Seismic site effects based on in situ borehole measurements in Bucharest, Romania

Within the NATO Science for Peace (SfP) Project 981882 “Site-effect analyses for the earthquake-endangered metropolis Bucharest, Romania” we obtain a unique, homogeneous dataset of seismic, soil-mechanic and elasto-dynamic parameters. Eight 50 m deep boreholes are drilled in the metropolitan area of Bucharest in order to obtain cores for dynamic tests and vertical seismic profiles for an updated microzonation map related to earthquake wave amplification. The boreholes are placed near former or existing seismic station sites to allow a direct comparison and calibration of the borehole data with actual seismological measurements. A database is assembled which contains Pand S-wave velocities, density, geotechnical parameters measured at rock samples and geological descriptions for each sedimentary layer. Using SHAKE2000 in a first step, we compute the spectral acceleration response and transfer functions obtained from the in situ measurements. The acceleration response spectra correspond to the shear-wave amplifications due to the sedimentary layers from 50 m depth (maximum depth) up to the surface. 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-east of Bucharest. 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. In Bucharest there are only a few sites, which were investigated with geophysical and geotechnical methods, to relate the local geology with seismic wave propagation properties (especially amplitude amplification). Therefore, the main purpose of NATO SfP Project 981882 is to obtain a unique, homogeneous dataset of soil-mechanic and elasto-dynamic parameters of the subsurface of Bucharest from 8 new boreholes to systematically model the so-called seismic site responses. 1. NATO SCIENCE FOR PEACE PROJECT 981882 1.1 State of the Art The zoning of the metropolitan area of Bucharest for seismic amplification pattern (microzonation) has been pursued since the 1977 disastrous event with great effort. Geophysical groups at the National Institute for Earth Physics (NIEP) and civil engineers at the National Institute for Building Research worked on this problem, as well as foreign (1) National Institute for Earth Physics, Bucharest-Magurele, Romania, (bala@infp.ro) (2) Universitat Karlsruhe (TH), Germany (3) University of Erlangen-Nurnberg, Germany International Symposium on Strong Vrancea Earthquakes and Risk Mitigation 191 institutions like the Universitat Karlsruhe (TH), the University of Trieste and the Japanese International Cooperation Agency. Their work resulted in an improved seismic database obtained from modern seismic observation networks as well as several borehole analyses. Based on these observations recent microzonation studies were done by e.g. Aldea et al. (2004), Cioflan et al. (2004), Kienzle et al. (2004) or Wirth et al. (2003). However, all these studies could cover only a fraction of the microzonation problem, because either seismic data alone (Wirth et al., 2003) or numerical modelling based on the assumed geological layering (Cioflan et al., 2004) was done. Sokolov et al. (2004) used spectral amplification factors and a probabilistic method to determine ground motion site effects in Bucharest. A major drawback of all studies is missing geophysical and geotechnical information from welldistributed boreholes in the Bucharest City area. New high-quality seismological data were acquired during the URS (URban Seismology) Project from October 2003 to August 2004. Within this project 32 state-of-the-art broadband stations were continuously recording in the metropolitan area of Bucharest (Ritter et al., 2005, Balan et al., this volume). This unique dataset provides important information on the seismic amplitude variation across the area. Additionally, there is a modern ground acceleration observation network (K2-network) which has been upgraded in the last years by the Universitat Karlsruhe (TH) and NIEP and which is now run by NIEP. From this strong motion network a database with valuable recordings was built. 1.2 Objectives of the Project The main objective of the NATO project is earthquake risk mitigation and better seismic safety of Bucharest, the capital of Romania, as there is a major gap in knowledge concerning seismic and geotechnical parameters in the shallow (< 50 m), unconsolidated soil and sediment layers. Therefore, eight new boreholes are drilled in the city. Six boreholes are placed at or close to URS station sites, two boreholes are placed in the proximity of K2 stations. The positions of the eight boreholes are carefully selected to fill information gaps in the central part of Bucharest City. By conducting seismic measurements in these boreholes and geotechnical analyses of the core samples, we want to determine dynamic parameters of in situ soils and rocks. These dynamic parameters will be used as input for linear and nonlinear waveform modelling to estimate the seismic amplitude amplification at specific sites in Bucharest. These modelled waveforms will be compared and calibrated with observations from seismic stations in the city. The results from the site-effect analysis will be gathered in an updated seismic microzonation map of Bucharest which will be disseminated to the public and especially to the end-users who will introduce our results in the future city planning (Ritter, 2006). 1.3 Technical Progress The boreholes sites were carefully selected according to the proposed plan and to the actual local situation in this case being a very populated area in a rapidly changing city. In 2006 two borehole sites were chosen in public places, belonging to the City Hall (“Titan 2” Park and “Tineretului” Park). The other two places were situated in private domains at the Ecologic University, near Dambovita River, and at the Astronomic Institute of Romania, near the Carol Park. The remaining boreholes were drilled in 2007 and are still under study. Here we report results from the 2006 campaign when the first four boreholes were successfully drilled in the central part of Bucharest City, and each sedimentary layer was thoroughly sampled. The drilling and the seismic measurements were done by a team from the Technical University of Civil Engineering Bucharest (UTCB) and CNRRS. The boreholes were protected with plastic tubes down to 50 m depth to ensure their stability for the in situ seismic