Estimation of site effects in Bucharest caused by the May 30-31, 1990, Vrancea seismic events

The Vrancea region seismicity, characterized by focal depths larger than 60 km and major events with magnitudes M-w greater than or equal to 6.9, is r esponsible for the most destructive effects experienced in the Romanian ter ritory, and may seriously affect high risk construction located on a wide a rea, from Central to Eastern Europe. This seismogenic volume must be taken into account both for seismic hazard analysis at the regional level (southe astern Europe) and national level (Romania and Bulgaria) as well as for mic rozonation studies of the highly populated cities located in the range of i nfluence of this source. Since about four destructive earthquakes occur eve ry century in Vrancea, the microzonation of Bucharest, the main city expose d to the potential damages due to these strong intermediate-depth shocks, r epresents an essential step towards the mitigation of the local seismic ris k. Two main approaches can be considered for the evaluation of the local se ismic hazard: (a) collection and extended use, for engineering purposes, of the recorded strong motion data, and (b) advanced modelling techniques tha t allow us the computation of a realistic seismic input, which can compensa te for the lack of strong motion records, actually available only for a few events that occurred in the last 20-30 years. Using a ground motion simulation technique that combines modal summation an d finite differences, we analyze, along a geologic profile representative o f the Bucharest area, the differences in the expected ground motion when tw o source mechanisms corresponding to the May 30-31, 1990, intermediate-dept h Vrancea earthquakes, typical events for the Vrancea seismogenic zone, are considered. All three components of motion are influenced by the presence of the deep alluvial sediments, the strongest local effect being visible in the transversal (T) one, both observed and computed. The details of the lo cal effects vary with varying the earthquake scenario, R and V components b eing very sensitive. Therefore, for a reliable determination of the seismic input all three components of motion (R, V and T) should be used.