Latest quakes move us west
Cape Campbell has moved 18 centimetres closer to Australia since the earthquake on August 16.
The 6.6 magnitude earthquake was described by Crown research institute GNS Science as "more concentrated" than the July 19 and 21 quakes. The July quakes moved the district 5cm to the east.
The August quake reversed that, moving parts of the district back to the west.
GNS Science researchers analysed data from 28 GPS stations around Marlborough and found that while the maximum movement was more than twice that of the 6.5 July 21 quake, the total area of fault rupture was much smaller.
The data, which looks at movements underground as well as on the surface, showed the largest land shift was recorded at Cape Campbell, where land moved 18cm to the west, the Crown research institute said.
Ken Gledhill, who is GNS Science's geonet and geohazards monitoring head of department, said yesterday that the 18cm movement recorded on the surface was smaller than up to 2 metres of movement that would have happened at the fault, about 10km deep across an area of some square kilometres.
"This is the effect on the surface of the earth."
Dr Gledhill said the 18cm move was "a reasonable amount".
"The depths down moves more."
He said the amount of movement that occurred in the August 16 quake was "pretty much" what scientists would expect from an earthquake of that size. Earthquakes forced larger, more rapid change on the surface of the earth, he said, compared with the forces that happened in slow mountain-building or erosion.
"We live in a very active country."
As part of further work by GNS researchers, pictures from a German "TerraSAR-X" satellite's flyover New Zealand would be compared with earlier photos to see how the earth has moved.
The July earthquakes were thought to have happened along the London Hill fault, and the epicentres were in Cook Strait.
The largest surface displacement in those earthquakes was to the east, in a large area between Blenheim and the Awatere Valley.
Horizontal movement on the seabed in southern Cook Strait, above the epicentre of the 6.5 magnitude quake, was up to 8cm.
Other parts of Marlborough would have been hit by smaller horizontal movements.
Most people would have been too distracted by the shaking from the earthquake to notice the horizontal movement. Dr Gledhill said a quake in Fiordland in 2009 had moved part of the area towards Australia by almost a metre.
- © Fairfax NZ News