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  OLDHAM 
  : 
  GREAT 
  EARTHQUAKE 
  OF 
  1897. 
  

  

  crust 
  of 
  the 
  earth, 
  S 
  S 
  being 
  the 
  surface. 
  Then 
  a 
  straight 
  line 
  drawn 
  vertically 
  from 
  

   F 
  to 
  E 
  is 
  the 
  seismic 
  vertical, 
  and 
  the 
  point 
  E 
  is 
  the 
  epicentre. 
  Where 
  the 
  centre, 
  

   instead 
  of 
  being 
  so 
  small 
  that 
  it 
  may 
  be 
  considered 
  a 
  point, 
  is 
  of 
  considerable 
  size, 
  

   then 
  the 
  epicentre 
  increases 
  in 
  size 
  to 
  a 
  corresponding 
  extent, 
  and 
  we 
  have 
  an 
  

   ntral 
  or 
  epifocal 
  area 
  or 
  tract. 
  

   2S. 
  From 
  the 
  centre 
  F 
  the 
  earthquake 
  waves 
  radiate 
  in 
  every 
  direction. 
  To 
  

   the 
  epicentre 
  they 
  travel 
  straight 
  upwards 
  ; 
  away 
  from 
  it 
  they 
  reach 
  the 
  surface 
  at 
  

   a 
  less 
  and 
  less 
  angle 
  as 
  we 
  get 
  further 
  away. 
  Let 
  F 
  A 
  represent 
  one 
  of 
  these 
  

   wave 
  paths 
  : 
  then 
  the 
  angle 
  E 
  F 
  A 
  is 
  the 
  angle 
  of 
  emergence. 
  

  

  29. 
  The 
  front 
  of 
  the 
  disturbance 
  travels 
  at 
  a 
  definite 
  rate, 
  and 
  if 
  the 
  rate 
  o^ 
  

   propagation 
  is 
  equal 
  in 
  every 
  direction 
  it 
  will 
  at 
  any 
  given 
  moment 
  have 
  reached 
  

   the 
  same 
  distance 
  from 
  the 
  centre 
  in 
  every 
  direction. 
  The 
  circles 
  drawn 
  in 
  fig. 
  v 
  

   may 
  be 
  taken 
  to 
  represent 
  the 
  wave 
  front 
  at 
  successive 
  equal 
  intervals 
  of 
  time 
  

   and 
  are 
  known 
  as 
  hotnoseists. 
  They 
  are 
  surfaces, 
  which 
  appear 
  as 
  lines 
  where 
  

   they 
  are 
  cut 
  by 
  the 
  ground 
  level, 
  and 
  if 
  we 
  had 
  a 
  perfectly 
  homogeneous 
  earth 
  

   and 
  a 
  perfectly 
  level 
  ground, 
  they 
  would 
  appear 
  as 
  circles. 
  In 
  practice 
  neither 
  

   condition 
  holds 
  good 
  and 
  the 
  homoseists 
  or 
  homoseismic 
  lines 
  are 
  irregular 
  in 
  

   form. 
  

  

  30. 
  As 
  the 
  earthquake 
  radiates 
  from 
  the 
  centre 
  it 
  becomes 
  less 
  and 
  less 
  violent, 
  

   and 
  it 
  is 
  possible 
  to 
  trace 
  lines 
  of 
  equal 
  violence 
  of 
  shock 
  which 
  are 
  known 
  as 
  

   isoseists 
  or 
  isoseismic 
  lines. 
  If 
  a 
  condition 
  of 
  perfect 
  homogeneity 
  prevailed 
  in 
  the 
  

   crust 
  of 
  the 
  earth 
  they 
  would 
  coincide 
  with 
  the 
  homoseists 
  ; 
  but 
  as 
  the 
  violence 
  

   of 
  the 
  shock 
  is 
  much 
  more 
  influenced 
  by 
  the 
  nature 
  of 
  the 
  surface 
  deposits 
  than 
  

   the 
  rate 
  of 
  propagation, 
  they 
  are 
  in 
  practice 
  found 
  to 
  pursue 
  an 
  even 
  more 
  

   devious 
  and 
  irregular 
  course. 
  

