Lack of aftershocks a good sign - expert

Posted 08 Jul 2011 by MediaStuff Popular
Posted in Earthquake Facts , Media
This post was added to the Stuff.co.nz website - click here to view this item

Cantabrians can be cautiously optimistic they may be through the worst of the earthquakes, a visiting United States seismologist says.

Professor Kevin Furlong, of Pennsylvania State University, said quake activity and energy release from the three biggest shakes were decaying steadily. But that did not mean there would not be any more large quakes.

He said the lack of big aftershocks for more than a fortnight reflected a general decay in activity.

"It's a nice decay. We had the first one in September, and that activity has decayed, and then the February quake, which decayed off but seems to have decayed faster," he said.

"Then with the June event, that has decayed faster than the February one.

"So the overall decay is there and each of the big aftershocks decayed faster than the one before. That is all very encouraging."

There was nothing in the data to suggest there could be another "spike" in activity.

The February 22 quake had not significantly reactivated the September fault zone, and the June shakes had not kicked the two previous faults back into life, Furlong said.

He believed the magnitude-5.6 and 6.3 quakes on June 13 were generated by another unknown fault up to 10 kilometres long running north-northwest/south-southeast from close to Sumner to near Port Levy and Pigeon Bay.

"It does appear that the fault that ruptured in June has a different orientation to the others in the sequence," he said.

"This was more just a little bit of thrust movement but mainly horizontal, with land on the eastern side of the fault moving to the north and the western side moving south."

The maximum slip on the fault was about one metre, with the western side also going up slightly, possibly by a few centimetres, he said.

There were still issues on whether the eastern end of the Greendale Fault came close to linking with the western end of the Port Hills Fault, he said.

"From my perspective, they almost join up, based on the aftershocks.

"The question is, what could that mean, based on the overall energy release since September."

Energy released by the June 21 magnitude-5.4 quake was not enough to make up the lower-than-expected seismicity in "the gap", and it might "need 10 of those" to do that.

"We can add up all the magnitudes of the quakes since September and convert that to seismic moment [energy release], a measure of the slip," he said.

"We have a plot of seismic moment which shows a big amount was associated with the Greendale Fault and smaller amounts with both of the other events [February 22 and June 13] and their aftershocks.

"But between the Port Hills and the Greendale you have a minimum, a bit like a valley, and as these aftershocks, like that 5.4 two weeks ago, occur they tend to fill that in.

"It is possible there just isn't the energy left there to be released."

The Canadian researchers involved in the seismic surveying of the "gap" are still analysing the data.

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