LOCAL SITE EFFECT OF HASHEFELA AND HASHARON REGIONS

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2 2 LOCAL SITE EFFECT OF HASHEFELA AND HASHARON REGIONS BASED ON AMBIENT VIBRATION MEASUREMENTS PROGRESS REPORT July, 2003 Report No 569/313/03 Dr. Yuli Zaslavsky, Dr. Avi Shapira, Marina Gorstein, Michael Kalmanovich, Vadim Giller, Ion Livshits, Alexander Shvartsburg, Galina Ataev, Tatyana Aksinenko, Dagmara Giller, Ilana Dan, and Nahum Perelman Prepared for The Steering Committee for National Earthquake Preparedness and Mitigation

3 3 CONTENTS LIST OF FIGURES AND TABLES 3 ABSTRACT 5 INTRODUCTION 6 GEOLOGICAL BACKGROUND 7 OBSERVATION AND ANALYSIS 9 SITE EFFECT ESTIMATION 13 DISTRIBUTION OF FUNDAMENTAL FREQUENCY AND AMPLIFICATION 20 SHEAR-WAVE VELOCITY ESTIMATION FROM AMBIENT VIBRATION 25 DISCUSSION AND CONCLUSIONS 29 ACKNOWLEDGEMENT 35 REFERENCES 37

4 4 FIGURES Figure 1. Geological map and location of the measurement points. Figure 2. Pilot and refined spectral ratios for two sites. Figure 3. Examples of average Fourier spectra and H/V spectral ratios obtained for two sites with the weak impedance contrast of thick (up to 600m) sediments. Figure 4. Examples of average Fourier spectra and H/V spectral ratios obtained for two sites with the weak impedance contrast and intermediate thickness (from 120m to 220m) of sediments. Figure 5. Influence of soft sediments and chalk with different thickness over dolomite on site response function: Site soft sediments - 40m, chalk 40m; Site 234 soft sediments - 65m, chalk 70m. Figure 6. Examples of average Fourier spectra and H/V spectral ratios obtained for two sites with thin soft sediments (30-40m) and the strong impedance contrast Figure 7. Examples of average Fourier spectra and H/V spectral ratios obtained for two sites with kurkar and clay and different depths to basement: a- 470m, b-630m. Figure 8. (a) Simplified geological section in E-W direction (line A-A in Fig. 1) and (b) average H/V ratios obtained for 6 points. Figure 9. Distribution of the fundamental frequency in the Coastal Plain and Hashefela areas. Figure 10. Distribution of maximum amplification level in the Coastal Plain and Hashefela areas Figure 11. Map of zones division in the Coastal Plain and Hashefela areas. Figure 12. Comparison of analytical response function and experimental spectral ratio for Point 210 (LD-19 well). Figure 13. Comparison of analytical response function and experimental spectral ratio for Point 188 (Tunis-5 well). Figure 14. Selection of the optimal model for Lachish 14 well (Point 346). Figure 15. Shear-wave profile estimated from ambient vibration measurements. Figure 16. Fragment of the structural map of top Judea Gr. and location of Yarkon 1 well (Point 218-1). Figure 17. Comparison between theoretical response function and experimental spectral ratio for Point 350 (Brur-3 well). Experimental ratio - red line; theoretical function, calculated assuming for kurkar layer: Vs =700 m/c - dashed line; Vs=350 m/sec black line.

5 5 Figure 18. Observed fundamental frequency plotted versus sediment thickness of deposits overlaying dolomites of Judea Gr. in the Hashefela region. TABLES Table 1. Stratigraphic scheme of the Hashefela region Table 2. The characteristics of the site response zones Table 3. Geotechnical properties of the rocks derived from investigations of the Lod- Ramla and Coastal Plain areas

6 6 ABSTRACT Resent destructive earthquakes have clearly shown that near-surface geological conditions play a mayor role in the level of ground shaking. Mapping predominant frequency of soil resonance and amplification permits identification of zones at risk in seismically-prone areas. The motivation and objectives of this study were to divide the Hashefela area into geographical zones, each one characterized by a fundamental frequency and amplification factor. In final version of this report we will adjust the overall soil-column model, which facilitates forecasting ground motion from future earthquakes, for each zone. Owing to the relatively low seismicity of the region, we concentrated our efforts on estimating site response by implementing the horizontal-to-vertical spectral ratio (H/V) of ambient vibration. Ambient vibration surveys were carried out at more than 300 sites; 160 of which were located either at, or very close to wells. The selection of an appropriate ensemble of windows of ambient noise for H/V spectral ratios facilitates successful removal of time variant source effects. We obtained a good correlation between the empirical and analytical evaluations of the fundamental frequency as well as for the amplifications level for shallow and deep soils with a multi layer distribution. In the studied areas the ground motion amplification is a factor 2-8 over the frequency range of 0.3 to 6 Hz. The amplification obtained by H/V ratios may be explained not only by peak in the spectra of the horizontal components but also by a trough in the spectra of the vertical components. We present maps that reflect the fundamental characteristics of site effects in the study area: dominant frequency and maximum relative amplifications. The extensive database of ambient vibration measurements carried out at drilling sites facilitated estimation of deep shear-wave velocity profiles by trial-and-error fitting of the calculated to the empirical transfer function.

