240 REPORT OF THE CALIFORNIA EARTHQUAKE COMMISSION. 
of its course is now occupied in large part by the railway tracks and structures of the 
Southern Pacific Company; and the exceptionally strong foundation necessarily pro- 
vided for the railway line probably explains why less damage was found here than one 
would at first have expected. At any rate, the greatest damage noted was the cracking 
of brick walls and the falling of cornices. . 
The space formerly occupied by Mission Bay has been partly filled to provide building 
sites, and of course the materials used in filling were deposited in water. The district 
is occupied in part by structures of great strength, such as railway tracks; in part it 
is devoid of buildings. Thruout the district, evidence was insufficient and inconclu- 
sive. Except near the former outlet of Mission Creek, and in the area further north 
formerly oceupied by the tidal marsh, the destruction produced does not denote inten- 
sity higher than Grade C. Apparently, therefore, land made by filling up spaces of open 
water is less dangerous, on the whole, than land made by depositing a thin rigid layer 
of filling upon a tract of marsh land. This, at least, is the lesson in San Francisco. The 
reasons for it are not very clear. Space forbids a discussion of theories which can not 
be adequately tested. It may be noted, however, that much of the material used in 
filling in areas of water has been broken rock derived from the grading down of neigh- 
boring rocky hills. 
Near the corner of Waller and Portola Streets, not far north of the head of Market 
Street, is a locality, less than a block in extent, where houses were shifted slightly on 
their foundations; their upper stories were moved farther eastward (downhill) than the 
foundations, as a result of shearing in the framework of the basement or of the first story 
of the buildings. (Plate 90p.) There also occurred minor bucklings and breaking of 
the thin asphalt pavement. The intensity, which belongs low in the range of Grade B, 
diminishes rapidly in all directions, and the district is surrounded by a band where the 
intensity is Grade C. Here a thin layer of sand reposes upon the slopes of a little upland 
valley between the low serpentine hills to the east and the high chert hills to the west. 
The effects are such as would be produced by a shaking downhill of this thin sand layer, 
with the structures which rest upon it. This seems the best explanation of high inten- 
sity in this district. Attention, nevertheless, must be directed to the fact that this, 
and three other districts shown on the map, No. 19, lie roughly along a straight line 
which nearly coincides with the western boundary of the serpentine body. At its north- 
west end, this boundary is known to be determined by a fault of considerable throw, 
constituting consequently a weak place in the crust of the earth here. It is not known 
how far southeast the fault extends, and it is not unlikely that it cuts entirely across the 
peninsula. The recurrence of these little districts of comparatively high intensity sug- 
gests that it continues as far south as Market Street, at least, and that such a zone of 
weakness was especially suited to the production of high intensity by the shock. This 
hypothesis can not be conclusively tested, but it is interesting and important enough 
to merit presentation and to receive attention in the event of future earthquakes. 
In support of the statement made in the foregoing pages that the intensity increases 
markedly as one approaches the fault, independently of the character of the ground 
and other factors, the following evidence is presented: 
Forty-eighth Avenue, between K and N Streets, is a district underlain by deep sand 
where extensive grading operations were undoubtedly necessary to convert an area of 
sand-dunes into streets and building lots. Here small, substantial frame dwellings were 
shifted bodily from 1 to 2 feet out of position, and the streets were slightly dislocated. 
Telegraph poles were thrown down or caused to lean over so much that only the tension 
of the wires kept them from falling completely, and lamp posts were overthrown. The 
dwellings suffered little structural damage, owing to their small, substantial character, 
and to their being built close to the ground; so that when shifted from their underpin- 
