﻿Geology. 
  513 
  

  

  Now, 
  the 
  seismographic 
  records 
  recently 
  obtained 
  by 
  Prof. 
  

   Milne 
  and 
  others 
  in 
  Japan 
  show 
  that 
  earthquakes 
  usually 
  begin 
  

   with 
  a 
  series 
  of 
  tremors 
  very 
  small 
  in 
  amplitude 
  and 
  very 
  rapid 
  

   in 
  period, 
  from 
  six 
  to 
  eight 
  occurring 
  every 
  second, 
  but 
  becom- 
  

   ing 
  slower 
  before 
  the 
  shock 
  takes 
  place. 
  These 
  may 
  last 
  for 
  

   many 
  seconds 
  or 
  even 
  several 
  minutes. 
  Following, 
  and 
  continu- 
  

   ous 
  with 
  them, 
  come 
  the 
  sensible 
  vibrations, 
  of 
  larger 
  amplitude 
  

   and 
  longer 
  period, 
  about 
  three 
  to 
  five 
  occurring 
  in 
  every 
  second. 
  

   One 
  or 
  more 
  of 
  these, 
  attaining 
  a 
  still 
  greater 
  amplitude 
  and 
  

   longer 
  period, 
  of 
  one 
  or 
  two 
  seconds 
  each, 
  constitute 
  what 
  are 
  

   generally 
  known 
  as 
  the 
  principal 
  shock 
  or 
  shocks. 
  The 
  earth- 
  

   quake 
  closes 
  with 
  vibrations 
  of 
  smaller 
  amplitude, 
  but 
  which 
  

   have 
  a 
  period 
  so 
  long 
  that 
  no 
  record 
  of 
  them 
  can 
  be 
  obtained. 
  

   The 
  earliest 
  tremors, 
  on 
  the 
  other 
  hand, 
  are 
  not 
  registered 
  on 
  ac- 
  

   count 
  of 
  the 
  smallness 
  of 
  their 
  amplitude, 
  and, 
  in 
  all 
  probability, 
  

   as 
  Prof. 
  Milne 
  suggests, 
  the 
  " 
  minute 
  movements 
  which 
  have 
  been 
  

   recorded 
  are 
  the 
  continuation 
  of 
  still 
  smaller 
  and 
  more 
  rapid 
  

   movements 
  which 
  .... 
  have 
  never 
  yet 
  been 
  rendered 
  visible." 
  

   It 
  is 
  to 
  these 
  supposed 
  rapid 
  vibrations 
  which 
  form 
  the 
  front 
  

   portion 
  of 
  an 
  advancing 
  earthquake, 
  that 
  Prof. 
  Milne 
  attributes 
  

   the 
  origin 
  of 
  the 
  earthquake-sounds. 
  We 
  may 
  conclude 
  from 
  

   these 
  observations 
  that, 
  initially 
  at 
  any 
  rate, 
  the 
  period 
  of 
  the 
  

   vibrations 
  increases 
  and 
  decreases 
  with 
  the 
  amplitude. 
  

  

  Now, 
  from 
  different 
  parts 
  of 
  the 
  area 
  over 
  which 
  a 
  fault-slip 
  

   takes 
  place, 
  there 
  must 
  proceed 
  vibrations 
  differing 
  greatly 
  in 
  

   amplitude, 
  and 
  therefore 
  also 
  in 
  period. 
  From 
  the 
  central 
  por- 
  

   tions 
  of 
  the 
  slip-area 
  will 
  come, 
  as 
  a 
  rule, 
  the 
  vibrations 
  of 
  largest 
  

   amplitude 
  and 
  longest 
  period 
  ; 
  while, 
  from 
  the 
  margins 
  there 
  

   will 
  proceed 
  minute 
  vibrations 
  of 
  a 
  period 
  so 
  short 
  that 
  they 
  may 
  

   be 
  perceptible 
  only 
  as 
  sound. 
  The 
  position 
  of 
  the 
  line 
  separating 
  

   the 
  marginal 
  and 
  central 
  parts 
  of 
  the 
  slip-area 
  will 
  depend 
  only 
  

   on 
  the 
  amplitude 
  of 
  the 
  vibrations 
  corresponding 
  to 
  the 
  period 
  of 
  

   the 
  lowest 
  sound 
  that 
  can 
  be 
  heard 
  ; 
  it 
  will 
  not 
  at 
  all 
  depend 
  on 
  

   the 
  amount 
  of 
  the 
  slip 
  at 
  the 
  center 
  of 
  the 
  area, 
  i. 
  e. 
  it 
  will 
  be 
  

   independent 
  of 
  the 
  intensity 
  of 
  the 
  shock. 
  — 
  p. 
  10. 
  This 
  theory 
  

   explains 
  (l) 
  the 
  fact 
  that 
  the 
  sound-area 
  is 
  not 
  concentric 
  with 
  

   the 
  disturbed 
  area, 
  and 
  the 
  sound-focus 
  is 
  nearer 
  the 
  surface 
  than 
  

   the 
  rest 
  of 
  the 
  seismic 
  focus 
  ; 
  (2) 
  the 
  fact 
  that, 
  in 
  great 
  earth- 
  

   quakes, 
  the 
  sounds 
  are 
  heard 
  only 
  within 
  a 
  comparatively 
  small 
  

   area 
  immediately 
  surrounding 
  the 
  epicentrum. 
  — 
  pp. 
  20, 
  21. 
  

  

  With 
  one 
  possible 
  exception 
  (that 
  of 
  Ben 
  Nevis), 
  the 
  earth- 
  

   quakes 
  of 
  1889 
  are 
  typical 
  examples 
  of 
  British 
  shocks 
  — 
  they 
  

   occurred 
  in 
  districts 
  where 
  earthquakes 
  are 
  rarely 
  felt, 
  and 
  their 
  

   disturbed 
  areas 
  are 
  circular 
  or 
  only 
  slightly 
  elliptical 
  in 
  form. 
  

   Turning 
  to 
  a 
  more 
  distinctly 
  seismic 
  area, 
  Switzerland 
  for 
  ex- 
  

   ample, 
  we 
  find 
  that 
  the 
  disturbed 
  areas 
  are 
  often 
  extremely 
  

   elongated, 
  the 
  longer 
  axes 
  being 
  parallel 
  to 
  those 
  of 
  the 
  neigh- 
  

   boring 
  Alpine 
  chain 
  ; 
  earthquakes 
  are 
  more 
  frequent, 
  their 
  in- 
  

   tensity, 
  as 
  a 
  rule, 
  is 
  greater, 
  and- 
  much 
  larger 
  areas 
  are 
  disturbed. 
  

   Different 
  stages 
  in 
  the 
  geological 
  history 
  of 
  a 
  district 
  are 
  

  

  