54 REPORT OF THE CALIFORNIA EARTHQUAKE COMMISSION. 
tions of neighboring parts of the alluvium would be nullified by it. When the alluvium 
is so soft and plastic that shearing forces are insignificant, the alluvium is flung back and 
forth by the reaction of the sides of the basin with effects apparently still worse than in 
the former case; and with the formation, near the sides, of elevations and depressions 
resembling wave-surfaces; but they are not true progressive waves, for the rigidity is too 
small for the surface waves described on page 47 to be formed; and the viscosity is 
too great to permit of gravitational waves, but the violent to-and-fro motion of the 
basin and the low rigidity produce, near the sides, elevations resembling a wave surface, 
and the motion of the soft alluvium is so quickly damped out by its viscosity that the 
form is fixt and remains after the disturbance is over. This condition was characteristic 
of the’small filled-in swamps of San Francisco, usually accompanied by a general lower- 
ing of the surface, due to the character of the refuse used for filling them; this material 
was so little consolidated that the surface has been steadily sinking for years (vol. 1, 
pp. 241-242), and its volume was materially reduced by the shaking of the earthquake. 
LARGE BASINS. 
The second class of alluvial basins are those which are too large to be lookt upon as 
moving as a whole; that is, they are larger than an eighth wave-length; in some cases 
they are many wave-lengths in breadth. They are represented in California by the large 
valleys, the Santa Clara, the San Joaquin, etc. We must picture them to ourselves as 
broad, shallow basins with irregular floors and containing material whose coefficient of 
rigidity varies considerably even in neighboring parts; this material is principally water- 
soaked sands and gravels, given a certain amount of rigidity by the weight of the material 
above it. As the elastic waves pass thru the underlying rock they enter the alluvium 
and are refracted upwards on account of the smaller velocity in the alluvium than in the 
rock. If the angle of incidence is sufficiently small, the amplitude of vibration in the 
alluvium will be larger than in the rock; for instance, if we assume the density of the 
alluvium to be 0.8 that of the rock, and the velocity of propagation one-fifth as great, 
then for normal incidence the amplitude of the refracted wave in the alluvium would be 
nearly double that of the incident wave in the rock, both for compressional and for dis- 
tortional waves. After entering the alluvium the waves would be reflected back and 
forth from the surface and bottom until they were damped out by the viscosity of the 
alluvium; for normal incidence the amplitude of the wave reflected from the bottom 
would be fifteen-sixteenths that of the incident wave, and then a large part of the motion 
would be kept in the alluvium. When the angle of incidence from the rock to the allu- 
vium is greater than zero, two reflected and two refracted waves are produced; and 
when this angle is not large the refracted wave in the alluvium would still have a larger 
amplitude than the incident wave in the rock. When reflected at the surface, the wave 
would have its phase changed by half a period, and if the length of its path to the floor and 
back again were a half wave-length, it would find itself in the same phase as the direct 
wave when it again reached the surface, and the resulting amplitude would be the sum 
of the amplitudes of the two waves. In many parts of the basins this relation of depth 
to wave-length must approximately have existed for some of the waves present in the 
earthquake disturbance. Repeated reflections would probably result in a surface am- 
plitude considerably greater than would occur at the surface of continuous rock; and the 
irregularities mentioned above would cause discordance of motion in points near together, 
and thus greatly increase the damage. 
A thin coating of alluvium over the rock would evidently move with the rock and 
would not have an especially large amplitude; it would be necessary for its depth to be 
something like an eighth of a wave-length to obtain the full effect; the small velocities 
