Seismic Microzonation Methods Accounting for Nonlinear Soil Response

The features of soil behavior in the near-fault zones of large earthquakes are discussed that should be accounted for in seismic microzonation, i.e., in evaluation of soil response in future strong earthquakes. We describe (1) the development of seismic zonation maps in Russia, USA, and Japan, (2) regional databases for ground motion estimation, (3) significant parameters of soil profiles, (4) the behavior of soils in the near-fault zones, (5) nonlinear damping mechanisms that attenuate motion at high and medium frequencies and only weakly affect low frequencies, (6) changes of elastic moduli of soft and dense soils during strong motion, (7) the influence of saturation of soils with underground water on characteristics of motion on the surface, (8) resonant and nonlinear phenomena in soil layers, (9) the influence of directivity effects in extended sources of large earthquakes on soil response and generation of high PGA in the near-fault zones. Considering these features, we conclude that seismic microzonation should be based on the calculation of acceleration time histories of future strong earthquakes, accounting for the soil response, as suggested by Aki and Irikura (1991). We should compute time series for given source-receiver pairs using existing methods and our current knowledge on the earthquake sources, propagation paths, and site conditions. Only complete data on the composition and physical parameters of soil layers, combined with adequate methods for calculation of soil response provide us with reliable estimates of the parameters of motion on the surface.