H/V SPECTRAL RATIOS TECHNIQUE - APPLICATION FOR BUCHAREST AREA

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1 International Symposium on Strong Vrancea Earthquakes and Risk Mitigation Oct. 4-6, 007, Bucharest, Romania H/V SPECTRAL RATIOS TECHNIQUE - APPLICATION FOR BUCHAREST AREA Bogdan Zaharia 1, Bogdan Grecu 1, Mircea Radulian 1, Mihaela Popa 1, Daniel Paulescu 1 ABSTRACT Bucharest is one of the most affected cities by earthquakes in Europe. Situated at km distance from Vrancea epicentral zone, Bucharest has suffered many damages due to high energy Vrancea intermediate-depth earthquakes. For example, the 4 March 1977 event produced the collapse of 3 buildings with 8-1 levels, while more than 150 old buildings with 6-9 levels were seriously damaged. The studies done after this earthquake had shown the importance of the surface geological structure upon ground motion parameters. Bonjer et al. (1999) used for estimation of the local response the seismic noise recorded at 16 stations in Bucharest. The H/V spectral ratios obtained for the 16 sites show a clear resonance peak between 1 and seconds and their amplitudes remain constant around the value of. In this study we extend the analysis carried out by Bonjer et al. (1999) using new noise data acquired in 00 in 0 sites in Bucharest area, noise data obtained during the URS experiment ( ) and earthquake recordings. Our main goal is to show the significance and limitations of the H/V ratios technique for the Bucharest case and the implications on the strategy to follow when assessing the seismic microzoning for Bucharest area. Key words: spectral ratio method, seismic microzonation, Nakamura s technique, site effects INTRODUCTION The behavior of the ground motion during an earthquake is generally well explained by the geological surface structure in the place where the phenomenon is studied. Past and recent observations have shown that the damage caused by strong earthquakes are more important in sedimentary basins than on hard rock structures. The city of Bucharest is situated in Romanian Plane, at km distance from Vrancea epicentral zone, area where earthquakes with high energy ( 3 earthquakes/100 years with M w >7) occur at intermediate depths (70 00 km). The geology of the city is characterized by 7 distinct sedimentary complexes (Mandrescu et al., 004), with different peculiarities and large intervals of thicknesses. These shallow Quaternary complexes were first identified and separated by Liteanu (1951) and then cited by different authors with minor changes (Lungu et al., 1999; Ciugudean and Stefanescu, 005; Hannich et al., 005). DATA USED AND APPLICATION OF H/V SPECTRAL RATIOS TECHNIQUE To compute the H/V ratios we used in this study a data set which consists of ambiental noise recordings of 15 minutes length and earthquake recordings. The noise measurements were carried out in June 00 at 18 sites within the city of Bucharest and two sites at Magurele. The noise recordings were recorded with a digital station Kinemetrics K equipped with a velocity sensor having the natural period of 5 seconds. We also used noise data recorded at 3 broadband stations deployed in Bucharest area within the Urban Seismology project (URS Ritter et al., 005). These stations were continously recording seismic data between October 003 and August 004. The earthquake data used in this study to compute the H/V 1 National Institute for Earth Physics, Bucharest-Magurele, Romania

