Site Effect and Seismic Hazard Microzonation Across the Town of Tiberias
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1 SITE EFFECT AND SEISMIC HAZARD MICROZONATION ACROSS THE TOWN OF TIBERIAS February, 2009 Report No 502/416/09 Principal Investigator: Dr. Y. Zaslavsky Collaborators: M. Gorstein, M. Kalmanovich, I. Dan, N. Perelman, D. Giller, G. Ataev, T. Aksinenko, V. Giller and A. Shvartsburg Prepared for Geological Survey of Israel 2 CONTENT LIST OF FIGURES .......................................................................................................................3 LIST OF TABLES .........................................................................................................................5 ABSTRACT ....................................................................................................................................6 GEOLOGICAL AND TECTONIC CONTEXT .........................................................................8 Stratigraphy and lithology .........................................................................................................10 BRIEF REVIEW OF SEVERAL EXPERIMENTAL METHODS FOR SITE EFFECT ASSESSMENT .............................................................................................................................13 MICROTREMOR RECORDING AND PROCESSING .........................................................15 Site response in Tiberias estimated by H/V spectral ratio from microtremor ...........................20 Comparison of H/V spectral ratios from microtremor and seismic events................................24 DISTRIBUTION OF THE RESONANCE FREQUENCY AND ITS ASSOCIATED H/V AMPLITUDE OVER THE STUDY AREA...............................................................................28 ESTIMATION OF SHEAR-WAVE VELOCITY MODELS AND RECONSTRUCTION OF SUBSURFACE STRUCTURE.............................................................................................32 SEISMIC HAZARD MICROZONATION................................................................................47 CONCLUSIONS...........................................................................................................................57 ACKNOWLEDGEMENTS.........................................................................................................58 REFERENCES.............................................................................................................................59 3 LIST OF FIGURES Figure 1. Tiberias - the New and Old together.................................................................................7 Figure 2. Geological map of the study area compiled from Schulman (1966) and Sneh (2008) with locations of the refraction profiles TB-1, TB-2 and TB-3 (Ezersky, 2008); R-1 and R-2 (Shtivelman, 1995) and profiles 1 and 2 for constructing cross sections.........................................9 Figure 3. Location of the measurement sites in the study area. Numbers indicate the sites used as examples. TB-1, TB-2 and TB-3 - refraction survey profiles (Ezersky, 2008); R-1 and R-2 – refraction survey profiles (Schtivelman, 1995); TVR, TVR2 and POR – accelerometer locations; Profile1 and Profile2 – profiles for reconstructing subsurface structure. ......................................17 Figure 4. Examples of seismometers locations during various sets of the site investigations in different geological conditions.......................................................................................................18 Figure 5. Examples of seismic station locations in Tiberias..........................................................19 Figure 6. Examples of average Fourier spectra (top) and H/V spectral ratios (bottom), which yield a single peak. The black line indicates a vertical spectral component; the grey line indicates the average of NS and EW horizontal components of motion.............................................................20 Figure 7. Examples of average Fourier spectra (top) and H/V spectral ratios (bottom), which yield two resonance peaks. The black line indicates a vertical spectral component; the grey line indicates the average of NS and EW horizontal components of motion........................................21 Figure 8. Fourier spectra (top) and H/V spectral ratios (bottom) obtained at sites located on the exposure of the Cover Basalt. f0 indicates the fundamental frequency of the measurement site; f is an artificial frequency. H/V ratios at site T62 obtained by processing and reprocessing of the measurement data are shown by dashed and solid lines respectively............................................22 Figure 9. H/V spectral ratio obtained at sites located on the outcropped Bira Fm. (T70 and T115) and alluvium (T10 and T50)...........................................................................................................23 Figure 10. Comparison between the H/V spectral ratios obtained near the Tiberias Town Hall (T156 and TVR) and at site T13 at different times........................................................................23 Figure 11. Accelerograms of the earthquakes recorded at site Tiberias Hotel (TVR2): (a) earthquake occurred in the Dead Sea (2004 02-11, 08:15, ML=5.1, R=120 km) and (b) earthquake occurred in the Dead Sea fault (2004 07-07, 14:35, ML=4.7, R=89 km);......................................25 Figure 12. Accelerograms of the earthquake occurred in the Dead Sea (2004 02-11, 08:15, ML=5.1, R=120 km) and recorded at site Tiberias Town Hall (TVR)...........................................26 4 Figure 13. Accelerograms of the earthquakes recorded at site Poriya Hospital: (a) earthquake that occurred in the Dead Sea basin (2004 02-11, 08:15, ML=5.1, R=120 km) and (b) earthquake that occurred in the Dead Sea fault (2004 07-07, 14:35, ML=4.7, R=89 km).......................................26 Figure 14. Comparison of different estimates of site amplification based on H/V spectral ratio techniques applied to earthquakes and microtremor recordings at sites TVR2 (Hotel) – (a); TVR (Town Hall) – (b) and Poriya Hospital– (c). ..................................................................................28 Figure 15. Distribution of the fundamental resonance frequency over Tiberias............................29 Figure 16. Distribution of amplitude associated with the fundamental frequency. .......................30 Figure 17. Comparison between the analytical transfer function (grey line) and experimental H/V spectral ratio (black line) obtained at two sites along TB-3 refraction profile. .............................33 Figure 18. Comparison between the analytical transfer function (grey line) and experimental H/V spectral ratio (black line) obtained at four sites along TB-1 refraction profile..............................34 Figure 19. Comparison between the H/V spectral ratios obtained from microtremor measurements in 1995 (grey line) and 2008 (black line) near refraction profile TB-2. ................36 Figure 20. Comparison between the analytical transfer function (grey line) and experimental H/V spectral ratio (black line) obtained at two sites along refraction profile TB-2. .............................36 Figure 21. Comparison between the analytical transfer function (grey line) and experimental H/V spectral ratio (black line) obtained well Kineret 6 (site 121).........................................................37 Figure 22. Comparison between the analytical transfer function (grey line) and experimental H/V spectral ratio (black line) obtained well Kineret 10-B (site 125)...................................................38 Figure 23. Schematic geological NS cross section beneath profile 1 ............................................42 Figure 24. H/V spectral ratio (black line) and analytical transfer function (grey line) for representative sites of profile 1 ......................................................................................................43 Figure 25. Schematic geological EW cross section along profile 2...............................................45 Figure 26. H/V spectral ratio (black line) and analytical transfer function (grey line) for representative sites of profile 2. .....................................................................................................46 Figure 27. Seismic microzoning map of Tiberias presenting zones of common site effect characteristics.................................................................................................................................49 Figure 28. Examples showing influence of thin upper soft layers on spectral accelerations computed for two sites located on the Cover Basalt. .....................................................................50 5 LIST OF TABLES Table 1. Stratigraphic table of the geological map of Tiberias (Sneh, 2008) ................................11 Table 2. Brief description of wells located in the Tiberias region .................................................12 Table 3. Parameters of earthquakes recorded by accelerometer stations used in this study. Distance is to the surface projection of the rupture........................................................................25 Table 4. Geophysical and analytical models for calculating transfer functions at points located along TB-3 refraction profile .........................................................................................................33 Table 5. Geophysical and analytical models for calculating transfer function at sites located along refraction profile TB-1. ..................................................................................................................35