  

  31. 
  The 
  actual 
  violence 
  of 
  the 
  shock 
  doubtless 
  decreases 
  outwards 
  from 
  the 
  

   epicentre, 
  but 
  its 
  destructiveness 
  does 
  not 
  necessarily 
  do 
  so. 
  At 
  the 
  epicentre 
  the 
  

   thrust, 
  if 
  we 
  are 
  dealing 
  with 
  a 
  wave 
  of 
  elastic 
  compression, 
  is 
  straight 
  upwards 
  ; 
  

   there 
  is 
  no 
  sidelong 
  push, 
  and 
  consequently 
  no 
  tendency 
  to 
  overturn 
  walls. 
  As- 
  

   we 
  recede 
  from 
  the 
  epicentre, 
  and 
  the 
  angle 
  of 
  emergence 
  decreases, 
  the 
  sidelong 
  

   motion 
  increases, 
  and 
  there 
  is 
  an 
  increasing 
  tendency 
  to 
  overturn 
  walls 
  and 
  

   buildings; 
  the 
  violence 
  of 
  the 
  earthquake 
  is, 
  however, 
  diminishing, 
  and 
  there 
  is 
  a 
  

   point 
  where 
  the 
  increasing 
  value 
  of 
  the 
  one 
  and 
  the 
  decreasing 
  value 
  of 
  the 
  other 
  

   lead 
  to 
  a 
  maximum 
  of 
  destructive 
  effect. 
  The 
  line 
  along 
  which 
  this 
  takes 
  place 
  

   is 
  the 
  meizoseismic 
  line 
  or 
  circle 
  ; 
  according 
  to 
  the 
  view 
  we 
  take 
  of 
  the 
  rate 
  at 
  

   which 
  the 
  violence 
  of 
  an 
  earthquake 
  diminishes 
  the 
  angle 
  of 
  emergence 
  to 
  which 
  

   the 
  meizoseismic 
  circle 
  corresponds 
  becomes 
  45 
  or 
  54" 
  44. 
  Theoretically, 
  then, 
  the 
  

   depth 
  of 
  the 
  focus 
  from 
  the 
  surface 
  should 
  be 
  equal 
  to, 
  or 
  rather 
  over, 
  half 
  the 
  

   diameter 
  of 
  the 
  meizoseismic 
  circle. 
  

  

  32. 
  Within 
  the 
  meizoseismic 
  circle 
  the 
  variation 
  in 
  destructiveness 
  of 
  the 
  

   earthquake 
  is 
  found 
  to 
  be 
  much 
  less 
  than 
  should 
  be 
  the 
  case 
  if 
  the 
  movement 
  

   were 
  purely 
  that 
  of 
  a 
  wave 
  of 
  elastic 
  compression. 
  This 
  is 
  in 
  accordance 
  with 
  

   what 
  has 
  been 
  said 
  regarding 
  the 
  impossibility 
  of 
  one 
  form 
  of 
  wave 
  motion 
  

   reaching 
  the 
  surface 
  unaccompanied 
  by 
  others. 
  In 
  practice 
  it 
  is 
  generally 
  found 
  

   that 
  there 
  is 
  an 
  area 
  over 
  which 
  there 
  are 
  comparatively 
  small 
  variations 
  in 
  the 
  

   destructive 
  force 
  of 
  the 
  earthquake. 
  This 
  lies 
  within 
  the 
  meizoseismic 
  circle, 
  which 
  

   is 
  not 
  always 
  recognisable, 
  while 
  outside 
  it 
  there 
  is 
  a 
  much 
  more 
  rapid 
  decrease 
  of 
  

   destruction. 
  This 
  inner 
  area 
  is 
  known 
  as 
  the 
  pleistoseismic 
  area. 
  

  

  