Unstable land risk to scarred region

Hundreds of millions of cubic metres of shattered rock on unstable hillsides still pose a significant hazard two years after the Kaiko¯ ura earthquake.

That is the warning from GNS Science, which says large aftershocks or heavy rain could trigger rockslides and change the course of rivers around North Canterbury and Marlborough, especially the Kowhai and Hapuku rivers on either side of Kaiko¯ ura. Bridges carrying State Highway 1 and the main rail line would be particularly vulnerable to being washed out by sudden landslides racing down those river valleys and disrupting the usual flow of water.

The highly complex magnitude-7.8 quake at 12.02am on November 14, 2016, tore apart 25 faults as it rippled from its epicentre close to Waiau in North Canterbury through Marlborough to the shores of Cook Strait.

As well as affecting lives and livelihoods, and severely disrupting transport arteries, it changed the landscape in what scientists have called one of the most complicated earthquakes yet studied. While erosion is already flattening fault scarps – a small step on the ground surface where one side of a fault has moved vertically – along the Kaiko¯ura coast, the landslide threat remains.

GNS Science engineering geologist Sally Dellow said that immediately after the quake, rivers were dammed by landslides. Several heavy rain events since had eroded those, ‘‘allowing backed-up water to drain away’’. ‘‘The biggest residual threat is the large amount of loose sediment in the hills and valleys of North Canterbury and Marlborough.

‘‘Future heavy rain or large earthquakes could continue to move big volumes into rivers, potentially causing a series of hazards all the way to the coast.

‘‘Possible threats include sudden changes in the courses of rivers and overtopping of flood banks,’’ she said.

GNS Science has mapped 24,000 landslides across about 10,000 sq km of the region, using before-and-after photographs, and satellite and radar images.

The biggest individual slide analysed blocked the Hapuku River. Dellow said this comprised 12 million cubic metres of rock collapsing into the river and forming a lake 400m long, 100m wide and 35m deep.

Engineering geologist Dr Chris Massey told Stuff close radar and satellite monitoring of the Hapuku and Kowhai river catchments was continuing.

The Kowhai River catchment has many smaller landslides while the Hapuku is dominated by the massive rockslide.

There was conservatively more than 200 million cubic metres of rock at risk of sliding in those catchments, he said.

The problem for the bridges across those rivers would be ‘‘if that sediment came down quickly and in large volume’’.

GNS Science earthquake geologist Dr Rob Langridge said erosion and farming were now modifying the immediate postquake landscape. The Papatea Fault ruptured about 5m vertically at Waipapa Bay. The fault formed a vertical wall. ‘‘Two years later, the wall has degraded and collapsed.’’

Dellow said the research was a window into what might happen in future earthquakes in Wellington and on the Alpine Fault.