Estimation of Site Effects in Bucharest Caused by the May 30–31, 1990, Vrancea Seismic Events

he Vrancea region seismicity, characterized by focal depths larger than 60 km and major events with magnitudes Mw ] 6.9, is responsible for the most destructive effects experienced in the Romanian territory, and may seriously affect high risk construction located on a wide area, from Central to Eastern Europe. This seismogenic volume must be taken into account both for seismic hazard analysis at the regional level (southeastern Europe) and national level (Romania and Bulgaria) as well as for microzonation studies of the highly populated cities located in the range of influence of this source. Since about four destructive earthquakes occur every century in Vrancea, the microzonation of Bucharest, the main city exposed to the potential damages due to these strong intermediate-depth shocks, represents an essential step towards the mitigation of the local seismic risk. Two main approaches can be considered for the evaluation of the local seismic hazard: (a) collection and extended use, for engineering purposes, of the recorded strong motion data, and (b) advanced modelling techniques that allow us the computation of a realistic seismic input, which can compensate 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 and finite differences, we analyze, along a geologic profile representative of the Bucharest area, the differences in the expected ground motion when two source mechanisms corresponding to the May 30–31, 1990, intermediate-depth 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 local effects vary with varying the earthquake scenario, R and V components being very sensitive. Therefore, for a reliable determination of the seismic input all three components of motion (R, V and T) should be used.