Array analysis of the ground velocities and accelerations from the 1971 San Fernando, California, earthquake

Abstract Profiles of ground velocity and acceleration, displayed as a function of epicentral distance, are analyzed for recordings of the 1971 San Fernando earthquake. Three long profiles (50 km) and three short profiles (2 km) are studied. Although there is considerable variation in waveforms and peak amplitudes observed along the long profiles, there are also many examples of coherent phases seen on adjacent stations. There are striking differences in the amplitudes and durations of ground velocity observed at stations located on hard rock sites as opposed to stations located within the large sedimentary basins of the Los Angeles area. Furthermore, the San Fernando Basin, which is adjacent to the source area, seems to respond quite differently from the Los Angeles Basin which is about 30 km from the earthquake source area. Ground acceleration profiles, however, show that there is no corresponding change in the duration or amplitude of high-frequency shaking with site characteristics. We infer that the excitation of surface waves within sedimentary basins is the reason that large peak velocities and displacements are observed for soft sites. The ground velocity waveforms are nearly identical along the three short profiles, which are all located within the Los Angeles Basin. Greater variation of waveforms and amplitudes are seen for ground acceleration along these short profiles, although strong phase coherence is still observed.