﻿164 
  OLDHAM: 
  GREAT 
  EARTHQUAKE 
  OF 
  I 
  897. 
  

  

  Chapter 
  X.— 
  THE 
  POSITION 
  AND 
  EXTENT 
  OF 
  THE 
  SEISMIC 
  

  

  FOCUS. 
  

  

  Having 
  completed 
  the 
  description 
  of 
  the 
  data 
  necessary 
  for 
  the 
  

   purpose, 
  we 
  may 
  now 
  proceed 
  to 
  a 
  discussion 
  of 
  the 
  nature, 
  situation, 
  

   and 
  depth 
  below 
  the 
  surface, 
  of 
  the 
  focus 
  from 
  which 
  the 
  earthquake 
  

   started. 
  

  

  All 
  earthquakes 
  are 
  of 
  the 
  nature 
  of 
  a 
  shock 
  or 
  disturbance 
  com- 
  

   municated 
  to, 
  and 
  propagated 
  through, 
  the 
  earth. 
  This 
  disturbance 
  

   may 
  originate 
  in 
  many 
  ways 
  : 
  some 
  small 
  earthquakes 
  are 
  believed 
  

   to 
  have 
  their 
  origin 
  in 
  the 
  falling 
  in 
  of 
  underground 
  caverns; 
  others 
  

   have 
  been 
  attributed 
  to 
  underground 
  explosions 
  of 
  steam 
  ; 
  others 
  

   again 
  to 
  volcanic 
  explosions 
  ; 
  but 
  the 
  commonest 
  cause, 
  and 
  that 
  to 
  

   which 
  all 
  great 
  earthquakes 
  appear 
  to 
  be 
  due, 
  is 
  a 
  sudden 
  relief 
  

   of 
  strain. 
  

  

  According 
  to 
  the 
  generally 
  accepted 
  theory 
  of 
  the 
  constitution 
  

   of 
  the 
  earth, 
  it 
  consists 
  of 
  a 
  highly 
  heated 
  central 
  mass, 
  which 
  is 
  

   continually 
  losing 
  heat, 
  and 
  consequently 
  contracting, 
  covered 
  by 
  

   a 
  comparatively 
  thin 
  surface 
  layer 
  which 
  has 
  already 
  cooled 
  as 
  far 
  

   as 
  present 
  conditions 
  permit, 
  and 
  is 
  not 
  capable 
  of 
  further 
  con- 
  

   traction 
  to 
  an 
  appreciable 
  degree. 
  Consequently, 
  as 
  the 
  central 
  core 
  

   contracts, 
  the 
  outer 
  shell 
  is 
  left 
  partially 
  unsupported, 
  and 
  so 
  thrown 
  

   into 
  a 
  state 
  of 
  strain. 
  The 
  relief 
  of 
  this 
  strain 
  may 
  be 
  gradual, 
  or 
  

   sudden 
  ; 
  in 
  the 
  latter 
  case 
  it 
  takes 
  the 
  form 
  of 
  a 
  fracture, 
  or 
  a 
  

   sudden 
  shift 
  along 
  a 
  preexisting 
  fracture, 
  and 
  is 
  accompanied 
  by 
  

   an 
  earthquake, 
  whose 
  extent 
  will 
  depend 
  on 
  the 
  amount 
  of 
  strain 
  

   which 
  has 
  accumulated 
  before 
  yielding 
  took 
  place. 
  Where 
  the 
  

   earth's 
  crust 
  has 
  no 
  great 
  power 
  of 
  resistance, 
  the 
  strain 
  will 
  

   not 
  accumulate, 
  and 
  earthquakes 
  will 
  be 
  frequent 
  and 
  small. 
  

   If, 
  however, 
  the 
  constitution 
  of 
  the 
  earth's 
  crust, 
  at 
  a 
  part 
  which 
  is 
  

   subjected 
  to 
  strain, 
  is 
  such 
  that 
  it 
  does 
  not 
  yield 
  readily, 
  the 
  strain 
  

   will 
  accumulate, 
  and, 
  when 
  it 
  becomes 
  too 
  great 
  to 
  be 
  borne 
  any 
  

   longer, 
  there 
  will 
  be 
  a 
  sudden 
  and 
  violent 
  relief, 
  accompanied 
  by 
  

   fracture 
  and 
  giving 
  rise 
  to 
  a 
  severe 
  earthquake. 
  Whatever 
  may 
  be 
  

  

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