﻿THE 
  SEISMIC 
  FOCUS. 
  IJ3 
  

  

  the 
  earth 
  and 
  a 
  few 
  miles 
  of 
  what 
  has 
  been 
  removed 
  we 
  may 
  be 
  able 
  

   to 
  infer, 
  with 
  a 
  greater 
  or 
  less 
  degree 
  of 
  certainty, 
  but 
  apart 
  from 
  

   this 
  all 
  we 
  can 
  know 
  is 
  the 
  extent 
  in 
  one 
  direction. 
  Even 
  here 
  the 
  

   information 
  may 
  ba 
  incomplete, 
  for 
  denudation 
  may 
  have 
  removed, 
  

   or 
  not 
  yet 
  exposed, 
  that 
  part 
  of 
  the 
  thrust-plane 
  where 
  its 
  dimensions 
  

   were 
  greatest 
  ; 
  or 
  the 
  continuation 
  may 
  be 
  covered 
  up 
  and 
  hidden 
  by 
  

   newer 
  deposits. 
  

  

  From 
  this 
  it 
  will 
  be 
  seen 
  that 
  in 
  attempting 
  a 
  comparison 
  we 
  can 
  

   only 
  take 
  one 
  dimension, 
  and 
  must 
  leave 
  area 
  out 
  of 
  consideration. 
  

   If 
  we 
  do 
  this, 
  it 
  is 
  not 
  necessary 
  to 
  go 
  beyond 
  the 
  great 
  ' 
  Faille 
  du 
  

   Midi 
  ' 
  in 
  Belgium, 
  a 
  thrust-plane 
  which 
  has 
  been 
  traced 
  for 
  120 
  miles, 
  

   for 
  an 
  instance 
  comparable 
  in 
  size 
  with 
  the 
  thrust- 
  plane 
  inferred 
  as 
  

   the 
  cause 
  of 
  this 
  earthquake. 
  

  

  There 
  remains 
  now 
  only 
  the 
  determination, 
  so 
  far 
  as 
  is 
  possible, 
  

   of 
  the 
  depth 
  below 
  the 
  surface 
  at 
  which 
  this 
  thrust-plane 
  lay. 
  In 
  

   attempting 
  this 
  there 
  is 
  only 
  one 
  method, 
  of 
  the 
  many 
  proposed 
  by 
  

   different 
  authors, 
  which 
  can 
  be 
  applied 
  in 
  the 
  present 
  case, 
  and 
  that 
  

   is 
  the 
  method 
  proposed 
  by 
  Dr. 
  Aug. 
  Schmidt 
  1 
  based 
  on 
  the 
  observed 
  

   rates 
  of 
  travel. 
  

  

  Fig. 
  18. 
  Schmidt's 
  modification 
  of 
  Seebach's 
  hyperbola. 
  

  

  The 
  principle 
  on 
  which 
  this 
  is 
  based 
  is 
  illustrated 
  by 
  fig. 
  18, 
  

  

  copied 
  from 
  Dr. 
  Schmidt's 
  paper. 
  In 
  this 
  the 
  outer 
  circle 
  represents 
  

  

  a 
  section 
  of 
  the 
  earth, 
  and 
  C 
  the 
  focus 
  of 
  an 
  earthquake, 
  the 
  depth 
  

  

  from 
  the 
  surface 
  being 
  much 
  exaggerated 
  to 
  bring 
  out 
  the 
  principle 
  

  

  1 
  Jahresheft. 
  Ver. 
  f. 
  vaterl. 
  Naturk. 
  in 
  Wuittemberg. 
  XLVI, 
  227 
  (1890). 
  

  

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  173 
  ) 
  

  

  