﻿168 
  OLDHAM: 
  GREAT 
  EARTHQUAKE 
  OF 
  1897. 
  

  

  earthquake, 
  there 
  would 
  be 
  no 
  thrust-plane 
  underlying 
  it, 
  and 
  the 
  

   focus 
  of 
  the 
  earthquake 
  would 
  have 
  to 
  be 
  regarded 
  as 
  a 
  complex 
  

   one. 
  That 
  is 
  to 
  say, 
  there 
  would 
  be 
  no 
  general 
  focus, 
  but 
  a 
  number 
  

   of 
  independent 
  ones, 
  along 
  each 
  fault, 
  and 
  the 
  magnitude 
  of 
  the 
  

   earthquake 
  experienced 
  would 
  be 
  due 
  to 
  the 
  simultaneous 
  occurrence 
  

   of 
  a 
  number 
  of 
  earthquakes 
  of 
  various 
  degrees 
  of 
  severity. 
  

  

  Whether 
  we 
  regard 
  the 
  focus 
  as 
  a 
  thrust-plane, 
  or 
  as 
  a 
  network 
  

   of 
  faults, 
  it 
  practically 
  covered 
  an 
  extensive 
  area. 
  The 
  hypothesis 
  of 
  

   a 
  thrust-plane 
  is 
  the 
  simplest 
  to 
  work 
  with, 
  as 
  also 
  the 
  most 
  probable, 
  

   and 
  it 
  is 
  that 
  which 
  has 
  been 
  adopted 
  in 
  the 
  following 
  pages. 
  1 
  

  

  Having 
  established 
  the 
  nature 
  of 
  the 
  focus 
  of 
  this 
  earthquake, 
  

   the 
  next 
  point 
  to 
  determine 
  is 
  the 
  approximate 
  horizontal 
  extent 
  

   and 
  boundary 
  of 
  the 
  thrust-plane, 
  or 
  of 
  that 
  portion 
  of 
  it 
  ever 
  which 
  

   movement 
  took 
  place. 
  This 
  reservation 
  is 
  important, 
  for 
  it 
  is 
  quite 
  

   conceivable 
  that 
  the 
  movement 
  may 
  have 
  taken 
  place 
  along 
  a 
  pre- 
  

   existent 
  plane 
  of 
  fracture, 
  whose 
  whole 
  extent 
  is 
  much 
  larger 
  than 
  

   that 
  portion 
  over 
  which 
  movement 
  took 
  place. 
  If 
  this 
  is 
  so, 
  it 
  is 
  

   obvious 
  that,though 
  we 
  may 
  be 
  able 
  to 
  determine 
  the 
  limits 
  of 
  the 
  latter 
  

   area 
  with 
  some 
  approximation 
  to 
  accuracy, 
  there 
  is 
  nothing 
  whatever 
  

   to 
  guide 
  ug 
  to 
  a 
  knowledge 
  of 
  the 
  whole 
  extent 
  of 
  the 
  old 
  fracture. 
  

  

  It 
  has 
  already 
  been 
  pointed 
  out 
  that 
  the 
  changes 
  which 
  accom- 
  

   panied 
  the 
  earthquake 
  can 
  be 
  traced 
  right 
  up 
  to 
  the 
  northern 
  edge 
  of 
  

   the 
  Garo 
  hills, 
  and 
  that 
  this 
  alone 
  might 
  lead 
  to 
  belief 
  that 
  similar 
  

   changes 
  took 
  place 
  under 
  the 
  alluvium 
  of 
  the 
  Brahmaputra 
  valley. 
  It 
  

   has 
  also 
  been 
  shown 
  that 
  there 
  are 
  independent 
  suggestions, 
  if 
  nothing 
  

   more, 
  of 
  changes 
  of 
  level 
  along 
  the 
  course 
  of 
  the 
  Brahmaputra 
  river. 
  2 
  

  

  1 
  There 
  is 
  one 
  hypothesis 
  which 
  might 
  explain 
  many 
  of 
  the 
  observed 
  effects, 
  and 
  that 
  is 
  

   the 
  injection 
  of 
  a 
  laccolite 
  or 
  sheet 
  of 
  molten 
  rock. 
  This 
  would 
  account 
  for 
  elevations 
  at 
  the 
  

   surface 
  and 
  for 
  unequal 
  elevations, 
  it 
  might 
  not 
  be 
  inconsistent 
  with 
  horizontal 
  shifting, 
  

   and 
  the 
  strain 
  on 
  the 
  overlying 
  crust 
  might 
  in 
  places 
  lead 
  to 
  fracture 
  or 
  faulting. 
  Our 
  

   knowledge 
  of 
  the 
  physics 
  of 
  igneous 
  intrusion, 
  small 
  though 
  it 
  be, 
  is, 
  however, 
  opposed 
  to 
  the 
  

   idea 
  that 
  such 
  an 
  intrusion 
  could 
  take 
  place 
  with 
  the 
  suddenness 
  necessary 
  to 
  account 
  for 
  the 
  

   earthquake, 
  and 
  when 
  we 
  consider 
  the 
  large 
  area 
  and 
  the 
  thinness 
  of 
  the 
  intrusion 
  required 
  

   to 
  fit 
  in 
  with 
  the 
  facts 
  observed 
  at 
  the 
  surface, 
  this 
  explanation 
  may 
  be 
  dismissed 
  as 
  physically 
  

   impossible. 
  

  

  2 
  Page 
  162 
  ff. 
  

  

  ( 
  168 
  ) 
  

  

  