7 7 INTRODUCTION Many methods have been used to characterize site amplification. The best approach is through direct observation of seismic ground motion, although such observations are limited to high seismicity areas and, also, by their high cost. An alternative method, recording only one station, consists of dividing the spectrum of the horizontal component by that of the vertical component of ambient vibrations (Nakamura, 1989, 2000). During the last decade, many sites in Israel have been investigated in an attempt to estimate the possible amplification of the seismic ground motion [Zaslavsky et al., 1995; Gitterman et al., 1996; Zaslavsky and Shapira, 2000; Zaslavsky et al., 2000; Shapira et al., 2001; Zaslavsky et al., 2002b,d,e and Zaslavsky et al., 2003]. All these studies are based on analysing ambient vibration and weak motion measurements incorporated with geological and geophysical information on the subsurface. We used various empirical methods to determine the site response functions including reference and non-reference techniques and referring to different sources of excitation earthquakes, explosions and ambient vibration. Appropriate ensembles of carefully selected windows of ambient vibration provide estimations of site response that are similar to those obtained from H/V spectral ratio of seismic events. However, there were cases where the Nakamura technique failed to yield conclusive results. This often happens when the ratio of the shear-wave velocity of the soil to the shear wave velocity of the underlying half space (bedrock) is higher than (amplification up to a factor of ~2) or when we are dealing with a complicated 3D structure of the underlying geology. Other examples are associated with poor excitation of the soil column due to weakness or remoteness of the microtremor sources. Thus, in many cases this poor behaviour of the Nakamura method could be foreseen and other methods should have been used. In other cases, where situation is better suited to the feasibility of the method, the results showed great similarity to the results obtained by other techniques and, thus, provide useful feedback to improve the reliability of the experimental results. In rare cases, the Nakamura technique even provided estimations of higher harmonics of the resonating soil column. It is interesting to note that large differences in the site response to seismic motion were observed over very short distances (several tens of meters). Site effects were also observed on rocky sites such as in the case where weathered and cracked granite bedrock showed amplifications by factor 4 in the frequency range of 6 to 7 Hz, well within the range of engineering interest for low-rise building [Zaslavsky et al., 2002a].

8 8 In order to tackle the task of mapping site effects using ambient noise measurements with a variable grid density in different areas across Israel, a team of new immigrant scientists with expertise in many different (seismology, geology, geophysics, data processing) was formed in the year So far the ambient noise survey has been carried out at more than 800 sites in the towns of Lod-Ramla (360 sites), Qiryat Shmona (290 sites) and, at Coastal Plain along a ~10 km wide strip between Ashqelon and Haifa (190 sites). The results of the investigations have already been published (see Zaslavsky et al., 2001; Zaslavsky et al., 2002c). As a continuation of the Coastal Plain mapping project, site effects were measured in the Hashefela and Hasharon areas (further named as Hashefela), which owing to its high population density, may be considered as a high seismic risk zone. The motivation and objectives of this study were to divide the Hashefela area into several geographical zones, each one characterized by fundamental frequency and amplification factor. In the final version of the report we shall adjust the overall soil-column model, which facilitates forecasting ground motion from future earthquakes, for each zone. The computation of the theoretical seismic response of the Hashefela area is essential for the estimation of specific seismic hazard and provides essential information for realistic earthquake damage scenarios. The applied methodologies take on added importance in regions with low seismicity but with high seismic risk, as the case of the Hashefela region. GEOLOGICAL BACKGROUND The investigated area, the Hashefela region, extends from Qirayt-Gat in the south to Binaymina in the north for about 120 km and is almost km wide (see Fig.1). The geological data of the region were collected from Gvirtzman (1969, 1970, 1984), Fleischer et al. (1993), Fleisher and Gafson (2000), and the geological map of Israel to a scale of 1:200,000 (Sneh et al., 1998). During the initial phase of compilation and determination of geothechnical data we gathered the information from 650 structural, water and oil wells from the database of the Geophysical Institute of Israel. The 160 wells were chosen for ambient noise measurements during the work planning stage. Quaternary rocks The Quaternary sediments outcropping in the Hashefela area are represented by alluvium, sand dunes of the Kurkar Group (see Tabl.1).

9 Figure 1. Geological map and location of the measurement points 9

10 10 Table 1. Stratigraphic sheme of the Hashefela region Lithology Thickness,m Formation Group Stage Age Sand,soil, loess, 0-40 Alluvium clay and gravel Holocene Sand dunes 0-20 Alternation of calcareous sandstone, Kurkar Pleistocene sand and red loam Clay Yafo Pliocene Limestone 0-50 Lakhish Saqiye Oligocene Marl chakly Beit Guvrin Oligocene Chalk Adulam Avedat Lower Eocene Marl,shale Taqie Mt. Paleocene Chalky limestone 0-70 Ghareb Scopus Maastrichtian Interstratification marl,chalk 'En Zetim Campanian Dolomites and limestones Bina Marlstone Dalyya Judea Turonian Chalks,marls and limestones Negba Cenomanian Dolomites and limestones Yagur Albian Quaternary Tertiary Cretaceous The Kurkar Group of Pleistocene age consists of alternating marine and eolian calcareous sandstones named kurkar, some reddish silty-clayey hamra, silts, clays, loose sands, loam and conglomerates. Occasionally the Kurkar Group unconformably overlies the Judea, Mt. Scopus, Avedat and Saqiye Group rocks. Two different representative provinces the west and east can be recognized in the Kurkar Group. The thickness of the Kurkar Group decreases from about 200m near the shoreline to 0m in the eastern part of the Hashefela area. Tertiary rocks The Saqiye Group overlies unconformably the Avedat and Mt.Scopus groups and consists of deep marine chalky marls of the Bet Guvrin Fm. and limestone of the Lakhish Fm. of Oligocene age, marls of the Ziqim and evaporates of the Mavqi im formations of Middle- Late Miocene age, and some volcanics such basalt flows of the National Park Volcanics (310- m thick at Site177 close to the Rishon Le Zion-1 well). The sedimentary rocks of Pliocene age are represented by homogenous clay and marly clay of the transgressional Yafo formation. The sedimentation of the Yafo Fm. was dominated by a westward tilting of about two degree, which produced an increase in thickness from a

