VIBRATORY MOVEMENTS AND THEIR EFFECTS. 45 
zone. Again, suppose two shocks originated at different centers, their waves, in general, 
would cross each other at an angle, and we might have circular or elliptical motion as a 
result of the combination of the two sets of compressional waves, of the two sets of dis- 
tortional waves, or of each set of compressional with the other set of distortional waves. 
Modifications of the waves on passing from one kind of rock to another would occur 
and give rise to still other combinations which would cause parallel rotations. 
With the hope of throwing light on the progress of the rupture along the fault-plane by 
determining the distribution of rotatory effects in the surrounding regions a special list of 
questions was sent out and many answers were received. They may be summarized as 
follows: at a distance, where the shock was but slightly felt, rotations were rarely noticed ; 
but where the shock was strong, even tho many miles from the fault, they were almost 
universal; a number of observers stated that the disturbance was first a simple vibration, 
and that the rotatory motion only appeared later; no one put the rotatory motion in the 
early part of the shock. Some, who did not notice rotations, stated that the direction 
of the motion changed during the disturbance. At a distance from the fault, where the 
movement was slow and gentle, the rotatory effect would not be very noticeable, but that 
it still existed is shown by the seismogram made at Carson City, where the intensity of 
the shock was greatly reduced. This general distribution of parallel rotations does not 
show how the rupture took place on the fault, but merely confirms the idea that the dis- 
turbance at any point was due to vibrations originating in many parts of the fault-plane; 
and the combinations of these vibrations would cause the variations in intensity and the 
rotations observed. The writhing motion of the steel smokestack at Mare Island (vol. 1, 
p. 212) must have been the result of a double vibratory motion of the ground combined 
into a parallel rotation; the elastic bending of the stack would cause a much greater 
vibration of the top than of the bottom; this explains the whole motion without the 
assumption of a tilting of the ground. 
In the first volume numerous examples are given of statues, monuments in cemeteries, 
chimneys, etc., which were rotated on their supports by the earthquake; many were 
turned thru an angle of 90° and some as much as 180° (vol. 1, p. 359), tho in the majority 
of cases the rotation was less than 20°. In the cemetery near San Rafael all except one of 
the rotated monuments were turned with the hands of a watch thru angles of 16° or less. 
Similarly, at Lakeport all the rotated chimneys were turned in the same direction (vol. 1, 
p. 188). This phenomenon has long been observed and occurs at the times of all violent 
earthquakes; it naturally suggests a rotation of the support; but, as has been seen, a 
more careful examination of this idea shows that it is entirely untenable; indeed, Charles 
Darwin long ago pointed out that if objects were turned on their supports by true rota- 
tions, the axis of each rotated object must be an axis of the rotation, which is a practical 
impossibility. The effort, therefore, was made to explain the rotation merely as the 
result of a to-and-fro vibration. What is necessary is to produce a moment around the 
vertical axis thru the center of gravity. 
Three suggestions have been made. First: Mallet’ suggested that the object may 
not bear uniformly on its support, but may only press on it in a few points, and as the 
pressure will in general be different at these points a moment around the center of gravity 
due to the frictional forces would be produced during a vibratory movement, resulting in 
a rotation. Altho it may be possible for small rotations to be brought about in this way, 
they are probably very small and unimportant; for it can easily be shown that if the 
frictional forces at the points of contact follow the ordinary laws of solid friction, namely, 
that the tangential forces are proportional to the normal pressures, then no moment 
around a vertical axis thro the center of gravity will be set up by the vibrations, and it is 
only in so far as the ordinary laws of friction are departed from that moments can be 

1 Dynamics of Earthquakes. Trans. Roy. Irish Acad. 1846, vol. xx1, pp. 51-105. 
