Seismic Microzonation of the Urban Area Using

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School of Civil Engineering, Sharif University of Technology, Tehran, Iran ... dynamic characteristics of the soil in question, such as the natural period, dominate ...
Seismic Microzonation of the Urban Area Using Microtremor Measurement (Case study: Kermanshah City) H. Sharafi School of Civil Engineering, Iran University of Science and Technology, Tehran, Iran

M. H. Baziar School of Civil Engineering, Iran University of Science and Technology, Tehran, Iran

S. M. Haeri School of Civil Engineering, Sharif University of Technology, Tehran, Iran

ABSTRACT One of the methods by which one can state dynamic characteristics of the soil an area, is the measurement and study of microtremors. Microtremors are small scale vibrations that occur in the ground from unknown sources and have an amplitude range of about 1 to 10 microns. Natural sources, like wind and sea waves or artificial sources like traffic or machine vibration may build up the microtremors in ground. When a microtremor passes along a deposit, it is altered by the seismic characteristics of that deposit. So measurement and analysis of these microtremors can show the dynamic characteristics of the soil in question, such as the natural period, dominate period, and to some extent amplification factor associated with the deposit. Due to importance of the vertical component of the earthquake (as been observed in the recent earthquake), in this project we have worked on the vertical component of the microtremors. Microtremors were measured on 113 sites located on soil deposits. These sites were chosen o profiles that cover the city in two directions.

KEYWORDS:

Microzonation, Spectra Analysis, Dynamic Period, Amplification Factor,

Microtremor.

INTRODUCTION The thing that is important in engineering view at the presence of a alluvium layer in a specific place during earthquake, is the ability to magnify or intensity average motions by the layer. Previous knowledge about the basement of this resonance, specially resonance periods that this phenomenon occurs are important parameters than can be beneficial during designing better strong structures. Magnification can be received by using the written records in different thickness of alluvium for one or several earthquakes.

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But unreality as the result of resource limitation having these records are impossible. To solve this problem there is more than there decade that micro vibration were suggested to find the soil magnification characteristic. To indentify the dominant earth period and study the dynamical behavior of alluvium in the city of Kermanshah during earthquake time the microtremors in 106 region has been measured. Regarding to world earthquake such Manjil earthquake (1990) , Bam earthquake (2003), it has been observed that a lot of damage in our country is the result of a large amount of vertical earthquake component. Study shows that there has not been any significant work on the vertical component of a earthquake and also the effect of existing alluvium. In the tests that has been done in Kermanshah city with measurement the vertical component on alluvium it is tried to work on the dominant period and magnification of alluvium for vertical and horizontal component[1],[4],[5].

INSTRUMENTS FOR MEASURING MICROTREMORS These instruments and accessories were used to measure microtremors in the city of Kermanshah. One microtremor constancy machine Kinemetrics SSR-1 model. Six long period shudder detector, Kinemetrics WR-1 model. A portable computer equipped with software Quick Look (QL) and Quick Talk (QT), that QL software used for observation and drawing of records. The microtremors measuring points and their distribution in the city In this study measuring microtremors was done using the net to dots with the distance of 1 km. these points were existing on the both sides of profile that were vertical on both sides. The profile distance from each other is 1 km and the distance between dots on the profile were 1 km[6],[7].

HOW TO MEASURE MICROTREMORS Because the environment has an effect on microtremors they are measured between 11 p.m and 6 a.m to reduce environmental noises during experimental. To control traffic the streets around measured place were closed [3].

METHOD OF ANALYZING ACQUIRED DATA FROM MICROTREMORS As we mentioned before measuring microtremors in each place of Kermanshah was measured using six shudder detectors and as the result six records found for each measured place. In fact three shudder detectors were for control and other were major detectors. These records actually shows the change in voltage of induced flow in shudder detector moving wire and because the induction flow voltage has direct relationship with the motion speed of wire of shudder detectors these records can be used to identify the dynamical characteristic of earth motion.

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Identifying the predominant period and maximum amplitude at microtremors are done by using Fourier amplitude spectrum or power spectral density in Kermanshah measurment we used Fourier amplitude spectrum analysis [9], [10], [17].

FOURIER AMPLITUDE SPECTRUM Finite conversion Fourier F(w) to acceleration can be obtained using this formula:

𝑖 = √−1 t is the time. The Fourier amplitude spectrum sum of squares of the real and imaginary parts of F(ω) so, 1

2 2 T  T   2 FS (ω ) =   a (t ) sinωt dt  +   a (t ) cosωt dt    0  0  

Because a(t) has the same unit as the acceleration dose. Fourier amplitude spectrum is very important in describing shudder movement in an earthquake[18],[19],[20].

