash is one in 10,000.

Geologists measured cracking at the cliff top, the volume of material that had fallen after the quakes and the shape and height of the cliffs to determine the risk.

The calculations showed some properties would likely have been covered in rocks during aftershocks from the February 22 quake, he said.

The risk would rapidly decrease in the years immediately after the quake, before reaching a "plateau" after 10 years.

The team planned to reassess risk levels in five years to see how they had changed and how the quakes had affected the stability of the cliff faces, Massey said.

- © Fairfax NZ News

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