11 11 few meters in the central Hashefela area to more than 2,000 m in the offshore region. The Yafo Fm. is overlaid diachronously by the Kurkar Group. The Avedat Group of Eocene age consists of massive soft silicified chalks and marl m thick related to the Adulam Fm., whereas its upper section is composed of massive soft chalks of the Maresha Fm. In the Coastal Plain area, the Avedat group occasionally overlies the Albian Talme Yafe Fm., the Judea Group and Senonian Mt. Scopus rocks. It is unconformably overlain by the Saqiye and/or the Kurkar Gr. In the Hashefela region, the Eocene sediments were deposited in a buried synclinorium situated between the Judea and Samaria heights and the Helez structure and they outcrops in the southestern part of the Hashefela area. Mount Scopus Group is representing by a marl-chalky facies of the En Zetim Fm., the silicified chalky limestone of the Ghareb Fm. and the limonitic shale of the Taqiye Fm. The upper boundary of the Mt. Scopus Gr. is overlain by flinty limestone of the Avedat Gr. The thickness of the Mt. Scopus Gr. varies from 0 to 250m. It outcrops in the northeastern part of the Hashefela area (upper reaches of the Hadera river) and in the Modi in area. Cretaceous rocks The Judea Group sequence has been divided into three parts consisting of massive dolomites and limestones of the Albian-Cenoman, Yagur Fm.; dolomites with interbedded marls of the Cenomanian Negba Fm., and limestone, dolomites and marls of the Turonian (Bina and Dalya) Fm. The Judea Group, represented generally by the Bina Fm., consists of hard white to gray limestone and dolomite; containing rudist and coral fragments with a thickness varying from 100 to 160 m. The Bina formation. outcrops along the foothills. The upper contact of the Bina Fm. is unconformably overlain almost everywhere by the En Zetim Fm. OBSERVATION AND ANALYSIS During the period July 2002 to April 2003 about 300 microtremor measurements were carried out in the Hashefela region (W ; E ; S ; N ). The work area is approximately 2300 km 2. Measurement sites were distributed along the lines, which are, in fact, extensions of those on the Coastal Plain from coastline to foothills. Measurement points were planned in close co-operation with Dr. Zohar Gvirtzman (the Geological Survey of Israel). The distribution of measurement points is shown in Figure 1. In order to obtain more reliable microtremor data the main part of the measurement sites comprised two observation points separated by tens of meters.

12 12 Ground motions were recorded using the multi-channel, PC-based, digital seismic data acquisition system GII-SDA (see Shapira and Avirav, 1995) designed for site response field investigations. The seismometers used are sensitive velocity transducers with a natural frequency of 1.0 Hz. Digital recordings use Hz band-pass filter and a sampling rate of 100 samples per second. Prior to and during the measurements we checked and determined the transfer function of the round motion data, i.e., particle velocity. One vertical and two horizontal seismometers (oriented north-south and east-west) are installed at each site. All seismometers are wired directly to the recording site. The horizontal-to-vertical spectral ratio [AH/V(f)] is 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)]. To obtain systematic and reliable results from the spectra of microtremors, we used many time windows that yielded many spectral rations that, in turn, were averaged. The average of the two horizontalto-vertical ratios is defined as the site amplification function: A ( f ) S ( f ) ( f ) n n 1 NS = i + 2n i= 1 i= SV 1 i S S EW V ( f ) ( ) i f i The critical assumption in all experimental site response estimations is that the reference motion represents the true input to the soil site. Even when the surface-rock sites are used as the input (reference site) to the basin, in the case of earthquake excitation, amplification factor may be underestimated at the basin site by a factor of 2 to 4 depending on frequency and site (Steidl at al., 1996; Zaslavsky et al., 2002a). In Nakamura s method the reference site is the vertical motion of the ambient vibration and therefore we cannot separate ambient noise from the true input of their wavefield. Moreover, in our opinion, excitation of resonance vibration from ambient noise in multi-layers medium is stochastic process. Therefore, it is very important to select the appropriate ensemble of windows of the ambient vibration in the spectral estimation procedure. The estimates of the spectral ratio for Points 240 and 236 obtained from pilot analysis and refined analysis are plotted in Figure 2. We should point out again here that the transfer function may be obtained from the spectral ratio of input and output of linear system, but not every spectral ratio is really a transfer function.

