﻿174 
  DR. 
  C. 
  DAVISON 
  ON 
  THE 
  PEMBROKE 
  EARTHQUAKES 
  [May 
  1 
  897,. 
  

  

  In 
  less 
  than 
  a 
  quarter 
  of 
  an 
  hour, 
  the 
  first 
  after-shock 
  (d) 
  took 
  

   place. 
  As 
  so 
  little 
  of 
  the 
  disturbed 
  area 
  lay 
  on 
  land, 
  it 
  is 
  impossible 
  

   to 
  determine 
  the 
  position 
  of 
  the 
  focus, 
  except 
  that 
  the 
  whole 
  or 
  the 
  

   greater 
  part 
  of 
  it 
  must 
  have 
  lain 
  beneath 
  the 
  sea. 
  If 
  it 
  were 
  con- 
  

   nected 
  with 
  the 
  transverse 
  fault 
  aa, 
  the 
  chief 
  displacement 
  of 
  the 
  

   series 
  was 
  evidently 
  followed 
  by 
  a 
  slip 
  farther 
  to 
  the 
  south. 
  

  

  The 
  next 
  after-shock 
  (e) 
  had 
  so 
  few 
  observers 
  that 
  its 
  connexion 
  

   with 
  this 
  fault 
  is 
  also 
  uncertain. 
  All 
  that 
  can 
  be 
  asserted 
  is 
  that 
  

   its 
  focus 
  lay 
  north 
  of 
  that 
  of 
  the 
  principal 
  earthquake. 
  

  

  Then 
  followed 
  the 
  strongest 
  of 
  the 
  after-shocks 
  (/). 
  From 
  the 
  

   approximate 
  coincidence 
  of 
  its 
  isoseismal 
  4 
  with 
  the 
  isoseismal 
  7 
  of 
  

   the 
  chief 
  earthquake, 
  we 
  may 
  infer 
  that 
  it 
  was 
  caused 
  by 
  another 
  

   slip 
  along 
  the 
  transverse 
  fault 
  aa, 
  and 
  in 
  almost 
  exactly 
  the 
  same 
  

   region. 
  

  

  By 
  the 
  slip 
  which 
  produced 
  this 
  earthquake, 
  the 
  pre-existing 
  

   stress 
  along 
  the 
  fault 
  aa 
  was 
  practically 
  relieved. 
  But 
  the 
  displace- 
  

   ments 
  by 
  which 
  this 
  relief 
  was 
  effected 
  increased 
  the 
  stress 
  along 
  

   adjoining 
  faults, 
  with 
  the 
  result 
  that 
  the 
  fourth 
  after-shock 
  (g) 
  

   points 
  to 
  a 
  slip 
  along 
  another 
  surmised 
  transverse 
  fault 
  /3/3, 
  approxi- 
  

   mately 
  parallel 
  to 
  the 
  fault 
  a 
  a 
  and 
  about 
  6 
  miles 
  farther 
  west. 
  

   The 
  increase 
  of 
  stress 
  having 
  been 
  inconsiderable, 
  one 
  small 
  slip 
  

   was 
  enough 
  to 
  restore 
  equilibrium 
  along 
  this 
  fracture. 
  

  

  In 
  about 
  3| 
  hours 
  after 
  the 
  principal 
  earthquake, 
  there 
  occurred 
  

   a 
  slight 
  shock 
  (A), 
  which 
  is 
  clearly 
  connected 
  with 
  one 
  of 
  the 
  longi- 
  

   tudinal 
  systems 
  of 
  faults 
  running 
  east 
  and 
  west. 
  Pour 
  days 
  later, 
  

   on 
  August 
  22nd, 
  a 
  somewhat 
  stronger 
  shock 
  (Jc) 
  took 
  place, 
  which 
  

   may 
  be 
  traced 
  to 
  another 
  fault 
  of 
  the 
  same 
  system, 
  lying 
  a 
  mile 
  

   or 
  so 
  farther 
  north. 
  On 
  the 
  map 
  (p. 
  158), 
  two 
  faults 
  are 
  shown, 
  

   which, 
  if 
  continued, 
  would 
  occupy 
  the 
  required 
  positions 
  ; 
  for, 
  as 
  

   Mr. 
  Marr 
  informs 
  me, 
  they 
  are 
  both 
  thrust-planes 
  hading 
  at 
  a 
  high 
  

   angle 
  probably 
  to 
  the 
  south. 
  

  

  Earthquakes 
  of 
  1893. 
  — 
  The 
  first 
  two 
  shocks 
  of 
  Nov. 
  2nd, 
  1893, 
  are 
  

   evidently 
  connected 
  with 
  one 
  and 
  the 
  same 
  fault, 
  for 
  the 
  line 
  join- 
  

   ing 
  the 
  centres 
  of 
  the 
  isoseismal 
  6 
  of 
  the 
  principal 
  earthquake 
  (A) 
  

   and 
  of 
  the 
  disturbed 
  area 
  of 
  the 
  first 
  after-shock 
  (6) 
  is 
  approximately 
  

   parallel 
  to 
  the 
  longer 
  axes 
  of 
  the 
  same 
  curves. 
  The 
  direction 
  of 
  the 
  

   originating 
  fault 
  must 
  be 
  about 
  E. 
  15° 
  N. 
  and 
  W. 
  15° 
  S., 
  but 
  with 
  

   regard 
  to 
  its 
  hade 
  the 
  evidence 
  is 
  doubtful. 
  The 
  fault 
  ee 
  (see 
  map, 
  

   p. 
  158) 
  is 
  a 
  thrust-plane 
  hading 
  probably 
  to 
  the 
  north; 
  the 
  fault 
  ££ 
  

   is 
  also 
  a 
  thrust-plane, 
  but 
  probably 
  hading 
  in 
  the 
  opposite 
  direction. 
  

   As 
  traced 
  on 
  the 
  map 
  it 
  is 
  possibly 
  incomplete, 
  for 
  Messrs. 
  Marr 
  and 
  

   Boberts's 
  map 
  does 
  not 
  extend 
  beyond 
  the 
  eastern 
  end 
  shown 
  ; 
  but 
  

   its 
  course 
  so 
  far 
  is 
  parallel 
  to 
  the 
  fault-line 
  implied 
  by 
  the 
  seismic 
  

   evidence. 
  Neither 
  this 
  fault 
  nor 
  the 
  preceding 
  will, 
  however, 
  satisfy 
  

   the 
  required 
  conditions, 
  for 
  they 
  both 
  dip 
  away 
  from 
  the 
  common 
  

   axis 
  of 
  the 
  isoseismal 
  lines; 
  but 
  the 
  district 
  between 
  them, 
  as 
  

   Mr. 
  Marr 
  informs 
  me, 
  ' 
  is 
  simply 
  wattled 
  with 
  minor 
  faults,' 
  and 
  it 
  

   is 
  probably 
  to 
  movements 
  along 
  one 
  of 
  these 
  that 
  the 
  earthquake 
  

   of 
  1893 
  must 
  be 
  assigned. 
  

  

  