DISCRETE-TIME ANALYSIS OF SEISMIC SITE AMPLIFICATION

A discrete-time analysis of seismic site amplification in layered medi a for vertically propagating shear waves is presented. The discrete-ti me approach not only is simpler and more accurate than the previously suggested frequency-domain techniques but also provides better physica l insight into the problem. The discrete-time domain equations of wave propagation can be derived by using the upgoing and downgoing waves a s auxiliary variables, and require only three parameters for each laye r: the reflection coefficient at the bottom of the layer, the travel t ime of waves in the layer, and the damping. The discrete-time formulat ion results in simple analytical models for site amplification. When c ompared with the previous ones, these new models have important advant ages: (1) They use the physical parameters of the site; (2) for a sing le soil layer over bedrock subjected to vertically propagating waves, the model is exact; (3) amplitudes and phase characteristics of the si te are accounted for in the models; (4) the models can incorporate mul tiple reflections in the layer; and (5) the models result in simple ti me-domain filters for simulating ground motions. They also provide con venient tools to develop random vibration models, generate site-specif ic response spectra, and identify site characteristics from recorded m otions.