13 13 Figure 2. Pilot and refined spectral ratios for two sites

14 14 SITE EFFECT ESTIMATION A number of measurements were made either at or close to boreholes, where the thickness of layers and geotechnical description of the local sediments are available. Clearly, the main geological structure is reflected in the measurements results. In this section we give some examples. In Figure 3 we present average spectra for three components (two horizontal and vertical) and individual and average H/V spectral ratios from ambient vibration recorded at Points and 331. Lithological sections at both sites are represented basically by alternating high-velocity chalk, marl and limestone. Depth to the basement is 600 m and 450 m, correspondingly. An increase in the spectral levels of the horizontal components is clear at a frequency of about 0.45 Hz for Point (Fig 3a) and close to 0.55 Hz for Point 331(Fig 3b), while the spectra of vertical components are flat. Hence, the H/V spectral ratios curves show amplification with a factor 2.0 at frequencies of about 0.45 Hz (Point 324-3) and 0.55 Hz (Point 331). A comparison of average horizontal (NS and EW) and vertical components spectra from ambient vibration recorded at Points 270 and 281 and its H/V spectral ratios are shown in Figure 4. At these points, as in the previous example, a rock basement is overlain by chalk, marl and limestone. However, the thickness of the sedimentary layers is 200 m and 130 m. The general character of these spectra is that the spectral levels for vertical components exceed the levels for the horizontal components within the frequency range 0.2 to 10 Hz. It is worth pointing out that there is a narrow-bandwidth trough at a frequency close to 1.9 Hz at 1.2 Hz at Points 281 and Point 270, respectively in the spectral levels of the vertical components. Spectra ratios, therefore, show a prominent peak at about 1.9 Hz with amplification up to 3 for Point 281 (Fig. 4a) and near 1.2 Hz with amplification about 2 for Point 270 (Fig. 4b).

15 15 Figure 3. Examples of average Fourier spectra and H/V spectral ratios obtained for two sites with the weak impedance contrast of thick (up to 600m) sediments

16 16 Figure 4. Examples of average Fourier spectra and H/V spectral ratios obtained for two sites with the weak impedance contrast and intermediate thickness (from 120m to 220m) of sediments

17 17 The H/V ratios at Points and 234 are shown in Figure 5. For two points we can see a similarity among the individual functions not only in terms of the peak position and intensity, but also in the whole shape. The dominant feature of all spectral ratios at Point is a well defined peak at about 1.7 Hz with amplification factor up to 4. As shown, the average spectral ratio at Point 234 has a predominant peak near 1 Hz with amplification up to 6. In these sites the soft sediments and chalk over dolomite show a strong contrast and cause main peak. Figure 5. Influence of soft sediments and chalk with different thickness over dolomite on site response function: Site soft sediments - 40m, chalk 40m; Site 234 soft sediments - 65m, chalk 70m. Examples of the average Fourier spectra and individual and average H/V spectral ratios at Points 211-A and 231 are plotted in Figure 6. According to the borehole data these points of measurements are located on sand and loam with a thickness of 35 40m overlaying the carbonates of the Judea Gr. The impedance contrast is very strong and the spectral ratios

18 18 at these sites present significant peaks in the frequencies 2.0 Hz and 2.5 Hz with amplification above 6.0. Figure 6. Examples of average Fourier spectra and H/V spectral ratios obtained for two sites with thin soft sediments (30-40m) and the strong impedance contrast In Figure 7 we plot spectral ratios at points 329 and 359. These points are situated on kurkar and clay with a depth to basement of 470m and 630m, respectively. We can see that different combinations of sediments produce a predominant peak near 0.3 Hz with amplification factor up to 3. Figure 8a displays a simplified sketch of the geological section along line A-A, showing the locations of the points where ambient vibration measurements

19 19 were carried out (see Fig. 1 for location of this line on the geological map). Figure 8b reveals amplification factor of site response functions (fundamental mode) for these sites. From examination of this figure we can see, that spectral amplification varies from one station to another in both maximum amplification and the position of the predominant peaks. Further the west, resonance frequency decreases from 5 Hz to 0.35 Hz, corresponding to the increasing thickness of the sedimentary cover. Figure 7. Examples of average Fourier spectra and H/V spectral ratios obtained for two sites with kurkar and clay and different depth to basement: a- 470m, b-630m.

20 20 A a. A b. Figure 8. (a) Simplified geological section in E-W direction (line A-A in Fig. 1) and (b) average H/V ratios obtained for 6 points.

