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Neculai Mandrescu, National Institute for Earth Physics (Romania)
Neculai Mandrecu, National Institute for Earth Physics (Romania)
Mircea Radulian, National Institute for Earth Physics (Romania)
Gheorghe Marmureanu, National Institute for Earth Physics (Romania)
Bogdan Grecu, National Institute for Earth Physics (Romania)
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Compared with other European capitals, Bucharest is exposed to the greatest seismical hazard. Only during the last strong earthquake (March 4,1977; Mw=7.4), 1391 people died in Bucharest and the damages in the city represented 2/3 of the total losses, evaluated by the World Bank at about 2 billions of dollars. In Bucharest, half of the loss value was due to the collapse of 32 blocks of flats, having between 6 and 11 floors. Other several tens of tall buildings were hard damaged. It was generally assumed that the main cause of this damage was the proximity of the building period of oscillation to the fundamental period of resonance specific for the geological conditions beneath the city.
The main purpose of our study is to analyze the influence of the local natural conditions upon the soil response in Bucharest, in case of large Vrancea (Romania) subcrustal earthquakes. To this purpose, we use two principal datasets: (a) geological, geotechnical and geophysical information, including in situ measurements of shear wave velocities, and (b) strong motion records of the Vrancea intermediate depth earthquakes, occurred since 1977 and recorded at the seismic stations in Bucharest.
The results obtained in local response evaluation using the accoustic impedance contrast and amplification factors are confirmed by the spectral analysis of the accelerograms. The good correlation between the results coming from the two kinds of data are mainly due to the quasi-uniform geological peculiarities over the entire city area, with almost horizontal strata and insignificant lateral inhomogeneity. Based on our analysis we conclude that the seismic source, travel path and site response will result in similar effects over the entire city area in case of strong Vrancea subcrustal earthquakes; thus, for Bucharest urban area and strong Vrancea earthquakes, one can refer rather to regional than local effects. Our analysis outlines two main features of high significance for engineering practice: (1) unstationary of the dynamic amplification process of the ground motion from one event to another; this means that the information regarding the ground response provided by study of the weak and moderate-size earthquakes cannot be extrapolated to anticipate the local response in case of large Vrancea subcrustal shocks and (2) setting limits for depth investigation of the dynamic characteristics of the soil, in order to establish the local response at uppermost 30m is inadequate for Bucharest urban area. In our opinion in case of strong subcrustal Vrancea earthquakes all the Quaternary deposits are excited and react as a whole, causing amplifications of the intermediate and mid band period; the influence of the strata close to the surface is greatly diminished, the acceleration response spectrum having practically a single peak.
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