RELEVANCE OF H/V SPECTRAL RATIO TECHNIQUE FOR BUCHAREST CITY

摘要

We investigate the validity and relevance of the H/V spectral ratio technique in the particular case of Bucharest City which was strongly affected by the 4 March 1977 (M_w = 7.4) damaging earthquake. Our study shows that the coincidence of the predominant frequency response of the entire succession of the Quaternary unconsolidated deposits above the Fratesti layers (f approx. 0.7 Hz) underlying the Bucharest urban area with the main frequency component in the Vrancea seismic source spectrum for the largest shocks (M_w > 7) is critical in controlling the strong ground motion properties. When applied to noise data, the H/V spectral technique reproduces the predominant frequency for the sedimentary cover beneath the city and the relatively uniform distribution of this structure over the city's area. An analysis of data on earthquakes of different sizes shows that the resonance response of the sedimentary cover becomes effective only for major Vrancea shocks and is practically negligible for events below magnitude 7. Consequently, we recommend avoiding any extrapolations from small and moderate earthquakes to large shocks when making the seismic microzonation map for strong Vrancea earthquakes in the Bucharest area.