Drilling deep into the Alpine Fault

Alpine quake optimism faulted by geologists

By PAUL GORMAN - The Press | 5:00AM - Friday, 13 April 2012

CANTERBURY UNIVERSITY earthquake researchers believe the latest study on the frequency of major Alpine Fault earthquakes is too optimistic.

Scientists from GNS Science, Otago University's geology department, Western Washington University's geology department and Nevada University's seismological laboratory recently found that the southern section of the 650-kilometre-long fault, near Haast, appears to rupture on average once every 480 years.

Previous investigations had suggested the fault breaks about every 300 years. The last great quake, of about magnitude 8.0, was in 1717, suggesting the next one could be due soon.

Lead author Kelvin Berryman, of GNS Science, said the 15-year study — published in this month's Bulletin of the Seismological Society of America — had provided "potentially some breathing space, that we're not as close to rupture as is sometimes assumed".

However, those conclusions are being challenged by Canterbury University geologists, including two professors, head of department Jarg Pettinga and hazards and disaster management expert Tim Davies.

In a Perspective page article in today's Press, Davies said the "implication" of the research was "we can all relax and stop worrying about the Alpine Fault, because it might not cause a major earthquake for another 200 years".

Initial responses from the public to the study had been "largely sceptical" and agreed with the Canterbury University view that the report was "over-optimistic".

"Scientists and the media need to provide complete and accurate information to the public, but in this case the information appears to have been presented in an overly positive light," Davies said.

The information from trenches across the fault within 12km of Haast did not necessarily apply to motion on the rest of the fault.

Previous Alpine Fault quakes had ruptured only up to 400km of the fault, so might not have extended south of Haast to be recorded there, he said.

There was "strong evidence" that the 485-year average return period did not apply to the fault north of Haast.

"The evidence that the threat of a major alpine earthquake anywhere on the Alpine Fault has receded is, in fact, very weak."

The best approach was to accept the fault's next major quake could occur any time and to be prepared for that, Davies said.

Berryman had warned that the researchers' calculations were based on a rupture history of only the three most recent quakes they found evidence for — in 1717, 1230 (plus or minus 50 years) and 750 (plus or minus 50 years).

GNS Science researchers Kate Clark and Ursula Cochran yesterday received $311,000 from the Government's Natural Hazards Funding Platform to improve forecasting of large to great quakes on the southern part of the Alpine Fault.

http://www.stuff.co.nz/the-press/news/christchurch-earthquake-2011/6734489/Alpine-quake-optimism-faulted-by-geologists