Christchurch earthquake: Radar points to rupturing of single fault

By Martin Johnston

A buckled footpath in Christchurch city centre. Photo / Getty Images
A buckled footpath in Christchurch city centre. Photo / Getty Images

Christchurch's devastating 6.3 earthquake is thought to have been caused by the rupturing of a single fault, unlike last September's magnitude 7.1 quake.

The latest maps from GNS Science and from satellite radar images place the Tuesday quake's fault as a 17km subsurface rupture in an east-west direction between Taylors Mistake (a surfing beach southeast of Sumner) and Halswell.

GNS geophysicist Dr John Beavan said yesterday that the satellite images supplied by French scientists and based on radar reflections captured before and after the quake show the ground just south of the fault was moved by about 50cm in a mix of westerly and upward movement.

"This is the amount of ground displacement after the shaking has stopped. The shaking during the quake would have been greater."

The radar images place the fault slightly further north than the GNS map partly because the images from space represent the topmost edge of a sloping fault that is not thought to have broken through to the surface.

It is suspected the fault cuts north to south, 60 to 70 degrees from horizontal.

GNS research platform manager Dr Kelvin Berryman said Tuesday's quake was less complex than September's, in which up to four interconnected faults ruptured almost simultaneously.

Tuesday's quake was part of the aftershock sequence, but there was no obvious underground structure directly connecting that subsurface rupture with the four-part Greendale Fault near Darfield, 40km west of Christchurch, which caused the main shock on September 4.

Geologists had suspected there were buried and unrecognised faults in Canterbury. Some might not have moved for many thousands of years, but had been reactivated as stresses in the Earth's crust had been redistributed since last September.

"If you strip away the sediment and gravels of Christchurch and the Canterbury Plains you would see the bedrock looking like broken glass from millions of years of earthquake activity," Dr Berryman said.

The underlying geology of Canterbury was the western end of the Chatham Rise with many east-west trending faults. Many geologists believed modern-day tectonic plate motions in the South Island had re-awakened some of these very old faults, causing them to fail. The Greendale Fault was one of these.

- NZ Herald

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