Christmastime Means Quakes In Christchurch

Just when everything seemed like it was getting back to normal, this happens:

A patchwork of faults below Pegasus Bay is responsible for the large earthquakes shaking Christchurch more than six months after the last significant events.

The area of active faulting about 10km off the New Brighton coast and further afield was identified in an undersea survey by the National Institute of Water and Atmospheric Research before the June 13 quakes.

GeoNet project manager Ken Gledhill said it appeared thrust movement on two adjacent offshore faults generated yesterday's 1.58pm 10km-deep magnitude 5.8 quake and the 6km-deep magnitude 6.0 shake at 3.18pm.

After more information was analysed it was possible the quake magnitudes could be upgraded and their locations refined, he said.

Preliminary analysis indicated the small faults were oriented approximately north-south, quite a different direction to the Port Hills and Greendale faults.

Yesterday's quake sequence was similar in some ways to that on June 13, with a smaller foreshock and a larger mainshock, although the difference between the two biggest quakes' magnitudes yesterday was much less, he said.

"The interesting thing is, if you look at their focal mechanisms, both have similar mechanisms and are quite different to the February or June quakes.

"These [ruptures] are at quite a lot steeper angle – it's a thrust event, a pushing up. That doesn't suggest to me they are on the same fault but two different faults, but, of course, the usual caution applies in making such an early analysis."

...Aftershock probability forecasts from GNS Science seismologists had put the chance of such large quakes in the next year as small.

They had said there was about a one-in-two chance of a magnitude 5.5 to 5.9 quake and about a one-in-seven chance of a magnitude 6.0 to 6.4.

The best map on the Web to track these things seems to be here.

A terrible time in the eastern suburbs:

Cantabrians hoping Christmas celebrations would be a brighter end to a bleak year were yesterday dealing with more damage to homes, infrastructure and businesses following two magnitude-5.8 and 6.0 earthquakes in a swarm of large aftershocks yesterday.

The earthquakes, which struck at 1.58pm and 3.18pm yesterday, centred in faults below Pegasus Bay, abruptly ended six months of relative calm for the city and has further set back its recovery.

...Sumner residents were without water after one of the overnight shakes caused a main pipeline leak. The water supply was shut off, and repairs crews were hoping to get it back up by 10am.

This morning, Orion said there were 50 customers without power, and the network in New Brighton and Bexley will remain fragile for some time.

Parker said it was the city's goal to have everything back up before Christmas, but "of course that's going to be dependant on what seismic activity we have over the next 24 hours".

"We'll do everything that we humanly can to make Christmas day normal for our people as they deserve.

...Retailers who have struggled to survive were dealt a major blow as stores packed with Christmas shoppers were evacuated. Some face being shut on the busiest trading days of Christmas Eve and Boxing Day.

Liquefaction triggered by the jolts flooded streets and properties. There were large deposits in Bexley, Avondale, New Brighton, Burwood, and Parklands.