Canterbury-Earthquakes-Information-Paper-FINAL-Approved-PDG-KB.doc

Canterbury-Earthquakes-Information-Paper-FINAL-Approved-PDG-KB.doc

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The Canterbury Earthquakes: Scientific answers to critical questions The Canterbury region has had six months of unexpected and extremely difficult challenges as a result of a sequence of damaging and deadly earthquakes and the associated aftershocks. The result is significant uncertainty within the public about why the February 22 aftershock was so damaging and deadly compared to the larger magnitude event on September 4, 2010. In order to provide information on these and other questions, the Royal Society of New Zealand, together with the Office of the Prime Minister’s Science Advisory Committee, has convened a group of earthquake science experts including those from the Natural Hazards Research Platform, a collaborative research consortia hosted by GNS Science, to provide answers to critical questions based on current best scientific information and knowledge. An enormous scientific effort is underway but it is difficult to define all of the critical attributes of an earthquake quickly after the event. As a result, our understanding of these earthquakes will improve and much more information will become available to be used in further scientific analyses and advice in the weeks and months ahead. The information below is provided for the purpose of improved understanding of the science relating to the Canterbury earthquakes. Prediction Why didn’t scientists know about the faults that caused the two earthquakes? Prior to September 4 th , 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 4 th , 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 1 . 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 1 1 | P a g e OFFICE OF THE PRIME MINISTER’S SCIENCE ADVISORY COMMITTEE
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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
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  • Fall '18
  • Don Reed
  • Canterbury Earthquakes

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