H/V Spectral Ratios Technique Application in the City of Bucharest: Can We Get Rid of Source Effect?

The main issue of this paper is to show that contrary to many examples of monitored strong earthquakes in different urban areas, the intensity and spectral characteristics of the strong ground motion induced in Bucharest area, by Vrancea intermediate-depth earthquakes, is controlled by the coupled source-site properties rather than by the local site conditions alone. Our results have important implications on the strategy to follow when assessing the seismic microzoning for Bucharest city: we recommend the application of deterministic approaches rather than empirical techniques, like H/V spectral ratios. However, when applied to noise data, the H/V spectral technique succeeds to reproduce the predominant frequency response characteristic for the sedimentary cover beneath the city and the relatively uniform distribution of this structure over the city area. The same technique is clearly inadequate when small earthquakes are considered and our results strongly disagree with any strategy of extrapolation from small and moderate earthquakes to strong earthquakes for microzoning purposes.