﻿174 
  OLDHAM: 
  GREAT 
  EARTHQUAKE 
  OF 
  1897. 
  

  

  of 
  the 
  method 
  more 
  clearly. 
  The 
  small 
  circle 
  represents 
  a 
  coseismic 
  

   line 
  corresponding 
  to 
  the 
  moment 
  when 
  the 
  earthquake 
  first 
  reaches 
  

   the 
  surface 
  at 
  the 
  epicentre. 
  On 
  the 
  assumption 
  that 
  the 
  earth 
  wave 
  

   travels 
  with 
  equal 
  speed 
  in 
  every 
  direction, 
  the 
  earthquake 
  will 
  

   have 
  reached 
  every 
  point 
  of 
  this 
  line 
  at 
  the 
  same 
  moment, 
  and 
  the 
  

   portions 
  of 
  the 
  radii 
  BA, 
  B'A' 
  which 
  lie 
  outside 
  the 
  circle 
  represent 
  

   the 
  intervals 
  of 
  time 
  which 
  elapse 
  between 
  the 
  time 
  when 
  the 
  earth- 
  

   quake 
  first 
  reaching 
  the 
  surface 
  at 
  the 
  epicentre, 
  and 
  when 
  it 
  reaches 
  

   the 
  points 
  A 
  A'. 
  If 
  now 
  we 
  draw 
  a 
  straight 
  line 
  through 
  the 
  epi- 
  

   centre 
  at 
  a 
  tangent 
  to 
  the 
  outer 
  circle, 
  and 
  lay 
  off 
  along 
  it 
  distances 
  

   equal 
  to 
  the 
  arcs 
  of 
  the 
  outer 
  circle 
  contained 
  between 
  the 
  epicentre, 
  

   and 
  the 
  points 
  where 
  the 
  radii 
  from 
  C 
  cut 
  it, 
  and 
  from 
  each 
  of 
  them 
  

   draw 
  a 
  perpendicular 
  line 
  equal 
  to 
  that 
  portion 
  of 
  the 
  corresponding 
  

   radius 
  which 
  lies 
  between 
  the 
  two 
  circles, 
  then 
  by 
  joining 
  the 
  ends 
  

   of 
  these 
  perpendiculars 
  we 
  get 
  a 
  hodograph, 
  or 
  time 
  curve, 
  of 
  the 
  

   earthquake. 
  

  

  Now, 
  it 
  is 
  to 
  be 
  noticed 
  that 
  for 
  each 
  equal 
  interval 
  along 
  the 
  

   surface 
  of 
  the 
  earth 
  from 
  the 
  epicentre 
  outwards 
  the 
  time 
  interval 
  

   increases, 
  that 
  is 
  to 
  say, 
  the 
  apparent 
  rate 
  of 
  travel 
  decreases 
  until 
  

   we 
  reach 
  the 
  point 
  where 
  the 
  radius 
  which 
  runs 
  at 
  right 
  angles 
  to 
  the 
  

   seismic 
  vertical 
  cuts 
  the 
  surface. 
  Beyond 
  that, 
  the 
  time 
  intervals 
  

   begin 
  to 
  decrease 
  once 
  more, 
  or 
  in 
  other 
  words 
  the 
  apparent 
  rate 
  of 
  

   travel 
  increases. 
  As, 
  from 
  the 
  method 
  of 
  construction 
  of 
  the 
  hodo- 
  

   graph, 
  the 
  line 
  is 
  more 
  steeply 
  inclined 
  in 
  those 
  parts 
  where 
  the 
  ap- 
  

   parent 
  velocity 
  is 
  less, 
  and 
  less 
  steeply 
  where 
  it 
  is 
  greater, 
  the 
  hodo- 
  

   graph 
  will 
  be 
  divided 
  into 
  two 
  parts, 
  one 
  concave 
  upwards, 
  the 
  other 
  

   concave 
  downwards, 
  and 
  the 
  point 
  of 
  passage 
  from 
  one 
  curve 
  to 
  the 
  

   other 
  corresponds 
  to 
  the 
  place 
  where 
  a 
  line 
  from 
  the 
  focus, 
  at 
  right 
  

   angles 
  to 
  the 
  seismic 
  vertical, 
  strikes 
  the 
  surface. 
  

  

  Consequently, 
  if 
  we 
  have 
  the 
  hodograph, 
  the 
  depth 
  of 
  the 
  focus 
  can 
  

   be 
  determined 
  by 
  drawing 
  a 
  line 
  through 
  the 
  point 
  of 
  flexure 
  at 
  

   right 
  angles 
  to 
  the 
  seismic 
  vertical, 
  when 
  the 
  distance 
  cut 
  off 
  will 
  

   represent 
  the 
  depth 
  of 
  the 
  focus 
  from 
  the 
  surface. 
  

  

  In 
  other 
  words, 
  the 
  depth 
  of 
  the 
  focus 
  is 
  the 
  versed 
  sine 
  of 
  the 
  

   ( 
  174 
  ) 
  

  

  