21 21 DISTRIBUTION OF THE FUNDAMENTAL FREQUENCY AND AMPLIFICATION A data set of the ambient vibration measurements was used to construct the distribution maps of the fundamental frequency and maximum relative amplification in the Hashefela region. During our campaign of systematic ambient vibration measurements in the Hashefela, we took the opportunity to supplement with new data and revise available results from the Coastal Plain. Frequency and amplification maps, shown in Figs. 9,10 integrate all experimental data obtained in both the Hashefela and Coastal Plain areas. The Hashefela region shows a large variation in the fundamental frequency (from 0.3 up to 7 Hz) and amplification (from factor 2 up to 7). Comparison of the results with the geological structure confirms the general trend of the correlation between the dominant frequency and the depth of the reflector represented by carbonates of the Judea Group in the central and eastern part of the Hashefela region. Low frequency band responses ( Hz) occur in the central part gradually increasing up to 7-8 Hz near the eastern edge of the area, in the foothills. In the south and southeast of Hashefela region, where chalks and marls of Eocene age outcrop, we observe frequencies in the same band of Hz. In the west, in the Coastal Plain strip, calcareous sandstone of the Kurkar Gr. dominate site response, substituting at depths of m for the carbonates of the Judea Gr. Significant steps in the frequency domain (from Hz to 1-2 Hz) caused by reflector substitution, are observed along the Coastal Plain. Toward the coastline the frequency, correlating with the thickness of the sedimentary deposits above the calcareous sandstone of the Kurkar Gr., increases up to 4 Hz everywhere, except in the small area located on the upstanding block near the northeastern boundary of the investigated region. This small area is an object of interest because the reflector changes twice within a distance of 4-5 km. According to our measurements, a reflector represented by carbonates of the Judea Gr. is substituted by calcareous sandstone of the Kurkar Gr. Then, the top of the Judea Gr. rising up to depths of 700m and higher was again interpreted as a reflector. The map of the maximum relative amplification shown in Fig. 10 reflects the impedance contrast between the bedrock and the overlying sediments. Amplification of factor 2-3 prevails in both the Hashefela and Coastal Plain areas, but for different reasons. In the Coastal Plain (see Zaslavsky, 2002c) amplification factor 2-3 matches loose deposits lying on calcareous sandstone of the Kurkar group, while in the Hashefela, the velocity contrast is defined by chalk and marl of Campanian and Eocene ages overlying dolomites of the Judea Gr. Moderate amplification values from factor 3 to 4 we observe in limited areas at Coastal

22 22 Plain (Tel-Aviv, Ashdod up to factor 6) where the bedrock is represented by hard calcareous sandstone, as well as in the central part of the Hashefela region where the impedance contrast is created by combination of the rocks of the Kurkar Gr., clay of Saqie Gr., and marl and chalk of the Avedat and Mt. Scopus Groups. Quaternary sediments, directly overlaying carbonates of the Judea Gr., cause the higher amplification values (from 5 up to 7) we observe in the eastern part of the study area, within the belt between the towns of Ramla and Kefar Sava, near the Shomron hills. The main outcome of analysis of the 500 ambient vibration measurements (Coastal Plain area included) was to produce a map of zones, each characterized by a fundamental frequency and amplification factor. Generalized lithological structure and ranges of layer thickness for each selected zone, efficiently recognized in the ambient vibration measurements, are stated in Table 2. Table 2. The characteristics of the site response zones Region Coastal Plain reflector: calcareous sandstone Zone Resonance Amplifi- Bedrock Lithology and thickness variation of sediments frequency, cation (reflector) above reflector,m Hz factor depth,m loam and sand < loam and sand loam and sand loam and sand loam and sand or chalk and marl< Hashefela and Hasharon loam and sand 15-45, chalk and marl loam and sand 45-60, chalk and marl loam and sand 15-45, chalk and marl chalk and marl chalk and marl reflector: dolomite loam and sand 0-80, sandstone 0-60, clay chalk and marl loam and sand 0-80, sandstone 0-60,clay chalk and marl chalk and marl In the future we plan to adjust the overall model for each zone and to use this for the purpose of earthquake hazard assessment in the Hashefela region.

23 Binyamina boundary between two reflectors boundary of Bina Fm boundary of mesurement area Netanya Tel Aviv No resonance frequensy Ashdod Ashqelon Qiryat Gat Figure 9. Distribution of the fundamental frequency in the Coastal Plain and Hashefela areas

24 24 Binyamina boundary between two reflectors boundary of Bina Fm. boundary of mesurement area Netanya Tel Aviv Ashdod Ashqelon Qiryat Gat Figure 10. Distribution of maximum amplification level in the Coastal Plain and Hashefela areas

25 25 Area Zone Resonance Amplification frequency, factor Hz Binyamina Coastal plain Hashefela and Hasharon Netanya Tel Aviv boundary between two reflectors boundary of Bina Fm boundary of mesurement area Ashdod Ashqelon Qiryat Gat Figure 11. Map of zones division in the Coastal Plain and Hashefela areas

26 26 SHEAR-WAVE VELOCITY ESTIMATION FROM AMBIENT VIBRATION One of the major requisites for numerical prediction of site effects is to feed the models with reliable parameter values concerning geotechnical properties of the subsurface structure. The SHAKE (1971) and Joyner s (1977) programs, which we use for site response modeling, suppose sufficiently detailed knowledge of the thickness, density and shear-wave velocity of each layer of the soil column. Data on underground structure can be obtained from borehole data. At that point we collect all available information on S-wave velocities and suggest rough starting values. The S-wave velocity values, which after iterative procedure of trial-and-error fitting yield good agreement between calculated and observed response functions, are accepted as optimal and used further. As initial velocity values for Hashefela we used the results obtained in the previous investigations in the Lod-Ramla and Coastal Plain areas (Zaslavsky at al., 2001; Zaslavsky et al., 2002c). These results are summarized in Table 3. Table 3. Geotechnical properties of the rocks derived from investigations of the Lod- Ramla and Coastal Plain areas Material Vs m/s Depth 0-50 m ρ g/cm 3 Depth m Vs m/s ρ g/cm 3 Vs m/s Depth m ρ g/cm 3 Vs m/s Depth >200m ρ g/cm 3 Sand, Loamy Sand Loam Calcareous Sandstone Clay Chalk and Marl Dolomite In order to verify the applicability of the velocity models derived in the Lod-Ramla and Coastal Plain areas for the Hashefela region and estimate shear-wave velocity distribution with depth (for sediment thickness of more than 300m) 140 wells with different lithological structures and layer thickness were involved in the modeling process. We give below we some examples of comparison of the calculated transfer functions and experimental spectral ratios.

