SITE EFFECTS FROM AMBIENT NOISE MEASUREMENTS AND SEISMIC HAZARD ASSESSMENT IN NORTHERN

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

1 TABLE OF CONTENTS

LIST OF FIGURES AND TABLES 2 ABSTRACT 4 1. INTRODUCTION 5 2. LOCAL 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 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 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. the SHAKE program (Schnabel at al., 1972). From November 2001 to May 2002 we carried out measurements along 22 lines from Ashkelon to Haifa (Zaslavsky et al., 2002). We selected 18 of them to be perpendicular to the coastline and intersect the different geological units, and four lines along the coastline. The 6 distance between lines was approximately 5km. About 180 sites were instrumented for this period. Owing to financial support of the Steering Committee for the National Earthquake Preparedness and Mitigation we continued the investigation of site effect in the Shefela region. From July 2002 to April 2003 about 300 noise measurements were carried out. The size of the study area is approximately 2300 km2. The soil sites exhibit H/V peak amplitudes ranging from 1 to 8 within the frequency band 2-10 Hz. In this report we include investigation of seismic hazard to the northern part of Tel-Aviv. The performed work consists of some phases:  Empirical evaluation of potential enhanced ground motion in soft sediments using dense recording grid of ambient noise. Reliable prediction of site amplification in the investigated area and producing maps of the distribution of the fundamental frequency and amplification.

 Integrated analysis of noise measurement results and available geological, borehole and geophysical information to construction of systematic and coherent multi-layer soil- column models that consequently yield analytical site response function.

 Dividing the study areas into zones and characterize each of them by a generalized soil column model. Use SEEH procedure of Shapira and van Eck (1993) for assessing Uniform Hazard Site-Specific Acceleration Spectra for each zone. 2. Local Geology

The investigated area of 1 km wide and 2.8 km long stretches along the shoreline of northern Tel Aviv. Data on geology of the region are mainly collected from Gvirtzman (1965, 1969 and 1984), a new version of the geological map of Israel to a scale of 1:200,000 (Sneh et al., 1998), and information from water wells and our observations. 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”. They are indicated on the location map (see Figure 1). Alluvium sediments. Sediments of Holocene age are composed of sand, soil, gravel, clay and loess. These sediments are developed along river beds with thickness 0-15 m and are distributed in a south-north oriented strip along the eastern border of the study area. Sand dunes of Holocene age. These sediments belong to the Taarukha Mb. and are exposed along the coastline. Sand dunes usually overlay either sandstone or loam. The thickness of these sediments varies from 0 to 10 m. 7 Kurkar Group of Pleistocene age comprises marine and eolian calcareous sandstones named “kurkar”, some reddish silty-clayey “hamra”, silts, clays, loose sands, loam and conglomerates. According to Gvirtzman (1984) the Kurkar Group in the study area belongs to the western province and it is represented by the calcareous sandstones (“kurkar”) with intercalation of clayey-silty sandstones (mostly “hamra”) and finer grained sediments. The thickness of the Kurkar Group reaches 160-200m near the shoreline. The Kurkar Group unconformable overlays the Yafo clay (Pliocene age). In our study we focuses mainly on the upper and middle parts of the Kurkar Group from the Ashdod Mb. Characteristics of the Quaternary lithostratigrafic units represented in the study area are given in Table 1. Table 2 lists the borehole data utilized in our study.

Table 1. Lithostratigraphic column of the Quaternary sediments represented in northern Tel Aviv (based on Gvirtzman, 1970, 1984).

Thickness Member Number Lithology Abbreviation Stage (m) (Mb)

Sand, soil, loess, clay and 1 0 -15 Recent al gravel

2 Sand dunes 0-10 Taarukha qs Holocene Eolinete ''kurkar'' calcareous 3 10-15 Tel Aviv qk sandstone 4 Red sand and loam ''Hamra'' 1-5 Netanya qh Eolinete ''kurkar'' calcareous 5 15-40 Giv'at Olga qk

sandstone

6 Silty sand and loam ''Hamra'' 3-5 KfarVitkin qh

Eolinete and marine ''kurkar'' 7 10-40 Herzlia qk calcareous sandstone Pleistocene

8 Red sand and loam ''Hamra'' 0-10 Poleg qh

Eolinete and marine ''kurkar'', 9 calcareous sandstone and 20-60 Ashdod qk sand

8

Table 2. Water well data in the study area. N EW NS Name Depth(m)

1 179800 669990 L3300 45 2 180000 669950 L3301 38 3 180460 669790 L3302 65 4 180110 670180 HR 41 5 180527 670957 L342 55 6 180247 671167 L340 65 7 180360 671020 SH34/A 57 8 181100 670300 Jalil 42 9 181750 672150 Zafrir 1 75 10 181532 672500 HH3 51

