High frequency response of a sediment-filled valley for seismic waves

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Time-domain responses as well as frequency-domain responses of a sediment-filled valley for incident SH waves are studied with the emphasis of site responses for high frequencies(>1 Hz)which are of interest in earthquake engineering. We calculate them using a boundary integral equation method in which Green's functions are evaluated using the discrete wave number method. Calculating frequency-domain responses as well as time-domain responses of the sediment-filled valley, we discuss in details effects of inclined interfaces of the valley upon wave amplification patterns, propagation of Love waves and duration of the ground shaking in the valley.Some spectral ratios of surface ground motion to bedrock motion are also calculated based on the acceleration responses at several sites due to an artificial earthquake ground input motion which has relatively high predominant frequencies and they are compared with spectral ratios of the flat layer response to bedrock motion. It is shown that since Love waves generated by the inclined interface have a great influence on the responses of the valley, spectral ratios of sediment-filled valley become more broad-band than that of the flat layer response.Propagation characteristics of harmonic waves by subsurface irregularities is studied experimentally with the detailed measurements of a surface ground motion generated by forced vibration of a reinforced concrete model footing placed on a subsurface irregular ground. Each of three different footings was excited in three kinds of directions to generate different patterns of wave propagation. We evaluated particle orbits, coherence function and phase information from the cross-power spectrum between adjacent sites and constructed phase velocitv dispersion curves to investigate wave propagation characteristics on subsurface irregular ground. It is shown that dispersion data measured on less irregular subsurface ground are in good agreement with the theoretical values.