The ‘second’ christchurch earthquake

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Commentary by Dr Peter Stafford on the second Canterbury earthquake

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The ‘second’ christchurch earthquake

  1. 1. The  ‘second’  Christchurch  earthquake,  February  22,  2011    Dr  Peter  J.  Stafford  Willis  Research  Fellow  RCUK  Fellow  /  Lecturer  in  Modelling  Engineering  Risk  Department  of  Civil  &  Environmental  Engineering    Imperial  College  London    Note:  Dr  Stafford  was  born  in  Christchurch  and  has  family  and  friends  living  in  the  city.  He  completed  his  undergraduate  degree  and  his  PhD  at  the  University  of  Canterbury,  in  Christchurch.    Widespread  damage  has  been  caused  by  the  ground  shaking  induced  by  the  ML6.3  magnitude  earthquake  located  just  to  the  south  of  Christchurch.    The  event  is  much  closer,  and  much  shallower,  than  the  larger  ML7.1  earthquake  that  occurred  in  September  2010.  This  is  the  primary  reason  why  the  levels  of  ground  shaking  that  have  been  observed  are  so  severe.    Initial  reports  from  GNS  Science  in  New  Zealand  (a  government  research  institute  looking  at  Geological  and  Nuclear  Sciences  –  GNS)  indicate  that  the  levels  of  shaking  that  were  observed  in  and  around  Christchurch  are  significantly  greater  than  the  levels  that  structures  are  typically  designed  to  withstand.  It  is  therefore  no  surprise  that  many  structures  have  suffered  extensive  levels  of  damage,  and  that  some  have  collapsed.      http://www.geonet.org.nz/var/storage/images/media/images/news/2011/lyttelton_pga/57159-­‐1-­‐eng-­‐GB/lyttelton_pga.png    
  2. 2. Images  such  as  that  above,  taken  from  the  GeoNet  website,  suggest  that  peak  ground  accelerations  reached  as  high  as  1.9  times  the  acceleration  due  to  gravity  (although  it  is  not  clear  whether  these  are  recorded  in  the  horizontal  or  vertical  direction  –  horizontal  accelerations  are  those  most  relevant  for  estimating  damage  to  buildings).  By  any  measure,  accelerations  of  this  level  are  considered  to  be  very  large.    For  emergency  response  teams  it  is  now  crucial  to  ensure  that  survivors  currently  trapped  in  debris  from  collapsed  buildings  are  extracted  as  soon  as  possible.  Teams  of  people  will  be  working  through  the  night,  in  very  trying  conditions  involving  further  aftershocks,  but  the  likelihood  of  people  surviving  will  reduce  rapidly  with  time.    This  event  is  almost  certainly  an  aftershock  of  the  larger  event  that  occurred  in  September.  This  large  event  caused  damage  to  structures  throughout  the  city  that  was  thought  to  be  primarily  superficial  (affecting  what  structural  engineer’s  refer  to  as  non-­‐structural  elements,  e.g.,  facades,  in-­‐fill  panels,  non-­‐load-­‐bearing    walls  and  partitions  etc).  However  it  is  too  early  to  say  whether  some  of  the  collapses  that  have  occurred  have  resulted  from  undetected  damage  caused  by  the  first  major  event.    The  recovery  efforts  following  the  first  major  event,  while  sustained  and  concerted,  are  far  from  compete  and  a  lot  of  work  remained  in  order  to  bring  the  city  back  to  a  normal  state  even  prior  to  the  more  recent  event  that  occurred  this  morning.  The  problem  for  many  home-­‐owners  now  will  be  associated  with  the  uncertainty  regarding  how  and  when  their  homes  will  be  repaired.    The  New  Zealand  Earthquake  Commission  (EQC)  must  again  provide  financial  compensation  for  the  losses  sustained  by  the  earthquake  and  this  will  place  a  heavy  burden  upon  them.  With  estimates  of  direct  losses  on  the  order  of  2  billion  NZD  resulting  from  the  first  event,  it  is  likely  that  this  new,  more  damaging,  event  will  send  direct  losses  many  times  above  this  level.  And,  of  course,  the  indirect  cost  to  businesses  is  also  likely  to  be  very  significant.    Aftershocks  of  this  size  are  to  be  expected  following  a  major  earthquake.  As  a  general  ‘rule-­‐of-­‐thumb’  the  largest  expected  aftershock  in  a  sequence  will  typically  have  a  magnitude  that  is  roughly  one  unit  of  magnitude  lower  than  the  mainshock.  Note,  however,  that  a  unit  change  in  magnitude  relates  to  an  approximately  30-­‐fold  difference  in  the  amount  of  energy  that  is  released  from  the  earthquake  source.    Aftershock  events  are  defined  as  being  dependent  upon  the  mainshock.  However,  it  is  not  always  clear  whether  an  earthquake  is  really  an  aftershock  or  not.  The  relevance  of  this  point  is  that  in  many  countries,  insurance  policies  are  tied  to  covering  one  ‘act-­‐of-­‐God’  type  event  per  annum.  If  the  damage  caused  by  the  aftershock  is  deemed  to  be  part  of  the  main  event  then  one  may  be  covered  by  insurance,  if  the  event  is  deemed  to  be  independent  then  coverage  may  not  be  guaranteed.  The  implications  of  this  are  great  for  events  such  as  this  where  the  aftershock  is  going  to  be  responsible  for  significantly  greater  degrees  of  loss  than  the  mainshock  event.  Fortunately,  for  those  affected  by  this  event,  their  coverage  through  the  EQC  is  not  structured  in  this  way.      A  very  good  idea  of  the  types  of  events  that  the  citizens  are  experiencing  now  can  be  seen  from  the  website  below  that  plots  the  aftershocks  that  have  occurred  over  recent  times.  http://www.christchurchquakemap.co.nz/  

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