3. Observations

The recorded in urban areas of Israel can be considered as caused by human activity (cars, factories, etc.) and propagates as high-frequency waves (> 1 Hz) or by oceanic waves, which propagates over large distances in the low-frequency range (< 1 Hz). All the analyzed data were recorded using the multi-channel digital seismic data acquisition system designed for site response field investigations (see Shapira and Avirav, 1995). The data acquisition system includes: a multi-channel amplifier with band pass filters 0.2-25 Hz, GPS (for timing) and a laptop computer with a 16 bit/word analog-to-digital (A/D) conversion card with a sampling rate of 100 samples/s. The seismometers are velocity transducers (L4C by Mark Products) with a natural frequency of 1.0 Hz and damping at 70% of critical. All the equipment: sensors, power supply, amplifiers, personal computer and connectors are carried in a vehicle, which also serves as a recording centre. The seismometers are fixed on levelled metal plate placed directly on the ground. Prior to performing measurements, the individual seismometer constants (natural frequency, damping and motor constant) are determined using sine and step calibration signals, and then the frequency response functions of all channels are computed. This procedure allows evaluating change of natural frequency and motor constant (voltage sensitivity) during long time of measurements in harsh conditions in the free field. The seismic stations characteristics are presented in Table 3. In the final instrumental test, all seismometers are placed at the same location and in the same orientation to record the same waves. 9

Table 3. Seismic station characteristics.

Generator Site Sensor Frequency Damping Component Code Const. at 1 Hz Number Number Hz % V/m/sec 3405 Vertical V405 94.8 1.00 70 1-35, 37- Horizontal 3210 H210 115.6 1.00 67 40, (NS) 41, 43-50 Horizontal 3401 H401 109.2 1.00 65 (EW) 2753 Vertical V753 114.4 1.00 67 Horizontal 36, 40a, 42, 2746 H746 112.8 1.00 66 50a (NS) Horizontal 2745 H745 112.0 1.00 66 (EW) 3226 Vertical V226 119.2 0,98 72 51, 52, 53, Horizontal 3233 H233 124.2 1.00 69 54, 56, 59 (NS) Horizontal 3232 H232 122.8 1.00 69 (EW) 3225 Vertical V225 123.4 1.00 66 Horizontal 55, 57, 58, 3236 H236 119.5 0.99 75 60 (NS) Horizontal 3235 H235 115.0 1.00 75 (EW)

Normally the differences between the seismic channels are marginal even without “correcting” for the instrumentation response. In fact, at frequencies below the natural frequency of the seismometers, correcting for the instrument response may increase variability and scattering. We note that all channels (horizontal and vertical) have practically the same frequency response function and amplifiers are set to the same gain level. Hence, spectral ratio may be calculated on the recorded signals. Moreover, it is possible to assess the predominant frequencies in the H/V spectra also at frequencies much lower than the natural frequency of the seismometer. We carried out 60 ambient noise measurements from September to November, 2008. The work area is approximately 2.8 km2. The locations of measurement points are shown in Figure 1. Coordinates of the measurement points are summarized in Table 4. In Figures 2 and 3 we present examples of the locations of the seismic stations at different lithological units and in the urban area during the site investigations in the northern Tel Aviv area.

10

Figure 1. Locations of measurement points, refraction lines and boreholes in the study area.

11

Figure 2. Examples of locations of seismic stations in northern Tel Aviv at different lithological units: 1 – alluvium; 2 – sand dunes; 3 – hamra; 4 - kurkar; 5 - coast sand; 6 - soil

12

Figure 3. Measurements of ambient noise in northern Tel Aviv.

13

Table 4. The locations of measurement points.

Point Coordinates Point Coordinates

number EW NS number EW NS 1 181147 672119 31 180779 670965

2 180927 672366 32 180659 670104

3 180628 672376 33 180776 670344 4 180613 672116 34 180650 670395 5 180878 672076 35 179795 669986

6 180611 671597 36 179717 670016 7 180329 671904 37 180000 669968

8 180589 671850 38 180345 670435

9 180619 671370 39 180602 670558

10 180526 670961 40 180840 670661 11 180352 670622 41 180080 671103

12 180500 672091 42 180163 671102 13 180391 671588 43 180924 671097

14 180257 671556 44 180851 671324 15 180226 671423 45 180637 672532

16 180391 671356 46 180883 671808 17 180385 671184 47 181029 672009

18 180083 670161 48 181141 672399 19 180123 670391 49 180900 671634

20 179926 670366 50 180420 669774

21 180144 670621 51 180360 670877

22 179956 670561 52 180253 671176

23 179827 670187 53 180365 671012

24 180316 670127 54 180566 671149

25 181371 672130 55 180223 670793 26 181408 672386 56 180653 670882

27 180983 672417 57 180796 671200 28 181336 671918 58 180887 670903

29 181134 671601 59 180029 670824 30 180855 670791 60 180197 671240

14 4. Data processing

To study the characteristics of spectra of the ambient noise signals, we compute Fourier spectra and spectral ratios. The record length (time window) used for spectral calculations depends on the fundamental frequency. The basic criterion is to choose the minimal time window which yields spectra that practically do change when the record length is increased. We have concluded that at sites with fundamental frequencies of 1 Hz (or more) we should use a record of at least 30 sec. The selected time windows are Fourier transformed, using cosine- tapering (1 sec at each end) before transformation and then smoothed with a triangular moving Hanning window. More precisely, we apply “window closing” procedure (see Jenkins and Watts, 1968) for smart smoothing of spectral estimates so that any significant spectral peaks are not distorted. The H/V spectral ratios are obtained by dividing the individual spectrum of each of the horizontal components [SNS(f) and SEW(f)] by the spectrum of the vertical component [SV(f)]:

