Photo courtesy Eric Hertz, 2 Degrees.
Some of New Zealand’s older buildings need to be knocked down before another quake does it for us, says Euan Smith Professor of Geophysics at Victoria University’s Institute of Geophysics - and more so because a bigger jolt could be on the way (keep reading).
Professor Smith said the country needed to learn several lessons from the Saturday’s earthquake in Christchurch.
The first lesson, for local authorities and policy makers, concerns what happened to the unreinforced masonry (URM) structures in this earthquake, Professor Smith told the Science Media Centre.
“Engineers in New Zealand have been warning of the danger posed by URM for decades. Some communities have taken action and required property owners to demolish such structures or to take remedial measures, such as retrofitting anti-seismic strengthening, to bring the structures up to an agreed level of performance in expected earthquakes."
The damage to unreinforced masonry structures (URM), including older brick chimneys, was no surprise. The risk had been identified in many studies. For example, chimney collapse was a major contributor to damage to housing in the 1931 earthquake.
"It is time for the rest to act. Owners of unreinforced masonry buildings, and chimneys, everywhere in New Zealand should be given a reasonable period of time - say 10 years - to demolish them or make them safe in future earthquakes.
Action should be taken New Zealand-wide because nowhere is immune from earthquakes that could cause damage to these weak structures, Professor Smith said.
A trigger for a larger quake?
Implementing such a policy may be urgent.
"In 1929 there occurred, in west Canterbury, a magnitude 7 earthquake which turned out to be the first of a series of seven major, magnitude greater than 7, earthquakes over the next 13 years," Professor Smith said.
"The series included the second and third largest earthquakes in European times - the M 7.8 Buller and Hawke's Bay earthquakes. The series ended with two M 7.2 and 7.0 earthquakes in the Wairarapa in 1942.
"It is improbable that this occurrence of such large earthquakes in rapid succession was coincidental. It is more likely that the faults which broke during the series were all stressed and ready to break, and that the occurrence of successive earthquakes helped bring forward the occurrence of the next.
"There is no reason to think that such a series could not happen again. Equally there is no way of knowing whether or not Saturday's earthquake has provided a trigger for more large earthquakes in the next few years.
"But if there are other faults that are ready to go, can we risk taking no action on a known and easily remediated peril - URM - and hope that that next earthquake also happens when people are safe in their houses?"
Could have been deaths
Policy makers had only good fortune to thank for the low injury rate on Saturday, and lack of deaths.
“[If the quake had it happened at, say, 12:30pm on a Friday, then the wreckage in the CBD, and damage to and within commercial properties elsewhere, must surely have produced a very much higher casualty rate that would have included deaths,” Professor Smith said.
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