ISOSEISMALS: DISTRIBUTION OF APPARENT INTENSITY. 347 
of that which was clearly due to the character of the geological formations. This ques- 
tion has been touched upon incidentally in the discussion of the relation of the valleys 
to distribution of apparent intensity, but it will be of advantage to review the facts 
here more systematically, tho quite briefly. 
In southern Oregon and in northeastern California, in Modoc, Shasta, Lassen, and 
Plumas Counties, the shock was so uniformly feeble that there is no suggestion of locally 
high intensity due to any cause. The same general statement is true of northeastern 
California, in Del Norte, Siskiyou, Humboldt, and Trinity Counties; but in this region 
some of the faults, particularly that of Redwood Mountain, were not farther from the 
seat of disturbance than certain localities farther south, where abnormally high apparent 
intensity was developed on valley-bottoms. If the Redwood Mountain fault had been 
a locus of movement, there can be little doubt, altho the settlements in that region are 
few and scattered, that we should have heard of the severity of the shock. No evidence, 
however, has come to hand indicative of any exceptional severity on or near the line 
of that fault. 
Along the eastern front of the Sierra Nevada, from Honey Lake and the T aylorsville 
district to Tejon Pass, altho there are many extensive faults, and altho on one of these 
there occurred a movement which caused the Inyo earthquake of 1872, yet there is no 
suggestion of any local movement on any of these on the morning of April 18, 1906. The 
intensity of the shock along this general fault-zone was about IV of the scale; but the 
movement was a slow, gentle swing characteristic of a heavy distant shock. 
Similarly, the numerous faults which traverse California south of Tehachapi may be 
left out of consideration, no shock at all having been felt over the greater part of the 
region, and but feebly in those parts where it was felt. 
There thus remain of the faults in California practically only those that fall within 
the zone of destruction, to merit serious consideration. The most northerly of these 
is the Mount St. Helena fault described by Osmont.!. This fault has a northwest-southeast 
strike, and a throw of not less than 2,000 feet. It forms a well-marked and little- 
degraded scarp on the southwest side of the mountain and the date of its principal 
movement is within the Quaternary period. The projection of this fault to the north- 
west is not known; to the southeast it undoubtedly passes beneath the floor of Napa 
Valley, in the vicinity of Calistoga. Neither on the slopes of the mountain nor at Calis- 
toga was there any evidence of abnormally high intensity, and the necessary inference 
is, therefore, that there was no movement on the fault at the time of the earthquake. 
The southwest front of the Berkeley Hills, and the extension of the same geomorphic 
feature farther south, forming the southwest front of the higher Mount Hamilton Range, 
is with little question a fault-scarp, or series of scarps, of Quaternary date, now more 
or less dissected and degraded. The northern extension of the fault-zone beyond San 
Pablo Bay is not known. It probably contributed to the definition of the western side 
of the ridge between Sonoma and Petaluma, but apparently did not traverse the middle 
part of Santa Rosa Valley, since the study of that region by Osmont failed to reveal it. 
This fault-zone is of peculiar interest from the point of view of the present discussion, 
since it appears to have been the seat of disturbance of the earthquake of 1868. At 
that time the fault-trace was marked by a crack at the surface, which was traceable 
for 20 miles or more along the base of the scarp slope, altho the amount of the move- 
ment was probably quite small. The trace of the fault is approximately parallel to the 
San Andreas Rift, and is 18 miles distant from it. As has already been suggested, this 
fault would seem a priori more susceptible to the influences which would make for re- 
newed movement than most other faults of the region. But there is no evidence that 
any movement occurred upon it. The intensity showed no abnormal increase along 

1 Loc. cit. 
