﻿xxvi 
  OLDHAM 
  : 
  GREAT 
  EARTHQUAKE 
  OF 
  1897. 
  

  

  two 
  lines 
  be 
  drawn 
  at 
  right 
  angles 
  to 
  each 
  other, 
  as 
  in 
  fig. 
  vii, 
  and 
  divided 
  

  

  600' 
  

  

  

  I 
  

  

  

  

  

  

  

  500 
  

  

  

  

  

  

  

  

  

  400 
  

  

  i 
  

  

  

  

  

  

  

  300 
  

  

  \\ 
  

  

  

  

  

  

  

  200 
  

  

  \ 
  \ 
  

  

  \ 
  

  

  

  

  

  

  100 
  

  

  \ 
  

  

  X. 
  

  

  ekfic 
  

  

  

  

  

  

  

  ^Vaffoya,- 
  

  

  

  

  

  

  

  

  

  

  

  , 
  

  

  

  , 
  

  

  

  01234567 
  89 
  10 
  

  

  Fig. 
  vii 
  Curve 
  of 
  frequency 
  of 
  aftershocks. 
  

  

  into 
  parts 
  of 
  equal 
  length 
  which 
  shall 
  represent 
  equal 
  numbers 
  of 
  earthquakes 
  in 
  

   the 
  one 
  and 
  equal 
  periods 
  of 
  time 
  in 
  the 
  other 
  case, 
  and 
  a 
  series 
  of 
  points 
  is 
  plotted 
  

   such 
  that 
  their 
  distances 
  from 
  the 
  one 
  line 
  represent 
  the 
  number 
  of 
  shocks 
  felt 
  in 
  

   any 
  given 
  period 
  of 
  time—hour, 
  day, 
  week., 
  etc.— 
  and 
  their 
  distances 
  from 
  the 
  

   other, 
  the 
  number 
  of 
  such 
  periods 
  that 
  have 
  elapsed 
  since 
  the 
  great 
  shocks, 
  then 
  

   the 
  line 
  joining 
  these 
  points 
  will 
  come 
  close 
  to 
  a 
  parabola 
  which 
  becomes 
  asymptotic 
  

   with 
  the 
  two 
  lines 
  Jat 
  right 
  angles 
  to 
  each 
  other. 
  

  

  90. 
  This 
  will 
  be 
  better 
  understood 
  from 
  an 
  inspection 
  of 
  fig. 
  vi, 
  which 
  represents 
  

   the 
  curves 
  of 
  frequency 
  of 
  the 
  aftershocks 
  of 
  two 
  Japanese 
  earthquakes, 
  as 
  deduced 
  

   by 
  Prof. 
  Omori. 
  It 
  will 
  be 
  seen 
  that 
  the 
  frequency 
  of 
  the 
  aftershocks 
  is 
  great 
  

   at 
  first, 
  but 
  the 
  decrease 
  in 
  frequency 
  is 
  also 
  great 
  ; 
  as 
  the 
  absolute 
  frequency 
  

   diminishes 
  the 
  rate 
  of 
  decrease 
  also 
  diminishes, 
  till, 
  when 
  the 
  excess 
  over 
  the 
  

   ncrmal 
  becomes 
  small, 
  the 
  rate 
  of 
  decrease 
  becomes 
  also 
  very 
  small 
  and 
  the 
  curve 
  

   becomes 
  almost 
  parallel 
  to 
  the 
  line 
  representing 
  the 
  interval 
  of 
  time. 
  

  

  91. 
  The 
  actual 
  course 
  of 
  the 
  line 
  representing 
  the 
  frequency 
  of 
  the 
  after- 
  

   shocks 
  does 
  not, 
  however, 
  follow 
  the 
  curve 
  exactly, 
  for 
  among 
  the 
  aftershocks 
  

   there 
  will 
  be 
  some 
  of 
  greater 
  severity 
  than 
  the 
  average, 
  and 
  as 
  each 
  of 
  these 
  

   has 
  its 
  own 
  series 
  of 
  aftershocks, 
  superimposed 
  on 
  those 
  of 
  the 
  main 
  shock, 
  there 
  

   will 
  be 
  temporary 
  increases 
  of 
  frequency 
  above, 
  compensated 
  by 
  temporary 
  

   decreases 
  of 
  frequency 
  below, 
  the 
  general 
  average 
  represented 
  by 
  the 
  smooth 
  

   curve. 
  

  

  92. 
  Sympathetic 
  earthquakes.-The 
  existence 
  of 
  secondary 
  earthquakes 
  ot 
  the 
  

   second 
  class 
  has 
  never 
  been 
  positively 
  established, 
  but 
  is 
  probable. 
  They 
  are 
  

  

  