27 27 Measurement point 210 is located at borehole Ld-19 and its soil column consists of sandy loam of the Kurkar Gr. overlaying dolomites of the Judea Gr. Using corresponding S- wave velocity values from Table 3 gives good agreement in both the fundamental frequency and amplification factor, as shown in Fig. 12. Figure 12. Comparison of analytical response function and experimental spectral ratio for Point 210 (LD-19 well) Measurement point 188 is situated at borehole Tunis 5 and represented by sandy loam, calcareous sandstone and clay overlaying Judea Gr. carbonates. Also in this case we can see in Figure 13, that Vs=650 at a depth of more than 100 m is assumed for clay of the Yafo Fm. and provides good coincidence between the observed and calculated response functions. We calculated analytical response functions for sites 319, 320, 363, 359, 395, and many others, located at wells with a soil column including clay layers. The thickness of the clay layers varies from 30 m up to m. In the models with significant misfit between analytical and experimental response function we corrected Vs value depending on depth. Our conclusions regarding optimal Vs velocity model in-depth are shown in Fig.15.

28 28 Figure 13. Comparison of analytical response function and experimental spectral ratio for Point 188 (Tunis-5 well) In order to estimate vertical distribution of Vs for chalk layers, the analytical response functions were calculated at measurement points 201, 406, 281, 270, 266, 267, 276, 309 etc. closed to boreholes. The depth of chalk bedding at these drilling sites changes from tens of meters to about 600 meters. According to our investigations in Lod-Ramla shear-wave velocity of united layer of marl and chalk facies varies from 700 m/s at depths of 0-50m to 950 m/sec at depths of more than 200 m. Figure 14 illustrates our failed attempt to use Vs=950 m/sec for a chalk layer with 590m thick bedding at a depth of 10m (point 346, Lachish 14 well). The difference between the fundamental frequency of the experimental and theoretical transfer functions is obvious. By trial-and-error fitting we found that the Vs value giving satisfactory agreement is 1200 m/sec. Consecutive analysis of the ambient noise measurements at boreholes allowed us to suggest a velocity model for the chalk (see Figure 15).

29 29 Lachish 14 well 10 m 600 m Figure 14. Selection of the optimal model for Lachish 14 well (Point 346). Velocity-depth distribution for clay and chalk-marl derived by means iterative fitting procedures are depicted on Fig. 15.

30 30 0 Vs, m/sec Chalk Clay -200 Depth, m Figure 15. Shear-wave profile estimated from ambient vibration measurements DISCUSSION AND CONCLUSIONS After recent earthquakes, a priori estimations of site effects became a major challenge for efficient mitigation of seismic risk, because, in the case of moderate earthquakes, significant damage and loss of life has been directly related to local geotechnical conditions. As illustrated by a test performed in the Turkey Flat, located near the Parkfield section of the San Andreas Fault (Field and Jacob, 1993), the numerical approach requires a very good knowledge of the local structure responsible for site effects. Only in the ideal case is it possible to perform a geophysical and geotechnical campaign with, for instance, cross-hole tests in order to obtain a reliable S-wave velocity and density of different lithological units and thickness of each layer. On the other hand, the known experimental techniques obtain reliable estimates of site effects, when the data are several tens of good quality earthquake recordings at the sites. These techniques, however, are costly, particularly in regions such as Israel with relatively low seismicity.the H/V ratio, i.e. the ratio between the Fourier spectra of the horizontal and vertical components of ambient vibrations (Nakamura s method) has

31 31 proved to be a valuable tool in determining first modes of transfer functions of site effects in the Hashefela and Hasharon areas, if applied with care and appropriately. Most of all we require analytical models to improve the reliability and relevancy of the results obtained. Where reliable data on subsurface velocity structure are lacking, we derived them via iterative procedure of adjusting theoretical response function to experimental spectral ratio at measurement points close to drilling sites. In this way in the Coastal Plain investigations revealed that the reflector (half-space) in the area of Ashkelon to Binyamina is calcareous sandstone of the Kurkar group, while at Carmel coast, according to our observations, the reflector is carbonates of the Judea group. Velocity models obtained on the basis of the extensive database of microtremor recordings in the Lod-Ramla area, considering our conclusions in the Coastal Plain, were distributed over the Hashefela region. We give some interesting examples for discussion. Measurement point is located at well Yarkon 1 on the upcast side of the fault marked by borehole data (see Fig. 16). The depth of the Judea Gr. limestones is 285 m. Modeled fundamental frequency and maximum amplification factor are 0.6 Hz and factor 3, while the measurements give us a frequency of 0.4 Hz, which corresponds to the depth of more than 400 m. Figure 16. Fragment of the structural map of top Judea Gr. and location of Yarkon 1 well (Point 218-1) An analogous situation where the depth of the Top Judea Gr. determined by well data and the depth of reflector inferred from our measurements are considerably different, is

