Big Alpine Fault quake may be 'in near future'
PAUL GORMAN AND MICHAEL DALY
GNS Science and University of Nevada-Reno scientists have found that the southern part of the 800 kilometre-long fault _ which runs along the western edge of the Southern Alps from Marlborough to Milford Sound _ causes quakes of around magnitude 8 every 330 years on average.
Dating leaves and seeds from a river terrace at Hokuri Creek near Lake McKerrow in far northwestern Southland, just north of Milford Sound, revealed 24 Alpine Fault quakes between 6000BC and the present.
Other research has found the most recent was in 1717, meaning the next may be only 30 or 40 years away, based on averages.
Professor Richard Norris, from the geology department at Otago University, said the Alpine Fault had the highest level of probability for rupture of any fault in New Zealand.
``Westland obviously is at high risk, with widespread damage likely and roads, bridges and other transport links likely to be badly affected (as well as the tourist trade),'' he said.
The fault crossed the main West Coast road in many places, and with an estimated 8m displacement would completely destroy it.
''Intensities further east in places like Queenstown, Te Anau, Wanaka and Mt Cook will be high enough to cause landslips and do damage,'' Norris said.
''Further east in the major cities of Christchurch and Dunedin, the intensities will be lower but the duration of shaking could still be sufficient to damage poorly constructed buildings...and possibly cause some liquefaction.''
Places such as Nelson, Wellington and Invercargill could also expect to feel some shaking.
Project leader Kelvin Berryman of GNS Science said ''a major earthquake in the near future would not be a surprise''.
''Equally it could be up to 100 years away. The bottom line is, if not in our lifetimes then increasingly likely in our children's or our grandchildren's.''
The study's findings, published today in the journal Science, were new and internationally significant, Berryman said.
The site had provided one of the world's best ever records of regular fault rupture.
''Prior to this project, the ages of only the last four Alpine Fault earthquakes were well known.
''Long records with more than 20 earthquakes have been obtained from other faults around the world such as the San Andreas Fault in California, but they are very rare.
''The Alpine Fault is perhaps only the fifth such long record and it has revealed the most regular rupture behaviour yet reported.
Auckland University biostatics professor Thomas Lumley said the intervals between quakes on the Alpine Fault tended to be quite close to the average interval, with relatively little spread.
Most recurrence intervals longer than 295 years _ the position now _ were shorter than 400 years, and many were only slightly longer than 295 years.
''That is, most of the time when a quake hasn't happened for 295 years, it happens within the next century and often within the next half-century. ...The risks are high, but that's because it seems to be an unusually regular fault.''
- © Fairfax NZ News
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