ISOSEISMALS: DISTRIBUTION OF APPARENT INTENSITY. 251 
Lake Merced. — About 6 miles north of Mussel Rock, where the Merced beds disappear 
under eeolian sands, the disturbance seems to have been quite violent. An old railroad 
trestle, that crosses the northern end of Lake Merced in the narrowest place, was badly 
wrecked. This bridge was broken in two places, and the intermediate piece was out of 
line with both-ends. The direction of the offsets was very nearly due north and south. 
At one break the west piece was shoved 12 or 14 feet past the other section. The west 
end of the intermediate piece failed to join the section at the west bank by 6 or 7 feet. 
The west section that remained with the bank was from 4 to 5 feet lower vertically than 
the intermediate piece. The trestle was old, built of heavy timbers on a sharp curve, 
and not in use, which will in part account for its destruction. The swaying of this bridge 
destroyed a section of it 50 to 60 feet long. On the hillside where this trestle reaches 
the west bank of the lake, cracks parallel to the shore line suggest the cause of the destruc- 
tion of the bridge. The displacements here are larger than any along the main fault- 
line, and it is apparently entirely local, due to the slipping and settling of the west bank 
of the lake. 
Upon following around the north end of the lake to the road that runs to the Life 
Saving Station, a line of terra-cotta pipe, about 8 inches in diameter, was found. There 
was no large displacement found in this pipe, altho it had been cracked at many points. 
There is nothing in these phenomena to show that there was afault thru the Merced Valley. 
Just south of the bridge across Lake Merced, a sand-bar was forced up out of the lake, 
from water that was previously 6 or 8 feet deep. This bar is parallel to the west bank 
of the lake, and has a direction almost due north and south. This was probably caused 
by the same thing that wrecked the bridge; that is, the displacement and settling of the 
west bank of the lake at the time of the earthquake. 
THE AREA SOUTHWEST OF THE SAN ANDREAS FAULT. 
Difference of apparent intensity on the two sides of the fault. — On passing from the beds 
of the Merced series on the northeast to the southwest side of the fault, there is a marked 
difference between the distribution of small cracks and little earthslides. On the north- 
east side, in the Merced beds, these cracks and landslides are common, but on the south- 
west side they are entirely absent. This can hardly be taken to show that there was any 
difference in intensity on the two sides of the fault; it is probably the result of the differ- 
ence in the character and stability of the rocks. At other places north of the fault, but 
southeast of the Merced beds, this difference has not been noticed, probably because in 
that part of the area the rocks are nearly all Franciscan. 
On the south side of the San Andreas fault there are no towns affording an opportunity 
for judging the intensity of the shock. The gradation must of necessity be based upon 
something else. Landslides occur both near the fault-zone and at a distance from it, 
under somewhat similar geologic and topographic conditions. It seems to be a fair 
assumption that a landslide is indicative of a high intensity. 
Laguna Salada Valley. — In the valley of Laguna Salada, the Ocean Shore Railroad 
had a temporary trestle erected for making a fill in the valley up to required grade. 
This trestle was twisted and thrown out of line, and the earth sank along the newly filled 
roadbed. Similar things happened to newly filled roadbeds along the west edge of the 
Santa Clara Valley, near Baden and San Bruno. 
Along the base of the cliffs south of Laguna Salada, there were several small slides, 
some from the face of the hills and others in the newly graded roadbed. There were 
many small cracks along the tops of the cliff, parallel to its edge, showing that the face 
of the bluff was shattered, and that more earth might slide. One big rock pinnacle, 
which had been left above the roadbed as a landmark, and which had seemed a little 
dangerous before, was shaken down, 
