Scientists surprised at low level of liquefaction
Liquefaction occurred at only a few sites in Marlborough in the 6.6 magnitude earthquake, surprising scientists who expected to see much more, as had happened in Christchurch.
GNS Science senior scientist (earthquake geology) Russ Van Dissen said yesterday that while he was not a specialist in liquefaction, he had certainly expected to see more of it in Marlborough.
Liquefaction occurred when three things were there: water saturation, with a high water table; strong enough shaking; and sediments with the right grain size.
He said given those three factors, which Marlborough had, he had expected to see more liquefaction.
Only minor signs of liquefaction were apparent, mainly on the southern side of Lake Grassmere.
Marlborough man Jeffrey Hynes sent photos into the Marlborough Express showing liquefaction at the Wairau Bar.
Mr Hynes, from the Rangitane iwi, said that since the earthquakes he had checked several sites for damage or changes to the landscape, particularly those which have cultural significance to his iwi.
Near the Wairau Bar he had found deep cracks in the ground and evidence of liquefaction. This was the same area that had been affected in earthquakes in the 19th century, he said.
Dr Van Dissen said that in the Christchurch earthquakes in 2010 and 2011, the factors were all exactly right for "metres and metres" of material that "just wanted to liquefy".
"That was not the case in Marlborough."
It was possible that Christchurch was the exception, rather than the rule, for how ground materials would behave in an earthquake, Dr Van Dissen said.
He could not be definitive about why the expected liquefaction had not been found, he said. It was possibly related to how long the shaking went on for and how much energy was introduced to the water-saturated sediments.
A helicopter reconnaissance flight by GNS staff members last week assessed new ground damage after the August 16 Grassmere earthquake.
There was also only minor re-activation of previous landslides on the cliffs of White Bluffs and Cape Campbell to Lake Grassmere, with no new significant landslides observed.
A slight increase in the number and size of new and re-activated landslides on the coast south of Cape Campbell was observed.
The most significant finding from the flight was in the Haldon Hills area around the Flaxbourne River near Ward, where new cracks on ridge crests ("ridge rents") and terrace edges, many new landslides and large displaced boulders were noted.
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