Effects of Topography on Ground-Motion Recordings at Short Wavelengths: The Case of a Cliff in Cephalonia, Western Greece

Reliable ground-motion measurements are essential for seismic hazard assessment and require seismological stations to be installed in free-field conditions, away from structural interferences. However, global network analyses show diverse installation configurations that can affect measurements. Although topographic effects are natural, their influence can extend over several meters, making them closely related to operators’ installation choices. Although summit effects are well studied and known to cause strong amplifications, the impact of cliffs or escarpments remains less explored. In this study, 16 SmartSolo IGU-16HR 3C nodes were placed on either side of a 30-m cliff located in Cephalonia (western Greece), for 4–10 months, recording 307 low-to-moderate-magnitude seismic events. The main results reveal that de-amplifications recorded at the cliff base are greater than amplifications determined at the cliff top, particularly between 5 and 15 Hz. This study shows that amplification and de-amplification are primarily driven by topography, whereas their directional dependence is likely shaped by both topographic and lithologic factors. Furthermore, the anisotropy related to cliff fracturing interacts with the effects of source back azimuth. Amplification and de-amplification can be correctly predicted using the frequency-scaled curvature (FSC) and illuminated FSC proxies, provided that an exponential functional form is used for determining de-amplifications at the base of near-vertical cliffs. Furthermore, the azimuthal dependence observed, inducing strong de-amplifications depending on the direction observed, seems related more to the anisotropy of geologic formations than to the cliff topography itself. These effects should be considered when using data from stations in similar topographic settings. Neglecting them can bias ground-motion models and underestimate earthquake magnitudes, particularly for stations at the base of cliffs. From these considerations, we strongly recommend installing future seismological stations a few tens of meters away from the bases and ridges of even small cliffs or escarpments.