Second Christchurch fault much better behaved

PAUL GORMAN
Last updated 05:00 29/03/2011

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Christchurch earthquake

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A second fault that appears to lie directly under Christchurch has been much better behaved than the one that caused the magnitude-6.3 earthquake on February 22.

Visiting United States seismologist Professor Kevin Furlong points to growing evidence from recent aftershocks as likely proof the shallow fault exists.

Its path below the city cannot be pinpointed closer than within one or two kilometres because of inaccuracies in recording exact aftershock locations. Furlong believes the pattern from those aftershocks is becoming clear enough to draw an approximately west-south-west/east-north-east line for the fault, from close to Riccarton, running below the central city and the eastern suburbs and then off New Brighton. GNS Science did not want to comment on the proposed fault location.

Furlong said the structure seemed responsible for generating the central city Boxing Day quake of magnitude 4.9, a cluster of aftershocks from that, the 5.1 quake on March 20 and Saturday morning's 4.3 shake.

The fault was oriented roughly parallel to what was being called the Port Hills or Estuary fault about 5km further south, he said.

Based on the depth of the aftershocks, it appeared to be dipping below the ground steeply towards the southeast, like the Port Hills fault.

While the two faults seemed to share a similar structure, their behaviour was different, Furlong said.

"The good news is this [Boxing Day] fault seems to be popping off as a series of magnitude 4.5 to 5.0 quakes. If a fault of this length went all at once, it could be larger," he said. "The length is very close to that of the [Port Hills] fault that caused the February earthquake. So we have a fault about 10km long, but the fault seems to be behaving as individual segments 3km or 4km long – the one that popped off on Boxing Day, the one that popped off on Sunday night a week ago and the one on Saturday morning. That is a positive. It's not doing anything ominous. If you looked at this sequence independently of anything else, you might say they have triggered each other, but there's nothing to show we think it's building up to a big one. I am thinking that each event has essentially triggered the next one down the line, basically unzipping the fault, but in this case so far it looks like a small zipper. The key is that the 5.1 did not reinvigorate the fault that moved on Boxing Day."

GNS seismologist Dr Bill Fry said they were looking at whether stress changes from last September's and last month's quakes might increase chances of ruptures on nearby faults.

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'World-record' shaker

Heathcote residents experienced the world's strongest recorded up-and-down shaking during last month's earthquake.

Equipment in a Heathcote Valley School building measured peak ground accelerations more than 2.2 times that of gravity only three seconds into the magnitude-6.3 quake.

The maximum downward ground acceleration was about 0.9 that of gravity (0.9g). Horizontal ground motion hit close to 1.7g.

Canterbury earthquake engineer Dr John Berrill said the 2.2g acceleration upwards was a record. "The downward, negative acceleration peaks are all less than 1g, whereas five positive peaks are in excess of 1g. This suggests that the building was left behind, in the air, as the ground accelerated downwards beneath it, with the building then falling under its own weight at near 1g until it impacted on the ground. In the upwards direction, it goes at the acceleration of the underlying ground, greater than 1g."

In Japan's magnitude-9.0 quake, peak horizontal ground accelerations hit 2.7g and vertical motion hit 1.88g.

- The Press

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