﻿^°k 
  53*] 
  0F 
  AUGUST 
  1892 
  AND 
  NOVEMBER 
  1 
  893. 
  175 
  

  

  There 
  can 
  be 
  little 
  doubt, 
  I 
  think, 
  that 
  the 
  fault-slips 
  of 
  1892 
  

   affected 
  the 
  conditions 
  of 
  stress 
  along 
  this 
  neighbouring 
  fault, 
  so 
  

   that 
  the 
  displacements 
  along 
  it 
  occurred 
  earlier 
  than 
  they 
  might 
  

   otherwise 
  have 
  done. 
  There 
  is 
  no 
  evidence 
  of 
  any 
  preliminary 
  

   slips, 
  the 
  first 
  that 
  took 
  place 
  being 
  the 
  most 
  important 
  of 
  the 
  series. 
  

   As 
  in 
  the 
  principal 
  earthquake 
  of 
  1892, 
  the 
  slip 
  was 
  discontinuous 
  : 
  

   but 
  whether 
  the 
  focus 
  consisted 
  of 
  two 
  detached 
  portions, 
  or 
  

   whether 
  the 
  slip 
  was 
  repeated 
  at 
  the 
  same 
  or 
  a 
  different 
  part 
  of 
  

   the 
  fault, 
  is 
  doubtful, 
  for 
  the 
  records 
  of 
  the 
  relative 
  intensity 
  of 
  

   the 
  two 
  parts 
  of 
  the 
  shock 
  are 
  discordant. 
  The 
  principal 
  part 
  of 
  

   the 
  focus 
  may 
  have 
  been 
  as 
  much 
  as 
  10 
  or 
  12 
  miles 
  long, 
  but 
  the 
  

   length 
  of 
  its 
  eastern 
  margin 
  must 
  have 
  been 
  considerable, 
  and 
  

   much 
  greater 
  than 
  that 
  of 
  the 
  western 
  margin. 
  This 
  is 
  shown 
  by 
  

   the 
  position 
  of 
  the 
  boundary 
  of 
  the 
  sound-area 
  with 
  respect 
  to 
  

   that 
  of 
  the 
  isoseismal 
  4, 
  and 
  also 
  by 
  the 
  time-relations 
  of 
  the 
  sound 
  

   and 
  shock. 
  

  

  The 
  stress 
  being 
  more 
  completely 
  relieved 
  on 
  the 
  eastern 
  side 
  of 
  the 
  

   focus, 
  we 
  should 
  expect 
  the 
  next 
  slip 
  to 
  take 
  place 
  on 
  the 
  opposite 
  

   side. 
  And 
  this 
  we 
  find 
  to 
  have 
  been 
  the 
  case, 
  for 
  in 
  the 
  first 
  after- 
  

   shock 
  (o), 
  which 
  occurred 
  a 
  quarter 
  of 
  an 
  hour 
  later, 
  the 
  epicentral 
  

   area 
  was 
  shifted 
  to 
  the 
  W.S.W. 
  Though 
  much 
  less 
  in 
  amount 
  of 
  

   slip, 
  the 
  focus 
  must 
  have 
  been 
  of 
  almost 
  the 
  same 
  length 
  as 
  that 
  of 
  

   the 
  principal 
  earthquake, 
  and 
  partly 
  overlapping 
  it, 
  but 
  with 
  its 
  

   centre 
  lying 
  about 
  8 
  miles 
  farther 
  to 
  the 
  W.S.W. 
  

  

  Assuming, 
  as 
  is 
  very 
  probable, 
  that 
  the 
  second 
  and 
  third 
  after- 
  

   shocks 
  are 
  connected 
  with 
  the 
  same 
  fault, 
  the 
  next 
  slip 
  occurred 
  on 
  

   the 
  eastern 
  side 
  of 
  the 
  principal 
  focus 
  and 
  at 
  increasing 
  distances 
  from 
  

   it. 
  The 
  great 
  rock-mass, 
  once 
  set 
  in 
  motion, 
  thus 
  appears 
  to 
  have 
  

   oscillated 
  slowly, 
  first 
  to 
  one 
  side 
  and 
  then 
  to 
  the 
  other, 
  before 
  finally 
  

  

  Conclusion. 
  — 
  I 
  have 
  not 
  yet 
  been 
  able 
  to 
  study 
  in 
  any 
  detail 
  the 
  

   seismic 
  history 
  of 
  the 
  district 
  principally 
  affected 
  by 
  these 
  two 
  series 
  

   of 
  earthquakes. 
  Several 
  shocks 
  have 
  been 
  felt 
  in 
  Pembrokeshire 
  

   and 
  Caermarthenshire 
  during 
  the 
  present 
  century, 
  some 
  of 
  which 
  had 
  

   their 
  origins 
  elsewhere, 
  and 
  at 
  least 
  three 
  (in 
  1802, 
  1832, 
  and 
  1840) 
  

   possibly 
  in 
  the 
  latter 
  county. 
  For 
  more 
  than 
  50 
  years, 
  however, 
  

   there 
  appear 
  to 
  have 
  been 
  no 
  slips 
  of 
  any 
  consequence 
  along 
  the 
  

   fault-system 
  here 
  considered, 
  and 
  the 
  following 
  facts 
  are 
  therefore 
  

   worthy 
  of 
  notice 
  : 
  — 
  (1) 
  that, 
  after 
  this 
  prolonged 
  interval 
  of 
  repose, 
  

   the 
  earlier 
  movements 
  took 
  place 
  along 
  transverse, 
  and 
  the 
  later 
  

   along 
  longitudinal, 
  faults 
  : 
  (2) 
  that 
  the 
  three 
  faults 
  of 
  the 
  latter 
  

   series 
  lie 
  successively 
  the 
  one 
  to 
  the 
  north 
  of 
  the 
  other, 
  as 
  if 
  the 
  

   abrupt 
  displacement 
  of 
  a 
  rock-mass 
  over 
  one 
  thrust-plane 
  impelled 
  

   the 
  further 
  advance 
  of 
  those 
  immediately 
  below. 
  

  

  PLATE 
  XI. 
  

  

  Map 
  of 
  the 
  Pembroke 
  Earthquakes 
  of 
  August 
  1892 
  and 
  November 
  1893, 
  on 
  

   the 
  scale 
  of 
  40 
  miles 
  to 
  1 
  inch. 
  

  

  