Wednesday, May 22, 2024

New crevasse on Ongaonga farm may be very old

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Geomorphologist takes a closer look to analyse unusually large and dramatic slip.
A reactivation of a historical landslip is likely to have caused this crevasse, says GNS Science engineering geomorphologist Brenda Rosser. Photo: Matt Holden
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The crevasse that appeared on a Hawke’s Bay farm recently is likely to be the reactivation of a historical landslip in the area, possibly thousands of years old, according to a GNS Science expert.

The 80m by 20m crack was discovered on the Ongaonga property of Matt Holden and business partner Hamish Bibby on June 24, as the region was pounded by more than 200mm of rain, which came on top of Cyclone Gabrielle in February.

Holden was at a loss to explain the crevasse but said it did not  appear to be a traditional slip, which they had plenty of experience with. He questioned if it had been caused by an earthquake.

Video by Matt Holden. Supplied

GNS Science engineering geomorphologist Brenda Rosser said after viewing regional LiDAR, a high resolution topographical map of the land surface taken in 2020-21, it was evident there were multiple historic landslips in the area.

“We don’t know how old these are, but they possibly haven’t moved for several thousand years,” she said.

Rosser said it appears the latest land movement is the reactivation of a landslide, and not caused by an earthquake.

“Perhaps the toe of the landslide has been affected by down-cutting in the river, which may have destabilised it? 

“We are seeing a lot of these big old landslides reactivate this year, especially with all the rain and extreme events. It appears this lower, older landslide has retrogressed (enlarged at the head-scarp) causing the crack on the land.”

Rosser said it is a typical  deep-seated slump or slide, and “we definitely don’t need an earthquake to see landslides like this”.

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