DIRECTIONS OF VIBRATORY MOVEMENT, 
GENERAL NOTE. 
The data for the discussion of the directions of propagation and vibration of the earth- 
waves is for the most part unsatisfactory and leads only to a conviction of the complex- 
ity of the general problem of earth movement. Apart from the intrinsic complexity of 
the subject, there were two conditions which were adverse to the securing of exact and 
significant information. The first of these was the lack of provision for obtaining instru- 
mental records of earthquake shocks thruout California. There were very few seismo- 
graphs installed in the state and such as were in existence proved in large measure inade- 
quate for the purpose for which they were intended. The second adverse condition was 
the hour at which the earthquake began. At its beginning most people were asleep, 
and the confusion incident to so rude an awakening was not conducive to sharp observa- 
tion. The chief trouble, however, inheres in the intricate and confused nature of the 
earth movement itself. A brief statement of the different kinds of movement involved 
in the commotion of the earth may be of service in the formulation of clear ideas of the 
nature of the shock in general and of the question of direction in particular. 
Usually the principal movement of the ground in an earthquake is vibratory. In the 
California earthquake there was, however, a mass movement in opposite directions on 
the two sides of the San Andreas fault. This mass movement was, as has been shown 
by the work of the Coast and Geodetic Survey, distributed over a wide zone on either 
side of the fault and diminished more or less regularly with distance from it. The move- 
ment was not vibratory except to a very limited extent; but it gave rise to the displace- 
ment of objects on the surface quite similar to that caused by the vibratory movement. 
Thus, in attempting to deduce the directions of propagation and vibration of the earth- 
waves from the phenomena of displaced objects or persons, it is necessary to discrimi- 
nate between the effects due to the mass movement and the true vibration of the ground. 
But this discrimination is only possible to a very limited extent, partly because the 
borders of the zone within which the mass movement caused the displacement of objects 
and persons are unknown, and partly because the two kinds of movement overlapt, con- 
spiring to produce a single effect. 
When we come to consider the earth-waves generated by the movement on the fault, 
probably as an effect of friction, it must be at once apparent that these waves emanated 
from innumerable points on a plane, one dimension of which is about 270 miles and the 
other probably 20 miles or more. On this plane, if we judge from the course of the 
fault-trace, there were at certain places inequalities which offered exceptional resistance 
to movement, and at these the jar was exceptionally heavy and dominated the vibrations 
emanating from portions of freer movement. From all parts of the fault-plane, there- 
fore, waves of various amplitudes were propagated in all directions, and their paths 
intersected. The consequent interference would in part make for neutralization and 
in part for intensification of the vibratory movement. It is thus evident that the effects 
produced by the emergence of these waves at the surface, or by the propagation of those 
emanating from the more superficial portions of the fault along the surface, could be 
systematically disposed only if the following conditions obtained: 
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