32 32 observed at Nir am-4 well (measurement point 364). In the geological interpretation of the well, the Top Judea Gr. is at a depth of 237 m that corresponds to a frequency of 0.8 Hz. The experimental spectral ratio shows a very clear peak at 0.65 Hz. From this coincidence of experimental and calculated response functions may be reached only when the reflector depth is 300 m. The issue of kurkar velocity must be solved separately at each site, because the Kurkar group represented by alternating calcareous sandstones, hamra and sand is usually marked as a united layer in well descriptions. In the previous investigations in the Lod-Ramla and the Coastal Plain areas we showed that ambient noise measurements might be a way to obtain a general idea of the subsurface structure. Here we give an example of a kurkar issue solution in the Hashefela region. The upper 100 m of the lithological column of Brur-3 well are described as calcareous sandstone of the Kurkar group. Calculating the analytical response function with appropriate S-wave velocity for sandstone yields amplification factor 2.5 at frequency 0.65 Hz while the result of the measurements at point 350 close to the well are: amplification factor of 4.8 at frequency 0.6 Hz. We repeated the procedure assuming Vs equal to 350 m/s (sand) and reached the desired result (see Fig 17). A fairly thick layer of sediments above reflector (about 300m) explains the small alteration in fundamental frequency for two models. Figure 17. Comparison between theoretical response function and experimental spectral ratio for Point 350 (Brur-3 well). Experimental ratio - red line; theoretical function, calculated assuming for kurkar layer: Vs =700 m/c - dashed line; Vs=350 m/sec black line

33 33 Using the database of ambient vibration measurements in the Hashefela region, it is possible to show correlation between fundamental frequency and thickness of sediments overlaying dolomites of the Judea Gr. for a wide range of sediment thickness from ten meters up to 700 m. The graph of this dependence is plotted in Figure Fundamental frequency, Hz Sediment thickness, m Figure 18. Observed fundamental frequency plotted versus sediment thickness of deposits overlaying dolomites of Judea Gr. in the Hashefela region

34 34 The significant deviations of the data points in both frequency and thickness values from strict linear relationships, despite a correlation coefficient of 0.85, are probably a consequence of lithological inhomogeneties. Drawing a straight line through the data points assumes an averaging of lithology and shear-wave velocity depth dependence. A thickness derived from this correlation cannot be valid for a particular site. Hence, the practical relevance of this correlation for calculating of models at locations, where no borehole information is available, is questionable. In the final version of this report we plan to extend the database of measurements and take into consideration lithological composition of the sediments above reflector. Some important conclusions from the experiment discussed in the present study were reached: Maps of fundamental frequency and maximum amplification factor constructed on the basis of 500 ambient vibration measurements in the Hashefela and Coastal Plain areas exhibit amplification of factor 2-7 over the frequency range from 0.3 up to 7 Hz. The key parameter of ground motion amplifications is impedance contrast between soil deposits and underlying bedrock, therefore we observe the same amplification level of factor 2-3 in areas with a very different geological structure: loose deposits lying on calcareous sandstone of the Kurkar group at Coastal Plain and chalks of the Avedat group overlying dolomites of the Judea Group. The fundamental frequencies determined in the present study in the Hashefela region were found to correlate generally with the dip of the reflector represented by dolomites of the Judea Gr. from the east to the west. Contrarily, in the Coastal Plain, the increase in the fundamental frequency eastward matches the dip of the reflector represented by calcareous sandstone of the Kurkar Gr. from the west to the east that was confirmed by additional measurements in the Coastal Plain. A change in reflector occurs at a depth of top Judea Gr. of m. The boundary is marked on the maps by red line in the central part of the study area, as well as near the northwest corner. The uncertainties associated with the proposed subsurface models yield a too high a variability between the analytical site response functions. Hence, we found it useful to compare the possible analytical functions with those obtained

35 35 empirically. After a trial and error process, we obtained 1D models that yield response functions consistent with our H/V observations. These models were used to define and constrain the average shear wave velocities of the unconsolidated materials overlying the bedrock. In particular we inferred velocity-depth model for clay and chalk, which show variation of Vs from 500 m/s up to 700 m/s within a depth interval of m for clay; and from 700 m/s up to 1300 m/s in the depth range of m. Owing to the fact that the H/V spectral ratio techniques are relatively simple and inexpensive, we would strongly recommend that they be performed in site response investigations to support and verify theoretical calculations. Predictions based on models inferred only from geological and geophysical information may differ significantly from empirical estimates owing to the geological complexity of the site and the significant uncertainty associated with evaluating model parameters. Based on subsurface information from borehole data and the 500 site response measurements, we divided the study area into twelve zones, each characterized by resonance frequency and amplification of fundamental mode of site response function. These models will be used in earthquake scenarios for damage and losses assessments. The ground acceleration and elastic response spectra in the each zone will be presented as a result convolution analysis of rock ground motion with analytical transfer function of zone. It must be remembered that our results for microzonation studies were obtained using a very sparse grid (7 km 2 ) and can be used in earthquake scenarios for damage and losses assessments only on a large scale. Site effects assessment at small scales in urban areas requires much more dense grid (0.2*0.2 km 2 ). We suggest that future research efforts in investigated areas should have a two-fold focus: to identify sediment sites where known or suspected strong impedance contrast; to investigate ground motion characteristics for these sites using very dense grid.

