Tough building codes couldn’t save Christchurch from unknown fault line

Image courtesy of NZSEE www.nzsee.org.nz

One of the things that has come to light about the New Zealand earthquake centred near Christchuch is the existence of a previously unknown faultline. This was only identified last September. According to ABC NEWS Geoscience and Isotope Research Agency seismologist John Ristau views what happened in Christchurch as being different.

“What was a bit unusual was just to have the magnitude-7 earthquake in the first place in that area around New Zealand, because the Canterbury region is not known to be particularly seismically active,”  This is where the discovery of this fault is so important. “It was buried under deposits that were 20,000 years old and it hadn’t been mapped before. So the fault was always there, we just didn’t know it existed.”

Everyone is now waiting to see the reaction of the regulators. At the moment all efforts are focused on rescue and recovery for those who experienced the effects of the quake. Here’s what happened immediately after the quake-

A placarding system was used. Reported in the IPENZ fact sheet “Building Safety Evaluation” and developed by the New Zealand Society of Earthquake Engineering and the Department of Building Housing the system is a triage system much like that used in Hospital Accident and Emergency wards.

“• Red carded buildings are considered unsafe to enter.

• Yellow carded are considered suitable only for restricted use or access until repairs are completed.

• Green carded buildings are considered safe to enter and appear to be in much the same structural condition as prior to the earthquake.

• Green placards state that building owners are “encouraged to obtain a detailed structural assessment of the building as soon as possible” and “report any unsafe conditions” to the Territorial Authority”.”

So what next? – according to a fact sheet from the Institute of Professional Engineer’s in New Zealand:

“There is a mandatory requirement that old buildings be strengthened to provide resistance to earthquake loads to a minimum of 33% of the current design level. The New Zealand Society of Earthquake Engineering recommends a minimum of 67% of current design level.
• Building owners must recognise the need for them to undertake further work, even for green-labelled buildings to ensure the minimum legal obligations are met, but are recommended to consider strengthening to 67% of current design levels.
• The strengthening process should also increase the ability of the structure to sustain damage without collapsing and causing loss of life.”

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