We live in Christchurch, New Zealand, and there have been two significant earthquakes recently. One in September last year, and another in February of this year. The February earthquake was rather more damaging – it resulted in 181 deaths and many more injuries. A large number of homes have been damaged beyond repair and life is hard for many in the city who have been without water supply, electriciy or sewerage.

The magnitude 6.3 (Feb 2011) event caused more damage than the 7.1 (Sep 2010), because it was close to the city – which means that a greater percentage of the total energy released hit Christchurch itself. Part of the central city is still deemed a no-access “red-zone”, and half of the commercial buildings will be demolished.

More technical detail is available from Geological and Nuclear Sciences here, a good collection of photos by Ross Becker here, and a video by Civil Defence here. A more cheery view, of how things used to be, is here.

Crane about to demolish a house on our street, with chimneys and a balcony

LIQUEFACTION

The September quake happened in the middle of the night. Since the power went off and there were no street lights, we thought we would traipse outside to look at the stars, as you do, and were rather suprised to find our courtyard area flooded and muddy. In the morning we found extensive amounts of sand in various parts of the yard. These are called ‘sand boils’ or ‘sand volcanoes’. They occur where water and sand mix and are subjected to pressure from an earthquake. The water is forced up through a gap in the overlying soil and it carries the sand/silt with it.

Sand and silt that has come up through the cracked driveway

Sand Volcano

Then in February, we were taking a moment outside, after the initial quake, with camera in hand. So when one of the several major after-shocks caused liquefaction right in front us, we were able to capture a little of it.


Where it was not very deep, the mud dried into interesting patterns. In most places however it just stayed in big piles that had to be shoveled out.

Dried mud - where it was not very thick

The sand and silt from liquefaction has been a major problem for the waste water system too, which is still not fully functional. This little ‘poo-bot’ was inspecting the sewerage system in our street a week or so ago:

Robot, for looking at the sewer

LATERAL SPREAD

We had some lateral spread – essentially the banks eroding towards the river. Our neighbours, right next to the river, had the floor plate of their house split in two.

One of the cracks, parallel to the river bank, at Mona Vale. About 1 m deep.

We are still getting significant aftershocks. There was a magnitude 5.5 this morning.

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