﻿25*> 
  OLDHAM 
  : 
  GREAT 
  EARTHQUAKE 
  OP 
  1 
  897. 
  

  

  concerning 
  the 
  nature 
  of 
  the 
  movements 
  which 
  would 
  be 
  propa- 
  

   gated 
  from 
  the 
  earthquake 
  centre 
  will 
  be 
  necessary. 
  They 
  are 
  :— 
  - 
  

  

  (1) 
  Waves 
  of 
  elastic 
  compression, 
  or 
  condensational 
  waves. 
  

  

  (2) 
  Waves 
  of 
  elastic 
  distortion 
  or 
  distortional 
  waves. 
  

  

  (3) 
  Undulatory 
  waves, 
  whether 
  purely 
  gravitational, 
  elastic 
  or 
  

  

  elastic-gravitational, 
  set 
  up 
  at 
  the 
  surface 
  in 
  and 
  near 
  the 
  

   epicentral 
  region 
  and 
  propagated 
  along 
  the 
  surface. 
  

  

  The 
  existence 
  of 
  waves 
  of 
  the 
  first 
  two 
  classes 
  has 
  long 
  been 
  

   recognised 
  ; 
  they 
  are 
  propagated 
  through 
  the 
  body 
  of 
  the 
  earth 
  and 
  it 
  

   is 
  to 
  them 
  that 
  all 
  earthquake 
  effects 
  are 
  commonly 
  ascribed, 
  as 
  was 
  

   done 
  by 
  Prof. 
  Grablovvitz 
  in 
  his 
  discussion 
  of 
  the 
  records 
  in 
  Ischia. 
  1 
  

   The 
  existence 
  of 
  undulations 
  of 
  the 
  third 
  class, 
  first 
  suggested 
  by 
  

   Professor 
  Milne 
  in 
  1888, 
  2 
  has 
  not 
  yet 
  been 
  fully 
  accepted. 
  

  

  The 
  rate 
  at 
  which 
  these 
  waves 
  travel 
  is 
  governed 
  by 
  the 
  density 
  

   of 
  the 
  medium 
  and 
  its 
  elasticity. 
  The 
  rate 
  of 
  travel 
  of 
  the 
  wave 
  of 
  

   elastic 
  comparison 
  is 
  always 
  greatest, 
  that 
  of 
  the 
  distortional 
  waves 
  

   less, 
  while 
  the 
  only 
  form 
  of 
  elastic 
  surface 
  undulation 
  which 
  has 
  yet 
  

   been 
  investigated 
  has, 
  according 
  to 
  Lord 
  Rayleigh, 
  a 
  still 
  slower 
  

   rate 
  of 
  transmission. 
  

  

  Now, 
  as 
  pointed 
  out 
  by 
  Dr. 
  A. 
  Schmidt 
  of 
  Stuttgart 
  in 
  1888, 
  8 
  not 
  

   only 
  it 
  is 
  inconceivable 
  that 
  the 
  elasticity 
  of 
  the 
  materials 
  of 
  which 
  

   the 
  earth 
  is 
  composed 
  should 
  be 
  unaffected 
  by 
  the 
  very 
  different 
  

   conditions 
  of 
  pressure 
  and 
  temperature 
  existing 
  in 
  the 
  deeper-seated 
  

   as 
  compared 
  with 
  the 
  superficial 
  portions 
  ; 
  but 
  it 
  is 
  also 
  not 
  in 
  

   accordance 
  with 
  observed 
  facts, 
  for 
  were 
  such 
  the 
  case, 
  the 
  rate 
  of 
  

   travel 
  of 
  the 
  waves 
  would 
  inevitably 
  be 
  the 
  same 
  for 
  th~ 
  same 
  rock 
  

   and 
  the 
  same 
  region 
  in 
  the 
  case 
  of 
  every 
  earthquake, 
  — 
  a 
  conclusion 
  

   which 
  is 
  in 
  utter 
  want 
  of 
  accordance 
  with 
  observed 
  facts. 
  We 
  must, 
  

   accordingly, 
  allow 
  that 
  the 
  depth 
  below 
  the 
  surface 
  at 
  which 
  an 
  

   elastic 
  wave 
  is 
  propagated 
  has 
  an 
  important 
  influence 
  on 
  its 
  rate 
  of 
  

  

  1 
  See 
  p. 
  232. 
  

  

  2 
  Trans. 
  Seismol. 
  Soc. 
  Japan, 
  XII, 
  p. 
  112. 
  

  

  3 
  Wellenbevvegurng 
  und 
  Erdbeben 
  : 
  Jahresheft. 
  Ver. 
  f. 
  Vaterland. 
  Naturkunde 
  in 
  Wurt- 
  

   temberg, 
  1888, 
  pp. 
  248—270 
  ; 
  noticed 
  in 
  Nature; 
  LI 
  I, 
  631—633 
  (1895). 
  

  

  ( 
  250 
  ) 
  

  

  