In the paper "Bedrock Intensity Attenuation and Site Factors from San Fernando
Earthquake Records," by K. W. Campbell and C. Martin Duke, which appeared in the
February, 1974 issue of the Bulletin, a correlation is made between a measure of intensity
of recorded ground accelerations and four categories of soil, classified by increasing
softness. The authors present different numerical site factors for each category and conclude
that their results are sufficiently reliable to justify their use for purposes of zoning.
The site factors and the conclusion are based mainly on the data presented in their Figure
5 and the lines of specified slope fitted by least squares to the data points. The set of lines
in their Figure 6 present the authors' idealized relation between intensity, softness of
ground, and distance from the source. The upper four lines in Figure 6 are taken from
Figure 5
