Major study probes NZ's big landslide threat

The Kaikoura Earthquake triggered tens of thousands of landslides - including major slips like this that blocked State Highway 1. Photo / Mark Mitchell

Scientists have launched a sprawling, $8m investigation to better understand the many threats that landslides pose to New Zealand.

A new study, being led by GNS Science, will analyse the thousands of landslides triggered by the 7.8 Kaikoura Earthquake to find what might happen in major events in the future - and potentially help roading planners design safer routes.

The November 2016 quake generated tens of thousands of landslides across an area stretching 10,000 sq km2, including the major slips that covered State Highway 2 and nearly 200 which blocked large, steep valleys.

The largest had a volume of about 12 million cubic metres, with the debris travelling nearly three kilometres down slope to dam the Hapuku River.

While landslides cost New Zealand an average of $300 million a year, Kaikoura was exceptional.

The rebuilding of SH1 and the Canterbury-Picton rail line has cost in excess of $1 billion, tourism revenue has been affected and freight costs were pushed up sharply by the diversion of Christchurch-Picton road and rail traffic.

The longer-term effects of what's called sediment aggradation - where the debris moves downstream from the steeper in-land slopes to the sea - caused another cascading hazard that will posed threats to farms and infrastructure.

These hazards could persist for years, and possibly decades - as had been shown by a series of landslides that hit the Kaikoura region during ex-Tropical Cyclone Gita's deluge this week.

A major landslide that was observed in the countryside around Waiau following the 2016 Kaikoura Earthquake. Photo / Kate Pedley

A team of GNS scientists had flown to the area to make fresh inspections of the slips, many of which had likely been primed by the 7.8 earthquake.

"We need to understand what would have happened in that same rainfall had the earthquake not occurred," GNS engineering geologist Sally Dellow said.

Immediately after major earthquakes and in the years and possibly decades after, the numbers of landslides triggered by weather events increase significantly, and then decline with time after the event.

The new five-year programme, funded by MBIE's Endeavour Fund, will aim to quantify how long a landscape takes to "heal" after a major event such as the Kaikoura earthquake.

Dellow, part of a team of international scientists involved in the project, said a big focus was on greywacke - a type of sandstone found across much of New Zealand.

"A lot of the landslides in the Kaikoura earthquake were in greywacke terrain - and this terrain basically runs up the centre of the North Island and the South Island.

"So many of our key east-west communication routes, like Arthurs Pass, Lewis Pass, the Manawatu Gorge, Rimutaka Hill and Waioeka Gorge, all run through it, and understanding that terrain is really important for being able to keep our road and rail networks open."

There was a need to learn how earthquakes from major faults - particularly the Alpine Fault and the Wellington fault - could pose a heightened risk.

"There's a real seismic hazard - and what we can learn from Kaikoura will tell us about how the greywacke terrain in those other areas might be affected."

Ultimately, the programme could help guide infrastructure route selection and resilience, development, and emergency response.

"We can feed into land use planning - making sure that when land is developed, it's done appropriately so the risk is minimised, and we can look at the geotechnical aspects, like how we can cut slopes to make them less risky to fall down."

Internationally, investment in improving slope practices had paid off - in Hong Kong, this had halved the risk from landslides.