After the Christchurch earthquake. What we did not recognize in that sequence was that just because a seismic event one order of magnitude lower than that of the original large event had not happened did not mean it could not happen – we would just take most of six months to find out.
The sequences of aftershocks certainly kept reminding people that the sequence would take some time to ride out, with notable aftershocks on 04, 13 and 19 October 2010. All of these caused brief power outages throughout the central business district, liquefaction and an end to trading at numerous small businesses in older buildings. An aftershock on Boxing Day 2010 permanently closed the Whitcoulls bookshop building in Cashel Mall, and it was not hard to see why with extensive cracks in the facade. The same aftershock should have forced the evacuation of the C.T.V. building, but did not – with dreadful consequences.
Wellington and Kaikoura are experiencing a similar pattern now. The initial burst of magnitude 6 aftershocks has gone quiet. Like Christchurch during its earthquake sequence, Wellington and Kaikoura have not quickly had an aftershock an order of magnitude lower than the original magnitude 7.8 event. Due to a logarithmic increase in energy release for every whole order of magnitude a magnitude 5.0 is about 32 times more powerful than a magnitude 4.0 at the same location and depth; a magnitude 6.0 is about 32 times more powerful than a magnitude 5.0 at the same location and depth and so on. Thus the absence of an aftershock in the magnitude 6.8-7.0 range means that quite substantial energy is being locked up in the sequence. Because of the large area affected by the original earthquake, it is impossible to know where such an event could occur.
Only time will tell how this aftershock sequence plays out.
Wellington and Kaikoura need to keep this in mind as they move forward from 14 November 2016. Both places need to remember there is no substitute for everyone getting out safely from a building after an earthquake. With that in mind there are going to be some painful arguments over whether to pull down high risk buildings that might look beautiful but are structurally unsound. Politicians will debate the costs of earthquake strengthening, not wanting to put the tenants under undue financial pressure or pass on to ratepayers. Councils and the Government have a duty of care to their taxpayers/ratepayers, visitors and anyone else in their jurisdiction on the day of a major disaster. By shirking their responsibilities they can be exposing themselves and thus their rate/taxpayer base by default to potentially massive litigation and/or criminal proceedings.
That is not okay.
Wellington City Council has started a programme of identifying at risk buildings. Depending on how earthquake prone they are, the W.C.C. is assigning the landlords 10, 15 or 20 year deadlines to bring their assets up to an acceptable standard. The 10 year deadlines are for the most at risk buildings, some of which are now probably closed indefinitely or have owners scrambling for indepth assessments so they can determine how they proceed. The recent and ongoing seismic activity will also hopefully have jolted owners of buildings in the other two categories to bring forward their planned assessments.
And if anyone is still in doubt, this piece of advice from the Ministry of Business, Innovation and Employment is worth remembering:
“Buildings with less than one-third of the strength of a new building have about 10 to 20 times the risk of serious damage or collapse when compared to a new building.” mbie.govt.nz