Dr. Yuli Zaslavsky EMPIRICAL DETERMINATION of SITE

MICROZONING OF THE EARTHQUAKE HAZARD IN ISRAEL

PROJECT 5

EMPIRICAL DETERMINATION OF SITE EFFECTS FOR THE ASSESSMENT OF EARTHQUAKE HAZARD AND RISK TO BEIT SHEAN & AFULA

November, 2005 Job No 569/175/05

Principal Investigator: Dr. Yuli Zaslavsky

Collaborators:

Galina Ataev, Marina Gorste in, Dr. Avraham Hofstetter, Michael Kalmanovich, Dagmara Giller, Ilana Dan, Nahum Perelman, Tatyana Aksinenko, Vadim Giller, Ion Livshits, and Alexander Shvartsburg

Submitted to: Earth Sciences Research Administration National Ministry of Infrastructures & Ministry of Absorption

Contract Number: 24-17-017

CONTENTS

LIST OF FIGURES ...... 3 LIST OF TABLES ...... 5 1. INTRODUCTION...... 8 2. BRIEF REVIEW OF SEVERAL EXPERIMENTAL METHODS FOR SITE EFFECT EVALUATION...... 9 3. H/V SPECTRAL RATIO TECHNIQUE...... 13 4. GEOLOGICAL SETTING...... 14 4.1. BET SHEAN...... 16 4.2. AFULA...... 20 5. THE PLANNING OF MICROTREMOR MEASUREMENTS AND DATA ACQUISITION ...... 25 6. DATA ANALYSIS...... 34 7. DISTRIBUTION OF THE FUNDAMENTAL FREQUENCY AND ITS ASSOCIATED H/V SPECTRAL RATIO LEVEL ...... 35 7.1. BET SHEAN...... 35 7.2. AFULA...... 38 8. VALIDATION OF H/V RATIO USING SEISMIC REFRACTION MEASUREMENTS ...... 43 8.1. BET SHEAN...... 45 8.2 AFULA...... 48 9. ESTIMATION OF SUBSURFACE STRUCTURE USING H/V SPECTRAL RATIOS FROM MICROTREMOR...... 52 9.1. BET SHEAN...... 53 9.1.1. Profile A-A-A'...... 53 9.1.2. Profile B-B' ...... 59 9.2. AFULA...... 63 9.2.1. Profile A-A'...... 63 9.2.2. Profile B-B’ ...... 66 9.2.3. Profile C-C'...... 71 10. IDENTIFICATION OF FAULTS USING H/V SPECTRAL RATIO FROM MICROTREMOR ...... 72 11. PRELIMINARY SEISMIC ZONATION AND PREDICTION OF ACCELERATION RESPONSE SPECTRA FOR LINEAR AND NON-LINEAR BEHAVIOR OF SOIL SEDIMENTS ...... 80 11.1. BET SHEAN...... 81 11.2. AFULA...... 91 12. CONCLUSIONS ...... 95 ACKNOWLEDGMENT ...... 97 REFERENCES...... 98 3

LIST OF FIGURES

Figure 1. Very simplified geological section as an illustration of Nakamura's technique assumptions...... 13 Figure 2. A fragment of the Geological map of Israel (1:200 000) in Lower Galilee with the towns of Afula and Bet Shean...... 15 Figure 3. Geological map of Bet Shean (1:50,000) showing distribution of measurement points...... 17 Figure 4. Lithological section of Sade Nahun-2 and Ain Ashtadri T/6 wells...... 18 Figure 5. Geological map of the Afula area...... 21 Figure 6. Lithostratigraphyc section for Gan Tapukhim, Gidon-5 and Sarid-1 wells .22 Figure 7. Examples of of seismic station location in Bet Shean...... 28 Figure 8. Examples of seismic station location in Afula ...... 29 Figure 9. Setup of (a) horizontal and (b) vertical transducers for identity test of channels...... 30 Figure 10. Ambient noise recordings for the setup of (a) horizontal transducers and (b) their spectral amplitudes without application of "Instrument response removal" procedure...... 30 Figure 11. Ambient noise recordings for the setup of (a) horizontal transducers and (b) their spectral amplitudes with application of "Instrument response removal"...... 31 Figure 12. Ambient noise recordings for the setup of (a) vertical transducers and (b) their spectral amplitudes without application of "Instrument response removal" procedure...... 32 Figure 13. Ambient noise recordings for the setup of (a) horizontal transducers and (b) their spectral amplitudes with application of "Instrument response removal" procedure...... 33 Figure 14. Distribution of H/V resonance frequency (a) and its associated amplitude level (b) within the town of Bet Shean...... 37 Figure 15. Typical examples of the H/V ratios obtained in the town of Afula...... 38 Figure 16. Distribution of resonance frequency of the first H/V peak in Afula ...... 39 Figure 17. Distribution of amplitude level at first resonance frequency in Afula ...... 40 Figure 18. Distribution of resonance frequency of the second H/V peak in Afula .....41 Figure 19. Distribution of amplitude level at the second resonance frequency in Afula ...... 42 Figure 20. Velocity depth section along refraction line RL-1...... 45 Figure 21. Average H/V spectral ratios obtained at point 162, 202, 81 and 201 along RL-1 refraction line compared with the theoretical transfer functions...... 46 Figure 22. Velocity depth section along refraction line RL-2...... 47 Figure 23. Average H/V spectral ratio (the red line) for points 199 and 200 superimposed with two theoretical transfer functions calculated assuming different basalt layers as possible reflector. Black and blue lines correspond to reflector represented by upper basalt layer and lower basalt respectively...... 48 Figure 24. H/V spectral ratio and analytical transfer function for point 115 (Gan Tapukhim well)...... 49 Figure 25. (a) Velocity-depth section along refraction profile Af-1...... 50 Figure 26. (a) Velocity-depth section along refraction profile Af-2 (Afula Hospital); (b) H/V spectral ratio and analytical transfer function for point 295 located at refraction line Af-2 ...... 51 Figure 27. Velocity-depth section along refraction profile Af-3 ...... 51 4

