﻿PREFACE. 
  XXV 
  

  

  others 
  they 
  have 
  been 
  broken 
  through 
  by 
  fractures 
  and 
  one 
  side 
  raised 
  by 
  hun- 
  

   dreds 
  and 
  even 
  thousands 
  of 
  feet 
  with 
  respect 
  to 
  the 
  other. 
  All 
  these 
  deformations 
  

   involve 
  the 
  exercise 
  of 
  great 
  forces, 
  which 
  are 
  opposed 
  by 
  the 
  natural 
  rigidity 
  

   and 
  cohesion 
  of 
  the 
  rocks; 
  where 
  this 
  resistance 
  is 
  small 
  the 
  deformation 
  may 
  

   go 
  on 
  gradually 
  : 
  where 
  it 
  is 
  somewhat 
  greater 
  the 
  strain 
  will 
  accumulate 
  till 
  

   the 
  resistance 
  is 
  suddenly 
  overcome 
  and 
  an 
  earthquake 
  is 
  the 
  result. 
  The 
  

   greater 
  the 
  power 
  of 
  resistance 
  the 
  greater 
  will 
  be 
  the 
  strain 
  which 
  accumulates 
  

   before 
  the 
  final 
  relief 
  takes 
  place, 
  and 
  the 
  greater 
  the 
  resulting 
  earthquake. 
  

  

  85. 
  Tectonic 
  earthquakes 
  have 
  been 
  further 
  classified 
  according 
  to 
  the 
  form 
  

   of 
  the 
  disturbance 
  to 
  which 
  they 
  were 
  due 
  into 
  faulty 
  heave, 
  overthrust 
  and 
  fold 
  

   earthquakes. 
  Leaving 
  the 
  last 
  out 
  of 
  the 
  question 
  as 
  a 
  very 
  doubtful 
  class, 
  the 
  

   separation 
  of 
  the 
  others 
  appear 
  to 
  be 
  of 
  little 
  use 
  and 
  to 
  be 
  rather 
  fanciful 
  than 
  

   philosophical. 
  All 
  alike 
  are 
  the 
  result 
  of 
  the 
  sudden 
  relief 
  of 
  accumulated 
  strain, 
  

   and 
  the 
  exact 
  form 
  of 
  the 
  fracture 
  to 
  which 
  this 
  is 
  due 
  appears 
  to 
  be 
  an 
  unessential 
  

   detail 
  ; 
  added 
  to 
  which 
  the 
  different 
  forms 
  of 
  fracture 
  accompanied 
  by 
  dis- 
  

   placement 
  to 
  which 
  the 
  terms 
  fault, 
  heave, 
  and 
  overthrust 
  are 
  applied 
  pass 
  into 
  each 
  

   other 
  without 
  any 
  defined 
  line 
  of 
  demarcation. 
  The 
  term 
  fault 
  is 
  used 
  when 
  both 
  

   the 
  fissure 
  and 
  the 
  movement 
  are 
  more 
  nearly 
  vertical 
  than 
  horizontal 
  ; 
  in 
  a 
  heave 
  

   the 
  fissure 
  is 
  also 
  nearer 
  vertical 
  than 
  horizontal, 
  but 
  the 
  movement 
  is 
  more 
  side- 
  

   long 
  than 
  up 
  and 
  down 
  ; 
  while 
  in 
  an 
  overthrust 
  the 
  plane 
  of 
  the 
  fracture 
  is 
  nearly 
  

   horizontal 
  and 
  the 
  horizontal 
  displacement 
  consequently 
  greater 
  than 
  the 
  vertical. 
  

  

  86. 
  It 
  is 
  important 
  to 
  note 
  that 
  the 
  tectonic 
  earthquakes 
  are 
  in 
  no 
  way 
  con- 
  

   nected 
  with 
  volcanic 
  action, 
  and 
  the 
  fears 
  so 
  generally 
  felt 
  in 
  Assam 
  and 
  Northern 
  

   Bengal 
  that 
  the 
  earthquake 
  of 
  1897 
  would 
  be 
  followed 
  by 
  the 
  outburst 
  of 
  an 
  active 
  

   volcano 
  were 
  not 
  only 
  ungrounded, 
  but 
  opposed 
  to 
  all 
  that 
  is 
  known 
  of 
  the 
  action 
  

   of 
  the 
  natural 
  agencies 
  by 
  which 
  earthquakes 
  are 
  produced. 
  

  

  VIII. 
  — 
  Secondary 
  earthquakes. 
  

  

  87. 
  All' 
  great 
  earthquakes 
  are 
  accompanied 
  by 
  others 
  which, 
  as 
  they 
  are 
  to 
  a 
  

   certain 
  extent 
  the 
  result 
  of 
  the 
  principal 
  one, 
  may 
  be 
  called 
  secondary 
  earthquakes. 
  

   These 
  are 
  of 
  two 
  kinds, 
  the 
  first 
  of 
  which 
  agrees 
  more 
  or 
  less 
  closely 
  with 
  the 
  prin- 
  

   cipal 
  shocks 
  as 
  regards 
  the 
  position 
  of 
  the 
  focus, 
  though 
  succeeding 
  it 
  in 
  point 
  of 
  

   time, 
  while 
  the 
  shocks 
  of 
  the 
  other 
  kind 
  coincide 
  with 
  the 
  principal 
  one 
  in 
  point 
  of 
  

   time, 
  but 
  start 
  from 
  a 
  different 
  focus. 
  A 
  third 
  class 
  might 
  be 
  made 
  out 
  of 
  shocks 
  

   which 
  to 
  a 
  certain 
  extent 
  combine 
  the 
  characteristics 
  of 
  the 
  two 
  first 
  mentioned. 
  

  

  88. 
  Aftershocks. 
  — 
  The 
  shocks 
  of 
  the 
  first 
  sort 
  are 
  known 
  as 
  aftershocks; 
  

   they 
  succeed 
  the 
  great 
  earthquake, 
  at 
  first 
  with 
  great 
  rapidity, 
  but 
  the 
  intervals 
  

   gradually 
  get 
  longer 
  and 
  longer 
  till 
  the 
  average 
  number 
  of 
  earthquakes 
  becomes 
  

   normal 
  once 
  more. 
  The 
  cause 
  of 
  the 
  aftershocks 
  is 
  not 
  difficult 
  to 
  understand. 
  

   Every 
  great 
  earthquake 
  is 
  accompanied 
  by 
  more 
  or 
  less 
  displacement 
  in 
  the 
  

   interior 
  of 
  the 
  earth 
  ; 
  this 
  gives 
  rise 
  to 
  a 
  new 
  distribution 
  of 
  strains, 
  and 
  the 
  

   gradual 
  relief 
  of 
  these 
  new 
  strains 
  causes 
  a 
  succession 
  of 
  earthquakes, 
  each 
  of 
  

   which 
  again 
  causes 
  a 
  redistribution 
  of 
  the 
  strains, 
  and 
  the 
  process 
  goes 
  on 
  till 
  

   equilibrium 
  is 
  more 
  or 
  less 
  completely 
  restored. 
  

  

  89. 
  These 
  aftershocks 
  have 
  been 
  carefully 
  studied 
  in 
  Japan, 
  and 
  it 
  has 
  been 
  

   found 
  that 
  their 
  decrease 
  in 
  frequency 
  approximates 
  to 
  a 
  definite 
  law, 
  such 
  that, 
  if 
  

  

  