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Thursday, March 3, 2011

Aftershocks May Abate Faster Now

Aftershocks may abate faster now

Last updated 05:00 01/03/2011
An end to aftershocks may come sooner rather than later for nervous Christchurch residents.

The potentially good news comes from GNS Science, which says there has been a significant number of large aftershocks since last Tuesday's magnitude-6.3 quake.

Natural hazards manager Kelvin Berryman said there had already been three aftershocks of magnitude 5 or higher.

That was in line with the rough rule of thumb that the largest aftershocks were a magnitude lower than the main shock.

After September 4's magnitude-7.1 quake, scientists warned an aftershock of 6.0 or more was possible, but the threat had dropped as months passed.

However, seismologists recently estimated there was still about a 25 per cent chance of a magnitude-6.0 or higher earthquake in the year starting February 1.

"This quake [last Tuesday's] has been incredibly rich in aftershocks, and seems to be dying away very quickly. That number of [magnitude] 5s represents a very energetic aftershock sequence," Berryman said yesterday.

"The other one [September 4] was a bit low in energy.

"The rate of decay from last week is much sharper than at Darfield.

"I think the aftershocks from both events will eventually converge, but we could say they will continue for several months at least," Berryman said.

GNS Science earthquake hazard modeller Matt Gerstenberger said seismologists were investigating why there was such unusually high activity in the first 24 hours after the Tuesday quake.

Models showed that over the next week, Christchurch residents could expect up to 10 aftershocks of magnitude 4.0 to 4.9, and possibly two of magnitude 5.0 or higher.

Within three weeks that should drop to one magnitude 4.0 every three to four days.

After a year, models suggested there would only be one of that size each month, he said.

Another magnitude 6.0 or more quake was possible but very unlikely.

"The magnitude 6.3 aftershock on February 22 was not necessarily expected to occur so long after the September main shock," Gerstenberger said.

"However, larger aftershocks can happen later in the life of aftershock sequences."

- The Press

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