Figure 28. Analytical transfer function and H/V spectral ratio for points (a) 209 (Merhaviya well) and 25 (Gidon 5 well) ...... 51 Figure 29. Characteristic 2-D cross section along profile A-A' (Bet Shean)...... 54 Figure 30. H/V spectral ratios compared with the theoretical transfer functions for points 79, 80 and 82...... 55 Figure 31. H/V spectral ratio for points 2 and 1 compared with the theoretical transfer functions...... 55 Figure 32. Comparison of the average H/V spectral ratios with the theoretical transfer function for points 4, 7, 27 and 9...... 56 Figure 33. Comparison of the average H/V spectral ratio obtained at point 6 located at refraction line R-0016 with the theoretical transfer function...... 57 Figure 34. H/V individual spectral ratio for site 164 without site effect ...... 58 Figure 35. H/V spectral ratios and analytical transfer functions for points 10, 11, 108 and 191...... 59 Figure 36. Characteristic cross section beneath profile B-B' (Bet Shean)...... 61 Figure 37. Comparison between average H/V spectral ratios with calculated transfer functions for points, located in the souther part of profile B-B' ...... 62 Figure 38. Comparison between average H/V spectral ratios and analytical transfer functions for points located in the central part of profile B-B' ...... 62 Figure 39. Individual H/V spectral ratios for points located on outcrop of basalt...... 62 Figure 40. Comparison between average H/V spectral ratios and analytical transfer function for 46, 176 and 177 located in the northern part of profile B-B'...... 63 Figure 41. Geological cross section along profile A-A-A'(Afula). For location see Figures 16-19...... 65 Figure 42. H/V spectral ratios and analytical transfer functions for points located along profile A-A’...... 66 Figure 43. Geological cross section along profile B-B' (Afula) ...... 68 Figure 44. H/V spectral ratios and analytical transfer functions for points located along profile B-B'...... 69 Figure 45. Geological cross section along profile C-C' (Afula) ...... 70 Figure 46. Characteristic H/V spectral ratios obtained at measuring points along profile C-C' ...... 72 Figure 47. Map showing different interpretations of faults location in the Bet Shean area...... 73 Figure 48. H/V spectral ratio and theoretical transfer function for point 2 vs. point 80 (Type I of fault identification)...... 74 Figure 49. H/V spectral ratio and theoretical transfer function for point 81 vs. point 4 (Type II of fault identification)...... 74 Figure 50. H/V spectral ratio and theoretical transfer function for points (a) 109 vs. point 91; and (b) 85 vs. 87...... 75 Figure 51. H/V spectral ratio and theoretical transfer function for point 50 vs. 74.....76 Figure 52. H/V spectral ratio and theoretical transfer function for point 21 vs. 42.....76 Figure 53. H/V spectral ratio 13 vs. 204; and 72 vs. 36...... 76 Figure 54. H/V spectral ratio and theoretical transfer function for point 76 vs. point 161 (Type 3 of fault identification)...... 77 Figure 55. H/V spectral ratio and theoretical transfer function for point 99 vs. point 147 (Type 3 of fault identification)...... 77 Figure 56. Geological map showing faults mapped by microtremor measurements in the Afula area...... 78 Figure 57. Map showing zone division in Bet Shean ...... 82 5

