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Title:
Source and site effects implications on Bucharest (Romania) microzonation
Authors:
Radulian, M.; Grecu, B.; Mandrescu, N.
Affiliation:
AA(National Institute for Earth Physics, 12 Calugareni str., P.O. Box MG-2, Bucharest-Magurele, 077125, Romania ; ), AB(National Institute for Earth Physics, 12 Calugareni str., P.O. Box MG-2, Bucharest-Magurele, 077125, Romania ; ), AC(National Institute for Earth Physics, 12 Calugareni str., P.O. Box MG-2, Bucharest-Magurele, 077125, Romania ; )
Publication:
American Geophysical Union, Spring Meeting 2007, abstract #S33B-10
Publication Date:
05/2007
Origin:
AGU
AGU Keywords:
7200 SEISMOLOGY, 7212 Earthquake ground motions and engineering seismology
Abstract Copyright:
(c) 2007: American Geophysical Union
Bibliographic Code:
2007AGUSM.S33B..10R

Abstract

The Bucharest metropolitan area is one of the most vulnerable cities of the World to earthquakes. Although the city is situated at a distance of around 150 km from the epicentral area, the damage caused by many Vrancea intermediate-depth earthquakes was extremely severe. Historical information over one thousand years suggests a rate of 2-3 damaging earthquakes per century. For example, the 4 March 1977 event produced the collapse of 32 buildings with 8-12 levels, while more than 150 old buildings with 6-9 levels were seriously damaged. Since then the occurrence of 3 other earthquakes (1986 / M=7.1; 1990 /M=6.9; 2004 /M= 6.0) demonstrated that the Vrancea seismic activity is continuing, permanently threatening the Bucharest City area. The studies done after 1977 earthquake had shown the importance of the surface geological structure upon ground motion parameters and emphasized the need for new methods of quantifying the site effects. The main purpose of our study is to analyze the influence of the source and local site conditions upon the soil response in Bucharest, in case of large (M greater than 7) Vrancea subcrustal earthquakes. To this purpose, we use on one hand, geological, geotechnical and geophysical information, including in situ measurements of shear wave velocities and data from ambient noise measurements, small-to-moderate earthquakes and large events, on the other hand. The Neogene sedimentary cover, as obtained on the basis of borehole information, undergoes a slight descend from south to north, accompanied by an increase of the deposits thickness in the same direction from about 150 m to 350 m. The predominant periods of oscillation of subsurface layers over Bucharest territory range between 1.0 and 1.9 s, increasing from south to north, in correlation with the constant increasing of the thickness of the Quaternary cohesionless deposits. The dominant resonance in the period range of 1- 2 s is obtained both using ambient noise data and earthquake data. The spectral analysis of the recorded waveforms (response spectra, power spectral density, H/V spectral ratios) shows two predominant periods, one around 0.4 s, other around 1.4 s, independently of site position in the city area. They correlate very well with the local structure layering and parameters, as shown by the theoretical amplification curves. In addition, the seismic source radiation for the largest shocks matches the local structure resonance at 1-2 s, which makes the seismic response in this range to be exceptionally enhanced as compared with the amplification observed for smaller events. Therefore, the disastrous damage reported for the high-tall buildings in Bucharest is a result of both subsurface sedimentary structure and source radiation from Vrancea shocks with magnitude exceeding 7. Our results outline two main features of high significance for earthquake engineering practice: (1) inadequacy for the Bucharest city case of the standard procedure that limits the investigation depth in order to set soil dynamic characteristics to uppermost 30m; the local response during the large Vrancea earthquake is controlled by the entire package of Quaternary cohesionless deposits which are significantly thicker than 30m beneath Bucharest and (2) difficulty to delineate zones with different local amplifications; thus, for Bucharest urban area and strong subcrustal Vrancea earthquakes one can refer rather to regional effects than "local effects".
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