﻿Vol. 
  53.] 
  OE 
  AUGUST 
  1892 
  AND 
  NOVEMBEE 
  1893. 
  161 
  

  

  The 
  central 
  isoseismal 
  (intensity 
  7) 
  is 
  in 
  the 
  form 
  of 
  a 
  semi- 
  

   ellipse 
  whose 
  shorter 
  axis 
  is 
  21 
  miles 
  in 
  length, 
  and 
  whose 
  longer 
  

   axis 
  is 
  directed 
  north 
  and 
  south. 
  As 
  only 
  the 
  northern 
  half 
  lies 
  on 
  

   land, 
  it 
  is 
  somewhat 
  doubtful 
  whether 
  the 
  centre 
  of 
  the 
  area 
  lies 
  

   beneath 
  the 
  land 
  or 
  the 
  sea 
  ; 
  but 
  in 
  either 
  case 
  it 
  is 
  probably 
  not 
  

   far 
  from 
  the 
  coast, 
  at 
  a 
  point 
  about 
  3| 
  miles 
  S.S.E. 
  of 
  Pembroke. 
  

   The 
  land-area 
  included 
  within 
  this 
  curve 
  contains 
  about 
  450 
  square 
  

   miles. 
  The 
  form 
  and 
  position 
  of 
  this 
  isoseismal 
  corroborate 
  the 
  

   inference 
  previously 
  drawn 
  as 
  to 
  the 
  direction 
  of 
  the 
  originating 
  

   fault. 
  

  

  Owing 
  to 
  the 
  incompleteness 
  of 
  the 
  isoseismal 
  6, 
  there 
  is 
  some 
  

   uncertainty 
  as 
  to 
  the 
  direction 
  in 
  which 
  the 
  supposed 
  fault 
  hades. 
  

   Comparing 
  the 
  isoseismals 
  7 
  and 
  5, 
  however, 
  it 
  will 
  be 
  seen 
  that 
  on 
  

   the' 
  eastern 
  side 
  of 
  the 
  epicentre 
  they 
  are 
  separated 
  by 
  a 
  distance 
  of 
  

   52 
  miles, 
  while 
  on 
  the 
  western 
  side 
  the 
  distance 
  between 
  them 
  is 
  as 
  

   much 
  as 
  62 
  miles. 
  If 
  the 
  district 
  on 
  either 
  side 
  of 
  the 
  epicentre 
  

   were 
  fairly 
  similar 
  and 
  uniform 
  in 
  structure, 
  this 
  would 
  indicate 
  

   that 
  the 
  fault 
  hades 
  to 
  the 
  west. 
  The 
  great 
  extent 
  of 
  the 
  

   disturbed 
  area, 
  and 
  the 
  fact 
  that 
  so 
  much 
  of 
  it 
  to 
  the 
  west 
  is 
  covered 
  

   by 
  sea, 
  lessen 
  the 
  force 
  of 
  this 
  inference 
  so 
  far 
  as 
  it 
  depends 
  on 
  the 
  

   relative 
  position 
  of 
  the 
  isoseismals. 
  

  

  On 
  the 
  other 
  hand, 
  the 
  distance 
  between 
  the 
  isoseismals 
  4 
  and 
  5 
  is 
  

   much 
  greater 
  towards 
  the 
  north, 
  north-east, 
  and 
  east, 
  and 
  at 
  first 
  

   sight 
  this 
  appears 
  to 
  lead 
  to 
  a 
  contrary 
  conclusion 
  to 
  that 
  above 
  

   stated. 
  In 
  the 
  neighbourhood 
  of 
  the 
  epicentre, 
  the 
  distance 
  be- 
  

   tween 
  two 
  given 
  isoseismals 
  is 
  greater 
  on 
  the 
  side 
  towards 
  which 
  

   the 
  fault 
  hades 
  than 
  on 
  the 
  opposite 
  side. 
  But 
  at 
  great 
  distances 
  

   from 
  the 
  origin 
  the 
  inclined 
  position 
  of 
  the 
  fault-plane 
  has 
  a 
  less 
  

   unequal 
  effect 
  on 
  the 
  intensity 
  at 
  equal 
  distances 
  on 
  opposite 
  sides 
  

   of 
  the 
  fault, 
  and 
  the 
  consequence 
  is 
  that 
  two 
  distant 
  isoseismals 
  are 
  

   less 
  separated 
  on 
  the 
  side 
  towards 
  which 
  the 
  fault 
  hades 
  than 
  on 
  

   the 
  other. 
  It 
  is 
  clear, 
  however, 
  that 
  the 
  relative 
  position 
  of 
  the 
  

   isoseismals 
  is 
  governed 
  to 
  a 
  great 
  extent 
  by 
  the 
  structure 
  of 
  the 
  

   districts 
  through 
  which 
  the 
  earth-wave 
  had 
  to 
  pass. 
  The 
  intensity 
  

   appears 
  to 
  have 
  died 
  out 
  less 
  slowly 
  on 
  continuous 
  land- 
  areas 
  than 
  

   when 
  sea-areas 
  intervened, 
  and 
  thus 
  the 
  isoseismal 
  5 
  might 
  have 
  

   extended 
  still 
  farther 
  west 
  if 
  there 
  had 
  been 
  no 
  sea 
  to 
  traverse. 
  

   Thus, 
  judging 
  from 
  this 
  earthquake 
  alone, 
  a 
  westerly 
  hade 
  of 
  the 
  

   fault 
  is 
  more 
  probable 
  than 
  an 
  easterly 
  one. 
  

  

  As 
  the 
  intensity 
  of 
  the 
  shock 
  is 
  greater 
  on 
  the 
  side 
  of 
  the 
  fault 
  

   towards 
  which 
  it 
  hades, 
  1 
  it 
  follows 
  that 
  the 
  fault 
  must 
  intersect 
  the 
  

   ground 
  along 
  a 
  north-and-south 
  line 
  lying 
  a 
  short 
  distance 
  (perhaps 
  

   2 
  or 
  3 
  miles) 
  east 
  of 
  the 
  centre 
  of 
  the 
  isoseismal 
  7. 
  Referring 
  to 
  the 
  

   map 
  (p. 
  158), 
  it 
  will 
  be 
  seen 
  that 
  such 
  a 
  line 
  nearly 
  coincides 
  with 
  

   the 
  longer 
  axis 
  of 
  the 
  curve 
  bounding 
  the 
  area 
  over 
  which 
  the 
  earth- 
  

   sound 
  (b) 
  was 
  heard 
  2 
  minutes 
  before 
  the 
  principal 
  shock. 
  

  

  Nature 
  of 
  the 
  Shock. 
  — 
  Owing 
  to 
  the 
  time 
  of 
  its 
  occurrence, 
  many 
  

   <of 
  the 
  observers 
  were 
  roused 
  by 
  the 
  first 
  tremors 
  or 
  by 
  the 
  pre- 
  

  

  1 
  Geol. 
  Mag. 
  1896, 
  pp. 
  78-79. 
  

  

  