Aerial sweep reveals major cliff collapses
Earthquakes have caused cliffs along the Marlborough coastline from the Vernon Bluffs towards Lake Grassmere to subside and fall into the sea.
The slumps were photographed by GNS senior engineering geologist Graham Hancox who flew over the area last week.
"There are places where the ground has moved downwards 10 to 15 metres on cliff tops," he said. "These are significant movements of massive parts of ground."
Cracks appeared 5m to 10m back from cliff edges.
Debris which fell from the top of the Vernon Bluff east of the Wairau Lagoon had run down a gully and into the sea.
Areas of cliff had also failed between Mussel Point, at the end of Marfells Beach, and Cape Campbell, quite close to the London Hill fault.
These sorts of failures happened in "special earthquakes" such as the 1848 Marlborough earthquake and the Wairarapa quake which hit Wellington seven years later, Mr Hancox said.
These were relatively small landslides because they did not happen in an epicentre area and the quakes were not very big. However, the damage indicated what might occur with a larger event.
Coastal Marlborough cliffs mostly made up of Tertiary and Cretaceous Periods mudstone and sandstone could fall if strongly shaken, he said.
Seddon was the most vigorously shaken town in the recent quakes because it was closest to their epicentre, Mr Hancox said. However, people in Ward felt them most strongly because of the area's geology with bands of soft material alternating with harder ridges.
"This might not be finished," he said. "We are still in a waiting game."
There was a substantial gap between the Christchurch quake in February 2011 and the next big one in June.
"Always expect the worst and hope for the best," he said.
Victoria University geophysics professor Euan Smith said Seddon soils were generally 1.5m to 3m of silt covering 15m of coarse gravels and small rocks, underlaid by siltstone and mudstone. Blenheim sat on 2.5m to 3m of silt on top of coarse gravels with deeper silt in the area of old streams.
One of his students had recorded that the earthquake shaking was amplified around Blenheim compared with surrounding hills.
River gravels usually provided a stable foundation compared with river silts, Prof Smith said.
Mr Hancox has invited people to email him photos which show the geological impacts of quakes - email@example.com
- The Marlborough Express