ISOSEISMALS: DISTRIBUTION OF APPARENT INTENSITY. 337 
alluvium of the flood plain itself, particularly in its lower part near the sea, and by the 
lesser destruction in the surrounding higher country. The data regarding the intensity 
on the high ridge to the south are scant, owing to the fewness of habitations, but on the 
Pliocene terrane on the northeast side of the flood plain, there was a distinct drop in 
the degree of destruction, altho this terrane consists largely of strata which are only 
partially indurated and but little coherent. 
The apparent intensity of the lower part of the Eel River flood plain grades from 
X to IX, tho in general nearer IX than X. It is surrounded by a belt of country where 
the intensity grades from IX to VIII. This belt has a width of a few miles on the Plio- 
cene terrane to the northeast of the flood plain, and probably scarcely extends to the 
harder Franciscan rocks of the ridge to the south. The facts thus necessitate the recog- 
nition on the intensity map of an area of high intensity, including a range from X to 
VIII, in the midst of a region where the prevailing intensity ranges only from VIII to 
VII. This, as will be seen in what follows, is typical of all the more important alluviated 
valleys of the Coast Ranges, and indicates clearly that the apparent intensity for such 
situations is a function of the underlying formations. 
On the more limited flood plain of the Mattole River at Petrolia, the destructive effects 
were even more intense than at Ferndale, and in marked contrast to those apparent in 
the few scattered houses on the rocky upland. But little can be inferred from this con- 
trast, since Petrolia is situated on the projection of the fault-trace, and only a few miles 
~ beyond the most northerly point to which it has been mapped. 
The town of Willets is at the headwaters of a branch of the Eel River on a flat allu- 
viated valley-bottom several miles in extent. The situation and character of the valley 
are such as to suggest that it is a filled-in lake basin. The ground-water below the 
valley-floor stands within a few feet of the surface. The town is 26 miles from the 
coast at Mendocino City, and not less than 30 miles from the fault-trace; yet the ap- 
parent intensity was not less than IX of the scale, or equal to that which prevails on 
the hard rocks in the zone, the distal border of which is usually not more than 6 miles 
from the fault-trace and often much less. Between Willets and the coast the intensity 
had diminished from X in the vicinity of the fault-trace to less than VII. This rapid 
rise from less than VII in the territory immediately to the west, to IX on the valley- 
floor, with no evidence of other factors intervening, and no evidence of similarly high 
intensity on the rocky slopes surrounding the valley, again indicates that the apparent 
intensity is a function of the character of the valley-floor. 
A similar condition prevails in the valley in which Ukiah is situated, 20 miles to the 
south of Ukiah. The physiographic features of the valley are described by Mr. George 
McGowan in his report describing the effects of the earthquake at Ukiah. The town is 
about 27 miles from the fault-trace, and in this interval the intensity had diminished 
from X to less than VII. In Ukiah, which is on the old flood plain of the Russian River, 
near the middle of the valley-floor, the intensity rose to between IX and VIII. Here 
again, there can be little doubt as to the influence of the underlying formations upon the 
destructive effects of the shock. This conclusion is supported by the time at which 
the shock was felt. Ukiah is one of the few places where satisfactory time observations 
were obtained. 
At the International Latitude Observatory, Dr. Townley reports that he was 
awakened by the shock and lookt at his watch, finding the time (corrected) to be 
5" 12" 30°, and he is of the opinion that the shock commenced at 5" 12" 17°. This 
accords fairly well with the time the shock was due at Ukiah, and affords no suggestion 
that the local high apparent intensity may have been due to a local earthquake. 
Another valley area of high intensity is on the west side of Clear Lake, extending 
from Kelseyville to Upper Lake. Lakeport, in the central portion of this area is 36 
Z 
