THE EARTHQUAKE AS A NATURAL PHENOMENON. 11 
At the initial tract a small movement sufficed to relieve the local 
strain, and the motion was then arrested by friction, but the move- 
ment was renewed by reaction from other tracts, and it alternately 
started and stopped till the accumulated stresses had spent them- 
selves. There was a similar rhythmic sequence in other parts of the 
fault, the frequency of the alternations depending on local conditions ; 
and the total movement of dislocation at each point was accomplished 
by a series of steps and not by a single leap. 
The time consumed in these reactions was not infinitesimal. The 
rate of propagation of changes in strain was of the same order of 
magnitude as that of earthquake waves in general, and the rate of 
propagation of the initiation of movement on the old fault plane 
may have been somewhat slower because of the necessity of accumu- 
lating a certain amount of stress increment to overcome the adhesion. 
It is probable that the completion of the fault required more than 
one minute, and it may have required more than two minutes. It is 
even possible that the displacement had been completed at the ini- 
tial point before it began at the most remote. 
In the succession of slippings and stoppings at any point of the 
fault plane each separate slip communicated a jar or pulse to the 
surrounding rock, and this pulse was propagated in all directions. 
The earthquake at any locality in the neighborhood of the fault 
consisted of such pulses from different directions. The general dis- 
tribution of intensity indicates that the pulses weakened in trans- 
mission somewhat rapidly, whence it may be inferred that the particu- 
lar pulses constituting the dominant elements of the local earthquake 
were those from the nearer parts of the fault. 
If this conception of the earthquake is correct, the rhythm observed 
in the region of high intensity was a phenomenon distinct from the 
rhythm of harmonic waves. It was essentially a frictional rhythm, 
dependent on the relation of certain rock strains and rock stresses to 
the resistances afforded by adhesion and sliding friction. It was 
irregular not only because the intervals of local starting and stopping 
were unequal, but because it was derived from a considerable area of 
the fault surface, in which the local rhythms were neither harmoni- 
ous nor synchronous. 
The compounding of unevenly spaced pulses from different points 
of the fault plane caused both reinforcement and interference, intro- 
ducing a character analogous to beats in music, but without the regu- 
larity of musical beats. It also at times made oscillatory motions 
swifter in one direction than the other, so that reciprocative accelera- 
tions were not always symmetrically arranged. In less technical 
language, the motion was jerky and included abrupt phases that were 
almost blows. The compounding also introduced variety in the 
direction of motion, especially at the end, when for a short time the 