36 36 ACKNOWLEDGMENT Our cordial thanks for the financial support of the Steering Committee for the National Earthquake Preparedness and Mitigation. We wish to thank Dr. Zohar Gvirtzman for his willingness to share knowledge and information on geology of the region and, especially, for his contribution in the planning of measurement points. We are most thankful to Dr. A. Hofstetter and L. Fleisher for fruitful discussion. Thank are also due to I. Chelinski, D. Artzi and Y. Menahem for their assistance in preparing this report.

37 37 REFERENCES Field, E.H., and Jacob, K.H., Monte-Carlo simulation of the theoretical site response variability at Turkey Flat, California, given the uncertainty in the geotechnically derived input parameters, Earthquake Spectra, V 5, No. 9, Fleischer, L., Gelberman, E. and Wolff, O., A geological-geophysical reassessment of the Judea Group, IPRG Report No. 244/147/92. Fleischer, L., Gafson, R.,2000. Top Judea Group- digital structural map of Israel. Phase 3, part 2, IPRG Report No. 873/55/99. Gitterman, Y., Zaslavsky, Y., Shapira, A., and Shtivelman, V., Empirical site response evaluations: case studies in Israel, Soil Dynamics and Earthquake Engineering, 15, Gvirtzman, G., Shachnai, E., Bakler, N., and Ilani, S., Stratigraphy of the Kurkar group (Quaternary) of the Coastal Plain of Israel, GSI Current Research, , pg Gvirtzman, G., Subsurface data on the Saqiye Group in the Coastal Plain and Hashelphela regions, Israel, Geological Survey of Israel, OD/1/69. Gvirtzman, G., The Saqiye Group (Late Eocene to Early Pleistocene) in the Coastal Plain and Hashephela regions, Israel, Ph D. Thesis, The Hebrew University, Jerusalem. Joyner, W. B., A Fortran program for calculating nonlinear seismic response, U. S. Geological Survey, Open File Report Nakamura, Y., A method for dynamic characteristics estimation of subsurface using microtremor on the ground surface. QR of RTRI, Vol. 30, No.1, Nakamura, Y., Clear identification of fundamental idea of the Nakamura s technique and its applications, 12 th World Conference of Earthquake Engineering, Auckland, New Zeeland, Proceedings, No.0084, pp8. Shapira, A., and Avirav, V., PS-SDA Operation Manual, IPRG Report Z1/567/79, p.24. Shapira, A., Feldman, L., Zaslavsky, Y., and Malitzky, A., Application of a stochastic method for the development of earthquake damage scenarios: Eilat, Israel test case. Computational Seism., 32, Shtivelman, V., Using surface waves for estimating shear wave velocities in the shallow subsurface onshore and offshore Israel, European J. of Environmental and Engineering Geophysics, 4, Sneh, A., Bartov, Y., and Rosensaft, M., Geological Map of Israel 1: 200,000, Geological Survey of Israel.

38 38 Zaslavsky, Y., Gitterman, Y., and Shapira, A., Site response estimations in Israel using weak motion measurements, Proceedings of 5th International Conference on Seismic Zonation, Nice, France, pp Zaslavsky, Y., Shapira, A., and Arzi, A.A., Amplification effects from earthquakes and ambient noise in Dead Sea Rift (Israel), Soil Dynamics and Earthquake Engineering, 20(1-4), Zaslavsky, Y., and Shapira, A., Questioning nonlinear effects in Eilat during MW=7.1 Gulf of Aqaba Earthquake, Proceedings of XXVII General Assembly of the European Seismological Commission, Lisbon, Portugal, pp Zaslavsky, Y., Shapira, A., Gorshtein, M., Kalmanovich, M., Giller, V., Livshits, L., Giller, D., Dan I., Leonov, J., and Peled U Microzoning of the earthquake hazard in Israel. Seismic microzoning of Lod and Ramla, GII Report N 569/143/01. Zaslavsky, Y., Shapira, A. and Arzi, A.A., 2002a. Earthquake site response on hard rock empirical study. Proceedings of 5th International Conference on Analysis of Discontinuous Deformation, ICADD-5, Beer-Sheva, Israel pp Zaslavsky, Y., Shapira, A. and Kenigsberg, M., 2002b. Earthquake site response study for designed bridges in Israel, Proceedings of 12th Conference on Earthquake Engineering, London, UK, Paper Reference 059. Zaslavsky Y., Shapira, A., Gorshtein, M., Kalmanovich, M., Perelman, N., Giller, V., Livshits, L., Giller, D., Dan I., Aksienko, T., and Ataev, G., 2002c. Generalization of site effects for earthquake scenario applications: the Coastal Plain area, GII Report, N 595/274/02. Zaslavsky Y., Shapira, A., Gorshtein, M., Kalmanovich, M., Giller, V., Livshits, L., Giller, D, and Dan I., 2002d. Detailed distribution of ground motion characteristics detected by ambient noise measurements a case study in Lod and Ramla, Israel. The XXVIII General Assembly of the European Seism. Comm., Genoa, Italy, 1-6 September. Zaslavsky, Y., Begin Z. B., Gorstein, M., Kalmanovich, M., and Shapira, A., 2002e. On the correlation between surface geology and site response along the coastal plane of Israel, GII Report No. 595/222/02, GSI Report No. GSI/24/02. Zaslavsky, Y., Shapira, A., and Leonov, J., Empirical evaluation of site effects by means of H/V spectral ratios at the locations of strong motion accelerometers in Israel, Journal Earthquake Engineering, in review.