Dispute over quake versus aftershock

By AAP with AG Staff 28 February 2011
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Experts debate whether the Christchurch quake was really an aftershock – and if another big one will follow.

GEOLOGISTS DISAGREE ON WHETHER last week’s devastating 6.3 magnitude earthquake in Christchurch was an aftershock, or a new earthquake separate from the magnitude 7.1 quake in September 2010.

The Christchurch CBD was reduced to rubble, with almost 150 bodies recovered and the death toll expected to rise to over 200. The deadly tremor came more than five months after the 4 September quake, which seriously affected the city’s infrastructure, but left no fatalities.

Though the latest temblor has been labelled an aftershock, Robert Yeats an emeritus professor of Geology at Oregon State University in Corvallis, US, argues that it was totally new seismic activity.

Shocking sequence

“That wasn’t an aftershock,” he told NZ newspaper The Press on Monday. “It might be a separate earthquake, part of a sequence of earthquakes. It is quite far from the Darfield aftershock cloud, and its fault plane solution is different.”

Yeats said usually it would take many years before seismic activity could be considered a new quake rather than an aftershock of a previous one. “That’s a point of debate among seismologists. But you can’t paint all aftershock series with the same brush,” he says.

But scientists in New Zealand disagree with his conclusions.

Dr Kelvin Berryman an expert on natural hazards at GNS Science says last week’s quake was part of the aftershock sequence that had been occurring since the September quake which was centred near Darfield, 40 km west of the city.

Cloud of activity

“Aftershocks have been spreading both west and east since the magnitude 7.1 Darfield earthquake in September and this has resulted in increased stresses in the Earth’s crust in the Canterbury region,” he says.

An expanding “cloud” of aftershocks, particularly at both ends of the main fault rupture, was a familiar pattern with large earthquakes worldwide, he adds. The frequency of aftershocks would continue to decrease in coming weeks.

Over periods of many weeks, this reduction tended to be fairly regular, but there were often anomalies, as last week’s quake had shown, Berryman says.

Professor Rick Sibson a retired geologist, formerly of Otago University, told the NZ Press Association that he believes the most recent quake was an aftershock. The rule of thumb is that in the months following a large earthquake there is usually an aftershock of about one unit of magnitude less than the initial quake, he said. “But it’s semantic really; it’s big enough to be considered a main shock by itself.”

If last week’s quake was new activity, then the rule of thumb would suggest that Christchurch should expect an aftershock measuring between 5 and 5.3 in magnitude. “There’s no rule of thumb on when that might hit though. That’s the catch,” Sibson said.

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