Topographic irregularities such as hills affect the amplification characteristics of strong ground motion. Due to the interference between direct waves and scattered waves, the behavior of seismic waves passing through irregular terrain is complex. In this study, records of seismic ground motion were compared to simulated seismic waves, calculated from 3-dimensional FEM. The study objectives were to build an analytical model capable of accurately reproducing the observed data and to evaluate the amplification characteristics of strong ground motion in areas characterized by irregular topography.

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Amplitude variations of the strong-motion response-spectra are evaluated using 275 pairs of acceleration records observed at the strong-motion sites in the South Tohoku and the Kanto regions. Each pair of records consists of those observed at the same site due to the different two events of which magnitudes and hypocentral distances are almost the same. In the analysis, the amplitude variation (AV) is assumed to be summation of two types of variations: component- dependent variation (CDV) and event-dependent variation (EDV). To evaluate these variations separately, random effects model with the two statistical elements is applied. The analysis using above all pairs of data gives the logarithmic standard deviations (LSD) of 0.23, 0.43 and 0.49 for CDV, EDV and AV, respectively, in the short period range. LSDs for EDV and AV decrease to 0.38 and 0.44, respectively, when the data pairs from the sources in the different epicentral areas are excluded. This indicates that the strong motions from different epicentral regions show relatively large amplitude differences even if their magnitudes and distances are almost the same. Combining our results with the amplitude variations estimated by Nakamura and Yasiro (2000) enables to resolve EDV into the variation due to source radiations (SRV) and the variation due to rupture directivities (RDV). In the short period range, LSDs for EDV and SRV are evaluated as 0.32 and 0.20, suggesting that variation of the seismic source radiations such as stress-drop differences is more significant than that of rupture directivities.

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