Alpine Fault not so 'well behaved'

PAUL GORMAN
Last updated 05:00 20/03/2014
ALPINE FAULT: An 800 kilometre-long fault which runs along the western edge of the Southern Alps from Marlborough to Milford Sound.
GNS Science

ALPINE FAULT: An 800 kilometre-long fault which runs along the western edge of the Southern Alps from Marlborough to Milford Sound.

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The Alpine Fault may have generated more large earthquakes than previously thought, making it increasingly dangerous in future, new research says.

Scientists have traditionally compared the fault to a metronome because of its apparent regularity in causing earthquakes around magnitude 8.0 every 300 years or so on average.

But University of Canterbury researchers have discovered signs that West Coast quakes - mostly blamed on the Alpine Fault - may not be as ''well behaved'' and regular as assumed.

They found higher ''slip release rates'' than might be expected based on existing Alpine Fault records, of an average 7.1 metres +/- 2.1m of ground movement in each event, meaning either:

- Moderate to large partial ruptures of the fault, generating quakes of magnitude 6.5 or higher, occur between the ''great'' magnitude 7.9 or larger quakes; or:

- Some of the quakes previously attributed to the Alpine Fault actually occurred on other faults.

Evidence of offsets along the fault was uncovered in dense bush using lidar (light detection and ranging), which strips away vegetation to reveal the landscape.

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.

Researchers agree the last great Alpine Fault quake of magnitude 8.0-8.2 occurred in 1717, rupturing the ground's surface along more than 300km of the fault.

In comparison, the Greendale Fault that ruptured on September 4, 2010, is about 30km long.

In light of their findings, another large quake around 1600, previously thought to be another great shake of similar size to 1717, now appears to be a partial rupture of magnitude of 6.5 to 7.0, or was even perhaps from another fault.

The most recent quake causing serious shaking on the Coast was about 1826 of magnitude 7.5.

A major Alpine Fault quake would cause significant damage on the Coast, block road passes for months, destroy infrastructure and generate several minutes of ground motion in Christchurch leading to liquefaction.

The research by Dr Greg De Pascale was published in Geology.

De Pascale's work over four years included periods in thick bush searching for ruptures.

He said lidar data had allowed him and collaborators Associate Professor Mark Quigley and Professor Tim Davies to find features, including streams and rivers, offset by fault movement.

They discovered the 1717 great quake caused about 7.5m of offset over about 380km of the fault. Previous research relied on dating material from trenches dug across the fault, as well as rockfalls and lake records, to estimate the size and year of quakes.

De Pascale said the conclusions were crucial for risk awareness.

''It suggests Alpine Fault quakes may be more frequent than previously suggested and less like clockwork every 300 years or so.

''Previous understanding suggested that after the next major Alpine Fault earthquake we can recover and relax for another, say, 200 to 480 years after the aftershocks cease. Our new findings suggest this may not be the case.

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''The next major quake may only be a partial rupture on the Alpine Fault and so there may be another large earthquake following this. This suggests an increased hazard.''

- The Press

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