Dispute on whether latest quake is an aftershock

The rubble on Madras Street, Christchurch. Photo / Geoff Sloan
The rubble on Madras Street, Christchurch. Photo / Geoff Sloan

Specialists 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 September 4 quake.

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 September quake, which seriously affected the city's infrastructure, but left no fatalities.

Emeritus Professor of Geology at Oregon State University Robert Yeats, suggested last week's shake was a new quake.

"That wasn't an aftershock," he told The Press.

"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," he said.

Prof 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."

But scientists in New Zealand disagreed with his conclusions.

GNS Science natural hazards research platform manager Kelvin Berryman said last week's quake was part of the aftershock sequence that had been occurring since the September quake which was centred near Darfield, 40km west of the city.

"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," Dr Berryman said.

An expanding "cloud" of aftershocks, particularly at both ends of the main fault rupture, was a familiar pattern with large earthquakes worldwide, he said.

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, Dr Berryman said.

Retired Otago University Professor of Geology, Rick Sibson told NZPA the most recent quake was considered an aftershock.

The rule of thumb was that in the months following a large earthquake there would be an aftershock of about one magnitude size less than the initial quake.

"But it's semantic really - it's big enough to be considered a main shock by itself."

Based on the earthquake rule, Christchurch should now expect an aftershock measuring between 5 and 5.3 in magnitude.

"There was no rule of thumb on when that might hit though. That's the catch," Prof Sibson said.

Insurance Council chief executive Chris Ryan said regardless of whether the quake was an aftershock or not, it would not affect insurance payouts for those making claims.

"It's possible they might have to pay an additional excess if they make a second claim," he said.

"If it's a second event insurers have to pay a lot more because they have to pay an excess as well to the reinsurers, but it doesn't make any difference to ordinary insured people."

There was a general consensus that insurance premiums would rise for everyone in the country in order for insurance companies stay in business, Mr Ryan said.

So far there had been no indication that Canterbury residents hit twice by the quakes would have trouble getting insured again.

"There's no real reason for insurers not to insure because when they do that they walk away from their business, it's what they do.

"I think what might happen is there will be an increase possibly in premiums when people renew their policy."

But that could depend on where their property was located, Mr Ryan said.


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