Constraining site response and shallow geophysical structure by ambient noise measurements and 1D numerical simulations: the case of Grevena town (N. Greece)

摘要

A large number of earthquake studies using both empirical and theoretical approaches clearly depict the strong correlation of resulting damage and local geology. This correlation forms the scientific basis of the subsurface soil structure studies for site response evaluation, since adequate knowledge of the subsoil geotechnical and geophysical properties can lead to realistic seismic hazard estimation through appropriate modeling of strong seismic motion. In this framework, ambient noise analysis and one dimensional numerical simulation modeling have been performed for the town of Grevena (Northwestern Greece) in order to study both the subsurface soil structure, as well as its expected effect on seismic motions. The horizontal to vertical spectral ratio (HVSR) method was implemented on an almost uniform grid of 60 single station ambient noise measurements inside the urban area, while the noise array technique was applied at four selected sites of the study area. The HVSR curves show complex patterns, occasionally with double HVSR peak frequencies, with the higher one HVSR frequency showing a good correlation with the recent Holocene clay-dominant formation. Using these results, as well as information on surface geology and existing geotechnical data, a microzonation of the Grevena town is attempted, indicating zones of similar site response to ground motion. Numerical simulation of ambient noise with synthetic recordings suggests that the complex features observed in several HVSR curves could be attributed to the impedance contrast due to the quite shallow (Holocene clays to Pliocene–Pleistocene sands) and a deeper (Pliocene–Pleistocene sands to Oligocene–Miocene bedrock) sedimentary formation.