﻿PREFACE. 
  xxvn 
  

  

  independent 
  shocks 
  which 
  originate 
  from 
  their 
  own 
  centres 
  at 
  or 
  shortly 
  after 
  the 
  

   passage 
  of 
  the 
  main 
  shock, 
  and, 
  as 
  the 
  time 
  of 
  their 
  occurence 
  is 
  determined 
  by 
  

   that 
  of 
  the 
  greater 
  shock, 
  they 
  may 
  be 
  called 
  sympathetic 
  earthquakes. 
  

  

  93. 
  It 
  is 
  conceivable 
  that 
  a 
  portion 
  of 
  the 
  earth's 
  crust 
  may 
  be 
  in 
  such 
  a 
  state 
  

   of 
  strain 
  that 
  a 
  very 
  slight 
  disturbance 
  may 
  be 
  the 
  determining 
  factor 
  in 
  start- 
  

   ing 
  an 
  actual 
  rupture. 
  In 
  these 
  circumstances 
  it 
  is 
  possible 
  that 
  the 
  molecular 
  

   displacements 
  due 
  to 
  the 
  passage 
  of 
  an 
  earthquake 
  may 
  be 
  sufficient 
  to 
  deter- 
  

   mine 
  the 
  final 
  fracture 
  and 
  so 
  give 
  rise 
  to 
  a 
  sympathetic 
  earthquake 
  ; 
  that 
  is 
  

   to 
  say, 
  to 
  an 
  earthquake 
  which 
  is 
  unconnected 
  with 
  the 
  primary 
  one, 
  so 
  far 
  as 
  

   the 
  place 
  and 
  cause 
  of 
  origin 
  are 
  concerned, 
  but 
  is 
  so 
  far 
  dependent 
  on 
  it 
  that 
  the 
  

   moment 
  of 
  its 
  occurrence 
  is 
  decided 
  by 
  the 
  passage 
  of 
  the 
  earthwaves 
  of 
  the 
  

   greater 
  shock. 
  

  

  94. 
  There 
  are 
  many 
  recorded 
  cases 
  of 
  earthquakes 
  which, 
  occurring 
  at 
  the 
  

   same 
  time 
  as 
  greater 
  ones, 
  have 
  been 
  regarded 
  as 
  sympathetic, 
  but 
  it 
  must 
  be 
  

   confessed 
  that 
  the 
  connection 
  has 
  in 
  no 
  case 
  been 
  proved. 
  As 
  a 
  rule, 
  the 
  coinci- 
  

   dence 
  in 
  time 
  is 
  only 
  approximate, 
  and 
  where 
  it 
  has 
  been 
  close 
  enough 
  to 
  permit 
  

   the 
  supposition 
  of 
  a 
  causal 
  connection, 
  there 
  always 
  remains 
  the 
  possibility 
  of 
  

   the 
  coincidence 
  being 
  merely 
  accidental. 
  The 
  existence 
  of 
  sympathetic 
  shocks, 
  

   though 
  they 
  are 
  referred 
  to 
  and 
  described 
  in 
  text 
  books, 
  must 
  still 
  be 
  looked 
  upon 
  

   as 
  conjectural 
  rather 
  than 
  established. 
  

  

  95. 
  Sympathetic 
  aftershocks. 
  — 
  A 
  third 
  category 
  might 
  be 
  made 
  of 
  shocks 
  

   which, 
  though 
  they 
  start 
  from 
  a 
  centre 
  which 
  is 
  neither 
  identical 
  nor 
  contiguous 
  

   with 
  that 
  of 
  the 
  great 
  shock, 
  yet 
  follow 
  it 
  and 
  are 
  due 
  to 
  the 
  alterations 
  of 
  strains 
  

   caused 
  by 
  the 
  displacements 
  which 
  accompanied 
  the 
  principal 
  shock. 
  In 
  the 
  

   case 
  of 
  the 
  1897 
  earthquake 
  there 
  were, 
  during 
  the 
  autumn 
  months 
  of 
  the 
  year, 
  

   a 
  number 
  of 
  small 
  earthquakes 
  in 
  the 
  Darjiling 
  district 
  of 
  the 
  Himalayas, 
  which 
  

   evidently 
  originated 
  there, 
  and 
  did 
  not 
  spread 
  into 
  that 
  district 
  from 
  a 
  distant 
  

   centre. 
  The 
  Darjiling 
  district 
  lay 
  well 
  outside 
  the 
  epicentre 
  of 
  the 
  principal 
  

   shock, 
  yet 
  these 
  shocks, 
  which 
  originated 
  there, 
  and 
  were 
  much 
  more 
  frequent 
  

   than 
  the 
  normal 
  number 
  of 
  shocks 
  occurring 
  in 
  that 
  district, 
  must 
  have 
  been 
  

   indirectly 
  due 
  to 
  the 
  great 
  shock 
  ; 
  that 
  is 
  to 
  say, 
  the 
  displacements 
  of 
  the 
  earth's 
  

   crust 
  under 
  the 
  epicentre 
  of 
  the 
  great 
  shock 
  caused 
  a 
  change 
  and 
  increase 
  

   of 
  the 
  strains 
  in 
  the 
  Himalayas 
  outside 
  that 
  tract, 
  which 
  resulted 
  in 
  an 
  increased 
  

   number 
  of 
  earthquakes. 
  These 
  were 
  not 
  aftershocks 
  in 
  the 
  ordinary 
  sense 
  

   of 
  the 
  word, 
  for 
  they 
  did 
  not 
  originate 
  from 
  the 
  same 
  focus 
  or 
  from 
  an 
  extension 
  

   of 
  it, 
  while 
  they 
  were 
  equally 
  not 
  sympathetic 
  earthquakes 
  as 
  defined 
  above, 
  for 
  

   they 
  occurred 
  at 
  a 
  very 
  different 
  time 
  ; 
  as 
  they 
  combine 
  part 
  of 
  the 
  characteristics 
  

   of 
  these 
  two 
  classes 
  of 
  secondary 
  earthquakes, 
  they 
  may 
  be 
  distinguished 
  as 
  

   sympathetic 
  aftershocks. 
  

  

  