Site characterisation in Durrës (Albania) from a seismic microzonation perspective

Twenty-nine single-station noise measurements, processed through the Horizontal to Vertical Spectral Ratio (HVSR) technique, a Multichannel Analysis of Surface Waves (MASW) survey, and two 2D array measurements were performed in Durrës to provide useful elements for a geophysical subsoil characterisation. Results from noise measurements defined a zone eastwards of the historical centre, where the characteristics of shallow soil layers are responsible for modification to the seismic response. In particular, HVSR curves in this area showed amplitude higher than 3.4 at a period higher than 1 s. From the analysis of the collected data, four different groups of HVSR curves were recognised, whereas noise measurements were also used to retrieve a preliminary bedrock depth map. These results are potentially useful for correlating construction typologies and vibration period of the buildings with the site amplification and with the damages observed during the seismic sequence for which the highest magnitude event was the M_W 6.2, 26 November 2019 earthquake. Moreover, two shear waves velocity V_S profiles were obtained from a joint inversion of Rayleigh curves with ellipticity. In one case, it revealed the presence of a soil of very poor mechanical properties, which was classified as D soil according to the international codes (EC8, NEHRP); in the other case, the V_S profile allowed us to retrieve important information about the seismic bedrock to be used in numerical modelling for the third level of seismic microzonation.