ISOSEISMALS: DISTRIBUTION OF APPARENT INTENSITY. 330 
soil, containing a large amount of water and surrounded or partially surrounded by solid 
strata, will not oscillate with the same motion as the surrounding strata. Moreover, 
in the case of the frequencies ordinarily occurring in earthquake motion, the amplitude 
of the oscillation of such a semi-fluid mass is likely to be greater than that of the sur- 
rounding solid strata; also the reversal of motion or the acceleration during reversal 
is likely to be greater than in the case of solid strata. Finally the greater relative motion 
of such a soft or semi-fluid mass is not prevented by overlying strata of drier and more 
compact material. 
REVIEW OF THE DISTRIBUTION OF APPARENT INTENSITY. 
In the preceding pages, all the data significant of the distribution of the intensity of 
the shock of April 18, 1906, have been set forth in such detail as seemed to be warranted 
in a statement of fact. The general conclusions drawn from the data are represented 
graphically upon the intensity map No. 23. It is proposed here, however, to call atten- 
tion to some of the more interesting and instructive phases of the distribution of inten- 
sity, and briefly to discuss their significance. 
It is to be noted, in the first place, that the region over which the disturbance was 
felt, extending from the Pacific Coast to Central Nevada, and from southern Oregon to 
southern California, is one of varied physiography. In a consideration of the relation 
of the physiographic features to the distribution of intensity, it will be necessary to 
distinguish only two classes of features; viz., (1) the mountain and hill slopes, generally 
underlain by firm rocks and veneered for the most part with a thin mantle of regolith 
and soil; and (2) the valley-bottoms usually underlain by a relatively great depth of 
infilled alluvium in a little coherent condition, and for the most part saturated with 
ground-water. 
The color bands on the map, indicating the gradation of intensity, show very con- 
siderable irregularities, or departures from the smooth curves which might reasonably 
be expected to obtain as the expression of such gradation of absorption of energy in 
homogeneous materials. To some extent such irregularities may be ascribed to the 
known lack of homogeneity in the firm elastic rocks of which the earth’s crust is chiefly 
composed. But the irregularities referred to are too great to permit us to regard such 
lack of homogeneity in the underlying elastic rocks as an important factor in determin- 
ing them. The irregularities are clearly related to the distribution of the valley lands. 
GENERAL DISPOSITION OF THE ISOSEISMALS. 
If now, before entering upon a consideration of these irregularities, we endeavor to 
ignore them, and so obtain a general conspectus of the color bands representing the 
gradation of intensity, the following features come out fairly clearly : 
1. On the northeast side of the fault-trace, the zones of equal gradation of intensity 
show a tendency to belly out to the northeastward, opposite the middle portion of the 
fault-trace. This tendency is most pronounced in the grades from VII to II of the 
Rossi-Forel scale, and is apparent in all grades below IX. 
The irregularities above referred to, associated with the distribution of valley lands, 
confuse somewhat the perception of this tendency, but do not detract from its reality. 
2. As a partial statement of the same general fact, the color zones become distinctly 
narrower, and their boundaries converge, as they approach the coast north of Eureka. 
This feature of the distribution of intensity clearly suggests that the isoseismal curves 
close in and swing around the end of the fault, and that there is, therefore, no indi- 
cation of a submarine prolongation of the fault beyond its known extent on the main- 
land in Humboldt County. 7 