S NS  f  S EW  f  ANS  f   AEW  f  (1) SV  f  SV  f 

The average spectral ratio for each of two horizontal components is computed, if the curves of average spectral ratios of the two components are similar then the average of the two horizontal-to-vertical ratios is defined as:

1  n S  f  n S  f   A f     NS i   EW i  (2) 2n  S  f  S  f   i  1 V i i  1 V i 

The length of recorded ground motions (ambient noise) may affect the results and influence the reliability and applicability of the technique. According to Teves Costa et al. (1996), Teves Costa and Senos (2000), Dravinski et al. (2003) a total ambient noise recording of five minutes is sufficient. Lebrun et al. (2004) and Garcia- Jerez et al. (2006) advocated recording time of 10 minutes, while many authors (e.g., Parolai et al., 2001; Ferretti et al., 2007, and others) suggest that signals should be recorded for at least 30 minutes. From comparison between the average H/V curves obtained from ambient noise recordings of different durations we conclude that recording for about 1-2 hours provides reliable estimate of the H/V function.

15 5. The Stability of the Horizontal–to-Vertical Spectral Ratio of Ambient Noise

The scientific literature reports (Duval et al., 1998; Mucciarelli et al 2003, Zaslavsky et al., 2005) that data from continuous measurements of ambient noise during several months as well as repeated measurements in different days, month, and years show that the shapes of the average spectral ratios at the same site under the same conditions of measurements yield almost identical results. Nevertheless, in our opinion, the stability and reproducibility of H/V ratio must be confirmed in each experimental study before analyzing and interpreting ambient noise measurements. Examples of average Fourier spectra obtained at Point 21 in different month are displayed in Figure 4a. The NS and EW components of spectra at this point for both sets of records have a sharp peak near 3.0 Hz. At the same time, the vertical spectra are flat in frequency range 1.5-5.0 Hz. As demonstrated in Figure 4b, the evaluated spectral ratios for a given site are the same although the ambient noise measurements were made on different dates.

a b

02.11.08 8

04.12.08

Figure 4. Comparison of (a) average Fourier spectra of three components of motion and (b) individual and average H/V spectral ratios observed at point 21 in different months.

The next example (Figure 5a) shows H/V spectral ratios obtained at Point 6 from measurements made in October and in December 2008. Variations of individual functions are small and all curves are similar in shape. Here, the dominant feature of the spectral ratios is a 16 peak near 5.0 Hz with H/V ratio about 4.0. Comparison between average spectral ratios for two sets of measurements demonstrates the resemblance (Figure 5b).

a

b

Figure 5. Comparison of (a) individual and (b) average H/V spectral ratios from ambient nose observed at Point 6 in different month.

Figure 6a shows the average noise spectra at Point 55 obtained in different years (January 2002 and December 2009) but in similar weather. Comparing the spectra, we can see that for both sets of records the frequency range, is 2.0-5.0 Hz, where horizontal and vertical spectra deviate. The amplitudes of horizontal spectra increases at frequencies near 2.5 Hz and consequently the average spectral ratios (Figure 6b) have peak at this frequency. One can see that the shapes of the average spectral ratios are in a good agreement not only in the range of predominant frequencies but throughout the investigated frequency range from 0.5 to 20 Hz. Figure 7a shows average noise spectra at Point 60 obtained in different years. Slight variations in shapes of spectra obtained from both sets of records can be attributed to the changing level and spectra of noise related to urbanization of investigate site. Nevertheless, the NS and EW components at this site have a sharp peak near 2.0 Hz and the average spectra ratios as well (Figure 7b). We note that the average curves are identical.

17

a

b

Figure 6. Comparison of (a) average Fourier spectra for three components of motion and (b) average H/V spectral ratios from ambient noise observed at Point 55 in different years.

a

b

Figure7. .Comparison of (a) average Fourier spectra of 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. 18 6. Variations of spectra and H/V ratio shapes

