﻿42 
  OLDHAM: 
  GREAT 
  EARTHQUAKE 
  OF 
  1897. 
  

  

  Chapter 
  III.— 
  THE 
  ISOSEISMIC 
  LINES 
  AND 
  AREA 
  OVER 
  

   WHICH 
  THE 
  SHOCK 
  WAS 
  FELT. 
  

   The 
  determination 
  of 
  the 
  isoseismic 
  lines, 
  that 
  is 
  to 
  say, 
  lines 
  of 
  

   equal 
  violence 
  of 
  shock, 
  is 
  the 
  necessary 
  preliminary 
  to 
  determining 
  

   the 
  varying 
  rate 
  of 
  diminution 
  .of 
  seismic 
  energy 
  in 
  different 
  

   directions. 
  Strictly 
  speaking 
  there 
  should 
  be 
  little 
  difficulty 
  in 
  deter- 
  

   mining 
  these 
  lines, 
  as 
  the 
  violence 
  of 
  th'e 
  shock 
  may 
  be 
  regarded 
  as 
  

   a 
  function 
  of 
  the 
  amplitude 
  and 
  period 
  of 
  the 
  wave, 
  'or 
  more 
  simply, 
  

   of 
  the 
  maximum 
  rate 
  of 
  acceleration 
  of 
  the 
  wave 
  particle. 
  All 
  

   these 
  elements 
  are 
  capable 
  of 
  determination 
  from 
  the 
  records 
  of 
  a 
  

   complete 
  seismograph, 
  and, 
  in 
  the 
  absence 
  of 
  these 
  instruments, 
  a 
  

   tolerably 
  complete 
  substitute 
  may 
  be 
  found, 
  at 
  any 
  rate 
  when 
  an 
  

   earthquake 
  is 
  severe, 
  in 
  the 
  overturning 
  of 
  simple 
  objects, 
  such 
  as 
  

   pillars, 
  in 
  countries 
  where 
  they 
  are 
  sufficiently 
  numerous. 
  

  

  In 
  the 
  present 
  case, 
  however, 
  objects 
  suitable 
  for 
  giving 
  seis- 
  

   mometric 
  results 
  of 
  any 
  degree 
  of 
  accuracy 
  are 
  rare, 
  and, 
  owing 
  to 
  the 
  

   extent 
  of 
  the 
  country 
  over 
  which 
  the 
  earthquake 
  was 
  more 
  or 
  less 
  

   destructive, 
  they 
  have 
  not 
  all 
  been 
  examined 
  by 
  competent 
  obser- 
  

   vers. 
  It 
  has 
  consequently 
  been 
  necessary 
  to 
  fall 
  back 
  on 
  the 
  general 
  

   accounts 
  of 
  the 
  earthquake 
  and 
  its 
  effects, 
  and 
  to 
  draw 
  the 
  isoseis- 
  

   mic 
  lines 
  on 
  the 
  basis 
  of 
  the 
  effects 
  of 
  the 
  earthquake 
  on 
  masonry 
  

   buildings 
  and 
  on 
  human 
  sensations. 
  

  

  In 
  the 
  account 
  of 
  the 
  Charleston 
  Earthquake, 
  Captain 
  Dutton 
  

   made 
  use 
  of 
  the 
  Rossi-Forel 
  scale 
  of 
  intensities 
  and 
  drew 
  

   isoseismic 
  lines, 
  or 
  isoseists, 
  representing 
  ten 
  different 
  degrees 
  of 
  

   intensity. 
  In 
  a 
  closely 
  populated 
  and 
  civilised 
  country, 
  where 
  most 
  

   of 
  the 
  buildings 
  are 
  of 
  brick 
  or 
  stone, 
  this 
  was 
  possible, 
  but 
  the 
  area 
  

   over 
  which 
  this 
  earthquake 
  was 
  felt 
  is 
  largely 
  a 
  wild, 
  thinly 
  populated 
  

   country, 
  and 
  even 
  in 
  the 
  thickly 
  populated 
  parts 
  brick 
  and 
  stone 
  build- 
  

   ings 
  are 
  rare 
  and 
  widely 
  scattered. 
  As 
  a 
  result 
  it 
  has 
  been 
  found 
  

   impossible 
  to 
  attempt 
  to 
  define 
  more 
  than 
  seven 
  degrees 
  of 
  intensity 
  

   lying 
  within 
  their 
  isoseists, 
  which 
  may 
  be 
  defined 
  as 
  follows 
  : 
  — 
  

  

  1. 
  The 
  first 
  isoseist 
  includes 
  all 
  places 
  where 
  the 
  destruction 
  of 
  

   brick 
  and 
  stone 
  buildings 
  was 
  practically 
  universal. 
  

  

  (42) 
  

  

  