1921. J Angenheister.—A Study of Pacific Earthquakes. 217 
observations are available. When the European observations of the quakes 
of the 1st May, 1917, 26th June, 1917, and 30th April, 1919, are available 
it will probably be possible to do this. 
V. Velocity of Surface Waves and Elasticity of the Earth’s 
Upper Crust. 
We have attributed the difference in the velocity of the preliminary 
tremors P and S under continent and ocean to the difference in the elasticity 
of the upper crust. This difference in travelling-time is a maximum for 
6° epicentral distance, for which the waves dip to a maximum depth of 
50 km. under the surface of the earth. We may assume that under ocean 
the thickness of the crust is about 50 km. In this earth-crust P and S 
travel quicker under ocean than under continent. For epicentral distances 
greater than 6° the difference of travelling-times under ocean and continent 
will decrease, because P and S will travel for an increasing fraction of their 
path in the homogeneous fluid magma below, in which the velocity may 
be the same whether under ocean or continent. For an epicentral distance 
of 18° it can be shown that only 10 per cent, of the path is in the upper 
crust. 
For affirmation of this probable difference of elasticity under ocean 
and continent we may investigate the characteristics of the main phase 
of far-distant earthquakes. These waves travel along the upper crust 
and not through the magma below. 
We must distinguish between two kinds of waves, namely,— 
(1.) The transverse distortional waves L (i . In these waves, according 
to Wiechert, the vibration is horizontal and perpendicular to the direction 
of propagation. They have no vertical component,* and have a period 
depending only on the thickness, density, and elasticity of the upper crust. 
They may be likened to free horizontal vibrations of a jelly with both its 
upper and lower faces free to move. 
(2.) Rayleigh waves, L r . These are well marked on a seismogram, 
in that they start with a few waves of large period, which quickly settles 
down to a smaller, constant period. The displacement is in the general 
case elliptic in a vertical plane containing the direction of propagation. 
The maximum displacement due to these waves takes place generally 
during the earlier waves of decreasing period which are so noticeable in a 
seismogram. In the different records it is thus easy to identify the 
Rayleigh waves, and in practice the movement due to them shows itself 
practically all in the azimuth of the epicentre. It may be mentioned that 
the ratio of velocities of L r and L q waves is theoretically equal to 0-92, 
and, as will be seen later, this ratio is also obtained in practice. In the 
investigations two very strong earthquakes were taken, the Kurile earth¬ 
quake of the 1st May, 1915, and the Kamchatka earthquake of the 31st 
July, 1915. The observations of Apia and Tiflis were compared for these 
earthquakes, since these two stations have practically equal epicentral 
distances—about 8,000 km.—from both earthquakes. The waves travel to 
Apia for both quakes under deep sea of mean depth 4-5 km. To Tiflis the 
waves travel for the Kamchatka earthquake under a shallow sea of mean 
depth 150 metres for 200 km. of their path, and for the remainder under 
the Asiatic continent. For the Kurile earthquake the path to Tiflis is 
400 km. under shallow sea of 150 metres depth, 450 km. under the Sea of 
Okhotsk of about 1,000 metres mean depth, and for the rest of their path 
under the Asiatic continent. Thus for Apia and Tiflis we have large and 
* See G. W. Walker, Modern Seismology, c'.iap. vi, Longmans, 1913. 
