The current article focuses on the results obtained from the analysis of seismic events recorded by a dense array located on a rock site at Argostoli in the Cephalonia Island of Greece. The objective of the study is threefold: (1) to explore to what extent the non-direct, diffracted surface waves influence the seismic wavefield at a rock site, (2) to investigate the loss of coherency of ground motions, and (3) to compare the results with those from a previously studied similar array located at an adjacent small, shallow sedimentary valley (soft-soil site). Both arrays consist of 21 velocimeters encompassing a central station in four concentric circles with diameters ranging from 10 to 180 m at the soft-soil and 20 to 360 m at the rock site. The seismic datasets under consideration include 40 or more events occurring around the site at epicentral distances up to 200 km having magnitudes ranging from 2 to 5. The seismic wavefields at both sites are analyzed by using the MUSIQUE algorithm: the backazimuth and slowness of the dominant incoming waves are extracted and the Love and Rayleigh waves are identified. Lagged coherency is estimated from the dataset, and the results are averaged for station pairs located at four separation distance intervals, 10-20, 20-30, 30-40 and 80-90 m. The results indicate that coherency at the rock site is, generally, larger than that from the softsoil, especially at frequencies below 5 Hz. At soil site, lower coherency is observed for pairs along the valley-width direction while no such directional dependence is observed at the rock. The wavefield analysis shows that whilst about 40-60% of the seismogram energy at the softsoil could be associated to diffracted surface waves (Love and Rayleigh) appearing mainly from the valley-width directions, only about 20% of energy at the rock site could be characterized as diffracted surface waves. Comparison with the widely-quoted parametric models reveals that the observed decay of coherency at both sites has little correlation with those from the models. These significant differences observed between the results of the rock and soil array indicate that the spatial incoherency is largely site dependent and is likely to be closely related with the formation of locally generated wavefield.

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