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Site Effects from Ambient Noise Measurements and Seismic Hazard Assessment in Northern Tel Aviv

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Site Effects from Ambient Noise Measurements and Seismic Hazard Assessment in Northern Tel Aviv SITE EFFECTS FROM AMBIENT NOISE MEASUREMENTS AND SEISMIC HAZARD ASSESSMENT IN NORTHERN TEL AVIV FINAL REPORT March, 200‏9 Report No 519/401/08 Principal Investigator: Dr. Y. Zaslavsky Collaborators: M. Kalmanovich, Dr. M. Ezersky, M. Gorstein, N. Perelman, I. Dan, D. Giller, G. Ataev, T. Aksinenko A. Shvartsburg, and V. Giller Prepared for Geological Survey of Israel 1 TABLE OF CONTENTS LIST OF FIGURES AND TABLES 2 ABSTRACT 4 1. INTRODUCTION 5 2. LOCAL GEOLOGY 6 3. OBSERVATIONS 8 4. DATA PROCESSING 14 5. THE STABILITY OF THE HORIZONTAL-TO-VERTICAL SPECTRAL RATIO OF AMBIENT NOISE 15 6. VARIATION OF SPECTRA AND H/V RATIO SHAPES 18 7. DISTRIBUTION OF H/V RESONANCE FREQUENCIES AND THEIR ASSOCIATED AMPLITUDE LEVELS 21 8. VALIDATION OF H/V SPECTRAL RATIO BY SEISMIC REFRACTION SURVEY 24 9. SEISMIC HAZARD MICROZONZTION 28 10. DISCUSSION AND CONCLUSIONS 32 11. ACKNOWLEDGMENTS 36 12. REFERENCES 37 2 List of figures and tables Figure 1. Locations of measurement points, refraction lines and boreholes in the study area. Figure 2. Examples of seismic stations location in the northern Tel Aviv area at different lithological units: 1 – alluvium; 2 – sand dunes; 3 – hamra; 4 - kurkar; 5 - coast sand; 6 – soil. Figure 3. Measurements of ambient noise in northern Tel Aviv. Figure 4. Comparison of (a) average Fourier spectra of three components of motions and (b) individual and average H/V spectral ratios observed at Point 21 in different months. Figure 5. Comparison of (a) individual and (b) average H/V spectral ratios from ambient nose observed at Point 6 in different months. Figure 6. Comparison of (a) average spectra Fourier for three components of motion and (b) average H/V spectral ratios from ambient noise observed at Point 55 in different years. Figure 7. .Comparison of (a) average spectra Fourier for three components of motion (NS and EW – horizontal and V – vertical) and (b) individual and average H/V spectral ratios observed at Point 60 in different years. Figure 8. Examples of (a)Average Fourier spectra and (b) H/V spectral ratios at sites whose subsurface structure may be approximated by one-layer model. Figure 9. Examples of (a) average Fourier spectra and (b) H/V spectral ratios at sites ratio from ambient noise observed at Points 6 and 42. Figure 10. Examples of (a) average and (b) individual and average H/V spectral ratios for sites whose subsurface structure yields low impedance contrast between sediment and reflector. Figure 11. Example of significant variations of site effects over short distance. Figure 12. Distribution of the resonance frequency (a) and its associated H/V amplitude (b) in northern Tel Aviv area. Figure 13. Examples of P-wave (a) and S-wave records at refraction line TA-2. Figure 14. (a) Analytical transfer functions (blue dashed line) compared with average H/V spectral ratios(red line ); (b) Columnar section of Well 7; (c) vertical cross section along line TA1 and obtained at Points 51, 53 and 54. Triangles indicate measurement sites near TA-1. 3 Figure 15. (a) Columnar section of geological structure near Point 6, (b) vertical cross section along line TA-2 and (c) analytical transfer function (blue dashed line) compared with average H/V spectral ratio (red line) obtained at Point 6. Figure 16. H/V spectral ratios from different sites (black thin lines) in Zone II, the average H/V spectral ratio (red line) and generalized analytical 1-D transfer functions (blue line). Figure 17. Seismic microzonation map of northern Tel Aviv with respect to acceleration response spectra calculated by SEEH. Figure 18. Generalized Uniform Hazard Site-specific Acceleration Spectra for all zones in the study area. The black dashed line represents the acceleration design function according to the Israel Building Code (IS-413). Both functions adhere to the same probability of exceedence (10% in 50 years), same damping ratio (5%) and the same soil type. Figure 19. (a) Lithological section of well L-342; (b) Vs model determined by integrated analysis of H/V ratios and geophysical data; (c) lithological interpretation of the modeling results. Figure 20. Schematic geological cross sections along Profile 1 and Profile 2 reconstructed using analysis of H/V spectral ratio from microtremor. Tables Table 1. Lithostratigraphic column of the Quaternary sediments represented in northern Tel Aviv (based on Gvirtzman, 1970, 1984). Table 2. Water well data in the study area. Table 3. Seismic station characteristics. Table 4. The locations of measurements points. Table 5. The resonance frequency and its associated H/V amplitude in northern Tel Aviv area. Table 6. Location of the seismic refraction lines at northern Tel Aviv area. Table 7. Results of the seismic refraction survey. Table 8. Subsurface model along lines TA-1 and TA-2 inferred from seismic refraction survey and optimal 1-D model of soil column. Table 9. Soil column models for calculating generalized acceleration response spectra for zones. Table 10. S-wave velocity ranges for lithological units