ISOSEISMALS: DISTRIBUTION OF APPARENT INTENSITY. 227 
of the effects which serve to define the lower grades were obliterated with the structures 
in which they were developed. Where buildings were sparsely distributed, it was often 
hard to determine what grade of intensity was developed, for the evidence was scattering 
and heterogeneous. Nevertheless, the map is a pretty faithful representation of the dis- 
tribution of intensity, and quite justifies the scientific and economic conclusions of a 
general nature that are drawn from it here. 
On the map, color in northwest-southeast bars (A, B, C, D, E) represents districts 
marked by unequivocal evidence. Continuous lines indicate the position of well-deter- 
mined boundaries between areas affected by different grades of intensity. Color applied 
in northeast-southwest bars (a, b, ¢, d, e) represents districts in which the evidence 
was scanty or circumstantial; and dotted lines indicate the position of boundary lines 
which were determined but vaguely by the phenomena in the field. 
DETAILED DESCRIPTION OF THE EVIDENCE BY LOCALITIES.’ 
No district designated upon the map as exhibiting intensity of Grade E, so far as the 
writer could find, exhibited any destructive effects of a more violent kind than the fall 
of chimneys. The really typical measure of intensity for these localities was the cracking 
and falling of plaster. Without exception, these are places where the firmly cemented 
bedrock of the Franciscan formations is either exposed directly or covered with a very 
thin mantle of soil. This lowest grade of intensity does not, by any means, characterize 
all places where the firm bedrock is exposed at the surface. It was rather developed on 
the summit portions of the rocky hills. The tops of Telegraph Hill and Russian Hill are 
districts in which a large part of the chimneys withstood the shock. This was also the 
case with the upper slopes of the chert hills about the head of Market Street, at the center 
of the area. Scarcely any injuries resulted on the hills of the Potrero; and one or two 
small serpentine hills just north of Market Street were likewise immune. Similarly, the 
Hunters Point serpentine ridge was subjected to a shock of low intensity; at least, a 
hasty survey pointed to this conclusion, tho the evidence was sparse and not thoroly 
examined. San Bruno Mountain, however, was about as near to the zone of faulting as 
Point Lobos, where most of the chimneys were thrown. Intensity of Grade D is believed, 
therefore, to have been developed upon the summit of San Bruno Mountain. 
The general fall of chimneys, slight cracking of brick work, and such damage, denoting 
intensity of Grade D, characterizes the northeastern half, or possibly two-thirds, of the 
city and county, except in localities where special conditions, chiefly lithological, modify 
it. Districts of exposed bedrock on the flanks of the hills, and of sand and alluvium 
wrapped as a thin mantle about their lower slopes, exhibited this degree of damage. 
Consequently a large area was affected by this grade of intensity which does not, in general, 
require detailed discussion; no violent nor specially significant effects being produced. 
Where, however, the loose earth covering is thicker, the magnitude and frequency 
of damage increases. Market Street, between Second and Fourth Streets; Mission Street, 
between First and Third Streets; and Howard Street, between Second and Third Streets, 
together with the blocks in the neighborhood of Market Street on Montgomery and 
Kearney Streets, Grant Avenue, Stockton and Powell Streets, form a district in which the 
effects denote an intensity only a little short of Grade C. A large proportion of the build- 
ings were excellent structures which individually withstood the shock well. In conse- 
quence, it was difficult to draw a line in this region between districts marked by broken 
chimneys and cracked brick walls, and those where more serious damage was certainly 
developed. The resistant character of the excellent buildings and the thoro obliteration 
by the fire of evidence produced in poor structures, render the determination of the 
intensity as Grade D somewhat doubtful. 


1 The streets referred to in these descriptions are shown on mp 20 of the atlas. 
