220 REPORT OF THE CALIFORNIA EARTHQUAKE COMMISSION. 
DISTRIBUTION OF APPARENT INTENSITY IN SAN FRANCISCO. 
By H. O. Woop. 
INTRODUCTION. 
In presenting the results of this study, the subject-matter has been taken up as follows: 
First, brief mention is made of the physiographic features of the city. Map No. 4, of the 
atlas accompanying the report of the Commission, shows the location of the city and its 
physiographic environment, also a segment of the Rift and of the fault on which the earth- 
quake of 1906 was generated, and the position of a similar fault where the shock of 1868 
originated. The city lies between these two zones of faulting. Then follows a note on 
the general geology of the region, illustrated by a geological map, No. 17 of atlas, prepared 
by Professor Andrew C. Lawson, on which is shown the areal distribution of the more 
important rock formations and of the districts of ‘‘made” land. Then comes the de- 
scription and classification of typical destructive effects examined in the field. An 
intensity scale is discust, and its relationships to the Rossi-Forel and Omori scales are 
determined as well as possible. By critical comparison with Omori’s scale, approximate 
values are fixed for the grades in terms of acceleration. - Illustrating this discussion, 
map No. 19 of the atlas, showing the areal distribution of intensity in terms of an 
especially devised scale, presents graphically the results of the investigations in the city. 
The methods employed in the preparation of the map are set forth; also the manner in 
which the intensity scale was utilized. In map No. 18 are shown several geological cross- 
sections with corresponding intensity profiles. As vertical codrdinates of the latter, 
values of the grades determined approximately in terms of acceleration were utilized. 
Following the general discussion of the intensity is a detailed description of the evi- 
dence which characterized various localities and determined the intensity grades ascribed 
to them. 
Next are discust details of evidence in the localities where very high intensity prevailed, 
which are of general interest owing to the suggestions they offer, the problems they raise, 
or the warnings they proclaim. 
PHYSIOGRAPHY. 
The San Francisco peninsula rises with bold relief from the level of the sea to hill sum- 
mits varying in altitude between 100 and 1,800 feet, with the broad Pacific to the west of 
it, the waters of San Francisco Bay to the east, and the Golden Gate on the north. South- 
ward, trending slightly east, the peninsula runs for several miles, merging finally with the 
hills of the Santa Cruz Range which mark the eastern limits of the Santa Clara Valley. 
On the western shore, promontories such as Point Lobos, Mussel Rock, San Pedro Point, 
and Montara Point, where rock-cliffs rise out of the surf, alternate with stretches of smooth 
beach line. At the north, the hills come down to the shore, forming rocky points: Point 
Lobos, where the Cliff House stands; Fort Point, marking the narrowest part of the 
Golden Gate; and the minor promontories of Black Point and Telegraph Hill farther 
east. The eastern shore is marked by prominent rock ridges extending out into the Bay, 
while between these, reaching well back into the hills, are sharply limited valleys cut 
down to the level of the sea and filled with deep deposits of alluvium, thus forming a 
gently sloping floor from which the hills rise abruptly. Before the building of the city, 
tide marshes with their little tidal creeks occupied the floors of these valleys, near their 
mouths. 
