THE INFLUENCE OF THE FOUNDATION ON THE APPARENT INTENSITY. 55 
of transmission in alluvium would allow this condition to be satisfied even with a much 
smaller depth than was found in many parts of the large Californian valleys. 
Surface waves would also be a strong factor in causing damage on alluvium; these 
waves and the irregularities due to varying coefficients of elasticity are probably the prin- 
cipal causes of the increased damage on alluvial soils, even when of sufficient thickness 
to experience the accumulated amplitudes described above. 
The sides of the basins would exert no special influence except in their immediate 
neighborhood; but a certain amount of irregular reflection and refraction there would 
probably cause unusual intensity at some points. 
THE FOUNDATION COEFFICIENT. 
At Professor Lawson’s suggestion I have attempted to find some quantitative relation 
between the intensity of the shock on sands, marshy land, and solid rock. 
We shall first consider small basins within the limits of San Francisco. If we look at 
the profiles drawn by Mr. Wood and reproduced in map 18, we find the following esti- 
mates of the accelerations (a column has been added to the table giving the ratios of the 
accelerations on the various materials to that on the most solid rock. I have called 
this ratio the foundation coefficient) : 
TaBLeE 4.— The Foundation Coefficient (San Francisco). 





ee crerat j ite aes aioe ACCELERATION FounpDATIon 
mm, /sec.? COEFFICIENT. 
EF perpentine .. fail. da 250 1.0 
EF Made land 2 .« . ... 1,100 4.4 
EF Marsh . 5 cng actaaetr ra 3,000 12.0 
Sandstoneseea ee: 2) 250 to 600 1 to 2.4 
CD Made land .. . ~ . 2,900 11.6 
CD SANG epee weet eee 600 2.4 
CD SandstOnegmes sao os une 400 1.6 
AB Sand (Mission Valley) . 1,100 4.4 
AB Marsht= eros ca ee. 3,000 12.0 
AB Sundry solid rocks .. 250 1.0 

These observations are not all entirely independent; for instance, the marsh indi- 
cated in the first and last sections is the same marsh, as these two sections go thru it; 
and the first two sections cross on the sandstone of Telegraph Hill. The high intensity 
vat the southwestern extremity of section AB has not been considered because it is too near 
the fault; nor has the local strengthening of the intensity near the middle of section CD, 
which apparently is not to be explained merely by the nature of the terrane at these 
points. The very low intensity of only 250 millimeters per second per second on solid 
rock indicates a smaller intensity in San Francisco than further north, where we should 
have to go at least three times as far from the fault-line to find the intensity so low. 
The table, which gives only a very rough approximation to the coefficients, shows that 
the damage on small marshes may represent an acceleration as much as 12 times as great 
as on solid rock; on made land, from 4.4 to 11.6 times as great; on loose sand, from 2.4 
to 4.4; and on sandstone, from 1 to 2.4. Altho it has been well known that the apparent 
acceleration on soft land is much greater than on rock, the ratios obtained seem very 
much greater than had been suspected. 
The following table gives a list of places on alluvial soil in basins of considerable size, 
too large to be considered small basins, in the sense we have used that word. 
