THE EARTH MOVEMENT ON THE FAULT OF APRIL 18, 1906. 141 
Table 4.— Readings of Mean Sea-level on the Fixt Tide-staff. 
READING OF 
MEAN SBA- 
LEVEL ON 
TrpE-STAFF 


Feet 
, 1897 to Apr. ee ree 8.339 
1, 1898 to Apr. 5 ee oe eo! 8.298 
1899 to Apr. ¢ i ah ee 8.528 
1900 to Apr. et Oren ne 8.550 
1901 to Apr. Caley ae aki. es 8.430 
1902 to Apr. ¢ Sea at owes 8.584 
1903 to Apr. bE. bee. 8.509 
1904 to Apr. ee eT s 30 

, 1905 to Apr. soul) ee 8.659 
1, 1906 to Apr. ee ae ae 8.631 


The ten annual means show an unmistakable tendency to fall into three groups, as 
indicated by the means shown in the last column of the table. Within each group there 
is no apparent tendency to increase or decrease. Between the first and second groups the 
reading of mean sea-level increased 0.202 foot and between the second and third groups, 
it again increased 0.132 foot. Such an increase corresponds to a subsidence of, the zero 
of the tide-staff with reference to mean sea-level. An examination of the monthly means 
indicates that probably the subsidence occurred suddenly in each case, the movements 
taking place about June, 1899, and April, 1904. The record must not be considered as 
proving positively that these two subsidences took place. The changes are not clearly 
beyond the range of possible error in the determination of mean sea-level on account of 
irregular changes in the water surface due to causes not clearly understood, tho they are 
beyond the possible range of instrumental errors. 
The annual mean for the one year after the earthquake, 1906-1907, agrees with the 
two preceding annual means within less than 0.04 foot. In no other case in the table do 
three successive annual means agree so closely with each other as these three. Appar- 
ently, therefore, no change in the elevation of the zero of the tide-staff occurred at the 
time of the earthquake. 
As further evidence that no appreciable change in the elevation of the tide-staff took 
place on April 18, 1906, the following table is submitted. Corresponding months of two 
years, one before and one after the earthquake, are compared to avoid the effects of annual 
inequalities. The comparison indicates that no change took place in April, 1906. 
Table 5.—Monthly Mean Readings of Mean Sea-level on Tide-staff. 


1905-1906 1906-1907 DIFFERENCE 
Feet Feet 
ALE a ener LUND Sen eee eee 8.507 8.462 + .045 
TN, eee oe ee 8.416 8.506 — .090 
July 2a re SN. Pe 8.668 8.688 — .020 
TAD i) a ee ea 8.676 8.797 SPA 
PepUCInter flare 4 pissy oy « 8.648 8.632 + .016 
Octohermenn LoL Sas) 4! =: 8.690 8.442 + .248 
INON.GMD Derpetins. Ja -sdlrkkle ox 8.751 8.295 + .456 
Decembers = = 5. +.) 8.479 8.625 — .146 
Aiciog . BS geo nee 8.701 8.784 — .083 
CLUE Vor Toi oo ha 8.877 8.725 + .152 
March ee ves 8.934. 8.944 — .010 
Fa os Se, a 8.558 8.669 — .111 
Mean = + .028 