The spectral analysis of ambient noise provides important information that enables us to characterize the site response especially in places where the local geological conditions may be responsible for intense ground motions. Qualitative analysis of the distribution of parameters that characterize amplitude spectra and spectral ratios over the study areas shows a high correlation with geological conditions and may be used to roughly delimit areas with different subsurface structure prior to modeling. Figure 8 displays the average amplitude spectra of two horizontal (NS and EW), the vertical components of motion and individual and average spectral ratios obtained at Points 18, 21, 55, 17 and 16 located along the line southeast to northwest (point locations and their I.D. numbers are shown in Figure 1). We note that spectra of horizontal components (EW and NS) with regard to vertical one is observed only from 2.5 Hz to 3.5 Hz. A common feature to the presented examples is the appearance of a single peak in the H/V spectral function which also coincides with a peak in the amplitude spectrum of the horizontal motion. Such spectra correspond to a simple model of the subsurface where a soft layer overlay rock. In such a simple model, the frequency will correspond to the fundamental resonance frequency of the soil, be proportional to the S wave velocity in that layer and inversely proportional to its thickness. The height of the H/V peak is proportional to the seismic impedance between the soil and the rock. Hence, the variations in the spectra and spectral rations at the sites are directly associated with variations in the geological structure of the subsurface. Figure 9 illustrates the characteristics average spectra of ambient noise recorded at Points 6 and 42 and their individual and average spectral ratios. In the vertical spectra there are narrow-bandwidth troughs at the frequency near 5.0 Hz for Point 6 and near 2.0 Hz for Point 42. Hence, the general characters of the spectral ratios are clear amplifications (Figure 9b) at frequencies near 5.0 Hz (Point 6) and 2.0 Hz (Point 42). Figure 10 displays the average amplitude spectra of two horizontal (NS and EW), the vertical components of motion and spectral ratio (individual and average) obtained at Points 15 and 43. This example demonstrates that careful selection of ambient noise time windows and their analysis may reveal resonance effect of the soil layer even when the seismic impedance is low. Comparing the horizontal and vertical Fourier spectra of Point 15 one can see at the frequency range 2.0-3.0 Hz that the horizontal components are higher than the vertical. This feature, emphasized in the H/V spectral ratio, relates to amplification of ground motion at frequency near 2.5 Hz. For Point 43, in the vertical spectra there is a through near 7.0 Hz, which is responsible for the peak in the H/V ratio. 19

a b

Point 18

Point 21

Point 55 48C

Point 17

Point 16

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. 20 a b Point 6

Point 42 4242 42

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. a b

Point 15

Point 43

Figure 10. Examples of (a) average Fourier spectra and (b) individual and average H/V spectral ratios for sites whose subsurface structure yields low impedance contrast between sediment and reflector. 21 Figure 11 shows observed horizontal-to-vertical spectral ratios obtained from ambient noise recorded at Points 28 and 47, which are only 300 m apart. These sites demonstrate the great variability in site response over very short distances. Average spectra ratio for Point 28 shows a prominent peak at about 10 Hz with amplitude about 4.0 while the average spectral ratio of Point 47 shows prominent peak at near 4. 0 Hz and its amplitude increase up to 6. Point 28 Point 47

Figure 11. Example of significant variations of site effects over short distance.

One feature consistently observed in all the results obtained from ambient noise measurements is the predominance of vertical component over the horizontal components in the frequency range of 0.1-1.5 Hz. This "deamplification" feature is emphasized in the H/V spectra ratio in the frequency range 0.1-1.5 Hz, as we have described in our previous investigation of site effect in the Coastal Plan area (Zaslavsky et al., 2004). This strange phenomenon may be explained by specific interaction between the swell and the sea bottom, when vertical component motion is predominant.

7. Distribution of H/V resonance frequencies and their associated amplitude levels

Damage distribution due to large earthquakes is frequently controlled by site effect. The ambient noise measurements yield a series of parameters to estimate the expected ground motion during an earthquake. Fundamental resonance frequencies and their amplitude levels at all measurements points across the northern Tel Aviv are summarized in Table 5. In accordance with this table the resonance frequencies vary from 2.0 Hz to 12 Hz with H/V amplitude in the range of 2-6. The maps in Figure 12, created on the basis of measurement data (see Table 5), show distribution of the resonance frequency and maximum values of H/V spectral. These figures integrate all the experimental data obtained in this study. The dominant frequency in the eastern part of the area is 7-12 Hz and decreases to 2.0-2.5 Hz in the western part and related to the thickness of the loose sediments represented by alluvium, sand dunes, loam "hamra" and 22 heterogeneous unit "kurkar". The map of maximum spectral ratios (Figure 12b) reflects the variations of the impedance between the reflector (Calcareous sandstone, Kurkar) and the overlying sediments. At Points 47 and 49 we observe H/V ratios with amplitude up to 6. This observation could be associated with thickness of alluvium sediments accumulated along stream where now waste water flows. At Points 30 and 40 the amplitudes of H/V ratios decrease to factor 1 (no amplification) that could be connected with outcropping of calcareous sandstone.

Table 5. The resonance frequency and its associated H/V amplitude in northern Tel Aviv area.

