Voiv. 6, 1920 
GEOPHYSICS: H. F. REID 
559 
comes to the earth's surface within a short distance of the epicenter, 
and 95% within a distance of 90°. When we consider that the earth- 
quake originates at a fault and that the mass movement there may be 
horizontal, inclined, or vertical, we realize that the energy sent out in 
different azimuths, and indeed at different vertical angles, may be de- 
pendent on the direction of the fault and of the mass movement, but no 
information has yet been obtained on this subject. 
B. Regional Problems 
The accurate surveys made by the United States Coast and Geodetic 
Survey of parts of California before and after the earthquake of 1906, 
made it very plain that slow movements of the earth's crust during some 
scores of years finally brought about a rupture of the rock along the San 
Andreas Fault, and that the two sides of the fault flung back in opposite 
directions under the elastic forces set up in the rock by those movements. 
But no information was gained regarding small ruptures taking place 
preliminary to the main break, nor regarding the location of the many 
sudden movements after the shock, causing what are known as after- 
shocks. A methodical study of a limited area where small shocks are 
frequent and strong shocks are occasional would throw much light on 
the sequence of events leading up to the rupture producing a strong shock, 
on the way in which the rupture is gradually enlarged, on the depth to 
which it extends, and on the order according to which quiet is gradually 
restored. It seems not impossible that the location of the preliminary 
shocks might serve to block out the fault along which a rupture was about 
to take place. Davison's study of the shocks preliminary to the severe 
Mino-Owari earthquake of Japan in 1891, seems to encourage this idea. 
But it is important that the actual origins of the shocks should be deter- 
mined; it is insufficient merely to list the places where the shocks were 
felt; this last method has led to serious error in the past. By placing 
simple monuments in properly selected positions and fixing their relative 
positions and heights from time to time by careful surveys, the slow move- 
ments of the crust and the relation of these movements to earthquakes 
could be discovered. Information could also be gained regarding the re- 
lation of the geological structure and the physiography of a region to its 
seismic activity. 
The velocity of transmission of earthquake vibrations in the immediate 
vicinity of the origin and near the earth's surface, can only be satisfac- 
torily studied by placing a number of seismographs at comparatively 
short distances apart, and not far from the origin of the shock. A good 
determination of this velocity would add precision to many seismological 
problems. 
There are many other problems that come in this class. Microseisms, 
small vibrations continuing for hours, frequently interfere with the satis- 
