Plenary Lecture

Natural Hazards Induced by Large Intermediate Vrancea Earthquakes in SE Europe

Professor Andrei Bala
Co-author: Mircea Radulian
National Institute for Earth Physics
P.O. Box MG-2, Bucharest-Magurele, Romania

Abstract: Bucharest, the capital of Romania, with more than 2 million inhabitants, is considered after Istanbul the second-most earthquake-endangered metropolis in Europe. It is identified as a natural disaster hotspot by a recent global study of the World Bank and the Columbia University (Dilley et al., 2005). Four major earthquakes with moment-magnitudes between 6.9 and 7.7 hit Bucharest in the last 65 years. The most recent destructive earthquake of 4. March 1977, with a moment magnitude of 7.4, caused about 1.500 casualties in the capital alone. All disastrous earthquakes are generated within a small epicentral area – the Vrancea region - about 150 km north of Bucharest (Fig. 1). Thick unconsolidated sedimentary layers in the area of Bucharest amplify the arriving seismic shear-waves causing severe destruction. Thus, disaster prevention and mitigation of earthquake effects is an issue of highest priority for Bucharest and its population. The studies done after this earthquake had shown the importance of the surface geological structure upon ground motion parameters. New seismic measurements are performed in Bucharest area with the purpose of defining better elastic and dynamic properties of the shallow sedimentary rocks. Down-hole seismic measurements were performed in a number of 10 cased boreholes drilled in the Bucharest City area. Processing and interpretation of the data lead to the conclusion that shallow sedimentary rocks can be considered weak in the area, down to 150 - 200 m depth. Seismic wave velocity values and bulk density values presented in the paper associated with local geology are useful primary data in the seismic microzonation of Bucharest City. They are used as 1D models to derive transfer functions and response spectra for the stack of sedimentary rocks in several parts of Bucharest area, leading to a better knowledge of the local site amplification and associated frequency spectra. The last chapter is dedicated to the data acquired in 20 sites in Bucharest City and to compute the spectral ratio of the noise. The obtained ratios confirm the previous results, showing a dominant resonance in the period range of 1.25 - 1.75 seconds. The average period of these maxima is 1.47 ± 0.20 s, while the average amplitude is 2.5. Our results bring evidence of the applicability of the ambient noise measurements for the risk assessment studies. Fig. 1: General map of study region. Strong earthquakes occur at depth below the Vrancea area which corresponds roughly to the southeastern bend of the Carpathian mountains. The seismic waves cause severe damage in Bucharest, about 150 km to the south. The map shows the distribution of epicenters during the URS (URban Seismology) experiment. The cross-section in the upper right corner shows the distribution of hypocenters along a NW-SE cross-section (dotted line in the map).