Figure 58. Spectral amplification curves for the nonlinear DSS calculated using Joyner program (1977) for different zones of Beit Shean for peak input motion from 0.1 to 0.3 g...... 85 Figure 59. Comparison linear and nonlinear soil response for peak input motion of 0.1 g for different Zones of Beit Shean: (a) time domain records on rock and surface; (b) spectra Fourier for rock, linear and nonlinear models ...... 86 Figure 60. Comparison linear and nonlinear soil response for peak input motion of 0.2 g for different Zones of Beit Shean: (a) time domain records on rock and surface; (b) spectra Fourier for rock, linear and nonlinear models ...... 87 Figure 61. Map showing zones division in Afula ...... 91

LIST OF TABLES

Table 1. Brief description of wells located in the town of Bet Shean ...... 20 Table 2. The instrument characteristics of the measurement stations ...... 26 Table 3. S-wave velocity models obtained from refraction survey along RL1 line and soil column models ...... 46 Table 4. S-wave velocity model obtained from refraction survey along RL-2 refraction line and soil column models...... 48 Table 5. Initial geotechnical data and soil column model for point 115 (Gan Tapukhim well) ...... 50 Table 6. Geotechnical data for calculation of model for point 25 (Gidon 5 well)...... 52 Table 7. Ranges of S-wave velocities for litho-stratigraphycal units represented in the study area taken from refraction survey...... 53 Table 8. S-wave velocity models obtained from refraction survey and downhole measurements and soil column models for point 6...... 57 Table 9. Vs values for the of upper travertine layers derived from fitting the theoretical transfer functions to H/V ratios for points from 4 to 9 along the profile...58 Table 10. Site classification and optimal dynamic shear strength (DSS) used...... 83 Table 11. Soil column models typical for representative sites of zones, transfer functions and linear and nonlinear spectral accelerations (Bet Shean)...... 88 Table 12. Soil column models typical for representative sites of zones, transfer functions and linear (red line) and nonlinear (blue line) spectral accelerations (Afula) ...... 92 6

ABSTRACT

The long documented history of destructive earthquakes in Israel shows that the whole area is subject to strong earthquakes, which have, in the past, caused considerable damage and many casualties. Similar earthquake to that occurred in January 749 and destroyed structures of the Roman-Byzantine period in Bet Shean will shake this region with its residents, buildings and facilities. In the present study forming part of a project "Microzoning of the seismic hazard in Israel" we once again used for evaluating site effects and estimating their influence on seismic ground motion a three-step process. At the first step, microtremor measurements with a dense grid were carried out and interpreted in order to map the predominant frequency and maximum relative amplification of ground motion. At the second step, all available geological information and well data were collected and incorporated as an aid to construct subsurface models for different sites within the investigated area. Finally, one-dimensional analytical models were used to predict site-specific acceleration response spectra from future earthquakes. The horizontal-to-vertical spectral ratios of ambient noise were used to approximate the fundamental resonance frequencies of the subsurface and their associated amplitudes. About 210 and 300 sites were instrumented in the towns of Bet Shean and Afula, respectively. The soil sites exhibit H/V peak amplitudes ranging from 2 to 7 in the frequency range 0.9 Hz to 13 Hz for Bet Shean. H/V spectral ratios in the Afula area reveal two resonance peaks corresponding shallow and deep reflectors. The first resonance frequency varies from 0.35 Hz up to 12 Hz with amplitudes of 2-8 units. The second resonance peak shows amplitude of units 2-8 in the frequency range 1 Hz to 10 Hz. These results imply significant variations in the shear-wave velocities across the area and considerable variations of sediments thickness. S-wave velocity profiles derived from limited geophysical and borehole data enabled the calibration of the H/V spectral ratios at corresponding locations by analytical site response functions calculated for 1D subsurface model. Velocity models further were used for estimating subsurface structure at sites where information on soil column was scarce. Certain sharp differences in the H/V ratios have been interpreted as being associated with a subsurface discontinuity, i.e., an unmapped fault. The evaluated 7 subsurface models are introduced using the SEEH procedure of Shapira and van Eck (1993) to assess Uniform Hazard Site-Specific Acceleration spectra (probability of exceedence of 10% during an exposure time of 50 years and a damping ratio of 5%) for different zones within the town of Bet Shean and Afula. The shape of the linear spectra obtained for all zones differ significantly from those prescribed by Israeli Building Code (IS-413). The Code requirements essentially underestimate the accelerations in the period range from 2 to 0.1 sec. We have estimated nonlinear response spectra for all zones and not found significant difference between the linear and nonlinear approaches for site classes B and C at peak input motions of 0.1g to 0.2g. Based on these comparisons, a nonlinear approach is recommended for site classes C, D and E for input motion greater than a few tenths of the acceleration of gravity. 8

