Ground motion amplifications for Bucharest based on 3D geological model and assigned geophysical properties

The evaluation of seismic hazard at local scale, with the contribution of strong-motion data from a dense seismic network and insightful geological and geophysical data, is one of the key components in seismic risk mitigation. Significant efforts were made to record and predict the highly variable peak spectral amplification values of strong seismic motion in Bucharest, capital city of Romania, especially after the 4 March 1977 Vrancea earthquake with a moment-magnitude of 7.4, which resulted in 1424 victims in the city (90% out of the national casualties). Using a recently compiled geological database, which relies mostly on several hundreds of borehole measurements performed for the subway in Bucharest and a recent DEM for the area, this study establishes the positions of the main seven Quaternary layers beneath the city. In this paper we review studies referring to shear wave velocity (𝑉𝑆) measurements in the area of Bucharest – as key input for seismic site amplification models and microzonation maps, selecting and reprocessing some data in order to obtain a homogenized database. This contains mean weighted 𝑉𝑆 values for the uppermost 30 m, 50 m, 70 m and 100 m depth intervals. By mapping and interpreting the newly assembled geological model, as well as the assigned geophysical values (shearwave velocity), we begin to compute the spectral amplification values at surface, using the data recorded at the earthquake from 27.10.2004 at surface and in the depth. The spectral modelling is applied to deeper models than the uppermost 30 m, and considering the 50 m, 70 m and 100 m depth intervals, where we have now an important database for weighted mean shearwave velocities (𝑉𝑆) in the depth. The results attested the importance of this action and we present solid results that the values computed for the deeper models are closer to the computed surface values, especially for the depth intervals of 70 and 100 m depth. The spectral acceleration values as well as the PGA computed at surface are based first on new database of geological model and assigned geological emerging in the last years, as well as on an extended measuring of the strong motion values of an earthquake at surface and also in the depth down to 100 m depth.