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[Kiwithrottlejockey]

Threat of quake in Southern Alps wanes

By PAUL GORMAN - The Press | 5:00AM - Tuesday, 03 April 2012



EARTHQUAKE-WEARY South Islanders finally have some good news — the Alpine Fault could still be 200 years away from generating its next quake of about magnitude 8.0.

Scientists have found that the most active central and southern sections of the 650-kilometre-long fault appear to break on average once every 480 years, not about every 300 years as previous studies suggested.

The date of the last huge quake from the fault has been pinpointed as 1717 (plus or minus two or three years), which had meant another major quake might be not far away.

But with an average return period of about 485 years, that quake might not take place for more than 180 years.

The researchers warn, however, that their calculation is based only on a rupture history of the three most recent events they found evidence for — in 1717, 1230, plus or minus 50 years, and 750, plus or minus 50 years.

The work of the researchers from GNS Science, Otago University's geology department, Western Washington University's geology department and Nevada University's seismological laboratory has been published in the April issue of the Bulletin of the Seismological Society of America.

The Alpine Fault runs from Marlborough down the western edge of the Southern Alps and heads offshore close to the entrance to Milford Sound. It marks the boundary between the Pacific and the Australian crustal plates.

The researchers dug five trenches at three sites up to 12km apart on the southern part of the fault near Haast, the Okuru River and the Turnbull River.

The trenches allowed dating of exposed river terrace deposits displaced by quakes and also revealed wood and bark from tree ferns that could be radiocarbon-dated.

Analysis then revealed that in the Haast area since 688 there had been three "large-to-great earthquakes", causing up to nine metres of horizontal displacement and about 1m of vertical movement.

The maximum amount of movement suggested each of the quakes had caused several hundred kilometres of the Alpine Fault to break.

With an average recurrence period of 485 years, the chances were that the next large or great quake was "not imminent", with only 295 years having passed since the 1717 event, the paper said.

"Therefore, the Alpine Fault in South Westland may not be close to rupture, as is often speculated."



Lead author Kelvin Berryman, of GNS Science, said the study had been "15 years in the making".

"The results provide us with potentially some breathing space, that we're not as close to rupture as is sometimes assumed. It gives us food for thought."

The results of another major study of the fault, further south where it crosses the remote Lake McKerrow, were likely to be published this year, he said.

That investigation had looked at the last 20 events, well recorded in stream sediments by the lakeside.

"That further work will extend the record out much further," Berryman said.

"The bigger story is yet to come out."

http://www.stuff.co.nz/the-press/news/christchurch-earthquake-2011/6682204/Threat-of-quake-in-Southern-Alps-wanes