346 REPORT OF THE CALIFORNIA EARTHQUAKE COMMISSION. 
Another circumstance which weakens the suggestion of a local earthquake is the 
failure of the isoseismals below VII to carry out the suggestion by bulging into the Coast 
Ranges on the west or the flanks of the Sierra Nevada on the east. In the latitude of 
Los Banos, the high apparent intensity was confined to the valley-floor; altho farther 
south, near Cantua, this can not be affirmed. In view of the experiments of Professor 
Rogers, it seems probable that in the near future, by an active prosecution of such 
experiments coupled with close field observation, we shall arrive at an arithmetical 
expression for the coefficient which will enable us to reduce the apparent intensity of 
water-saturated alluvium to the true intensity due to vibration in homogeneous elastic 
rock. When that coefficient becomes available, it will perhaps be possible to determine 
whether or not the destructive effects exemplified in the San Joaquin Valley at Los 
Banos are referable to the conditions of the ground or to a local seismic disturbance. 
Until then the question must remain an open one. Analogy with other valley lands 
nearer the fault, where high apparent intensities are referable, both on the field evidence 
and in the light of Professor Rogers’ experiments, to local conditions, militates against 
the hypothesis of a quasi-independent earthquake. The remoteness of the region from 
the known fault and the high intensities on the flanks of the Coast Ranges indicated by 
the new landslides at Cantua, favor that hypothesis; but no positive conclusion can 
be reached at present. 
RELATION OF APPARENT INTENSITY TO KNOWN FAULTS. 
Altho the geology of California has been studied in detail at but few localities out- 
side of the gold belt of the Sierra Nevada, yet the general reconnaissance work that 
has been done by various geologists has brought to light many of the important faults 
in the state. Such as are known are indicated on map No. 1, without any attempt 
to discriminate between the varying degrees of certainty with which their existence has 
been determined. The map serves the double purpose of bringing together for the firs! 
time our knowledge of. the distribution of faults thruout the state, and of illuminat+ 
ing a brief discussion of the relation of apparent intensity to fault-lines. On 4 of these 
faults there have occurred 5 severely destructive earthquakes within the last 50 yeers. 
It thus behooves students of Californian seismology to become familiar with these st:uc- 
tural features of the state. A recent account of the Calabrian earthquake of September 
8, 1905, dealing particularly with the distribution of intensity,’ and the relation of that 
distribution to fault-lines known or inferred, gives an especial interest to the considera- 
tion of the faults of the Californian region at this time. In the preceding section of this 
report, it has been shown very definitely that abnormally high apparent intensities were 
developed on the valley-bottoms, and the cause of this has been referred in « general 
way to the incoherent and water-saturated condition of the materials underlying these 
valley-bottoms. In Calabria, in the account referred to, Professor Hobbs correlates 
the zones of exceptionally high intensity with lines of ancient faults, whih in some 
portions of the region are known on geological evidence to exist, and in others are sup- 
posed to exist because of the high intensities manifested. He does not recognize the 
character of the underlying formations as an important factor in producing different 
degrees of intensity, as inferred from destructive effects at the surface. in this respect 
his conclusions do not harmonize with those arrived at in the study of the California 
earthquake of April 18, 1906. It thus becomes a matter of interest to ascertain what, 
if any, influence was exercised by the known faults of California, other than that which 
was the seat of disturbance, upon the distribution of apparent intensiy, independently 

*W. H. Hobbs, The Geotectonic and Geodynamie Aspects of Calabria end. Northern Sicily. 
Leipzig, 1907. 
