On Re-occurrence of SiteSpecific Response

The period and amplitude variations of local peaks in the Fourier amplitude spectra of free-field strong ground motion recorded at five stations in San Fernando Valley of metropolitan Los Angeles, California, are described, searching for peaks that reoccur during different earthquakes. The data suggest that some local peaks reoccur (about 50 % of the time), during shaking by small local earthquakes (peak ground velocities, vmax, 10–20 cm=s: During large strong motion amplitudes \u85vmax. 20 cm=s; these peaks are shifted towards longer periods (by nonlinear response of soils) or disappear. The data also suggest that densification and settlement of soil, minutes and hours following th