﻿xvi 
  OLDHAM: 
  GREAT 
  EARTHQUAKE 
  OF 
  1897. 
  

  

  nothing 
  in 
  front 
  to 
  restrain 
  the 
  movement 
  of 
  the 
  wave-particle 
  and 
  it 
  can 
  con- 
  

   sequently 
  range 
  further 
  out 
  from 
  its 
  normal 
  position. 
  The 
  effect 
  of 
  this 
  greater 
  

   freedom 
  and 
  range 
  of 
  motion 
  where 
  the 
  wave 
  comes 
  out 
  to 
  a. 
  free 
  surface 
  is 
  very 
  

   marked, 
  and 
  will 
  be 
  referred 
  to 
  in 
  the 
  chapters 
  dealing 
  with 
  earth 
  fissures 
  and 
  

   landslips. 
  

  

  44. 
  To 
  this 
  greater 
  freedom 
  of 
  motion 
  at 
  and 
  near 
  the 
  surface 
  of 
  the 
  ground 
  we 
  

   must, 
  at 
  any 
  rate, 
  to 
  a 
  large 
  extent, 
  attribute 
  the 
  more 
  noticeable 
  and 
  destructive 
  

   character 
  of 
  the 
  shock 
  at 
  the 
  surface 
  of 
  the 
  ground. 
  Of 
  the 
  nature 
  of 
  the 
  move- 
  

   ment, 
  all 
  that 
  can 
  be 
  said 
  here 
  is 
  that 
  it 
  is 
  characterised 
  by 
  an 
  extreme 
  com- 
  

   plication 
  and 
  intricacy 
  of 
  the 
  path 
  of 
  the 
  wave-particle. 
  At 
  one 
  and 
  the 
  same 
  time 
  

   it 
  is 
  subjected 
  to 
  wave 
  motion 
  of 
  varying 
  amount 
  and 
  period, 
  and 
  of 
  each 
  of 
  the 
  

   three 
  distinct 
  types 
  described 
  in 
  paras. 
  9 
  to 
  21. 
  

  

  45. 
  In 
  spite, 
  however, 
  of 
  this 
  complexity, 
  a 
  prevailing 
  direction 
  can 
  often 
  be 
  

   recognised, 
  which 
  may 
  or 
  may 
  not 
  change 
  during 
  the 
  period 
  for 
  which 
  the 
  earth- 
  

   quake 
  lasts. 
  As 
  a 
  rule 
  there 
  is 
  no 
  relation 
  between 
  the 
  direction 
  of 
  this 
  movement 
  

   and 
  the 
  direction 
  in 
  which 
  the 
  epicentre 
  lies, 
  that 
  is, 
  from 
  which 
  the 
  shock 
  is 
  

   travelling. 
  In 
  the 
  case 
  of 
  very 
  severe 
  earthquakes 
  it 
  may 
  happen 
  that 
  man 
  v 
  

   of 
  the 
  buildings 
  and 
  monuments 
  overthrown 
  point 
  in 
  a 
  direction 
  towards, 
  or 
  away 
  

   from, 
  the 
  epicentre. 
  Where 
  this 
  is 
  so 
  they 
  may 
  be 
  used 
  for 
  determining 
  the 
  

   position 
  of 
  the 
  focus, 
  but 
  the 
  method, 
  as 
  referred 
  to 
  in 
  Appendix 
  D, 
  requires 
  both 
  

   care 
  and 
  skill 
  in 
  its 
  application. 
  

  

  46. 
  So 
  far 
  reference 
  has 
  only 
  been 
  made 
  to 
  what 
  can 
  be 
  observed 
  without 
  

   instrumental 
  aid. 
  If 
  the 
  same 
  disturbance 
  is 
  studied 
  with 
  the 
  aid 
  of 
  seismographs 
  

   it 
  is 
  found 
  that 
  the 
  duration 
  is 
  about 
  double 
  of 
  what 
  can 
  be 
  felt, 
  and 
  that 
  it 
  

   follows 
  the 
  same 
  law. 
  That 
  is 
  to 
  say, 
  it 
  first 
  increases 
  as 
  the 
  epicentre 
  is 
  receded 
  

   from, 
  and 
  then 
  decreases, 
  only 
  a 
  smaller 
  and 
  smaller 
  fraction 
  of 
  the 
  whole 
  

   disturbance 
  being 
  of 
  a 
  character 
  which 
  can 
  have 
  any 
  effect 
  on 
  the 
  instrument. 
  

   What 
  takes 
  place 
  may 
  in 
  fact 
  be 
  likened 
  to 
  the 
  changes 
  that 
  take 
  place 
  in 
  the 
  band 
  

   of 
  ripples 
  set 
  up 
  in 
  a 
  pond 
  by 
  a 
  stone 
  falling 
  into 
  it. 
  As 
  they 
  spread 
  outward 
  

   the 
  width, 
  not 
  only 
  of 
  each 
  individual 
  wave, 
  but 
  of 
  the 
  whole 
  band, 
  increases, 
  and 
  

   then, 
  if 
  the 
  pond 
  is 
  large 
  enough, 
  one 
  wave 
  after 
  another 
  may 
  be 
  seen 
  to 
  grow 
  

   so 
  long 
  and 
  flat 
  that 
  it 
  ceases 
  to 
  be 
  discernible, 
  and 
  finally 
  the 
  pond 
  resumes 
  its 
  

   originally 
  undisturbed 
  condition. 
  

  

  47. 
  This 
  spreading 
  out 
  of 
  the 
  waves 
  is 
  very 
  conspicuous 
  in 
  the 
  records 
  ob- 
  

   tained 
  by 
  the 
  delicate 
  instruments 
  specially 
  designed 
  to 
  detect 
  the 
  movement 
  

   which 
  takes 
  place 
  at 
  distances 
  beyond 
  where 
  the 
  earthquake 
  ceases 
  to 
  be 
  re- 
  

   corded 
  by 
  ordinary 
  seismoscopes. 
  In 
  these 
  the 
  preliminary 
  tremors 
  which 
  have 
  a 
  

   duration 
  to 
  be 
  measured 
  in 
  seconds 
  within 
  the 
  seismic 
  area, 
  outrun 
  the 
  larger 
  waves 
  

   by 
  periods 
  which 
  may 
  range 
  up 
  to 
  half 
  an 
  hour. 
  They 
  are 
  probably 
  due 
  to 
  

   elastic 
  vibrations 
  propagated 
  through 
  the 
  body 
  of 
  the 
  earth, 
  and 
  in 
  the 
  case 
  of 
  

   the 
  earthquake 
  of 
  1897 
  it 
  was 
  possible 
  to 
  recognise 
  that 
  they 
  present 
  two 
  phases 
  

   which 
  may 
  be 
  interpreted 
  as 
  being 
  due 
  to 
  the 
  arrival 
  of 
  the 
  condensational, 
  and 
  

   of 
  the 
  slower 
  travelling 
  distortional, 
  waves 
  respectively. 
  

  

  48. 
  This 
  division 
  of 
  the 
  preliminary 
  tremors 
  into 
  two 
  phases 
  had 
  not 
  been 
  

   definitely 
  recognised 
  before 
  1897, 
  but 
  it 
  seems 
  probable 
  that 
  it 
  may 
  be 
  found 
  in 
  

   the 
  records 
  of 
  other 
  earthquakes 
  when 
  they 
  come 
  to 
  be 
  re-examined 
  in 
  the 
  light 
  

   of 
  the 
  distinction 
  recognised 
  in 
  the 
  case 
  of 
  the 
  1897 
  earthquake. 
  It 
  lends 
  such 
  

  

  