THE INFLUENCE OF THE FOUNDATION ON THE APPARENT INTENSITY, 
THE GREATER DAMAGE ON ALLUVIUM. 
Experience shows that the damage done by destructive earthquakes is much greater on 
alluvial soil than on solid rock. A glance at the isoseismal map No. 23 will show how well 
this was exemplified by the California earthquake. Probably the best example we have 
is the city of San Francisco itself, which is built variously on solid rock, on sand, on natural 
alluvium, and on “made ground.” ‘The description of the destruction done in the city 
(vol. 1, pp. 220-245; maps, Nos. 18 and 19) shows that within its limits the character of 
the foundation was a far more potent factor in determining the damage done than near- 
ness to the fault-line. This is not a question of the transmission of vibrations, for, on 
account of the higher elasticity of solid rock, it would transmit vibrations far better than 
alluvium ; and indeed, as the alluvium occupies limited and comparatively shallow basins 
in the rock, the vibrations are always transmitted from a distance thru rock; and the 
question really to be answered is: How are the vibrations modified in a basin of alluvium 
so as to make them more destructive than without this modification? By analogy the 
well-known experiment of the ivory balls has been invoked to explain the fact. If the 
first of a row of ivory balls in contact receives a sharp blow, it transmits the shock to the 
next ball, but remains almost stationary itself; the shock is thus transmitted from ball to 
ball, and the last one, having nothing before it, flies off. It is said that the surface of allu- 
vium having nothing above it, and having little cohesion, experiences a much stronger 
vibration than a rock-surface under similar circumstances. But the analogy does not 
seem to me a good one, for the lack of constraint of objects above the surface is the same 
whether we are dealing with rock or with alluvium; and it is only in so far as a lack of 
cohesion in the alluvium would permit its surface to be thrown into the air that a difference 
in the two substances might be supposed to make itself evident; but in the cases we are 
considering, the shock is not nearly strong enough to produce such an effect ; and besides, 
structures built on rock are not usually firmly attacht to it; they would be thrown up- 
wards just as easily as tho they rested on alluvium, if subjected, in the two cases, to the 
same vibratory acceleration. 
Notre. — When a transverse wave, in which the vibrations are parallel with a free surface, is reflected 
from the surface, the amplitude at the surface is twice as great as that of the incident wave; the ampli- 
tude varies periodically with the distance from the surface in such a way that it equals the large surface 
amplitude at distances of any even number of times \/4 cos 7; where ) is the wave-length, and 7 is the 
angle of incidence; and it is zero at distances of any odd number of times the same expression. With 
transverse waves not parallel with the surface or with longitudinal waves the problem is much more 
complicated; it would still resemble the simpler case, but the variations of intensity would be less marked. 
The strong surface motion would extend some distance into the medium; this is probably why observa- 
tions in mines have shown practically the same intensity of movement as at the surface; the depth of the 
mines is only a fraction of \/4 cos 7. 
THE THEORY OF MR. ROGERS’S EXPERIMENT. 
With the object of throwing light on this subject, Mr. J. F. Rogers made some very 
interesting experiments (vol. I, pp. 326-335), in which sand containing various amounts 
of water and held in a wooden box was caused to vibrate to and fro and the movement of 
the top of the sand compared with the movement of the box. The outside forces are 
B 49 