2 International Symposium on Strong Vrancea Earthquakes and Risk Mitigation 85 ratios consisted of 17 Vrancea intemediate-depth earthquakes with magnitudes between 4.1 and 6.0. The earthquakes were recorded by the K stations deployed within Bucharest area in the framework of the cooperation between National Institute fro Earth Physics and University of Karlsruhe (Bonjer and Rizescu, 000). For this study we used only the K stations which were situated in the same place with URS stations. BFG site BTM site Figure 1. Examples of H/V ratios for ambient noises during June 00 measurements The apllication of Nakamura s technique (1989) to estimate H/V spectral ratios for June 00 measurements (examples are given in Fig. 1) shows, with no exception, a dominant peak between 1 and seconds period. The average period of the maxima of these peaks is T= 1.47 ± 0.0 seconds. The amplitude of these peaks varies slowly from.05 at BVC site to.95 at BDL site (Fig. ), while only two sites (BAD and BFF) exhibit peaks with amplitude smaller than. The remarkable similarity of the amplitudes and the shapes of the peaks, suggests that there are no significant lateral variations and impedance contrasts within the subsoil of Bucharest. The distribution of the fundamental period of resonance determined by the H/V ratios (Fig. ) indicates an increase of the period from south to north, which correlates well with the increase of the thickness of sedimentary layer towards northern part of the city bad bff ere bf bot bdl plv bgm vic acd bst bcu bfg btm bhm buh bvc u3 ucb mtr iba bad bff ere bf bot bdl plv bgm vic acd bst bcu bfg btm bhm buh bvc u3 ucb mtr iba u u Figure. Distribution of the fundamental period of resonance (left) and of the amplitude of the H/V dominant peaks (right) during June 00 measurements To check our results, we used the the sedimentary layer parameters obtained by Bala et al. (006) and we compute the fundamental period by applying the following formula T=4h/β, where β is the S-wave velocity (m/s) and h the layer thickness (m). In our computations we considered one layer (made of the first 6 sedimentary complexes, see Table 6 in Bala et al., 006) which overlies the half-space (the Fratesti layer). The shear-wave velocity in the sedimentary layer was calculated using the following formula: v n i= 1 s = n i = 1 d i d v i si (1)

3 86 B. Zaharia et al. where d i and v Si denote the thickness (in meters) and the shear-wave velocity (in m/s) of the i-th layer, in a total of n layers, existing in the same type of stratum (d i and v Si were determined by borehole measurements), while for the depth we considered 3 cases: the minimum depth h min = 100 m when we chose for each of the 6 complexes the minimum depth, the average depth h avg = 140 m when we chose an average depth for each complex, and the maximum depth h max = 180 m when we chose the maximum depth for each complex (see Table 6 in Bala et al., 006 ). We obtained the following fundamental periods: T 1 = 1.09 s (for h min ), T = 1.53 s (for h avg ) and T 3 = 1.97 s (for h max ). It can be noticed that the average period obtained from H/V ratios and T are very close. These results are also in good agreement with the study of Mandrescu et al. (004) which showed that the computed predominant period of oscillation, T, of the surface layers over Bucharest territory ranges between 1.0 and 1.9s with an increase from southern to northern part of the Bucharest. The period of oscillation T was estimated using the same relation, T=4h/β. The same technique was applied to the noise data recorded in URS experiment and our analyses confirm the previous results (examples are given in figure 3). A prominent peak response in the period range from 1s to s is visible in all cases, independently site. No further resonance peaks with amplitudes greater than 1.5 URS07 site URS3 site are visible at periods greater Figure 3. Examples of H/V ratios for ambient noises during URS experiment than 0.1 s, except for the site URS3 where a secondary peak can be identified at around 9 Hz. These results do not contradict the general uniform subsoil structure of the Bucharest city. Finally the H/V spectral ratios method was applied to small-to-moderate earthquakes in order to test if the results obtained in such a way come close to the results obtained using ambient noise (examples are given in Fig. 4). The H/V spectral ratios present a remarkably similarity in the range of lower frequency (under 1 Hz). In all sites the H/V spectral ratios are dominated by a clear resonance between 0.5 and 1 Hz (1 and s) which corresponds with the resonance obtained from noise data. The average period of the maxima of these peaks is T= 1.8 ± 0.17 seconds. The amplitude of these peaks varies from 3.7 to 7.5. Note that in the case of earthquake data a secondary peak may appear, probably related to source contribution. STATION BAP - Average H/V RATIO STATION BGM - Average H/V RATIO 8 8 H/V SPECTRAL RATIO 6 4 H/V SPECTRAL RATIO FREQUENCY (Hz) FREQUENCY (Hz) Figure 4. Examples of H/V ratios for earthquakes

