Updates On The Canterbury Fault Lines

Posted 04 Jun 2011 by Swiggs Popular
Posted in Earthquake Facts

Three confirmed new faults and two other areas of likely active faults have been identified near Christchurch by scientists examining seismic hazards around the city.

Extensive surveying over the past two months has uncovered a 25-kilometre-long fault under the sea off Kaiapoi. Two faults run almost directly below central Christchurch and two patches containing possible small faults – one off the Brighton coast and the second just north of Port Levy and Pigeon Bay.

But the outlook may not be as grim as it sounds. Scientists delivered a largely optimistic message for the people of Canterbury at a briefing yesterday. Despite their findings they say: The tsunami risk from the large offshore fault is believed to be very low because it is unlikely to generate a quake higher than magnitude 7.0. Also, its past movement has been mostly horizontal rather than vertical.

If you go to this Post here by Sue wells it explains the risk of further earthquakes.

Also go to the GNS website following this link for their detailed evaluation

There is no sign that aftershocks are spreading offshore onto, or close to, it or other already recognised Pegasus Bay faults.

The offshore faults are very slow moving, among the slowest moving in the country.

The faults underneath Christchurch are not particularly large and appear to have very long intervals between rupturing.

However, they also say the findings released yesterday are preliminary and that more research is needed.

The most important missing piece of the puzzle are results from the underground surveying of what has become known as "the gap".

Survey lines in the aftershock-rich area between the eastern end of the Greendale Fault and the western tip of the Port Hills fault were completed only last Saturday.

Canterbury University geological sciences Professor Jarg Pettinga , who has directed the seismic surveying work, said the analysis was under way but it would take another four weeks or so before those findings could be released.

Earthquake Recovery Minister Gerry Brownlee said the media briefing was a chance to update the public on everything scientists had been able to deduce so far about the Canterbury quakes.

Findings from the intensive research programme would inform future building codes.

National Institute of Water and Atmospheric Research principal scientist Philip Barnes said a survey by the research vessel Kaharoa of the sediments and bedrocks below the Pegasus Bay seabed in March covered about 800 square kilometres.

It confirmed the location of offshore faults near the Ashley River, and revealed the existence of a roughly east-west oriented fault off the coast near Kaiapoi that was in several segments and about 25km long. While it had the potential to generate a quake of between magnitude 6.0 and 7.0, there was "no strong evidence" aftershocks were migrating offshore onto any fault structures in Pegasus Bay, he said.

Pettinga said the sub-surface Port Hills fault, responsible for the deadly February 22 magnitude 6.3 quake, showed up on a survey along the beach from Waimairi Beach to the tip of Southshore, as did another less clear fault further north.

A north-south survey line down Barbadoes St also showed an underground fault below about Moorhouse Ave.

Natural Hazards Platform manager Dr Kelvin Berryman, of GNS Science, said that fault appeared to be the "home" of the damaging Boxing Day quake.

The three faults were lying roughly parallel in an east-north-east/west-south-west orientation underneath the Christchurch area.

Barnes said small active faults were likely in an area offshore on the same line as the Port Hills fault where aftershocks had been centred.

The second area off the north coast of Banks Peninsula was also thought to contain small faults.

But no aftershocks had been recorded there.

Rock layers below the sea had made it very difficult to survey in that location.


EDIT: Below is the update of aftershocks from September 4th 2010 to September 2012
quakes 0912.jpg


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