﻿THE 
  SEISMIC 
  FOCUS. 
  175 
  

  

  angle 
  whose 
  arc 
  is 
  the 
  distance 
  of 
  the 
  point 
  of 
  flexure 
  of 
  the 
  hodo- 
  

   graph 
  from 
  its 
  centre. 
  

  

  As 
  we 
  have 
  seen 
  in 
  chapter 
  IV, 
  the 
  hodograph 
  of 
  this 
  earthquake 
  

   shows 
  just 
  those 
  features 
  which 
  it 
  should 
  according 
  to 
  hypothesis, 
  

   and 
  the 
  point 
  of 
  change 
  from 
  one 
  curve 
  to 
  the 
  other 
  is 
  situated 
  at 
  

   about 
  280 
  miles 
  from 
  the 
  centre. 
  A 
  distance 
  of 
  280 
  miles 
  on 
  the 
  

   surface 
  of 
  the 
  earth 
  corresponds 
  to 
  the 
  arc 
  of 
  an 
  angle 
  of 
  a 
  little 
  over 
  

   4 
  , 
  and 
  the 
  versed 
  sine 
  of 
  4 
  amounts 
  to 
  9*3 
  miles. 
  

  

  It 
  must, 
  however, 
  be 
  remembered 
  that 
  this 
  result 
  is 
  to 
  be 
  taken 
  

   merely 
  for 
  what 
  it 
  is 
  worth. 
  The 
  method, 
  however 
  accurate 
  in 
  

   theory, 
  is 
  subject 
  to 
  too 
  many 
  sources 
  of 
  error 
  in 
  practice 
  to 
  be 
  thor- 
  

   oughly 
  depended 
  on 
  in 
  the 
  present 
  case, 
  as 
  the 
  time 
  observations 
  are 
  

   too 
  few 
  and 
  too 
  imperfect 
  to 
  allow 
  the 
  hodograph 
  to 
  be 
  drawn 
  with 
  

   perfect 
  certainty. 
  

  

  A 
  more 
  important 
  source 
  of 
  error 
  may 
  be 
  consequent 
  on 
  the 
  form 
  

   of 
  the 
  focus. 
  If 
  we 
  suppose 
  that 
  yielding 
  first 
  took 
  place 
  in 
  the 
  cen- 
  

   tre 
  of 
  the 
  thrust-plane 
  and 
  spread 
  outwards 
  from 
  thence, 
  no 
  great 
  

   change 
  would 
  result 
  in 
  the 
  hodograph, 
  as 
  the 
  rate 
  of 
  spread 
  of 
  the 
  

   fissure 
  would 
  probably 
  be 
  much 
  the 
  same 
  as 
  that 
  of 
  the 
  propagation 
  

   of 
  the 
  earthquake 
  disturbance. 
  But 
  if, 
  as 
  is 
  conceivably 
  possible, 
  

   there 
  was 
  a 
  preexistant 
  fissure, 
  yielding 
  might 
  have 
  taken 
  place 
  

   practically 
  simultaneously 
  at 
  more 
  than 
  one 
  point 
  and 
  spread 
  from 
  

   more 
  than 
  one 
  centre 
  over 
  the 
  focal 
  area. 
  In 
  this 
  case 
  the 
  hodo- 
  

   graph, 
  as 
  drawn, 
  would 
  no 
  longer 
  give 
  the 
  true 
  distance 
  of 
  the 
  point 
  

   of 
  flexure 
  from 
  its 
  centre 
  ; 
  an 
  unknown 
  amount 
  would 
  have 
  to 
  be 
  cut 
  

   off 
  from 
  the 
  right 
  hand 
  side 
  of 
  the 
  curve 
  and 
  the 
  resulting 
  depth 
  

   focus 
  correspondingly 
  diminished. 
  

  

  There 
  is, 
  however, 
  one 
  consideration, 
  not 
  yet 
  referred 
  to, 
  which 
  

   ' 
  would 
  have 
  an 
  opposite 
  effect 
  and 
  would 
  more 
  than 
  counteract 
  any 
  

   diminution 
  in 
  the 
  deduced 
  depth 
  of 
  focus 
  resulting 
  from 
  the 
  supposi- 
  

   tion 
  referred 
  to 
  in 
  the 
  last 
  paragraph. 
  In 
  fig, 
  18 
  and 
  its 
  explanation 
  it 
  

   is 
  assumed 
  that 
  the 
  earthquake 
  travels 
  at 
  the 
  same 
  rate 
  in 
  every 
  di- 
  

   rection 
  and 
  at 
  all 
  depths. 
  It 
  is 
  certain, 
  however, 
  that 
  the 
  waves 
  

  

  ( 
  i75 
  ) 
  

  

  