Why didn’t scientists know about the faults that caused the two earthquakes? Prior to September 4th, there were no surface signs of the Greendale Fault or the fault that generated the Lyttelton aftershock and there was no evidence for seismicity on these faults (i.e. ‘foreshocks’). Seismic surveys have located some ‘hidden’ faults across parts of the Canterbury Plains, but these particular regions had not been surveyed for this purpose. An oil-gas seismic survey had been carried out but did not reveal any convincing evidence for the presence of the Greendale Fault. Following September 4th , there was significant aftershock activity in the area of the Lyttelton Fault and around many faults in the region but there was no clear indication that a larger earthquake was imminent there. It was predicted that aftershocks from the September 2010 earthquake might reach magnitude 6, and some smaller aftershocks had already occurred under Christchurch city. Why wasn’t some warning given about the possibility of a big and damaging aftershock under the city? Warnings were given over the risks from large aftershocks [http://www.nzherald.co.nz/nz/news/article.cfm?c_id=1&objectid=10671602]. The prediction of aftershocks of approximately magnitude 6 is based on statistical analysis of historical earthquakes (Bath’s Law), which states “the average difference in magnitude between a mainshock and its largest aftershock is 1.2, regardless of the mainshock magnitude”. A quick survey of some of New Zealand’s largest historical earthquakes conforms to this average, although there is significant variability. The 6.3 aftershock is not outside the average range. The isolated and smaller aftershocks that occurred under the city CBD do not necessitate the presence of a larger fault capable of generating larger earthquakes, however this is possible. Seismic and aerial surveys are targeting this area in the near future to provide constraints on the geometry, extent, and...
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