Sunday, July 24, 2011

Liquefaction and life

I had a blog follower (okay my Mother) ask me to explain liquefaction - or how the earth underneath a big part of Christchurch could turn liquid in an earthquake.

I don't think I can do a much better job of a technical explanation than Wikipedia, but here is an experiment that might help with understanding.

Take some tupperware or a mason jar.  Put dirt at the bottom.  Then add water.  Let it sit for a day or so until everything settles.

You probably have dirt at the bottom and water on top, right?  The two have settled into their separate spaces.

Now, shake it up really, really, really, hard.  What do you have?  Mud.  Or muddy water - depending on what makeup of water and dirt you put in there.

This is what happened to Christchurch.

Christchurch sits next to the ocean, has lots and lots of underground water sources, and where you get close to the ocean - a higher sand content to the soil.  When everything gets shaken up in an earthquake (or one of the ongoing aftershocks), mud happens.  This also messes with the pressure balance and in some places -- the now-liquid soil shoots up to the surface.

Here is one picture we took of liquefaction residue on the side of a neighborhood street.

Many people who lived in the eastern suburbs of Christchurch are now getting their houses (even whole neighborhoods) 'red-zoned'.  This means that the land under their houses is considered too susceptible to future liquefaction, and the government is condemning not just their houses - but their land.  No building will ever be allowed on that land.  The government is paying out the value of their land after the insurance company has paid them for the value of their destroyed house and valueables.  They can go build (or buy) somewhere else.

This is happening to approximately 5,000 homes - or 5% of the houses in Christchurch.

The people in the red zone are somewhat lucky.  They, at least, are getting resolution.

The people who are not so lucky are the people who are in the 'white' zone.  These are people who lived on the hills outside Christchurch and have had their homes decimated - but not red zoned.  They technically could build on their land again, but they might be unable to find fire or earthquake insurance if they do.

Last night, I had much of this explained by our next door neighbor.

First, he explained that the 5 point something earthquake that we felt on Friday was nothing to worry about.  Apparently the earthquakes that roll slowly and last a longer time are generally located out in the plains and aren't anything that will cause much damage.  It is the sudden jolt earthquakes that are right under Christchurch and more bothersome.

The February 22 earthquake was this type of earthquake.  It jolted their home in the hills, took out their power, water, sewage and every piece of glass or ceramic in their house, and rendered it completely unliveable.  But because they are in the Port Hills, they are not in the liquefaction danger zone, and therefore not red-zoned.  They don't expect their house/insurance/earthquake issues to be decided much before Christmas.

As a University employee, he and his wife were able to rent University owned housing.  First, they lived in our house.  Our house, however, only has one living space and doesn't have cable.  So, on the times he could get his rugby matches on the television, his wife would also have to watch them.  So, they moved to the larger house next door (which has two living spaces - and cable) when that became available.  And they are quite thankful for it.


  1. Much better explanation than Wikipedia. The picture and examples help. Thanks! :-)

  2. P.S. Condolences to your neighbor and all who are so inconvenienced. It's nice to get the human interest stories we wouldn't hear otherwide.