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A civil defence report says there is a 65 per cent chance of the Alpine Fault rupturing in the next 50 years.
The Canterbury region's revised civil defence emergency management plan is expected to be discussed by territorial authorities in Christchurch tomorrow.
The report, which ranks the risks and likelihood of various emergency situations, says there is a 65 per cent chance the fault could cause a magnitude 8 quake in the next 50 years. Scientists have previously warned that damage from such a quake would be significant across the South Island.
On the West Coast it would block road passes for months, destroy infrastructure and generate several minutes of ground motion in Christchurch leading to liquefaction.
The latest report said in the Mackenzie the damage would be "catastrophic", and in Timaru and Waimate "major".
The percentages were based on historical evidence of past earthquakes and their frequency, with data supplied by GNS Science and Canterbury University.
According to the management plan, the risk assessment profile ranks the consequences of impact for the Timaru and Waimate districts at level 4 (extensive injuries, high-level building and infrastructure damage, major financial loss).
For the Mackenzie district, it was level 5 (deaths, most buildings extensively damaged and huge financial loss).
The Alpine Fault runs hundreds of kilometres along the western Alps from Marlborough to Milford Sound, where it heads offshore, and marks the Pacific and Australian plate boundary.
Dr Andrew Gorman, associate professor of Otago University's geology department, said the probability "sounded about right".
"The thing with earthquakes is we can't predict exactly when they're going to occur, but history gives us a good guide," he said.
"The Alpine Fault has been active every 200 to 400 years, and the last known event was 1717. That suggests it is likely to be active within our lifetime." He said the fault's rupture would knock out most of the South Island's main transport routes, while hydro infrastructure could also be affected.
"A lot of the South Island would effectively shut down."
However, South Canterbury's most common civil defence risk was flooding or heavy rainfall, which ranked as a likelihood of A ("highly likely"), but with a consequence ranking of 3 ("moderate building and infrastructure damage, high financial loss").
Neville Reilly, the Canterbury region's civil defence controller, said the plan put in a lot of consideration for "each district, not just the region as a whole". It had gone through a public submission process earlier this year.
"I think people are more aware of the need for community response and resilience in civil defence emergencies. Across the region .... we've had the catastrophic earthquakes, but also more recently, highly damaging storms and rain events," Reilly said.
"We need to maintain constant vigilance. People have short memories. It's important for us that we maintain high awareness."
- The Timaru Herald