162 REPORT OF THE CALIFORNIA EARTHQUAKE COMMISSION. 
the most common. The latter, by reason of their greater elasticity, are usually much 
better adapted to withstand the wracking movement of an earthquake shock than are 
brick and masonry walls. The intensity, as inferred from a region of wooden buildings, 
would, therefore, in general appear to be less than that for a region of brick or masonry 
structures. Even among the latter, and among the brick chimneys of wooden houses, 
which were so generally used as indicators of intensity, there is great variation in strength 
due to the variation chiefly in the character of the mortar used in their construction. 
Along river bottoms and on valley floors, particularly where the ground water is 
abundant, structures were much more susceptible to damage than similar structures 
founded on the firm rocks of the valley slopes. This apparently high intensity of the 
-shock in the valley lands was in part due to an actual slumping of the ground, which 
wracked the buildings independently of any elastic- vibration communicated to them 
from the ground. 
Finally, in grade X of the scale, fissures in the ground are taken as a criterion of the 
highest grade of intensity, when in reality such fissures have under different conditions 
very different values from this point of view. The fissures which extend down into the 
earth’s crust, and are due to its actual rupture on a fault-plane, are of course significant 
of the highest degree of disturbance usually experienced in earthquakes; but those cracks 
and fissures which occur in valley bottoms, due to the slumping of soft material toward 
the stream trench, or those cracks which are associated with landslides, in those cases 
where the landslide was imminent and was merely precipitated by the earth jar, are super- 
ficial phenomena and do not necessarily indicate so high a degree of intensity as that 
marked X on the scale. It therefore becomes necessary to discriminate such fissures, 
and this was not always done in the reports sent in to the -Commission. 
These various imperfections in the scale used for grading the intensities would of course 
be minimized if the entire field were examined and reported upon by one observer. The 
personal equation would in that case, for practical purposes, be constant. But when 
the observations were made over so vast a field by a great number of persons, so diversely 
qualified for the work, the errors are necessarily numerous. 
Added to this are the large gaps in the records, due to the scant population in the more 
mountainous parts of the region affected. In these thinly populated tracts, there is not 
only an absence of individual observations at the time of the earthquake, but also a lack 
of structures which would reveal to subsequent examination the effects of the shock. 
It will thus be apparent that any effort to map the distribution of the intensity of the 
earthquake can only yield rough approximations to the actual facts. Yet such approxi- 
mations have their value, and the Commission has not been discouraged by the imperfec- 
tions of the method from applying it to the full extent permissible under the cireum- 
stances. The results are given graphically on the isoseismal map which accompanies 
this report. (Map No. 23.) 
In compiling this map, it has been found best to plot the intensities upon the basis 
of a literal interpretation of the Rossi-Forel scale, as regards damage to structures. It 
results from this that in the river bottom the curves represent zones of equal destructive 
effects, but not necessarily zones of equal intensity, interpreted in terms of acceleration 
of the vibratory movement of the earth. In these tracts we are confronted with the 
question as to whether the locally high destructive effects were wholly due to the char- 
acter of the ground, as in part they certainly were, or whether these may not be ascribed 
in part to local auxiliary faults in the earth’s crust which did not appear as ruptures at 
the surface. This question will receive special consideration in the sequel, when the 
facts have been more fully set forth. 
It is now proposed to describe somewhat in detail the effects of the earthquake which 
serve as the basis of the isoseismal map, beginning at the north and going southerly. 
