Seismic microzonation of the broader Chania basin area (Southern Greece) from the joint evaluation of ambient noise and earthquake recordings

In the present work, a combination of single station and multi-sensors array measurements of ambient noise have been performed in the urban area of Chania and its southern basin, in order to determine the main subsurface geophysical model properties and assess the local site-effect on seismic motions. Furthermore, Horizontal to Vertical Spectral Ratio on Earthquake recordings (HVSRE) has been employed to validate the ambient noise results. Simulation of ambient seismic noise has been also used in an effort to cross-validate the experimental methods applied. The results indicate a highly-varying subsurface model due to the complex geological setting. Horizontal to Vertical Spectral Ratio (HVSR) reveals a low frequency resonance peak associated with the seismic contrast of the overall Quaternary/Neogene sediment cover with the underlying bedrock. A second higher HVSR frequency peak, usually disregarded in standard HVSR analysis, is observed for most HVSR data. This peak is found to be associated with the strong velocity contrast of the low velocity Quaternary sediments and the underlying stiffer Neogene deposits, using both empirical correlations and appropriate modelling. The available array measurements verify the clear separation of these Quaternary deposits and the underlying Neogene sediments in the southern Chania basin, as well as the more loose surficial Neogene formations and the underlying stiffer layers within the main city complex. Simulated ambient noise signals processed with the same HVSR settings verify the findings of the experimental approaches. Earthquake recordings processed with the HVSRE method show a good agreement with the expected amplification pattern of the final geological-geophysical structural model of the broader Chania area.