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Friday, March 4, 2011

Christchurch fault risk 'crucial' to rebuild

Christchurch fault risk 'crucial' to rebuild

PAUL GORMAN
Last updated 05:00 03/03/2011
 
 
A full risk assessment of all faults in the Christchurch area is crucial before any multibillion-dollar rebuild of the city, an earthquake expert says.

Canterbury University geologist Mark Quigley, who is currently away from Christchurch, said historical averages of likely aftershocks did not give the full picture necessary for building a new city.

Quigley, who ran a series of public lectures, attended by thousands of people, after the September 4 quake to explain the science behind the shake, said his house and property had been damaged and was without sewerage, water and power.

For aftershock prediction, seismologists often used Bath's Law, which said the average difference in magnitude between a main shock and its largest aftershock was 1.2, regardless of the magnitude of the main quake, Quigley wrote in a blog posting on his website.

That average had held good for recent big New Zealand quakes.

Before February 22, the difference from the September 4 magnitude 7.1 quake and the largest aftershock was 1.5 in magnitude. Unfortunately, that had changed to a difference of only 0.8 in magnitude after the 6.3 aftershock, making that arguably of higher magnitude than would be expected.

"While we can use historical examples to help us predict possible aftershock magnitudes, each sequence can be different, depending on the length or, more accurately, the potential rupture area of faults throughout the area, the strength of the faults, how close they are to their breaking points, and how things like stress transfer and fluid pressures associated with the main shock or other aftershocks influence these faults," he said.

"This illustrates how important it is to know the location and length of other faults in the vicinity of Christchurch and offshore before we even discuss putting billions of dollars into a rebuild. This can be done relatively inexpensively with existing technology."

Quigley slated "predictions" of aftershocks by some, including Aucklander Ken Ring, who bases his weather and quake forecasts on the phase and closeness of the Moon.

Nobody had predicted recent Canterbury quakes, he said.

"Vague quotes about dates of `increased' activity plus or minus several days, without magnitudes, locations and exact times, do not constitute prediction. Ken Ring's probability of getting a prediction correct based on perigee-apogee, new moon-full moon for 2010 was 63 per cent," he said.

On average, New Zealand had about 330 earthquakes of magnitude 4.0 to 4.9 each year, 26 of 5.0 to 5.9, two of 6.0 to 6.9 a year and one of 7.0 to 7.9 every three years, Quigley said.

"If unspecific about magnitude and location, then your chances of `predicting' an earthquake that is likely to be locally felt and recorded is greater than 90 per cent," he said.

"Should we evacuate an area every time the Moon is on its closest approach, is full or new, is moving rapidly, is at its maximum declination or is crossing the equator? Imagine the fear and frustration of such an approach.

"This would require several evacuations a month of `unspecified areas' to other `unspecified areas'. This is ludicrous."

Ring could not be contacted yesterday.

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