Associated Fundamental Associated Point Fundamental Point H/V frequency H/V number frequency (Hz) number amplitude (Hz) amplitude

1 3.5 2.5 31 5.0 3 2 4.5 3.5 32 4.0 4.0 3 2.5 2.5 33 7.0 2.0 4 3.0 2.3 34 3.5 4.0 5 6.0 3.0 35 3.0 2.0 6 5.5 3.5 36 2.5 3.0 7 2.5 3.0 37 3.3 3.0 8 4.5 4.0 38 4.0 3.5 9 4.5 2.5 39 3.5 2.5 10 3.3 3.5 40 - 1 11 3.5 4.0 41 2.5 2.5 12 2.5 3.0 42 2.3 3.0 13 3.0 2.5 43 7.5 2.0 14 2.5 2.5 44 3.5 3.0 15 2.5 2.0 45 2.5 2.5 16 3.0 2.0 46 4.0 5.0 17 3.0 3.0 47 4.0 5.5 18 3.5 3.0 48 3.5 3.0 19 3.5 3.5 49 3.5 6.0 20 2.0 2.5 50 3.7 3.5 21 3.5 3.5 51 3.5 3.5 22 2.5 3.8 52 3.0 3.0 23 2.3 3.0 53 3.5 3.0 24 3.5 4.0 54 4.3 4.0 25 12 2.5 55 2.8 3.0 26 7.0 2.0 56 3.5 3.0 27 4.0 3.0 57 3.5 2.0 28 9.5 2.5 58 8.7 2.8 29 10 2.0 59 2.0 2.5 30 - 1 60 2.4 3.0

23

Hz a 8

5

6

7

2

0

0

0 4

3

2

6

7

1

0 No 0 0 Resonance frequence

6 b

7

0

0

0 0

6

7

2

0

0

0

180000 181000

6

7

1 0 5

0 0

4

3

6

7

0

0

0 2 0

No amplification 180000 181000

Figure 12. Distribution of the resonance frequency (a) and its associated H/V amplitude (b) in northern Tel Aviv area.

24 8. Validation of H/V spectral ratio by seismic refraction survey

Many authors assume ambient noise is primary composed of surface waves. Others claim that the H/V spectrum of ambient noise is dominated by the upward propagation of the SH wave through the layered media. The authors of this study demonstrated, through many previous studies where noise measurements were made near refraction surveys, that the fundamental frequency and its corresponding H/V amplitude derived from analysis of ambient noise are practically the same as fundamental frequency and its corresponding amplification level derived from the computed transfer function of SH waves at low strains propagating through a relatively simple 1-D model of the site, known from geotechnical and geophysical surveying. In this study we carried out two seismic refraction lines on purpose to obtain the velocity distribution of compression (VP) and shear waves (VS) with depth. The position of the refraction lines (TA) are indicated in Figure 1 and summarized in Table 6. The data acquisition were carried out using Summit II Plus seismic recorder, manufactured by DMT, Germany, with 96 channels where 48 of which were used for both lines TA-1 and TA-2. Vertical 10 Hz geophones

(for VP-wave) and horizontal 8 Hz geophones (for S-wave) were inserted into the earth with distance separations of 5 m (for line TA-1) and 4m (for line TA-2). Digipulse seismic source was operated in accordance with the Generalized Reciprocal Method (Palmer, 1986): data were acquired using 5 shots: at the ends of lines, at a distance of about 70-100 meters from each end and the center of each line. Examples of P and S-wave field records are presented in Figure 13.

Table 6. Location of the seismic refraction lines at northern Tel Aviv area.

Line Station Line length Coordinates

number number m Easting Northing

TA-1 S1 0 180374 670843

TA-1 48N 235 180478 671056 TA-2 S1 0 180522 671507 TA-2 48N 188 180582 671686

Data collected from two seismic refraction lines providing information on the P- and S- wave velocities and sediment thickness within the accuracy and resolution of the geophysical technique are given in Table 7. One can distinguish at TA-1 section two layers with different Vs velocities: the uppermost layer from the surface down to 18-26 m has shear wave velocity of 310 25 m/sec and the second layer under 18-26 m Vs is of 1120 m/sec (see Figure 14). At TA-2 section three layers can be distinguished. The uppermost layer from the surface down to 7-10 m has shear wave velocity of 270 m/s. The second layer in the depth range of 7-10 m to 21-25 m is characterized by shear wave velocity of 640 m/sec. Finally lowermost layer under 21-25m has shear wave velocity of 980m/s (see Figure 15).

Figure 13. Examples of the P-wave (a) and S-wave records at refraction line TA-2.

Table 7. Results of the seismic refraction survey.

Depth range VS Depth range Vp Layer Layer m m/sec m m/sec Line TA-1 1 0 to 10 500 1 0 - 18 310 2 10 to 18-26 1530 2 Under 18-26 1120 3 Under 18-26 2190 Line TA-2 1 0 - 7-10 270 1 0 to7-10 460 2 7-10 to 18-26 640 2 7-10 to 18-26 1800 3 Under 18-26 980 3 Under 18-26 2280

26 The key parameters to analytically evaluate site effects by using computer codes such as SHAKE (Schnabel et al., 1972) are the S-wave velocity of the unconsolidated sediments, thickness of each layers density and specific attenuation in different lithological units as well as S-wave and density of the hard rock acting as a seismic reflector. Data collected from two seismic refraction profiles provide information on the velocity and thickness of sediments at points located very close to refraction lines and velocity the seismic reflector. These data summarized in Table 7. Densities and specific attenuation in different lithological units we selected on base of literature sources (see Table 8). These data we used as trial model. The stochastic optimization algorithm (Storn and Price, 1995) is applied in order to fit analytical transfer function to observed average H/V spectral ratio. Figures 14 shows vertical cross section along line TA1 (lower frame) and analytical transfer functions compared with average H/V spectral ratios obtained at Points 51, 53 and 54 (upper frames). The prominent feature of these curves is a wide band of frequencies which is fair agreement with the 1-D theoretical transfer functions. The average averaged H/V spectral function for Point 6 near the AT-2 line is shown in Figure 15. The analytical transfer function and the corresponding H/V spectral ratio of Point

6 show peak at the fundamental frequency 5.5 Hz. We note that both thickness and VS after fitting practically coincide with those obtained from two seismic refraction lines (Table 6). There is a clear disagreement between lithological column and velocity-depth section for both wells.

