[en] A theoretical approach is used to show the importance of differences in earthquake-generating mechanisms. Data from underground nuclear explosions show the importance of slight variations in travel path. The influence of the site is studied using two approaches. The first uses data recorded at the same site during a number of different earthquakes. The second fixes the earthquake, in this case the San Fernando earthquake, and examines all sites that are one-sigma sites to see if there is a common generic site type that significantly amplifies the ground motion. It is concluded that generally fixing the site does not reduce the variation of the data. At site-resonant frequencies, the spectral response is greater than envelope spectra of Regulatory Guide 1.60. Small near-field earthquakes have significantly greater high-frequency content than is cntained in Regulatory Guide 1.60. No generic site condition could be determined that consistently amplified the ground motion from the San Fernando earthquake