AbstractWhile the accurate estimation of ground‐motion amplitudes across the entire frequency band of engineering interest is not possible at the present time, the excitation and propagation of long‐period strong‐ground motion can be understood with existing seismological methodology. In the Los Angeles Basin, the long‐period strong ground motion excited by the San Fernando earthquake is dominated by the presence of surface waves, whose gross amplitude and frequency content are easily attributable to physical properties of the earthquake source and source‐station propagation paths. Observed measures of the long‐period strong ground motion of the Kern County earthquake relative to the San Fernando earthquake at two sites in the Los Angeles Basin which recorded both shocks can be predicted with considerable accuracy by a simple earthquake source model. This source model is extrapolated to represent the maximum credible earthquake likely to affect the Los Angeles area, taken to be a repeat of the Fort Tejon (1857) earthquake along the San Andreas fault. The measures of long‐period strong ground motion in the Los Angeles Basin estimated for it agree well with the comparable measures of Earthquake A‐2, intended to represent the same situation. For the purpose of aseismic design of long‐period structures, Earthquake A‐2 is a reasonable, if not all inclusive, estimate of the long‐period strong ground motion in the Los Angeles Basin generated by a magnitude 8+ earthquake along the San Andreas fault north an
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