4 International Symposium on Strong Vrancea Earthquakes and Risk Mitigation 87 CONCLUSIONS The range of fundamental periods (1 seconds) obtained using H/V spectral ratios for noise and earthquake data corresponds with the resonance period of the sedimentary layer as determined by geological and geotechnical data. The resonance is visible in all cases indicating a relative uniform structure beneath Bucharest city, except an increase of the fundamental period from south to north, in the same direction as the increase of the thickness of the cohesionless Quaternary deposits. The resonance period is close to the resonance of high tall buildings in Bucharest (8 0 floors) which are primarily affected by Vrancea strong earthquakes. ACKNOWLEDGEMENTS These results are based on the data recorded in the framework of the bilateral cooperation between Geophysical Institute, University of Karlsruhe (Germany) and National Institute of Earth Physics (Romania) in the framework of CRC 461 Project and URS Urban Seismology in Bucharest, Romania Project. The work is part of the CERES Project 4-53/ financed by the Ministry of Education and Research of Romania. The authors express their gratitude to Klaus P. Bonjer and Neculai Mandrescu for very useful scientific discussions, comments and suggestions. REFERENCES Bala, A., V., Raileanu, I., Zihan, V., Cigudean, B., Grecu, 006. Physical and dynamic properties of the shallow sedimentary rocks in the Bucharest metropolitan area, Romanian Reports in Physics, Vol. 58, No., p.1-50, Bonjer, K.-P., M. C., Oncescu, L., Driad, M., Rizescu, A note on empirical site responses in Bucharest, Romania. Vrancea Earthquakes: Tectonics, Hazard, and Risk Mitigation, Editors: Wenzel, F., Lungu, D., Kluwer Academic Publishers, Bonjer, K.-P., Rizescu, M. (000), Data Release of the Vrancea K Seismic Network. Six CD s with evt-files and KMI v1-,v-, v3-files. Karlsruhe-Bucharest, July 15, 000. Ciugudean, V., I., Stefanescu, Engineering geology of the Bucharest city area, Romania, 005, submitted to IAEG -006, paper no. 35 Hannich, D., K.-P., Bonjer, H., Hoetzl, D., Lungu, V., Ciugudean, T., Moldoveanu,, C., Dinu, D., Orlowsky, 005. Evaluation of soil parameters through Vertical Seismic Profiling (VSP) in Bucharest, Romania, paper submitted to SOIL DYNAMICS AND EARTHQUAKE ENGINEERING, Elsevier Science Ltd, Oxford. Liteanu, G., Geology of the city of Bucharest. Technical Studies, Series E, Hydrogeology, Bucuresti, No.1 (in Romanian) Lungu, D., A., Aldea, T., Moldoveanu, V., Ciugudean, M., Stefanica, Near Surface Geology and dynamic properties of soil layers in Bucharest, in Vrancea Earthquakes: Tectonics, Hazard and Risk Mitigation, Contributions from the First International Workshop on Vrancea Earthquakes, Bucharest, Romania, 1 4 November Editors F. Wenzel, D. Lungu, O. Novak, Kluwer Academic Publishers, p , Mândrescu, N The Vrancea Earthquake of March 4, 1977 and the Seismic Microzonation of Bucharest, Proc. nd Inter. Conference Microzonation, San Francisco, 1,

5 88 B. Zaharia et al. Mândrescu, N., M., Radulian, G., Mărmureanu, 004. Site conditions and predominant period of seismic motion in the Bucharest urban area, Rev. Roum. Geophysique 48, p , Nakamura, Y., A method for dynamic characteristics estimation of subsurface using microtremor on the ground surface. Q.R. of R.T.R.I., 30-1, p Ritter, J.R.R., S.F., Balan, K.-P., Bonjer, T., Diehl, T., Forbinger, G., Marmureanu, F., Wenzel, and W. Wirth W., 005. Broadband urban seismology in the Bucharest metropolitan area, Seismological Research Letters, 76, no. 5.