'Soft rocks' prevented earthquake damage

23:22, Aug 16 2009

Rocks that lurched rather than snapped saved the southern South Island from massive damage in Wednesday night's Fiordland earthquake.

Civil Defence and seismologists say the 7.8 magnitude quake under Resolution Island at 9.22pm on Wednesday had the potential to cause extensive damage and threaten life.

Prime Minister John Key said New Zealanders could "thank their lucky stars" for such a close escape.

GNS Science principal scientist Martin Reyners said the magnitude of the quake and its shallowness, only 12 kilometres deep, would normally be a dangerous combination.

But the country's biggest quake for 80 years occurred in a remote location and in "soft rocks" that muffled some of its power.

The quake was centred where the Australian crustal plate was diving under part of the South Island on the Pacific plate.

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It ripped open a six-metre thrust between the plates throughout an area about 60km along the Fiordland coastline and down to about 50km below the surface, Reyners said.

It occurred close to the offshore Alpine Fault but was not on it, and it was of a magnitude similar to that expected when the Alpine Fault next breaks, which some say is overdue.

"It's a good reminder to New Zealanders that we have had earthquakes this size in the past 1931 in Hawke's Bay and 1929 in Buller and sooner or later we will have one in a more populated area," he said.

"It started about 30km deep and ruptured upwards. Because it happened between the plates, it's a place where the rocks are what we call reasonably soft.

"So with the earthquake, when it ruptured it was more of a 'lurch' than a 'snap'. People would have felt the low-frequency shaking that's the rolling rather than high-frequency shaking, which is the shaking that causes damage to buildings.

"There was a DOC (Department of Conservation) team on Resolution Island, where the fault would have ruptured just about under their feet. As I understand it, they weren't tossed out of bed."

A team of seismologists travelled to the area yesterday to install six seismographs along the coast between Dagg Sound and Puysegur Point, Reyners said.

"We have a couple of permanent stations in the area Deep Cove and one at Puysegur Point but we want to install a few more stations as we want to get a better fix on the geometry of the fault that broke."

AFTERSHOCKS FELT

Strong aftershocks continued to rock Fiordland this morning following Wednesday night's 7.8 earthquake.

Two earthquakes measuring over five in magnitude hit the area early today.

The first, measuring 5.3 in strength jolted Southland residents at 4.13am.

It was centred 120km west of Tuatapere and was 30km deep.

The second measured 5.6 in magnitude and hit at 5.27am.

It was centred 130km north west of Tuatapere and was 12km deep.

Aftershocks would continue for a week or more.

Seismologists had had an inkling that a big quake was due in southern Fiordland.

"What we had observed is over the last 10 or so years we had had a lot of earthquakes in northern Fiordland, from Doubtful Sound and northwards. We knew that eventually the plate motion along the Fiordland coast would have to equalise the strain, so this has filled a bit of a gap," he said.

"It is the biggest earthquake in the world this calendar year, and seismologists get very excited by that."

NO ESCAPE

A Te Anau woman who left the town after an earthquake six years ago had moved back only to be hit by another quake on Wednesday night.

The woman and her husband left the town after an earthquake, measuring 7.1 on the Richter scale, struck the region in 2003.

The pair have since moved back but were among thousands of Southlanders rocked by the magnitude 7.8 quake on Wednesday night.

Despite being terrified of earthquakes, the woman said Wednesday's quake was nowhere near as bad as the 2003 jolt and the couple had no plans to leave Te Anau again.

While the 2003 quake had struck with a bang, and was accompanied by a terrible, loud noise, Wednesday's quake was silent and had more of a rolling action, she said.

The 2003 quake had been so strong it had flung the couple's outdoor furniture around the backyard, whereas Wednesday's quake, despite being higher magnitude, had caused no damage, she said.

The couple, who were entertaining friends at home when the quake struck, said they heard no sirens or alarms after the earthquake, but had checked in with neighbours and friends to make sure everyone was OK.

Support from the community had been excellent and everyone worked together to make sure people were safe, she said.

"The support around the area is just amazing," she said.

"We're very happy."

-  with NZPA

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