Tuesday 7 September 2010

Kiwi geologists explore earthquake heart dates 16,000 years

New Zealand's Institute of Geological and Nuclear Sciences has started to analyse Saturday's 7.1 earthquake. Today the have released stunning footage of the first fly-over, showing the newly-discovered 22 km fault line that came to the surface.

The monster quake produced the strongest ground-shaking ever recorded in New Zealand - the highest measurement of 1.25 times the strength of gravity, was recorded near the epicentre.

GNS scientists say that fault appears to have been inactive for at least 16,000 years.

"There was a 4m of horizontal displacement in alluvial terraces that were deposited about 16,000 years ago at the end of the last glaciation," Dr Kelvin Berryman, from GNS Science said. "When the last ice-age ended, rivers brought large amounts of gravel from the high country and distributed it throughout Canterbury, many meters thick in some places."

"Before Saturday, there was nothing in the landscape that would have suggested there was an active fault beneath the Darfield and Rolleston areas," Kelvin Berryman said. "Geologists have no information on when the fault last ruptured as it was unknown until last weekend. All we can say at this stage is that this newly revealed fault has not ruptured since the gravels were deposited about 16,000 years ago."

"It is highly likely there are other 'hidden' faults around New Zealand which may be capable of producing large earthquakes in the future," Berryman said. "The fault had been accumulating stress for thousands of years. Faults fail catastrophically when stresses exceed a certain threshold."

Geologists at GNS Science are now examining drill-hole data held by Ecan and on-land seismic recordings made by oil and gas exploration companies to learn more geological history of the area where the earthquake occurred.

"One of the things they are keen to learn is if the fault extends beyond the known surface rupture," Berryman says. "The earthquake's effects in terms of levels of damage and liquefaction were consistent with existing knowledge about earthquakes of this magnitude."

"In the context of New Zealand's overall seismic hazard, Saturday's earthquake was a low probability event. We've known earthquakes are possible on the Canterbury Plains, but they are infrequent."

Seismologists believe the major earthquake risk to Christchurch still comes from known faults in North Canterbury, in the Canterbury foothills, and from the Alpine Fault which extends up the spine of the South Island.

Saturday's earthquake produced the strongest ground-shaking ever recorded in an earthquake in New Zealand. The highest ground-shaking measurement of 1.25 times the strength of gravity was recorded at Greendale near the epicentre.

Similar recordings were made on other 'strong-motion' instruments within 15km of the epicentre. Data from these instruments, which are located throughout New Zealand, is used by engineers and is incorporated in building codes.

"The instruments in Canterbury have produced extremely valuable data that will be of interest to scientists and engineers internationally."

Dr Berryman said preliminary computer modelling had indicated there had been negligible effect on the stress regime of the Alpine Fault, 90km to the west of Christchurch.

River bank collapses into Rakaia Gorge during aftershock.

As at 3pm on Monday, seismologists at GNS Science had processed at least 85 aftershocks since the main quake on Saturday. Of these, seven have been between magnitude 5.0 to 5.4; 42 have been between magnitude 4.0 to 4.9; and 36 were between magnitude 3.5 and 3.9.

Today, the North Island regions of Hawkes Bay and the Wairarapa experienced two earthquakes, measuring 5.2 and 3.6 on the Richter scale.

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