Timaru fault lines are 'not very active'

A leading geologist says the majority of fault lines in South Canterbury and Timaru are not necessarily very active, but there are still a lot of unknowns.

Dr Andrew Gorman, of the Otago University's geology department, said that although most of the fault lines near South Canterbury were classed as being active only "once in every 5000 to 10,000 years", there were a lot of other variables to consider.

"Part of the problem is that we simply don't know the location of many of them," he said.

"It's true that many of them have not been active in a long time, geologically speaking, but the same could have been said for Christchurch prior to 2010."

Dr Gorman said Christchurch's "unique setting" meant the effects of the 6.3 magnitude earthquake were exacerbated.

"The circumstances of the geology, particularly in the area north of Banks Peninsula and the Port Hills, and the fact it was shallow, made the impact of the earthquakes that much stronger," he said.

"Liquefaction caused as much, if not more, damage than the actual shaking."

Dr Gorman said that, in general, Timaru's bluestone rock foundation would make it less susceptible to liquefaction than much of Christchurch, which is built on estuarine sediments near sea level.

"However, in Lyttelton, which is also built on solid volcanic rock like Timaru, it was the shaking that really affected the town, while the liquefaction mostly affected the harbour. This would be the most likely scenario for Timaru as well," he said.

Dr Gorman said the real focus was on the Alpine Fault, which was classed as being active one in every 200 to 400 years; the last known event was in 1717.

Should the Alpine Fault rupture, it could produce up to a magnitude 8 earthquake. "The biggest damage caused by the Alpine Fault wouldn't necessarily be buildings but roading, agricultural and tourism infrastructure."

The Timaru Herald