An attenuation formula was derived for the Arias instrumental intensity on bedrock based, in part, on the source spectrum function obtained by Aki. The constants in the formula were calibrated for the San Fernando earthquake by using eight bedrock spectra derived from surface accelerograms. The calibrated formula was then used to compute the Arias bedrock intensities at most of the sites of the ground-level accelerographs and seismoscopes that recorded the earthquake. Maxima accelerations from accelerograms and spectral accelerations from seismoscope records were then used to compute the Arias intensities at the surface by using an empirical relation obtained by Arias. After the instrument sites were classified into four groups, (1) crystalline rock, (2) sedimentary rock, (3) shallow alluvium, and (4) deep alluvium, surface-to-bedrock intensity ratios were correlated with these site classifications which leads to four surface attenuation curves constrained to have the same slope. From the constant differences between these curves, it is possible to define site factors to be applied to bedrock intensity in order to estimate surface intensity for zoning purposes. Conversely, these factors can be applied to surface intensities for deriving bedrock attenuation curves for other earthquakes in which the geology of the instrument sites is only generally known. The site factors relative to unweathered, unfractured crystalline rock outcrops are 1.80, 3.63, 3.74, and 5.12 for classifications 1 through 4, respectively.