336 REPORT OF THE CALIFORNIA EARTHQUAKE COMMISSION. 
3. The zones of equal gradation from IX to V of the scale are narrower at the southern 
end of the fault-trace than at the northern end, and they close around the end much 
more closely. This fact is suggestive of less depth of disturbance at the southern end 
of the fault. But the general disposition in the south of the zones ranging from V to 
II is not essentially dissimilar to those in the north. 
4, The disposition of the isoseismal curves along the coastal territory between Point 
Arena and Shelter Cove indicates that the trace of the fault on the sea-floor lies but a 
few miles off shore, and that its course partakes of the nature of a very obtuse sigmoid 
curve, approximately parallel to the trend of the coast. It follows from this inference 
that the fault observed in Humboldt County is continuous with that extending from 
the vicinity of Point Arena southeastward. No facts have come to light which weaken 
this conclusion, altho all the facts have been diligently sought for. 
5. On the southwest side of the fault, the territory upon which it has been possible 
to trace the isoseismals, particularly those ranging above VI, is very much smaller than 
on the northeast side. In so far as the territory available is representative of the entire 
southwestern crustal block, it appears, chiefly. from the isoseismals covering portions 
of San Mateo, Santa Cruz, San Benito, and Monterey Counties, that the intensity dimin- 
ished much more rapidly to the southwest than it did to the northeast. This interest- 
ing fact suggests that, of the two crustal blocks differentially displaced on the fault, the 
southwest block was perhaps the more passive. It may, however, indicate that the 
apparent intensity, as interpreted from effects on structures and objects, is a function 
of the character of the underlying rocks; since on the southwest side of the fault-trace 
there are extensive areas of highly elastic granitic rocks, while on the northeast side of 
the fault-trace these granitic rocks are deeply buried by sedimentary formations and 
appear nowhere at the surface west of the Sierra Nevada. 
6. The zones of equal gradation of intensity, ranging from X to VII, are fairly evenly 
spaced, tho broadening with diminishing intensity; from VII to VI the zone is notably 
broader, particularly in the northern portion of the region affected; and from VI to II 
the broadening of the zones is very marked. 
RELATION OF APPARENT INTENSITY TO VALLEYS. 
The generalizations above set forth are independent of the irregularities in the iso- 
seismal curves associated with the valleys. We may now inquire into the relationship 
which obtains betWeen the valleys and the distribution of apparent intensity. 
The most northerly locality where this relationship appears is on the flood plain of 
the Kel River, near the coast, in Humboldt County. The lower part of the Eel River 
Valley has been carved by stream erosion out of a synclinal trough of Pliocene strata 
having a thickness of over a mile. The syncline is flanked by older and much harder 
sandstones which are probably of Franciscan age. On the south of the valley these 
older sandstones constitute a bold mountain ridge, stept with terraces, which terminates 
in Cape Mendocino. The north side of the ridge has an east and west trend, and the 
Pliocene strata extend well up on its flanks. There is no suggestion of a fault on this 
side of the ridge, the trend being determined by the axis of the synclinal fold. The other 
side of the flood plain has a less regular northwest-southeast trend, converging upon 
the south side in the vicinity of Rio Dell. The flood plain is thus bounded by a wide 
trumpet-shaped but asymmetric contour terminating in lagoons and sand beaches south 
of Eureka. The depth of the alluvium of the flood plain is not known, but the features 
of the region suggest that it is undergoing subsidence and the alluvium may be several 
hundred feet thick. On this flood plain the apparent intensity was notably higher 
than on the surrounding slopes. This is shown by the extent of destruction in Fern- 
dale and other towns situated upon it, and by the rupturing and deformation of the 
