ISOSEISMALS: DISTRIBUTION OF APPARENT INTENSITY. 241 
ning, they had but a few inches to fall. Still, it is the opinion of the writer that the in- 
tensity developed here was little, if any, short of the maximum on the made land in the 
city, tho the conditions were not such as to permit so great damage. 
On Ocean Avenue and X Street, near where the former outlet of Lake Merced flowed, 
fissures were developed in the street and in the sands on either side, and water was 
squeezed out so as partly to flood the roadway. Drain pipes were unearthed and bent 
or twisted. 
From the former outlet of Lake Merced, where W Street meets the Grand Ocean 
Boulevard, or Great Highway, southward along the ocean, low cliffs of soft rock — the 
Merced sandstones and shales — rise abruptly from the beach. These mount gradually 
as we go southward, until at Mussel Rock they attain a height of 500 feet. All along 
this line of cliffs, and for a short undetermined distance inland, the rock masses were 
cracked, broken, and traversed by narrow fissures. These effects grow more and more 
numerous and of greater and greater magnitude until, a short distance north of Mussel 
Rock, the fault is reached. A short distance south of X Street, a small, substantial 
frame dwelling, built upon a good foundation under the cliffs by the beach, was almost 
overturned. South of this there were no structures along the beach except the seaward 
end of the Lake Merced Tunnel, an hydraulic arch which was slightly broken, tho em- 
bedded in the rocks of the Merced formation. All along the faces of these cliffs, much 
material fell or slid down to the beach. 
CONCLUSIONS. 
This investigation has clearly demonstrated that the amount of damage produced 
by the earthquake of April 18 in different parts of the city and county of San Francisco 
depended chiefly upon the geological character of the ground. Where the surface was 
of solid rock, the shock produced little damage; whereas upon made land great violence 
was manifested. Other things being equal, there was a decrease of intensity from the 
southwest toward the northeast, as the distance from the zone of faulting increased. 
Other conditions, however, exerted a controlling influence. There was, for instance, 
much greater contrast, in the destructive effects produced, between the summit of Tele- 
graph Hill and the vicinity of the Ferry Building, about a quarter of a mile apart and 
at practically the same distance from the fault, than there was between the damage 
produced near the Ferry Building and along the trace of the fault itself. (Consult the 
intensity map and profiles.) In this part of the zone of destruction, change in distance 
from the fault clearly did not influence the gradation in intensity, so much as did change 
in the character of the ground. 
ADDENDA. 
Subsidence of made land.—The unstable character of the made land on the water- 
front of San Francisco has long been known. This instability made itself manifest in 
a progressive subsidence which, in the course of years, rendered it quite difficult to main- 
tain the grade of the streets. An effort was made by Mr. C. E. Grunsky, when he was 
city engineer, to determine the rate of this subsidence, and the following extract from 
his report as city engineer for the year 1902-3 is not without interest in connection with 
the violent disturbance of the ground caused by the earthquake in the areas of made 
land: 
Examination has again been made to determine the amount of sinking in those 
improved portions of the city where subsidence has heretofore been observed. The result 
of this examination appears from the following table, in which is also given the subsidence 
which occurred during the preceding year. 
R 