Table 8. Subsurface model along lines TA-1 and TA-2 inferred from seismic refraction survey and optimal 1-D model of soil column.

Seismic refraction survey Optimal 1-D model of soil column

TA-1 Measuring V , Thickness V Thickness Damping S S Density point m/sec m m/sec m % 51 26 330 23 1.6 3 53 310 26 310 26 1.6 3 54 18 310 19 1.6 3 Half space 1120 - 1120 - - -

TA-2

270 10 270 10 1.6 3 6 640 14 640 15 1.8 2 Half space 980 - 1100 - - - 27

a

7 7 b Well 7 7 c 10 40 70 100 130 160 190 220 0 0 0 0 0 --1010 -10 -10 Vs=310 m/s -10 -10 --2020 -20 -20 -20 -20

--3030 -30 -30 -30 -30 Vs=1120 m/s

-40-40 -40 -40 -40 -40

-50 Loam, sandy loam -50 -50 -50 -50 -50 Loam,Loam, sandy sandyloam loam CalcareousCalc.sandstone and sandsandstone and sand --6060 Calc.sandstone-60 and sand Fig.16a Fig.15aFig.16a Fig.15a 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 obtained at Points 51, 53 and 54. Triangles indicate measurement sites near TA-1.

7

a b 0 6 10 40 70 100 130 160 190 c 00 0 Vs=270 m/s -10 -10-10 -10

-20 -20-20 -20 Vs=640 m/s

-30 -30-30 -30

-40 -40-40 -40 Vs=980 m/s

-50 -50-50 -50

-60

Figure 1Fig.16a5. (a) Columnar section of geological structure near Point 6, (b) vertical cross section along line AT-2 andFig.15a (c) analytical transfer function (blue dashed line) compared with average H/V spectral ratio (red line) obtained at Point 6. 28 Nakamura (1989, 2000) hypothesized that site response could be estimated from ambient noise. Many studies show that the H/V ratio coincides with response function of near surface structure to incident shear wave. However, other authors (for example, Satoh eat al. 2001) conclude that only the predominant peak of H.V ratio is correlated with the fundamental resonance frequency. Our numerous studies as well as this study showed that the Nakamura method can predict, in addition to the fundamental frequency, amplifications that are similar to those modelling technique.

9. Seismic hazard microzonation

The amplification of the ground motion during earthquakes due to geological site conditions makes it necessary for urban areas to perform detailed seismic hazard assessment. Under seismic hazard microzonation we understand subdividing of an area into zones with respect to geological characteristics of the sites and thus characterize the site specific seismic hazard at different locations. We use H/V measurements of ambient noise together with available geological and geophysical information to construct subsurface models for the investigated regions. These models may be used for seismic hazard microzonation across the study areas. The grouping of H/V observations is done manually taking into consideration the fundamental frequency, amplitude and the shape of H/V spectral functions. For example, the assemble of H/V spectral ratios from different sites in Zone II in northern Tel Aviv, the average spectra ratio and analytical 1D response function are depicted in Figure 16.

