On the role of stiff soil deposits on seismic ground shaking in western Liguria, Italy: Evidences from past earthquakes and site response

In this work, we analyze the macroseismic fields of the main historical earthquakes occurred in western Liguria (northwestern Italy) in order to identify possible relations between the isoseismal patterns and the local geological setting. Anomalies in the spatial distribution of macroseismic intensities are often attributed to site effects. We observed that, in the region of study, such anomalies are mainly located in areas characterized by outcropping Pliocene soil deposits, mainly made up of stiff silty and marly clays. To investigate the influence of such deposits on ground response, both horizontal-to-vertical spectral ratio measurements and numerical site response analyses were carried out. For all sites analyzed, the spectral ratio curves show marked amplification peaks between 2 and 5 Hz. Following such empirical evidence, equivalent linear ground response analyses were performed in order to examine the relation between the measured resonance frequencies and the response of a set of soil columns presenting Pliocene marly-clayey deposits at their top. To this end, a database of geotechnical and geophysical properties, including > 50 reference soil profiles, was compiled. This database was used to define alternative numerical models considering three different ranges of thickness of the Pliocene marly-clayey deposits (between 5 and 35 m, 30 and 65 m, 60 and 105 m). Results show marked amplification effects in the ranges 1-3 Hz, 2-5 Hz, and 3-10 Hz depending on the thickness of the investigated soil deposits. These frequency ranges are in agreement with those observed from the experimental measurements, which are therefore proved adequately representative of the resonance frequencies of the Pliocene marly-clayey deposits. Therefore, based on our findings, such deposits can be considered an essential factor in producing the “anomalies” observed in the isoseismal patterns of the macroseismic fields of the historical earthquakes occurred in western Liguria.