﻿RATE 
  OF 
  PROPAGATION. 
  69 
  

  

  and 
  extent 
  of 
  the 
  seismic 
  vertical 
  in 
  Chapter 
  X. 
  But 
  the 
  time 
  

   observations 
  themselves 
  have 
  to 
  be 
  taken 
  into 
  consideration 
  in 
  this 
  

   discussion, 
  and 
  for 
  this 
  reason 
  their 
  description, 
  and 
  the 
  investi- 
  

   gation 
  of 
  the 
  rate 
  of 
  travel 
  of 
  the 
  earthquake, 
  are 
  undertaken 
  here, 
  

   although 
  this 
  course 
  involves 
  a 
  certain 
  assumption 
  of 
  what 
  is 
  only 
  

   established 
  in 
  subsequent 
  chapters. 
  

  

  In 
  calculating 
  the 
  distances 
  of 
  the 
  stations 
  at 
  which 
  we 
  have 
  

   time 
  reoords 
  from 
  the 
  epicentre, 
  it 
  is 
  necessary 
  to 
  assume 
  this 
  as 
  a 
  

   point 
  ; 
  but, 
  as 
  will 
  be 
  seen 
  later 
  on, 
  this 
  was 
  a 
  large 
  area, 
  and 
  it 
  

   is 
  consequently 
  necessary 
  to 
  determine 
  what 
  point 
  to 
  accept 
  as 
  

   that 
  of 
  departure 
  of 
  the 
  earthquake 
  wave. 
  Two 
  courses 
  are 
  open, 
  

   either 
  to 
  take 
  a 
  central 
  point 
  in 
  the 
  focal 
  area 
  or, 
  as 
  all 
  our 
  stations 
  

   lie 
  to 
  the 
  west, 
  one 
  near 
  its 
  western 
  limit. 
  In 
  either 
  case, 
  certain 
  

   errors 
  are 
  possible. 
  

  

  Supposing 
  the 
  earthquake 
  to 
  have 
  been 
  due, 
  as 
  suggested 
  in 
  

   Chapter 
  X, 
  to 
  the 
  formation 
  of 
  'a 
  great 
  thrust-plane, 
  the 
  movement 
  

   probably 
  commenced 
  near 
  the 
  centre 
  and 
  spread 
  outwards. 
  Now 
  the 
  

   rate 
  at 
  which 
  the 
  fissure 
  spread 
  may 
  have 
  been 
  the 
  same 
  as, 
  greater, 
  

   or 
  less 
  than, 
  that 
  at 
  which 
  the 
  earthquake 
  waves 
  travelled. 
  In 
  the 
  

   latter 
  case, 
  if 
  we 
  assume 
  a 
  starting 
  point 
  at 
  the 
  western 
  limit 
  of 
  the 
  

   focal 
  area, 
  it 
  is 
  evident 
  that 
  all 
  the 
  calculated 
  distances 
  will 
  be 
  less 
  

   than 
  the 
  true 
  distances 
  which 
  the 
  earthquake 
  wave 
  had 
  travelled, 
  

   for 
  the 
  wave 
  would 
  have 
  reached 
  the 
  assumed 
  centre 
  before 
  the 
  

   disturbance 
  starting 
  from 
  that 
  point 
  was 
  originated. 
  If, 
  on 
  the 
  other 
  

   hand, 
  the 
  formation 
  of 
  the 
  fissure 
  started 
  at 
  more 
  points 
  than 
  

   one, 
  or, 
  if 
  it 
  spread 
  at 
  a 
  greater 
  rate 
  than 
  that 
  of 
  travel 
  of 
  the 
  

   earthquake 
  wave, 
  then 
  all 
  distances 
  calculated 
  from 
  the 
  centre 
  of 
  

   the 
  focal 
  area 
  would 
  be 
  greater 
  than 
  the 
  true 
  distance 
  travelled 
  

   by 
  the 
  earthquake 
  wave. 
  The 
  probability 
  is 
  that 
  the 
  rate 
  of 
  

   spread 
  of 
  the 
  fissure, 
  and 
  of 
  the 
  travel 
  of 
  the 
  earthquake 
  wave, 
  were 
  

   practically 
  the 
  same 
  ; 
  and 
  in 
  this 
  case 
  we 
  may 
  reckon 
  from 
  a 
  

   point 
  near 
  the 
  western 
  end 
  of 
  the 
  focal 
  area 
  when 
  deducing 
  

   the 
  rate 
  of 
  travel 
  of 
  the 
  disturbance. 
  An 
  assumed 
  centre 
  in 
  

   Lat. 
  25 
  45' 
  N., 
  Long. 
  90 
  15' 
  E., 
  has 
  accordingly 
  been 
  used 
  

  

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  69) 
  

  

  