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). 29 The site specific acceleration spectra of a given site may be obtained by convolution of the analytical response function with observed and/or synthesized strong motions. In a series of previous studies we successfully applied the procedure developed by Shapira and van Eck (1993) to assess the site specific uniform hazard acceleration response. That procedure which we term SEEH (Stochastic Estimation of the Earthquake Hazard) is based on the stochastic method developed and used by Boore (1983), Boore and Atkinson (1987), Boore and Joyner (1991) among others. In brief, the SEEH process starts by performing Monte Carlo simulations of the expected seismic activity in seismogenic zones that may affect the study area. The seismicity and other regional parameters that characterize earthquake hazards in Israel are presented and discussed by e.g. Hofstetter et al. (1996), Shapira and van Eck (1993), Shapira and Hofstetter (1993), Shapira (2002), Sellami et al. (2003), Begin (2005) and Begin et al. (2005). These studies were used to specify the seismogenic zones affecting the region and their seismicity in the form of frequency-magnitude relationships. Considering the uncertainty in estimating the frequency of occurrence, we generate several possible earthquake catalogues (artificial catalogues) for a long time span (thousands of years). These simulations are followed by synthesizing S wave ground motions at the investigated site from each of the earthquakes in the artificial catalogues using the stochastic method (e.g. Boore 1983, 2000). The site response due to the propagating waves from the base-rock to the site's surface are computed given the properties and structure of the subsurface at the analysed site. These wealth of synthetic free surface motions, representing what may happen in a long time span are used to compute the corresponding acceleration response spectra for a 5% damping ratio. In the final stage of SEEH all generated response spectra are assembled for estimating the spectral accelerations which correspond to a prescribed probability of exceedance and yield the uniform hazard, site specific acceleration response spectrum. The uncertainties associated with the values assigned to different parameters in the computations are considered by performing Monte Carlo simulations throughout the SEEH process. This involves the uncertainty in Q-values, magnitudes, seismic moments, stress drop and attenuation derived from previous studies performed in the region. The seismic hazard functions, i.e., the Uniform Hazard Site-specific Acceleration spectrum are computed for 10% probability of exceedence in an exposure time of 50 years and a damping ratio of 5%. Seismic microzonation map (Figure 17) presenting zones of common site effect characteristics were prepared for northern Tel Aviv. Each zone is characterized by a generalized seismic hazard function representative of the sites within that zone. The acceleration response spectra for all zones are shown in Figure 18. The soil column models leading to these generalized functions are given in Table 9. 30 For comparison, we also plot the design spectra required in the same area by the current Israel Standard 413 (IS-413) and for ground conditions that meet the BSSC (1997) soil classification scheme. It is very important that for study area of only 2.8 km2, considerable difference between response spectra of different zones is observed. If for Zones I, II and IV IS- 413 code underestimates response spectra calculated by SEEH method in the period range 0.15- 0.5 sec; for Zones III and V the Israel code overestimates the calculated spectra in all period range

6

7

2

0

0

0

6

7

1

0

0 0

Zones:

f= 2-3 Hz A=2-3

f= 3-4 Hz A=2-4

f=4-7 Hz A=2.5-4

6

7

0

0

0

0 f=3.5-4 Hz A=5.5-6

f=7-12 Hz A=2-3

No site effect

180000 181000

Figure 17. Seismic microzonation map of northern Tel Aviv with respect to acceleration response spectra calculated by SEEH. 31

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.

Table 9. Soil column models for calculating generalized acceleration response spectra for zones.

Zone Thickness Vs Density Damping m m/sec g/cm3 % 14 300 1.7 3 1 38 500 1.8 2 Half space 1120 2.2 - 23 310 1.7 3 2 Half space 1120 2.2 - 10 270 1.6 3

3 15 640 1.9 2

Half space 1100 2.2 -

6 160 1.6 4

7 240 1.7 3 4 9 550 1.8 2 Half space 1100 2.2 6 300 1.7 3 5 9 640 1.8 2 Half space 1100 2.2 - 32 10. Discussion and conclusions

Studies carried out earlier in the Coastal Plain (Zaslavsky et al., 2005) show that we have two types of rock layers that seem to dominate the site response in this area: the hard limestone and dolomite of the Judea Group and relative shallow calcareous sandstone of the Kurkar Gr. Significant increase in the fundamental frequency of H/V ratio from 0.2-0.3 Hz to 2- 3 Hz is caused by reflector substitution. The calcareous sandstone distributed along the coastline in a strip of about 10 km(?) wide is proven to be a main reflector in the study area. S-wave velocity obtained from the seismic refraction surveys for Kurkar and normally used in microzoning studies is of 600-750 m/sec (for example, Zaslavsky 2008c). Only once, seismic refraction survey carried out near the Reading (Stivelman, 1998) for seismic hazard assessment (Zaslavsky and Peled, 1998) yielded S-wave velocities of 1170m/sec. In the present study we again obtained S-wave velocity of the calcareous sandstone 1000-1110 m/sec, thereby extending the area of abnormally high velocity reflector and correspondingly increasing impedance contrast with overlying sediments. At the Coastal Plain we have many cases where analysis of microtremor measurements provides an opportunity for us to understand the subsurface structure at boreholes with unclear lithological description via modeling and correlation with available geophysical data. We present in Figure 19 an example of well L-342, whose lithological section consists of unseparated sand and calcareous sandstone. 1-D model derived by fitting of analytical function to the H/V ratio at well location yields three layers with Vs equal to 310, 550 and 1000 m/sec. Based on correlation with the stratigraphy of the region (see Table 1) we suggest the following lithological annotation for these layers:  Sand alternating with calcareous sandstone and loam “hamra” have Vs=310 m/sec;  Calcareous sandstone alternating with sand and loam has Vs=550 m/sec;  Calcareous sandstone as a fundamental reflector occurs at a depth of 40 meters and has Vs=1000 m/sec. Two schematic east-west geological sections shown in Figure 20 illustrate results of the microtremor measurement analysis (for their position see Figure 1). For comparison lithological section of three wells located along profile BB are shown in this figure. One can see a significant discrepancy in the subsurface structure. In similar manner, analyzing H/V spectral ratios and geological-geophysical information we determined velocity ranges for all lithological units in the study area. They are given in Table 10. It is important to bear in mind that the correlations derived are valid in the study area only and could not be used at any other place 33

Figure 19. (a) Lithological section of well L-342; (b) Vs model determined by integrated analysis of H/V ratios and geophysical data and (c) lithological interpretation of the modeling results.

