10 THE SAN FRANCISCO EARTHQUAKE AND FIRE. 
MOTIONS CONSTITUTING THE EARTHQUAKE. 
The earthquake occurred between 10 and 15 minutes after 5 crclock 
a. m., standard time of the one hundred and twentieth meridian. As 
time w T as consumed in the propagation of the shocks, the moment of 
beginning at airy place depended in part on the distance of the place 
from the zone in which the disturbance originated. As this zone was 
hundreds of miles in extent it is probable that the time of beginning 
was not the same at all points along it. For similar reasons the char- 
acter and sequence of the motions constituting the earthquake were 
not the same at any two localities, and the differences in the vicinity 
of the zone of origin may have been very great. In and near San 
Francisco the principal part had a duration of about one minute; it 
was preceded by comparatively faint tremors for several seconds, and 
it was followed by minor tremors. During the stronger part the 
motion was chiefly in horizontal directions and oscillatory, but its 
rhythm was irregular in period and emphasis. On firm ground in the 
same region the range of motion is believed to have been more than 2 
inches, but was not measured, there being no seismograph capable of 
recording it. On soft ground the range of oscillatory motion may 
have been several times greater, and it was also greater in the 
immediate vicinity of the fault trace. 
The following paragraphs present my conception of the essential 
character of the motions constituting the earthquake in the region 
of its destructive intensity. In the endeavor to make a brief and 
clear statement the conception is expressed somewhat badly, with 
little reference to the various uncertainties of fact and theory by 
which it is actually qualified. 
The earthquake fault has a length of 300 miles, possibly 400. 
Nothing is known of its depth. It coincides with a plane of earlier 
faulting, on portions of which there have been movements within a 
few decades. The fact of recurrence on the same plane shows that 
the rock faces in contact had not become welded, so that the molec- 
ular force which there resisted motion was less than the cohesion 
of solid rock and may have been little stronger than adhesion. A 
tract of the crust including the fault plane had come to be affected 
by a system of slowly increasing shearing strains, and the associated 
stresses were the forces directly causing the fault. When the stress 
component coincident with the fault plane at some point became 
greater than the adhesion (or cohesion) a local slipping took place. 
This caused a redistribution of strains and stresses, the local relief 
of strain being followed by increase of strain and stress in all adjoin- 
ing tracts of the fault plane, with, the result that the adhesion was 
overcome in those tracts and the area of incipient faulting thereby 
enlarged. Thus from the initial tract the lesion was propagated 
as a sort of wave through all the fault plane. 
