Angenheister.—A Study of Pacific Earthquakes. 
219 
1921.] 
waves) for an epicentral distance of 6° travel 18 per cent, more rapidly 
under ocean than under continent. For the records of Sydney and 
Pulkowa somewhat smaller ratios are obtained, but this may be explained 
by the fact that the path to the former place is partly under shallow r 
water. 
The higher velocity of the L q and S waves under ocean we may 
attribute to either an increase of rigidity {rj) or a decrease of density (p) r 
/_ 
since v = v 
P 
The geological and gravitational evidence, however, appears to point 
not to a decrease of density under ocean but to an increase. Thus from 
the equation for y we deduce that the rigidity of the earth’s crust under 
ocean is at least 44 per cent, greater than under continent. 
VI. Absorption of Energy of Earthquake-waves. 
For both quakes discussed in the preceding section the amplitude of 
ground-movement in the maximum of the main phase is only about half 
as much in Samoa as in Tiflis. Even if we admit that the determination 
of ground-movement is difficult, this difference is much greater than the 
error of measurement. It must therefore be assumed either that the 
energy is radiated with different intensity in different directions, or that 
the absorption of energy under ocean is greater than under continent. 
The centres of both quakes are on the north-west side of the great North 
Pacific deeps. Moreover, earthquakes on tectonic faults, according to 
observations felt in the neighbourhood of the quake, appear to have a 
shaking area of more or less elliptical form, but it is not known whether 
the distribution of energy to great distances from the epicentres is also 
unsymmetrical. If the whole difference in motion at Apia and Tiflis (of 
equal epicentral distance) depends only on the difference of absorption 
under ocean and continent, and if also we assume that energy is sent out 
equally in all directions, we obtain 
— a 0 Asa 
Isa ■— Iq6 
„ A 
Ti 
and Iii — I, e 
where A is the epicentral distance, a 0 is the coefficient of absorption under 
ocean, u c under continent, Sa indicates Samoa, and Ti Tiflis. 
Now, A Sa = A Ti , practically. 
Isa A(a c —«o) 
= e 
L Ti 
The energy of a harmonic-wave oscillation is proportional to the 
square of the velocity and to the amplitude, and the mean value can be 
A, where A is the amplitude and T the periodic time. 
shown to be 
2r 2 
rp 2 
For the same period, therefore, Isa/I-Ti = A 2 s a /A 2 xi ; and, combining, 
2 Ati 
with the preceding equation, a 0 — « c = - log e —.(1). 
The amplitudes used in the comparison were the maximum horizontal 
displacements of the main phase. In both Tiflis and Samoa the displace¬ 
ment for the first maximum was practically in the azimuth of the epicentre 
of the earthquake, showing that we are practically dealing in this case with 
Rayleigh waves. In Tiflis the maximum horizontal displacement of the 
quake of the 1st May, 1915, was 3-82 mm., and took place in a direc¬ 
tion approximately N. 23° E., whereas the azimuth of the epicentre is- 
