How the Kaikōura earthquake contributed to Gita's landslides

Monday, 26 February 2018

Gravel and water poured into David McIlroy's house at Rosy Morn as former cyclone Gita passed through.

Scientists are trying to work out how big a factor the November 2016 Kaikōura earthquake was in last week's landslides during the passage of former cyclone Gita.

GNS Science engineering geologist Sally Dellow said the supposition was the 7.8-magnitude quake had made landslides – some of which closed State Highway 1 north and south of Kaikōura – more likely.

More than 200,000 cubic metres of debris came down across State Highway 1 at a single site near Okiwi Bay, north of Kaikōura.

About 200 millimetres of rain fell in the catchments around Kaikōura in 18 hours during the storm. Based on detailed rainfall figures and analysis from hydrologists working in the area, such an event had a return period of 150 years or more, Dellow said.

Researchers would be acquiring data using LiDAR – which uses pulsed light to help generate precise, three-dimensional information about the shape of the Earth and its surface characteristics – to study the detailed topography after Gita.

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That would be compared with LiDAR from before and immediately after the earthquake.

'That will tell us where the sediment has come from, where it's ended up, how thick it is,' Dellow said. 'We will be able to trace that sediment through the landscape.'

In October 2017, GNS started a five-year, $8.2 million project under the Ministry of Business, Innovation and Employment's Endeavour programme to develop predictive tools for managing the risks from landslide and sediment hazards generated by earthquakes and post-earthquake rain and aftershocks.

'Evidence from previous earthquakes in New Zealand and overseas suggests that the frequency of landsliding after a large earthquake is significantly higher than before the event. This is because strong earthquakes cause slope cracking and generate a lot of landslide debris,' the project outline said.

'The debris generated by the Kaikōura Earthquake, when mobilised, will create new hazards, including further landslides, landslide dams and dam failures, rapid aggradation, and increased river channel instability, as the debris cascades from hill slope to sea.

'These hazards may persist for decades and therefore represent a prolonged risk that must be considered by the impacted communities and stakeholders.'

NZ Transport Agency earthquake recovery manager Tim Crow said about 300,000 cubic metres of material needed to be cleared from 60 highway sites on either side of Kaikōura after Gita. The largest at Okiwi Bay, north of Kaikōura, had around 200,000cu m of material.

South of Kaikōura at Rosy Morn – where two houses filled with debris – about 5000 cu m of material had washed down the hill.

The amount of damage caused by Gita to the highway through Kaikōura was the most by a storm or cyclone since 1975, when former cyclone Alison brought down 83 slips north and south of the town.

Dellow said researchers would be looking closely at the damage caused by Alison, compared to that of Gita.

Last Thursday, scientists also flew over the largest of the landslide dams resulting from the Kaikōura Earthquake – on the Hapuku River – to see what impact Gita had on it.

The dam was among about only 10 remaining of the original 196 landslide dams created by the earthquake, Dellow said. The surviving dams were slowly getting smaller. Hapuku hadn't failed catastrophically and was episodically declining in size.

The Kaikōura Earthquake generated tens of thousands of landslides in a 10,000-square-kilometre area of North Canterbury and Marlborough. The most intense landslide damage was concentrated in a 3500sq km area around the areas of fault rupture.