VIBRATORY MOVEMENTS AND THEIR EFFECTS. 
CHARACTER OF THE MOVEMENTS. 
When the rupture occurred on the fault-plane, it is probable that the movement did 
not begin at the same moment at all parts of the plane; it probably started in some 
limited region, and the stress, being relieved by the break there, was concentrated upon 
nearby points which gave way, and thus the rupture spread from point to point until it 
extended over the whole fault-plane; and it is also probable that the whole movement at 
any point did not take place at once, but that it proceeded by very irregular steps. 
‘We can determine roughly the time which would have been required for the rock to 
come back to its natural position of equilibrium if it had vibrated freely without friction. 
The period of vibration of the rock, distorted by simple shear, as explained on page 50, 
is given by the expression 7) =4 H-Vp/n; where H, the distance from the fault-plane 
to which the distortion extends, may be taken as 6 km. (8.7 miles), p is the density of the 
rock, say, 2.6; and mis the coefficient of rigidity, say 2 x 10‘ dynes per square centimeter 
(2,900,000 pounds per square inch). With these values of the constants we find the 
total period to be about 8.7 seconds, or the time for the rock to move from its original 
displacement to its position of equilibrium one-fourth of this, or 2.2 seconds. This is 
found from the equation of the free vibration of the rock, in which case the straight line 
at right angles to the fault is distorted so as to be concave toward its position of equi- 
librium; but the observations in fig. 5 (page 16) show that the rock was distorted with 
the convex side toward the position of equilibrium. If therefore the break had been 
sharp, with no friction at the fault-plane, we should have had vibrations containing 
higher harmonics, so that the rock at the fault-plane would have made rapid but short 
vibrations back and forth during the 2.2 seconds necessary for it to reach the equilibrium 
position. This, however, was not what actually occurred; small slips took place at 
different parts of the fault-plane, and as the results of these successive slips and the great 
friction, some 30 to 60 seconds were required before the rock came to rest; and even 
then certain parts of the rock were apparently still held in a strained condition by strong 
friction, and from time to time gave way, producing the aftershocks which are listed in 
another part of the report. 
The more or less sudden starting and stopping of the movement and the friction gave 
rise to the vibrations which were propagated to a distance. The sudden starting of the 
motion would produce vibrations just as would its sudden stopping; and vibrations are 
set up by the friction of the moving rock, exactly as the vibrations of a violin string are 
caused by the friction of the bow; the string vibrates altho the bow is drawn steadily 
across it; or as vibrations are set up in a finger-bowl when a wet finger is drawn along 
the edge; in this case we can see the vibrations transmitted to the water in the bowl. 
Vibrations once started are propagated as elastic waves in the rock and consist in 
general of compressional waves like simple waves of sound, in which the vibratory move- 
ment of any particle is in the direction of propagation; and of transverse waves like those 
of light, in which the movement of the vibrating particle is at right angles to the direction 
of propagation. As a compressional wave advances, the mass of rock thru which it 
passes is subjected to successive compressions and extensions. 

1 See p. 21. 
39 
