﻿PREFACE. 
  XV 
  

  

  40. 
  Leaving, 
  for 
  the 
  moment, 
  the 
  consideration 
  of 
  the 
  preliminary 
  tremors 
  

   and 
  of 
  the 
  long 
  slow 
  afterwaves, 
  and 
  confining 
  attention 
  to 
  the 
  central 
  parts 
  of 
  

   the 
  disturbance, 
  which 
  constitutes 
  the 
  sensible 
  shock, 
  it 
  seems 
  necessary 
  to 
  dis- 
  

   tinguish 
  the 
  character 
  of 
  the 
  movement 
  in 
  small 
  and 
  great 
  shocks. 
  In 
  those 
  

   which 
  nowhere 
  reach 
  a 
  degree 
  of 
  violence 
  sufficient 
  to 
  cause 
  alarm, 
  and 
  much 
  less 
  

   any 
  actual 
  damage, 
  it 
  is 
  probable 
  enough 
  that 
  the 
  motion 
  is 
  largely 
  due 
  to 
  

   elastic 
  vibration 
  propagated 
  directly 
  from 
  the 
  focus 
  to 
  the 
  surface 
  through 
  the 
  

   substance 
  of 
  the 
  earth. 
  In 
  the 
  case 
  of 
  severe 
  earthquakes, 
  this 
  form 
  of 
  motion 
  

   is 
  also 
  present, 
  but 
  overpowered 
  by 
  waves 
  of 
  a 
  different 
  character, 
  which 
  are 
  prob- 
  

   ably 
  analogous 
  to 
  the 
  elastic 
  surface 
  undulations 
  of 
  Lord 
  Rayleigh, 
  and 
  originate 
  

   at 
  the 
  surface, 
  within 
  the 
  epifocal 
  area, 
  whence 
  they 
  spread 
  outwards 
  in 
  every 
  

   direction. 
  

  

  41. 
  A 
  peculiarity 
  of 
  this 
  sensible 
  earthquake 
  motion 
  is 
  that, 
  as 
  the 
  disturbance 
  

   radiates, 
  its 
  duration 
  at 
  first 
  increases 
  and 
  then 
  appears 
  to 
  diminish. 
  This 
  was 
  

   very 
  noticeable 
  in 
  the 
  case 
  of 
  the 
  earthquake 
  of 
  1897 
  ; 
  within 
  the 
  epicentre 
  the 
  dura- 
  

   tion 
  of 
  the 
  great 
  shock 
  did 
  not 
  exceed 
  two 
  minutes 
  ; 
  at 
  Calcutta 
  it 
  was 
  certainly 
  

   5 
  or 
  6, 
  and 
  the 
  maximum 
  duration 
  seems 
  to 
  have 
  been 
  at 
  about 
  500 
  miles 
  from 
  

   the 
  centre, 
  reaching 
  some 
  seven 
  minutes. 
  From 
  here 
  the 
  reported 
  duration 
  

   diminishes 
  till 
  at 
  the 
  furthest 
  limit, 
  in 
  Ahmedabad, 
  it 
  is 
  reported 
  to 
  have 
  been 
  but 
  

   a 
  second 
  or 
  two. 
  This 
  does 
  not, 
  however, 
  mean 
  that 
  the 
  disturbance 
  of 
  the 
  

   ground 
  only 
  lasted 
  for 
  that 
  period, 
  but 
  that, 
  except 
  for 
  a 
  few 
  seconds, 
  the 
  move- 
  

   ment 
  was 
  too 
  slow 
  and 
  gentle 
  to 
  be 
  felt. 
  