Table 10. S-wave velocity ranges for lithological units represented in the study area.

Thickness Vs Number Lithology Member Abbreviation. (m) (m/s) Sand, soil, loess, 1 0 -15 Recent al 150-250 clay and gravel 2 Sand dunes 0-10 Taarukha qs

Eolinete ''kurkar'' 3 10-15 Tel Aviv qk calcareous sandstone 250-350 Red sand and loam 4 1-5 Netanya qh ''Hamra'' Eolinete ''kurkar'' 5 15-40 Giv'at Olga qk calcareous sandstone Silty sand and loam 500-750 6 3-5 Kfar Vitkin qh ''Hamra'' Eolinete and marine 7 ''kurkar'' calcareous 10-40 Herzliyya qk sandstone Red sand and loam 8 0-10 Poleg qh ''Hamra'' 900-1120 Eolinete and marine 9 ''kurkar'' calcareous 20-60 Ashdod qk sandstone and sand 34

West East A TA-2 A' 30 13 30 14 29 20 6 49 20 10 Sea 10 BSL BSL 0 0 (m) (m) -10 -10

-20 -20

-30 -30

-40 -40 Scale: Vert./Horiz.=1/4 -50 -50 0 100 200 300 400 500 600 700 800 900 1000 1100 1200 D i s t a n c e , m

West East TA-1 B' B 6 30 60 52 7 30 53 5 58 30 20 10 31 20 56 10 10 Sea BSL BSL (m) 0 0(m) -10 -10

-20 -20

-30 -30

-40 -40 Scale: Vert./Horiz.=1/4 -50 -50 0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 D i s t a n c e , m

Lithological sections of water wells: Alluvium Vs=170-250m/s Loam, sandy loam Sand alternatig with calc.sandstone, sand dunes, loam "hamra", (Taaruha, Tel Aviv, Calc.sandstone and sand Netanya Mbs.) Vs=250-350 m/s

Water well Calc.sandstone alternatig with sand and loam (Givat Olga,Kf. Vitkin Mbs.) Vs= 500-750m/s Investigated site

Calc.sandstone (Herzliyya, Ashdod Mbs.) V= 900-1200 m/s TA-1 Reflaction line

Top reflector

Figure 20. Schematic geological cross sections along Profile 1 and Profile 2 reconstructed using analysis of H/V spectral ratio from microtremor.

Damage distribution during large earthquakes is frequently controlled by site effects. Subsoil impedance contrasts can significantly amplify the shaking level, as well as increase the duration of strong ground motion. This study again demonstrates the significant added value of performing ambient noise measurements over a dense grid of measuring sites and improves possibilities to calibrate and correlate seismological information with information from other sources, e.g., surface geology, seismic refraction surveys, borehole data, geotechnical information and more. Such an integrated study is likely to produce a systematic assessment of the seismic hazard. These results are intended for use of local authorities, town planners, and civil engineers. Therefore, it is very important to apply our efforts to evaluate effects of local site 35 conditions on ground motion and damage that would allow considerably reducing losses from strong earthquakes in the future. The conclusions may be summarized as follows: 1. The different seismic behavior of the geological units covering the northern Tel Aviv city is observed by the fundamental frequency and its H/V level obtained from ambient noise recorded in sixty sites across the investigated area. 2. The site response variations may be significant over short distances, thus we strongly recommend that prediction of different seismic shaking characteristics during large earthquakes should be based on the experimental site response functions obtained over a relatively dense gird of measurement points. 3. The shape of the horizontal-to-vertical spectral ratios determined in this study, as well as the good agreement between two horizontal components strongly suggest that site effects in this region, are of 1-D nature. This verify our assumption that maps of distribution of the fundamental frequency and its associated H/V level present a reliable picture of site effects that may be expected in this region during future earthquakes. 4. The analytical transfer functions calculated based on the data from seismic refraction survey matches fairly the H/V ratios curves deduced from ambient noise measurements. 5. Seismological and geotechnical data are highly complementary and that neither should be used alone for a seismic hazard assessment. Reliable estimations are those obtained by combining different approaches, supplemented with geotechnical (geophysical) and geological data. 6. Each analytical model should be tested by seismological measurements for its ability to predict actual site effect. Using analytical methods only is unsafe and may lead to wrong estimation of seismic hazard. 7. The application of methodology for using ambient noise records as an aid in carrying out earthquake hazard microzonation is very important, especially in Israel, where seismic activity is relatively low while seismic risk is high owing to the high population density and rapid urban development. 8. The results described in this study, as well as obtained earlier, calls for need to continue microzoning the seismic hazard across Israel. 36 8. Acknowledgments

Many thanks are due to Dr. R. Hofstetter for reviewing the text giving interesting suggestions. Thanks are due to Ms. L. Feldman, Ms. B. Reich, Ms. C. Ben Sasson, and Ms. V. Avirav, for assisting with data collection. We also thank Mr. Y. Karmon and Mr. Y. Menahem for their assistance in preparing report.

37 References

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