PROCESS OF MICROTREMORS RECORDS Analyzing microtremors and identifying frequency in Nakamura method is based on the spectrum ratio of horizontal component in a vertical component and vice versa. One of the benefits of these methods is elimination of resource and machinery effect. To produce spectrum and their ratio on records the processing steps are described below: Choosing six twenty seconds window of written records. These windows are chosen in a manner that lack resource effect as much impassible and as result they can be best a symbol of microtremors. Among six windows the best window that is the representative of dynamic parameter of soil at that point [1],[13]. Applying band pass filter with edge frequency of 0.05 and 20 Hertz, applying band pass filter arises from this fact that basically in urban measurement the frequency is five to ten Hertz and above and under the effect of urban sources[12],[13]. Applying 10% Hening window to reduce the Gips phenomenon during calculation of spectrum amplitude. Calculation at amplitude spectrum in each window for three components V,NS,EW using sudden Fourier conversion. Average of amplitude spectrum for six windows selection (six twenty seconds) to reduce the amplitude of by passing collar and strength then the amplitude of microtremors. Smoothing the produced spectrum for three component using moving average filter. Combine the horizontal amplitude spectrum together is done by using this equation [6],[8],[14].

(H

2 NS

2 + H EW

)

1 2

HNS: North-South horizontal component; HEW: East-West horizontal component.

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After producing spectrum, spectrum ratio of combined horizontal component to vertical was calculated using Nakamura method. Provide spectrum ratios vertical component to combined horizontal component for natural period earth of vertical motions are recognized. Spectrum graph of three components V, NS, EW accompanying with combined spectrum ration H/V and V/H for each point has been produced. By obtaining the dominant period of horizontal and vertical motion, alluvium of different earthquake width has been received. For example let’s look at one point out of 113 measured points and all the processing steps were given in the Figures 3,4,5,6,7. It needs to mention that in these figures the first graph is horizontal component NS, second graph is EW and third figure is the vertical component of microtremors and are described below.

COMPARING MEASURED RECORDS IN DAY AND NIGHT FOR A SPECIFIC POINT To insure the correct function of SSR-1 machines, microtremors were measured in a specific point in day and night. With comparing these two records it was inferred that the measured record in a day has a greater frequency than in night. In another word frequency of measured records in a day comparing with the record in night is more likely to shift to higher frequency. The reasons for this cause are traffic and environmental noise in a day. Fig. 8 is the record for point G12 that was measured in a night and fig. 9 is its record that has been measured in day. In both cases because of traffic and environmental noise they have higher frequencies than measured record in a night. In another word frequency has been shifted to a higher frequency.

RESULTS A cording to processing steps that was described above the dominant period for horizontal motion and vertical motion and coefficient for relative and absolute intensity coefficient for horizontal and vertical motion of alluvium has been received. The acquired period and intensity coefficient in the production of earthquake width map in Kermanshah city was used the dominant period of points for horizontal and

H

V

vertical motion has been received. By using V and H spectral ratio of microtremors measured on the sediment using Nakamura method (1998) for horizontal motion for deposit, the results are as follow: The dominate period of the ground in different parts of the city were calculated for horizontal and vertical components. Dynamic amplification factor for vertical and horizontal ground motions have been qualitatively carried out for each site. Seismic microzonation maps of Kermanshah city for vertical and horizontal vibrations and different period range have been produced. The collected information can be used for site selection from different structures such as high-rise building in the city of Kermanshah and can be used for city planning and future development of the city. Also the relative intensifying coefficient of points in four graphs with relative intensity of (0-1) very low, (1-2) low, (2-4) average and higher than 4 high for horizontal and vertical motion has been classified. These classification has been brought in a form of widths for horizontal and vertical motions. The numbers inside parentheses mean magnification coefficient of alluvium intensity.

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Also width with different period was produce in a range of (0-0.25), (0.25-0.5), (0.5-0.75), (0.75-1), (1-1.5),(1.5-2) and (2 or higher) for horizontal motion and in period range of (0-0.25), (0.25-0.5), (0.50.75), (0.75-1) and (1 or higher) for vertical motion that different widths were brought in figures 10 to 13.

Figure 1: the record obtained microtremors at Point Figure 2: recording at the point A2 after base line there were no processing. A2 that correction and instrumental correction . .

Figure 3: The record obtained microtremors at Point A2 after filtering.

Figure 4: Fourier spectrum records for three NS,EW,V components

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Figure 5: The Fourier spectrum after smoothing.

