September 21 2014, updated 7:34pm

Fault awakens after 16,000 years of inactivity

BY PAUL GORMAN
Last updated 05:00 07/09/2010

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Scientists believe it is at least 16,000 years since the last rupture of the hidden fault that caused Saturday's magnitude 7.1 earthquake and generated the strongest ground-shaking ever recorded in New Zealand.

By last night more than 90 significant aftershocks had rattled Canterbury since the shock that produced major damage to parts of Christchurch and the region.

Yesterday's frequent quakes were generally between four and five in magnitude. Some were felt as sharp jolts because they were as shallow as 4km below the surface and only 10km from the centre of Christchurch.

Since 4.36am on Saturday and up to 6.30pm yesterday, there had been seven aftershocks of between magnitude 5 and 5.4, 45 between 4 and 4.9, and 39 between 3.5 and 3.9.

The new fault trace that ripped across Canterbury farmland on Saturday morning was initially believed to be 13km long but has now been re-drawn as 22km long, with an extra strand extending to close to Rolleston.

Land, roads and shelter belts in some places have been offset by nearly 4m where the fault crosses them.

Until now the fault has been hidden under the gravels of the Canterbury Plains with no features on the surface.

GNS Science earthquake scientist Dr Simon Cox said many of the aftershocks appeared to be centred around the newly discovered Rolleston segment.

"The aftershock swarm is focused further over that way – maybe there's something more there that we're not just seeing yet?"

A name has not been chosen for the fault yet, although Cox said the "Rolleston Fault" was possible.

Shaken Cantabrians received some good news yesterday about the risk of major earthquakes on other faults in the region.

GNS natural hazards manager and earthquake geologist Dr Kelvin Berryman said computer modelling had shown Saturday's quake had had "negligible effect" on the Alpine Fault on the western edge of the Southern Alps.

The Alpine Fault is expected to be the source of "The Big One", which could be up to magnitude eight.

Berryman said the river terraces that were deposited about 16,000 years ago at the end of the last glaciation had hidden the fault.

"Before Saturday, there was nothing in the landscape that would have suggested there was an active fault beneath the Darfield and Rolleston areas.

"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."

The fault had been accumulating stress for thousands of years. Scientists were now keen to learn if the underground fault extended beyond the visible surface rupture.

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Greendale, not far from the epicentre of the main quake, recorded the strongest earthquake ground-shaking ever in New Zealand of 1.25 times the strength of gravity, Berryman said.

Duty GNS Science seismologist John Ristau said the earthquake aftershocks would continue for some time but the predicted large one of magnitude six might not occur.

"The more time that goes by, the less likely it is going to happen, but you're still not out of the danger zone."

Cox said the Rolleston branch of the new fault trace was a subtle feature and had not been immediately obvious.

- The Press

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