302 REPORT OF THE CALIFORNIA EARTHQUAKE COMMISSION. 
1. That the fault-plane were uniformly even or systematically uneven. 
2. That the rock affected both by the rupture and by the vibrations were homo- 
geneous thruout. 
3. And that the stress which gave rise to the rupture were uniform for the entire 
extent of the fault. 
It is fairly certain that none of these conditions actually did obtain; and we might, 
therefore, predict that the disposition of the effects of the shock, and particularly of 
the heavier portions of the shock, from which directions might be inferred, would be 
irregular, tho distribution of the intensity in the aggregate might be fairly symmetrical. 
This conclusion has been reached on the tacit assumption that there is but one kind of 
earth-wave or vibratory movement. But it is highly probable on theoretical grounds, 
and the theory is supported by experiment, that the vibration of the earth generated 
at the fault resolves itself into two quite distinct waves having quite different rates of 
propagation and direction of vibration. One of these is the longitudinal wave, so called 
because the vibrations are parallel to the direction of propagation, and the other is the 
transverse wave in which the vibrations are normal to the direction of propagation. 
The rate of propagation of the longitudinal waves in highly elastic rocks is nearly double 
that of the transverse waves. It will thus be evident that at any locality within the 
zone of disturbance an object may be shaken or displaced by the emergence of the longi- 
tudinal wave at that point, and that the movement due to the emergence of the transverse 
wave may be superimposed upon this either before or after it has come to rest. The 
resultant effect will be accordingly difficult to interpret as to the direction of the vibra- 
tion for either wave. When, however, the locality in question is sufficiently far removed 
from the fault, the interval between the emergence of the two waves may be sufficiently 
long to permit of the effect of the first being noted before that of the second is super- 
imposed. 
In the case of the California earthquake, the movement of the ground was compli- 
cated by the fact that both longitudinal and transverse waves were propagated in direc- 
tions nearly parallel to the surface from the superficial portion of the fault, and these 
for many miles out from the fault might be expected to give rise to movements discord- 
ant with those due to the arrival of similar waves from the deeper portions of the fault. 
It would thus seem, from the considerations thus far presented, that regularity in the 
disposition of the effects of the shock upon which a judgment as to the direction of the 
vibration nfight be based, was about the last thing to be expected. In other words, it 
would seem, on a priori grounds, to be a hopeless task to plot upon a map of California 
the direction of propagation and vibration of the earth-waves. The hopelessness of 
the task is intensified when certain other considerations are taken into account. For 
example, there were secondary short surface-waves of low speed and high amplitude 
observed in many parts of California, which are quite different from the high-velocity 
waves thus far discust. These undoubtedly had an important effect in the displacement 
of objects and persons, and so influenced judgments as to the direction of movement. 
Similarly on the alluvial bottoms of the rivers the ground lurched consistently toward 
the stream trench, whatever the orientation of the latter might be; and the phenomena 
arising from such movement gave rise to judgments as to the direction of the earth-waves 
which were of course erroneous. 
Added to all this was the general fact that those who contributed reports from various 
parts of the state to the general account of the earthquake in many cases based their 
judgment as to the direction of the shock upon the displacement of portions of structures, 
such as chimneys, or of objects within buildings. This kind of evidence was in most 
cases untrustworthy, and could lead to reliable conclusions only when treated critically 
and statistically so as to obtain a general result. Even the displacement of buildings 