Figure 7: The record for point G12 that was measured in a night

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Figure 6: Corrected records at the point A2 based on time

Figure 8: The record for point G12 that was measured in day

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F Figure 9: Miccrozonation of o the Kermannshah area on the basis of different d ampplification for horizzontal movem ment

F Figure 10: Miicrozonation of the Kermaanshah area onn the basis off different ampplification for verrtical movemeent

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Figure 11: Microzonnation of the Kermanshah K a on the baasis of variatioon of area the variety predominantt period for hoorizontal movvement

Figure 12: Microzonnation of the Kermanshah K a on the baasis of variatioon of area the varietty predominannt period for vertical v moveement

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REFERENCES 1. Aki, K. (1988) “Local Site effects on Strong Ground Motion”, Earthquake Engineering & Soil Dynamic. II, PP. 103-155. 2. Haeri, S.M., Kholafaie, M. (1994) “Local Site Effects in the City of Rasht During the Manjil Earthquake of June 20, 1990, Iran”, Proc. of the Second Int. Conf. on Earthquake Resistant Construction and Design, Berlin. 3. Haeri, S.M., Hajialilue, M. (1999) “vertical And Horizontal Seismic Microzonation by Microtremor Measurment”, Proc. Of the Third int. Conf. on Seismology and Earthquake Engineering May 17-19, 1999, Tehran, I.R.Iran 4. Ghayamghamian, M.R., Mogi H., Kawakmi, H. (1995) “Mirotremor Data Analysis for Seismic Microzonation in Earth of Tehran”, Earthquake geotechnical Engineering, Ishihara (ed.) 1995 Bakema, Rotterdam, ISBN 90540578 5. Ghayamghamian, M.R., kawakmi, H. (1998) “Segmental Cross-Spectrum in Mirotremor Spectral ratio Analysis”, Structural Safety And Reliability Shiraishi, Shinozuka & Wen(eds) Balkema, Notterdam, ISBN 9054109875 6. A-M. Duval, J-P. Mneroud, S.Vidual & P-Y.Bard (1995) “usefulness of Microtremor Measurments For Site Effect Studies” 10th Europe Conference on Earthquake engineering, Duma(ed.) Balkema, Notterdam, ISBN 9054105283 7. Bour M., Fouissac D., Dominque P. & Martin C. (1998) “on the use of Microtremor Recording in Seismic Microzonation” Soil Dynamic and Earthquake Engineering 17(1998) 465-747 Elserier Science Ltd. Printed in Great Britain 0267-726179818 8. Lemo J., Chavez-Garcia J. (1994) “Are Microtremors Useful in Site Response Evaluation” Bulltin of Seismological Society of America, Vol. 84, No 5, PP 1350-1346m October 9. Bouckvalas G., Krikeli I. (1991) “Effect of local soil Stratigraphy on microtremor Measurements”, Proc. Second Int. Conf. on recent Advance geotechnical Earthquake Engineering and Soil Dynamic, March 11-15, St. Louis, Missouri, Paper No 8-21 10. Bard, P.Y. et al. (1996) “Seismic Zonation Methodology for The City of Nice”, Progress Report. 11. Japan Working Group for TC-4 Committee (1993) “Seismic Zoning on Geotechnical Hazard”, Japan Society of Soil Mechanics and Foundation Engineering. 12. Ishihara, K., Ansal, A.M. (1982) “Dynamic behavior of soils, soil amplification and SoilStructure interaction”, Final Report For Working Group D.,UNDP/UNESCO Project on Earthquake Risk Raduction in the Balkan Region. 13. Bard, P. Y., (1995) “Effects of Surface Geology on Ground Motion Results and Remaining Issues”, The new methods of Microzonation and sitedependent seismic Design, International Instiute of Earthquake Engineering and Seismology, Iran. 14. Nakamura Y. (1989) A method for dynamic characteristics estimation of subsurface using microtremor on the ground surface’’ QR of RTRI, Vol. 30, No. 15. Kagami, H., S. Okada, K. Shiono, M. Oner, M. Dravinski and A.K. Mal (1986) Observation of 1 to 5 second microtremors and their amplif ication to earthquake engineering. Part III. A twodimensional study of site effect in S. Fernando Valley, Bull. Seism. Soc. Am., No.76, pp.1801- 1812.

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16. Kanai, K. and Tanaka T. (1961) On Microtremor VIII, Bull.Earthq. Res. Inst., Tokyo university, Vol.39, pp. 97-114. 17. M. Bour, D. Fouissac, P. Dominque and C. Martin (1998) “On the use of microtremor recordings in seismic microzonation” Soil Dynamics and Earthquake Engineering, Volume 17, Issues 7-8, 12 October, pp. 465-474. 18. Sankar Kumar Nath, (2005) “An initial model of seismic microzonation of Sikkim Himalaya through thematic mapping and GIS integration of geological and strong motion features.” Journal of Asian Earth Sciences, Volume 25,Issue2,May, pp. 329-343. 19. S. Mukhopadhyay and P. Bormann, Low cost seismic microzonation using microtremor data: an example from Delhi India. Journal of Asian Earth Sciences, Volume 24, Issue 3, December 2004, pp. 271-280. 20. Ansal, A. Editor (2004) Recent Advances in Earthquake Geotechnical Engineering and Microzonation, Kluwer Academic Publishers

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