Wellington, Sept 6 NZPA
Canterbury’s magnitude 7.1 earthquake may turn out to be precursor to New Zealand being hammered by a series of big quakes.
Victoria University geophysicist Euan Smith said there was a potential for big quakes to trigger other big quakes.
“The 7.1 magnitude quake that rocked Canterbury on Saturday morning came from a previously unknown fault, but if recent history is anything to go by, it could be one of a series of 7-plus magnitude quakes, some of which are still to come,” said Prof Smith.
Until last year, seismologists regarded the past 30 to 40 years as a quiet period in New Zealand as there had been no big on-land earthquakes.
Such large earthquakes, sometimes came in clusters lasting 10 to 20 years, which were then followed by a quiet period.
In the “busiest” period since European settlement, there was a cluster of New Zealand earthquakes which began in 1929 when a magnitude 7 earthquake, also in west Canterbury, turned out to be the first of a series of seven major (magnitude greater than 7), earthquakes over the next 13 years.
“That series included the second and third largest earthquakes in European times – the magnitude 7.8 Buller and Hawke’s Bay earthquakes,” Prof Smith said.
There were five big quakes by 1935 and the series ended with magnitude 7.2 and 7.0 earthquakes in the Wairarapa in 1942, which left the lower end of the North Island with the kind of damage now being seen in Canterbury.
“It is improbable that this occurrence of such large earthquakes in rapid succession was coincidental,” said Prof Smith.
“It is more likely that the faults which broke during the series were all stressed and ready to break.
“The occurrence of successive earthquakes helped bring forward the occurrence of the next.”
Prof Smith said there was no reason to think that such a series could not happen again.
“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.
“If there are other faults that are ready to go, can we risk taking no action?”.
On average, New Zealand can expect a magnitude 6 quake once a year, a magnitude 7 quake once a decade, and a magnitude 8 quake once every century, according to GNS Science.
But in recent years it has recorded a 7.1 shake in Fiordland (August 22, 2003), a 7.2 quake in the Puysegar Trench off the Fiordland coast (November 22, 2004) and a 7.8 shake in Fiordland (July 15 2009).
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