﻿On 
  the 
  Frequency 
  of 
  Earthquakes. 
  507 
  

  

  we 
  get 
  a 
  curve 
  of 
  frequency 
  similar 
  to 
  the 
  Italian, 
  except 
  that 
  the 
  

   maximum 
  and 
  minimum 
  are 
  reversed, 
  the 
  greatest 
  number 
  of 
  

   shocks 
  corresponding 
  to 
  the 
  period 
  when 
  the 
  load 
  is 
  being 
  lightened 
  

   most 
  rapidly, 
  indicating 
  that 
  these 
  shocks 
  are 
  due 
  to 
  a 
  general 
  

   movement 
  of 
  elevation 
  rather 
  than 
  depression, 
  a 
  conclusion 
  in 
  accord 
  

   with 
  field 
  observations 
  of 
  other 
  great 
  earthquakes. 
  In 
  addition, 
  

   the 
  shocks 
  which 
  occurred 
  during 
  the 
  period 
  1885-90 
  were 
  taken 
  

   out, 
  as 
  representing 
  a 
  more 
  normal 
  activity, 
  though 
  still 
  one 
  in 
  

   which 
  aftershocks 
  form 
  fully 
  half 
  of 
  the 
  record, 
  and 
  the 
  curve 
  

   was 
  found, 
  as 
  might 
  have 
  been 
  expected 
  from 
  the 
  character 
  of 
  the 
  

   record, 
  to 
  combine 
  the 
  features 
  of 
  the 
  Mino-Owari 
  aftershocks 
  with 
  

   those 
  of 
  the 
  Italian 
  curve 
  of 
  frequency, 
  of 
  earthquakes 
  prevailingly 
  

   of 
  the 
  so-called 
  ' 
  tectonic 
  ' 
  type. 
  

  

  These 
  results 
  are 
  of 
  twofold 
  geological 
  interest. 
  In 
  the 
  first 
  

   place 
  they 
  confirm 
  the 
  conclusion 
  drawn 
  from 
  a 
  study 
  of 
  the 
  Cali- 
  

   fornian 
  earthquake 
  of 
  1906*, 
  that 
  the 
  great 
  earthquakes 
  differ 
  from 
  

   the 
  ordinary, 
  not 
  merely 
  in 
  degree 
  but 
  in 
  kind. 
  They 
  indicate 
  that 
  

   in 
  the 
  latter 
  the 
  main 
  stress 
  is 
  compressive, 
  probably 
  due 
  to 
  settle- 
  

   ment, 
  and 
  in 
  the 
  former 
  to 
  elevation 
  or 
  tension, 
  a 
  conclusion 
  which 
  

   is 
  in 
  accord 
  with 
  the 
  fact 
  that, 
  in 
  those 
  cases 
  in 
  which 
  it 
  has 
  been 
  

   possible 
  to 
  compare 
  accurate 
  measurements 
  made 
  before 
  and 
  after 
  

   the 
  earthquake, 
  the 
  comparison 
  has 
  indicated 
  an 
  expansion, 
  eleva- 
  

   tion, 
  or 
  both, 
  of 
  the 
  area 
  affected 
  by 
  the 
  disturbance. 
  

  

  The 
  second 
  point 
  of 
  interest 
  is 
  that 
  the 
  figures 
  give 
  a 
  means 
  of 
  

   estimating 
  the 
  rate 
  of 
  growth 
  of 
  the 
  strain 
  which 
  produces 
  earth- 
  

   quakes. 
  If 
  we 
  accept 
  the 
  hypothesis 
  that 
  earthquakes, 
  in 
  the 
  limited 
  

   sense 
  of 
  their 
  orchesis, 
  are 
  due 
  to 
  the 
  relief 
  by 
  fracture 
  of 
  a 
  growing 
  

   strain 
  when 
  this 
  has 
  reached 
  the 
  breaking 
  point, 
  it 
  can 
  be 
  easily 
  

   shown 
  that 
  a 
  variable 
  strain, 
  acting 
  in 
  alternate 
  periods 
  in 
  increase 
  

   or 
  decrease 
  of 
  the 
  general 
  growth 
  of 
  strain, 
  while 
  leaving 
  the 
  average 
  

   rate 
  unaltered, 
  will 
  give 
  rise 
  to 
  a 
  corresponding 
  variation 
  in 
  the 
  

   frequency 
  of 
  shocks 
  in 
  each 
  period 
  ; 
  and, 
  besides 
  that, 
  there 
  is 
  a 
  

   simple 
  relation 
  between 
  the 
  magnitudes 
  of 
  the 
  two 
  stresses, 
  to 
  

   which 
  the 
  strains 
  are 
  due, 
  and 
  the 
  variations 
  from 
  the 
  mean 
  fre- 
  

   quency 
  of 
  earthquakes. 
  A 
  calculation 
  on 
  these 
  lines 
  shows 
  that 
  

   the 
  growth 
  of 
  strain, 
  for 
  Italy, 
  is 
  such 
  that, 
  accepting 
  the 
  pub- 
  

   lished 
  estimates 
  that 
  an 
  area 
  of 
  the 
  earth's 
  crust 
  of 
  the 
  magnitude 
  

   of 
  Italy 
  would 
  crush 
  under 
  its 
  own 
  weight 
  if 
  left 
  unsupported 
  to 
  

   the 
  extent 
  of 
  1/400 
  of 
  the 
  force 
  of 
  gravity, 
  the 
  breaking 
  strain 
  

   would 
  be 
  reached 
  in 
  about 
  3| 
  years, 
  starting 
  from 
  a 
  condition 
  of 
  

   no 
  strain. 
  The 
  aftershocks 
  of 
  the 
  Mino-Owari 
  earthquake 
  give 
  a 
  

   little 
  less 
  than 
  half 
  this 
  figure, 
  which 
  is 
  again 
  reduced 
  to 
  from 
  five 
  

   to 
  six 
  months 
  if 
  account 
  is 
  taken 
  of 
  the 
  difference 
  between 
  the 
  

   resistance 
  of 
  rock 
  to 
  tension 
  and 
  to 
  compression. 
  These 
  figures 
  are 
  

   given 
  for 
  what 
  they 
  are 
  worth 
  ; 
  at 
  the 
  least, 
  they 
  are 
  of 
  interest 
  as 
  

   being 
  the 
  first 
  authentic 
  estimate 
  which 
  it 
  has 
  been 
  possible 
  to 
  

   make 
  of 
  the 
  time 
  required 
  to 
  prepare 
  for, 
  and, 
  thence, 
  of 
  the 
  rate 
  

   of 
  growth 
  of 
  the 
  particular 
  tectonic 
  process 
  involved 
  in 
  the 
  

   production 
  of 
  earthquakes. 
  

  

  * 
  Q. 
  J. 
  G. 
  S. 
  vol. 
  lxv. 
  (1909) 
  p. 
  14. 
  

  

  

  