Ground-motion amplitude across ridges

Local amplification and wave diffraction on an elongated ridge near Sourpi in central Greece were studied by the analysis of seismic records of local and regional earthquakes. Data were obtained during field work especially designed for this purpose. These data were analyzed in the frequency and time domains. In the frequency domain, spectral ratios show amplifications of 1.5 to 3 at the ridge top relative to the base of the ridge. The horizontal components of motion are more amplified than the vertical component and the observed spectral ratios seem stable for different earthquake locations. Theoretical spectral ratios, calculated by the indirect boundary element method, are dependent on earthquake location but are in general agreement with the observed spectral ratios. Another dataset, from Mont St. Eynard in the French Alps, showed similar characteristics with spectral amplitudes on the top of the ridge up to four times those on the flank. These relative amplifications are within the range predicted by numerical simulations. The numerical simulations also show that the topographic effect involves the emission of diffracted waves propagating from the top toward the base of the ridge. The use of a seven-station array on the ridge at Sourpi made it possible to identify such waves. The analysis was performed with wave separation methods using singular value decomposition and spectral matrix filtering. Our results show agreement between experimental data and theoretical results supporting the use of numerical simulations for estimation of purely topography-induced amplification on ridge tops. Our results also show that such amplification is moderate for the ridges under study.

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