Seismic ground motion characteristics in the Bucharest area: sedimentary cover versus seismic source control

It is crucial for the engineering-geological purposes to explore and properly understand the role of the shallow sedimentary cover on the seismic motion characteristics in Bucharest area in parallel with the Vrancea seismic source influence. The information of the thickness of the sedimentary cover was collected from borehole sites located within Bucharest area. The noise measurements and all the available accelerograms recorded in the city area for moderate-size and strong Vrancea intermediate-depth earthquakes are analyzed in correspondence with the sedimentary cover topography. The resonant period of oscillation, deduced from the sediment thickness and average shear-wave velocity, roughly correlates with the peaks in the H/V spectral ratios of ambient noise and earthquakes. On the other hand, the source seems to primarily control the kind of motion. Thus, the moderate-size earthquakes cannot excite the resonant oscillation associated with the sedimentary cover (1-2 s). As our work shows, this is really important only for the largest shocks (Mw > 7). The analysis of the entire instrumental data set recorded up to now in Bucharest urban area and its neighborhood suggests a rather weak variability of the peak motion values and spectral amplitudes over the study area, in agreement with the relatively small variability of the local shallow structure. (authors)