  

  42. 
  In 
  the 
  area 
  of 
  propagation 
  of 
  an 
  earthquake, 
  this 
  movement, 
  which 
  can 
  be 
  

   felt, 
  and 
  to 
  which 
  the 
  damage 
  done 
  is 
  due, 
  is 
  quite 
  superficial. 
  It 
  has 
  been 
  found 
  

   by 
  experiment 
  in 
  Japan 
  that 
  an 
  excavation 
  of 
  only 
  20 
  feet 
  in 
  depth 
  is 
  enough 
  to 
  

   reduce 
  the 
  wave 
  motion 
  of 
  an 
  earthquake 
  to 
  a 
  very 
  small 
  fraction 
  of 
  what 
  it 
  is 
  

   at 
  the 
  surface. 
  Another 
  fact, 
  pointing 
  to 
  the 
  same 
  conclusion, 
  is 
  the 
  rarity 
  

   with 
  which 
  an 
  earthquake 
  is 
  felt 
  underground 
  in 
  mines, 
  even 
  where 
  these 
  are 
  

   shallow. 
  In 
  the 
  rare 
  cases 
  where 
  an 
  earthquake 
  has 
  been 
  felt 
  underground, 
  like 
  

   that 
  referred 
  to 
  in 
  Appendix 
  H 
  to 
  the 
  following 
  report, 
  it 
  is 
  probable 
  that 
  the 
  

   mine 
  lay 
  within 
  the 
  core 
  of 
  the 
  earthquake, 
  and 
  that 
  the 
  motion 
  felt 
  was 
  due 
  to 
  

   those 
  molar 
  displacements 
  of 
  the 
  ground 
  which 
  have 
  been 
  referred 
  to 
  in 
  para. 
  33. 
  

   The 
  1897 
  earthquake 
  was 
  not 
  felt 
  underground 
  in 
  the 
  mines 
  of 
  the 
  Bengal 
  

   coalfields 
  j 
  it 
  was 
  felt 
  and 
  caused 
  some 
  damage 
  in 
  the 
  coal 
  mines 
  at 
  Makum 
  in 
  

   Upper 
  Assam 
  ; 
  but 
  these 
  mines 
  are 
  really 
  over 
  ground, 
  being 
  driven 
  into 
  the 
  

   side 
  of 
  a 
  hill 
  and 
  not 
  descending 
  below 
  the 
  general 
  surface 
  level, 
  the 
  conditions 
  

   are, 
  therefore, 
  not 
  strictly 
  comparable 
  to 
  those 
  of 
  a 
  mine 
  sunk 
  in 
  open 
  and 
  

   tolerably 
  level 
  country. 
  

  

  43. 
  If 
  the 
  felt 
  earthquake 
  wave 
  is 
  of 
  the 
  nature 
  of 
  Lord 
  Rayleigh's 
  elastic 
  

   surface 
  undulations, 
  the 
  restriction 
  of 
  the 
  shock 
  to 
  the 
  surface 
  is 
  in 
  accordance 
  

   with 
  the 
  theory, 
  for 
  at 
  a 
  short 
  distance 
  the 
  range 
  of 
  motion 
  would 
  become 
  too 
  

   small 
  to 
  be 
  felt 
  below 
  the 
  surface. 
  It 
  may 
  also 
  be 
  explained, 
  in 
  the 
  case 
  of 
  purely 
  

   elastic 
  condensational 
  or 
  distortional 
  waves, 
  by 
  the 
  greater 
  play 
  which 
  is 
  possible 
  

   at 
  the 
  surface 
  than 
  in 
  the 
  body 
  of 
  the 
  earth. 
  While 
  the 
  wave 
  is 
  being 
  transmitted 
  

   through 
  the 
  rocks 
  which 
  constitute 
  the 
  crust 
  of 
  the 
  earth, 
  the 
  movement 
  of 
  

   each 
  wave-particle 
  is 
  restrained 
  by 
  the 
  resistance 
  of 
  those 
  in 
  front, 
  to 
  which 
  it 
  

   has 
  to 
  communicate 
  its 
  motion 
  : 
  but 
  when 
  the 
  wave 
  reaches 
  the 
  surface 
  there 
  is 
  

  

  