represented in the study area. 4 Abstract The necessities for detailed mapping of the earthquake hazard in urban areas, stems from the fact that geological inhomogeneity dominate the spatial distribution of the intensity of damage and amount of casualties. In most of the cities around the world, including the cities in Israel, direct information from strong motion recordings is unavailable. The great variability in the subsurface conditions across a town/city and the relatively high cost associated with obtaining the appropriate information about the subsurface, strongly limits proper earthquake hazard quantification. The investigated area of 1 km wide and 2.8 km long stretches along the shoreline of the Northern Tel Aviv. The quaternary sediments of the Kurkar Group, alluvium and sand dunes outcropped in the area. Kurkar Group are represented by, hamra and heterogeneous geological unit “kurkar”. .The possible site amplification effect, using ambient noise surveys, was estimated at 60 sites. The soil sites exhibit H/V amplitudes ranging from 2 to 6 in the frequency range 2 to 12 Hz. Limited data on sediments thickness and velocities are available from two refraction lines and boreholes. Transfer functions calculated for models based on these data are used to validate H/V ratios at corresponding locations and justified further utilization of velocity structure as a starting model for other sites, away from refraction lines and boreholes. Their layer thicknesses are sought, yielding calculated transfer functions to match the observed H/V curves by considering resonance peak. Along the coastal plain, site effects may vary significantly over very short distances, even in cases associated with the same geological units. For example, joint analysis of the measurement results and geological together with geophysical data revealed that S-velocity range for the geological unit “kurkar”, which consists of alternating marine and eolian calcareous sandstones with reddish silty-clayey sandstones (mostly “hamra”) and finer grained sediments, is from 300 m/sec up to 1120 m/sec. The evaluated subsurface models are introduced using SEEH procedure of Shapira and van Eck (1993) to assess Uniform Hazard Site-Specific Acceleration Spectra at the investigated sites. We divided the study area into zones and characterized each of them with a generalized soil column model. Thus the seismic hazard zonation map obtained are closely tied to site effects actually measured, and therefore may lead to realistic site-specific seismic hazard assessment in the northern Tel Aviv, in spite of the borehole, refraction data and other subsurface information paucity. 5 1. Introduction Seismic wave amplifications in alluvial deposits, common in urban areas, have contributed to damage and loss of life in number earthquakes in the resent past. The resonance frequency of soft layers upon which many town in Israel are built is often in the same frequency range as urban structures. "Double resonance" of both the site and the building can occur then. There are many examples from different countries were double resonance was the main reason of dramatic structural damage. Many techniques have been presented to evaluate site effects. There is no doubt that the best evaluation of site effect is based on dense strong motion observations using spectral ratios of seismic records from sedimentary sites and bedrock reference site, because the non linear effect is included. In most cases, mainly in regions where the seismic activity is relatively low as in Israel, this type of analysis is usually impractical. The horizontal-to-vertical spectral ratio of ambient noise was presented by Nakamura (1989, 2000) and now is widespread tool for the study of ground motion amplification. Many authors among them Lermo and Chávez-García (1994), Seekins et al. (1996), Toshinawa et al. (1997), Chávez-García and Cuenca (1998), Enomoto et al. (2000) and Mucciarelli and Gallipoli (2004), show that the H/V spectral ratio technique can be a useful tool for the assessment of ground motion characteristics on soft sediments especial when no nearby reference sites are available. However, other authors (for example, Bonilla et al. 1997; Horike et al. 2001, Satoh et al. 2001) conclude that whereas the predominant peak of H/V ratio is well correlated with the fundamental resonance frequency, the amplitude of this peak is not necessarily the amplification level as obtained from sediment-to-bedrock spectral ratio of earthquake records. In a recent comprehensive study by SESAME European project (Atakan et al., 2004; Bard et al., 2004) it has been shown that H/V spectral ratio from ambient noise can be used to obtain reliable information about the amplification of sedimentary layers. Our measurements of ambient noise in urban environments in Israel (more than 4500 sites) show (Zaslavsky et al., 2000; Shapira et al., 2001; Zaslavsky et al., 2005, 2008a, b) that in sites with relatively high level of ambient noise and impedance contrast of shear wave velocity between rock and soil (velocity ratio more than 3.0) there is a good agreement between H/V amplitudes of the fundamental mode (sometimes also second mode) and the theoretical transfer function calculated by e.g.
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