﻿308 
  Scientific 
  Intelligence. 
  

  

  A 
  fault-slip 
  does 
  not 
  of 
  necessity 
  take 
  place 
  concurrently 
  all 
  

   over 
  the 
  focus 
  or 
  instantaneouly 
  at 
  any 
  one 
  point 
  of 
  it. 
  But, 
  as 
  

   a 
  general 
  rule, 
  it 
  certainly 
  occupies 
  a 
  very 
  short 
  interval 
  of 
  time, 
  

   and 
  at 
  places 
  near 
  the 
  epicentre 
  the 
  duration 
  of 
  the 
  sensible 
  part 
  

   of 
  an 
  earthquake 
  must 
  be 
  mainly 
  due 
  to 
  the 
  size 
  of 
  the 
  focus 
  and 
  

   the 
  finite 
  velocity 
  of 
  the 
  earth-waves. 
  

  

  The 
  seismic 
  focus 
  is 
  practically 
  a 
  surface 
  inclined 
  to 
  the 
  horizon, 
  

   and 
  is 
  often 
  of 
  great 
  length 
  in 
  a 
  horizontal 
  direction. 
  In 
  its 
  sim- 
  

   plest 
  form 
  there 
  will 
  be 
  a 
  central 
  region 
  of 
  the 
  fault- 
  surface 
  where 
  

   the 
  relative 
  displacement 
  of 
  the 
  two 
  rock-masses 
  is 
  a 
  maximum, 
  

   and 
  this 
  will 
  be 
  surrounded 
  by 
  a 
  region 
  in 
  which 
  the 
  relative 
  dis- 
  

   placement 
  is 
  small, 
  and 
  gradually 
  dies 
  away 
  towards 
  the 
  edges. 
  

   As 
  the 
  vibrations 
  of 
  great 
  amplitude 
  are 
  also 
  of 
  long 
  period, 
  we 
  

   may 
  consider 
  that 
  from 
  all 
  parts 
  of 
  the 
  focus 
  there 
  start 
  together 
  

   vibrations 
  of 
  various 
  amplitude 
  and 
  period, 
  the 
  large 
  and 
  slow 
  

   undulations 
  coming 
  mostly 
  from 
  the 
  central 
  region, 
  and 
  the 
  small 
  

   and 
  rapid 
  vibrations 
  of 
  those 
  which 
  bound 
  it. 
  It 
  is, 
  I 
  believe, 
  

   from 
  these 
  marginal 
  regions, 
  and 
  especially 
  from 
  the 
  upper 
  and 
  

   lateral 
  margins, 
  that 
  the 
  more 
  sensible 
  sound-vibrations 
  chiefly 
  

   come. 
  I 
  will 
  now 
  endeavour 
  to 
  show 
  how 
  the 
  phenomena 
  of 
  

   earthquake-sounds 
  can 
  be 
  explained 
  on 
  this 
  theory. 
  

  

  If 
  there 
  is 
  no 
  discontinuity 
  in 
  period 
  between 
  the 
  large 
  vibra- 
  

   tions 
  which 
  come 
  from 
  the 
  center 
  of 
  the 
  area 
  and 
  the 
  sound-vibra- 
  

   tions 
  from 
  the 
  focal 
  margins, 
  it 
  is 
  evident 
  that 
  among 
  the 
  vibra- 
  

   tions 
  must 
  be 
  included 
  those 
  which 
  produce 
  the 
  deepest 
  sound 
  

   that 
  can 
  be 
  heard 
  by 
  the 
  human 
  ear. 
  The 
  rumbling 
  character 
  of 
  

   the 
  sound 
  may 
  be 
  due 
  partly 
  to 
  its 
  neighbourhood 
  to 
  the 
  lower 
  

   limit 
  of 
  audibility, 
  partly, 
  no 
  doubt, 
  to 
  irregularities 
  in 
  the 
  fault- 
  

   surface 
  producing 
  local 
  vibrations 
  in 
  the 
  initial 
  disturbance. 
  To 
  

   the 
  latter 
  cause 
  may 
  also 
  be 
  owing 
  the 
  intermittent 
  sounds 
  heard 
  

   by 
  some 
  observers. 
  

  

  But 
  whether 
  there 
  be 
  any 
  slight 
  fluctuations 
  in 
  amplitude 
  and 
  

   period, 
  or 
  not, 
  it 
  is 
  evident 
  that 
  the 
  average 
  intensity 
  of 
  the 
  sound 
  

   must 
  gradually 
  increase 
  until 
  the 
  shock 
  is 
  felt 
  ; 
  and 
  as 
  soon 
  as 
  it 
  

   is 
  over, 
  or 
  nearly 
  over, 
  must 
  gradually 
  die 
  away. 
  At 
  the 
  same 
  

   time 
  the 
  greater 
  amplitude 
  of 
  the 
  vibrations 
  from 
  near 
  the 
  central 
  

   part 
  of 
  the 
  focus 
  will 
  render 
  audible 
  vibrations 
  of 
  longer 
  period 
  

   than 
  those 
  which 
  come 
  from 
  the 
  margins 
  ; 
  and 
  thus 
  the 
  depth 
  of 
  

   the 
  sound 
  will 
  increase 
  and 
  decrease 
  with 
  the 
  intensities 
  of 
  the 
  

   sound 
  and 
  shock. 
  Especially 
  will 
  this 
  be 
  the 
  case 
  with 
  the 
  vibra- 
  

   tions 
  from 
  the 
  neighbourhood 
  of 
  that 
  part 
  of 
  the 
  focus 
  where 
  the 
  

   initial 
  amplitude 
  is 
  greatest 
  ; 
  and 
  the 
  strongest 
  vibrations 
  and 
  

   loud 
  explosive 
  crashes 
  audible 
  to 
  some 
  persons 
  should 
  therefore 
  

   be 
  observed 
  concurrently. 
  

  

  Similar 
  reasoning 
  leads 
  to 
  the 
  conclusion 
  that 
  the 
  intensity 
  of 
  

   the 
  sound 
  should 
  increase 
  with 
  that 
  of 
  the 
  shock 
  in 
  different 
  

   earthquakes, 
  and 
  that 
  the 
  sound 
  should 
  be 
  deeper 
  with 
  strong 
  

   shocks 
  than 
  with 
  weak 
  ones. 
  But 
  while 
  the 
  marginal 
  vibrations 
  are 
  

   limited 
  in 
  period 
  and 
  amplitude, 
  those 
  from 
  the 
  central 
  parts 
  of 
  

   the 
  focus 
  have 
  a 
  much 
  wider 
  range. 
  With 
  very 
  weak 
  shocks 
  the 
  

  

  