Local amplification and wave diffraction on an elongated ridge near So
urpi in central Greece were studied by the analysis of seismic records
of local and regional earthquakes. Data were obtained during field wo
rk especially designed for this purpose. These data were analyzed in t
he frequency and time domains. In the frequency domain, spectral ratio
s show amplifications of 1.5 to 3 at the ridge top relative to the bas
e of the ridge. The horizontal components of motion are more amplified
than the vertical component and the observed spectral ratios seem sta
ble for different earthquake locations. Theoretical spectral ratios, c
alculated by the indirect boundary element method, are dependent on ea
rthquake location but are in general agreement with the observed spect
ral 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 amplific
ations are within the range predicted by numerical simulations. The nu
merical 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 w
ith wave separation methods using singular value decomposition and spe
ctral matrix filtering. Our results show agreement between experimenta
l data and theoretical results supporting the use of numerical simulat
ions for estimation of purely topography-induced amplification on ridg
e tops. Our results also show that such amplification is moderate for
the ridges under study.