Canty shake shows The Gap under stress

SARAH-JANE O'CONNOR
Last updated 05:00 20/11/2013
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The latest earthquake to jolt Cantabrians shows there is enough stress in the region to create further shakes, says a leading scientist.

The aftershock at 11.36pm on Monday was recorded at magnitude 4.6 and located 10km southwest of Christchurch, between Halswell and Prebbleton, at a depth of 8km.

The region southwest of Christchurch, around Prebbleton and Lincoln, has been dubbed The Gap. It is a physical void between the east of the Greendale Fault, which ruptured in September 2010, and the western end of the Port Hills Fault, which caused widespread damage in February 2011.

Pennsylvania State University geoscientist Professor Kevin Furlong, who was working in Christchurch at the time of the major earthquakes, said the latest aftershock in The Gap "raises the ongoing question of what to make of earthquake activity in this area that was neither part of the September rupture nor the February rupture".

"The ongoing activity there indicates the region still has a sufficient level of stress to drive earthquake activity," Furlong said, although to date the aftershocks in the area had been moderate.

Furlong said the Canterbury earthquake sequence was being studied globally and the information gained will "drive interpretations of other events".

"That is important for improving earthquake science, but provides little solace for Cantabrians."

GNS Science seismologist John Ristau said an aftershock in that region was not unusual, and fitted the pattern of aftershocks.

"It's not so much that the earthquakes get smaller and smaller with time," he said. Rather, "they get less frequent".

Whether there was still energy to be released from The Gap was "a matter for debate", Ristau said.

University of Canterbury geologist Professor Jarg Pettinga said that while the shake was the "first significant event for quite a while", there was nothing surprising about an aftershock there.

"It's an area which has been quite rich in relatively small and moderate sized aftershocks," he said. "There's nothing unusual about it from that point of view."

GeoNet had received more than 4200 "felt" reports on the quake by yesterday afternoon.

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