Shocks may be 'last gasp' of Feb 22 fault

MARK CORNELL/Fairfax NZ

Aftershocks in January 2012

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Large aftershocks distressing Christchurch residents may be the last gasps of the Port Hills Fault that ravaged the city on February 22 last year.

Between midnight and 7.30am yesterday, 21 earthquakes, including magnitude-5.1 and magnitude-5.5 shakes, gave many people a sleepless night.

The quakes were all centred in a patch of Pegasus Bay about 5 kilometres off New Brighton.

A National Institute of Water and Atmospheric Research marine survey in April identified the presence of small, splintered faults in the area and further south near Port Levy, each capable of generating quakes of between magnitude 5.0 and 6.0.

Researchers also discovered a larger fault running out to sea off Kaiapoi that some believe could produce a quake of about magnitude 7.0.

GNS Science natural hazards platform manager Kelvin Berryman said yesterday that one theory was the Port Hills Fault, or splinters of it, were responsible for the aftershocks hitting the city since December 23.

"I'm sort of comfortable in thinking this could be some residual activity from the eastern end of the February 22 fault that didn't rupture all the way out there," Berryman said.

"We did see that there was a small patch of that fault that does go offshore that may not have had much activity until lately."

The "patch" had been "very productive" in generating aftershocks since the two big quakes on December 23, he said.

"These [magnitude] 5.5s are pretty substantial but only a quarter, or a third, of the energy of the [magnitude] 6.0 you had," he said.

Berryman said GNS Science researchers would work this week to come up with some answers on what was causing the swarm of quakes.

United States seismologist Kevin Furlong, a visiting professor at Canterbury University last year from Pennsylvania State University, said all the quakes since December 23 were similar, produced by slip on short, steeply dipping faults oriented roughly east-northeast/west-southwest.

"The magnitudes of the events are not particularly alarming, but what is of particular interest is that we are having something that is more swarm-like; that is, a multiple number of similar-sized events in a region, but not all on the same structure or fault."

Stress modelling for the September 2010, February and June 13 quakes showed the area east of the city had experienced a small increase in stress supporting quake behaviour, Furlong said.

"The stress changes are relatively small. They in themselves don't cause the earthquake," he said.

"But if a region is suitably stressed prior to the stress change, then these small amounts of stress change can advance or trigger the subsequent events.

"Basically, the situation that Christchurch finds itself in is a region – and by this I mean the entire region surrounding the city – that was suitably stressed prior to the September event, and since that time we have seen a sequence of events that pretty much follow the patterns of where stresses were slightly increased by the September event.

"What has controlled the actual space-time pattern of events is not totally clear, but in general it has been pretty much an eastward march of earthquakes."

Some "good news" was that each of the big quakes appeared to be acting independently, he said.

"When the February event occurred, it did not reinvigorate the Greendale Fault. Similarly, the June event did not reinvigorate either of the two previous faults, and the recent suite of events has not really lit up any of the previous regions, including the June rupture," he said.

"It seems to indicate that each event is effective in reducing the stresses on its fault such that they are no longer in a triggerable situation.

"Good news in all of this is that the activity is moving east. After the June event, since it was on a structure of different orientation – more north-south – it was unclear whether the behaviour was changing. These events seem to indicate a return to the slow but sure eastward march.

"None of this really will make Cantabrians feel better about things, but what makes this a bit intriguing is that it has continued in such a systematic way for so long.

"I guess my arm-waving explanation is that the entire region was sufficiently stressed, and there were numerous fault segments, none of them particularly large, that were near failure conditions."

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