Bucharest, the capital of Romania, is affected by strong intermediate-depth earthquakes originating in the Vrancea seismic area. This city is located in the centre of the Moesian Platform, a large sedimentary basin (450 by 300 km) that reaches 10 km deep. Intensities up to VIII-IX have been reported in the city after four earthquakes with Mw>7 in the last century. The recorded ground motion is characterized by predominant long periods and has been interpreted as the combined effect of source mechanism and local geological conditions.
In order to better understand the effects of the geological structure beneath Bucharest, non-invasive geophysical techniques have been applied on seismic recordings (single-station and array) of both ambient vibrations and low-magnitude earthquakes. Using a 35km diameter array of broadband seismometers, we analyzed the surface-wave field at very low frequencies (0.05 – 1 Hz). Estimates of Love and Rayleigh wave dispersion curves (fundamental and higher modes) and Rayleigh wave ellipticity were first retrieved and analyzed. In addition, the variation of the SH-wave fundamental frequency of resonance was mapped across the area. This information has been used together with the available borehole logs (<300 m) and geological interpretations of deep structures to characterize the shear-wave velocity structure down to 7 km of depth and to create a comprehensive 3D geophysical model of the basin underlying Bucharest.
Finally, we investigated the contributions of different seismic surface waves, such as those produced at the edges of the basin or by multipath interference (Airy phases of Love and Rayleigh waves), using the array processing technique MUSIQUE. We focused on frequencies between 0.1 and 1 Hz to better understand the complex nature of the H/V ratios in this range.
The results represent the starting point to simulate past strong events and will be further used to estimate the expected ground motion for different earthquake scenarios.