Tuesday, March 1, 2011

Christchurch Earthquakes

At 12.51 on Tuesday the 22nd of February 2011, I was standing in front of the computer checking my twitter feed. Then began a gentle swaying motion that lasted about 10 seconds. I immediately posted a tweet recording the event and clicked to the quake drums on the Geonet website to find the epicentre. I always do this as our big worry here is the risk posed by a tsunami triggered by a nearby local earthquake. Geonet took until 1pm to update their drums, longer for the Christchurch drum. But within three minutes someone from Christchurch posted on twitter.

Within the next few hours the true scale of the event unfolded, a catastrophe for the city. The loss of life and devastation are just terrible. We wait with trepidation for the casualty lists as although we live five hours drive away it is without doubt we will have lost friends and acquaintances.

Those of us in the science community were well aware of the potential risks to buildings and infrastructure posed by the soils of central and eastern Christchurch. The localities at risk are displayed on hazard maps. However, all of New Zealand faces a tectonic or volcanic threat of some degree so we tend to ignore the dangers in day to day living.

The quakes impact is very relevant to Okarito as we too live with the potential for liquefaction. Okarito lagoon features several circular islands that may be the result of large scale sand blows.

During the development struggles 10 years ago, the Community hired a geotech scientist to advise whether the easternmost undeveloped sections were suitable building sites. He concluded that the land was no better or worse than most of eastern Christchurch. It wasn't a yes or no answer, simply saying they do so over there so why not here?

Fortunately, the majority of Okarito houses are built on gravel and coarse beach sand, a more stable substrate than the saturated fine sands under the undeveloped sections. The global financial crisis has taken care of Okarito's eastern sections in the short term but the issue remains.

Everybody is chastened by the tragedy unfolding in Christchurch. Our local risks seem more real now and we wonder how we will cope during and after an event. I expect our civil defence preparations will become a little more polished in the near future.

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