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Thursday, April 7, 2011

Seismic probe after latest jolt

Seismic probe after latest jolt

Last updated 05:00 08/04/2011
A multimillion-dollar investigation below Christchurch has begun close to the centre of yesterday morning's aftershock.

Researchers started the seismic survey on the beach at Southshore yesterday.

Over the next two days, they will run a survey line  at least 6km long  up the beach to the Waimairi Beach Surf Life Saving Club.

Their equipment will transmit and then receive signals from down to about 2km, allowing them to map any faults hidden in rock under several-hundred metres of sediments.

Canterbury University geological sciences department Professor Jarg Pettinga said it was possible the work near Southshore might reveal the Port Hills Fault that caused the February 22 earthquake, as well as a suggested fault further north some believe generated the Boxing Day quake.

The magnitude 4.0 quake at 3.57am yesterday was centred near the Avon-Heathcote Estuary, about 11km deep.

Visiting United States seismologist Professor Kevin Furlong said it appeared to be in much the same location as the March 20 magnitude 5.1 quake. That aftershock seemed to be on a fault that ran through the centre of the city and had also generated the Boxing Day magnitude 4.9 shake.

By late yesterday, the GeoNet website had received nearly 400 "felt reports" from the 3.57am quake.

There have been 1050 aftershocks since the magnitude 6.3 quake on February 22.

Pettinga said yesterday's data was promising but would take time to analyse.

"We're trying to get some idea of the extent of the faults and if they extend up to within one or two kilometres of the ground surface."

Surveying the beach would help define the eastern end of the faults, which did not appear to go far offshore.

"The aftershock activity does seem to stop not far out off the coast, which indicates the faults might not extend beyond there," Pettinga said.

Once the beach work was completed this weekend, the researchers would move into the central city to carry out a north-south survey along one of the main streets east of Colombo St.

No other survey lines were planned further west in Christchurch unless they were thought necessary after the city work, he said.

More surveying would be carried out south-west of the city early next month.

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