1. INTRODUCTION

Most examples from several destructive earthquakes during the two past decades, for example, in Mexico-City, 1985 (Singh et al., 1988; Reinoso and Ordaz, 1999), Spitak, Armenia, 1988 (Borcherdt et al., 1989), California, Loma Prieta, 1989 (Hough et al., 1990) and Northridge, 1994 (Hartzell et al., 1996), Kobe, Japan, 1995 (Iwata, et al., 1996), Kocaeli (Izmit), Turkey, 1999 (Ozel et al., 2002) Algeria, 2003 (Hamdache et al., 2004) have clearly shown that local site conditions can greatly increase ground shaking during an earthquake. Therefore an accurate estimation of the seismic ground motion across the cities is of prime importance for urban developments and mitigation of seismic risk. In 2001 a special team was formed in the Geophysical Institute of Israel to map site effects in different areas across in Israel. Up to now, seismic mirozonation studies in the towns of Lod and Ramle, Qiryat Shemona, Kefar Sava, Dimona, Arad, the Coastal Plain, Hashefela and Qrayot (in the final stage) regions have been carried out. Detailed results are presented on the webpage www.relemr-merc.org. Choice of next object for microzonation study was dictated by the following concurrence of circumstances: The long history of earthquakes does not leave place for doubts that sooner or later an earthquakes as large as, or larger than the earthquake that occurred in January 749 and destroyed structures of the Roman-Byzantine period in Bet Shean will shake this region together with its residents, buildings and facilities. We know that effect of local site conditions on ground motion plays a major role in the shaking levels and hence should be seriously incorporated into seismic hazard estimations at a specific site. This is particularly important for both Bet Shean and Afula where we have strong impedance contrast between the soft sediments and the underlying bedrock represented by the basalts of Pliocene and Miocene ages. In addition, over the area of study the soil conditions change significantly from place to place. We are fully confident that shaking levels of future earthquakes can be predicated with sufficient details in order to consider in the seismic design of buildings and other structures. We believe that some of destructions might have been avoided if more information regarding resonant frequencies of the ground had been available. 9

In the present study forming part of a project "Microzoning of the seismic hazard in Israel" we again used a three-step process for evaluating site effects and estimating their influence on seismic ground motion (Zaslavsky et al., 2005). At the first step, microtremor measurements with a dense grid were carried out and interpreted in order to map the predominant frequency and maximum relative amplification of ground motion. In the second step, all available geological information and well data were collected and incorporated as an aid to construct subsurface models for different sites within the investigated area. Finally, one- dimensional analytical models were used to predict site-specific acceleration response spectra from future earthquakes. The application of this methodology makes possible reliable assessment of disaster from different earthquakes, especially in the regions where big earthquakes present a long return period, but which exhibit a high seismic risk according to historical reports, population distribution and its socio-economic importance. This report presents the results of main tasks performed within the framework of the project on evaluation of ground shaking characteristics in Bet Shean and Afula.

2. BRIEF REVIEW OF SEVERAL EXPERIMENTAL METHODS FOR SITE EFFECT EVALUATION

Among the empirical technique for site response estimating were summarized and discussed (Field and Jacob, 1995; Kudo, 1995, Lachet et al., 1996; Satoh et al., 2001 and others). Areas of high seismicity present opportunities for determining the site response functions through analysis of recorded ground motion during an actual strong event by comparison with recordings at a nearby reference site located on rock (Jarpe et al., 1988, 1989; Darragh and Shakal, 1991; Satoh et al., 1995; Hartzell, 1998; Reinoso and Ordaz, 1999 and others). The method for assessment site response function by ratio between the spectrum observed on a site of interest and the spectrum from the same source recorded at reference site have involved since Borcherdt (1970) and usually refers as the Standard Spectral Ratio (SSR). In Israel, where the seismic activity is relatively low, using strong motion data for analysis of site effect is impractical (Zaslavsky et al., 2003c; in Hebrew). Many investigators (McGarr et al., 1991; Field et al., 1992; Liu et al., 1992; Carver and Hartzell, 1996; Steidl et al 1996; Zaslavsky et al., 2000b; Lebrun et al., 2004 and many others) use SSR that to evaluate site response functions from moderate or weak motion recording of earthquakes. 10

The basic assumption of SSR method is that signal recorded in reference site (usually a bedrock outcrop site) presents the input motion of the base of the soil layers site. In many cases (Steidl et al., 1966; Zaslavsky et al., 2002) the weathered and cracked bedrock can have a site response of their own. It my be note that use these surface rocks as a reference sites often lead to underestimating of the amplification by factor 2-4 in frequency range which is within the range of engineering interest. Computation of spectral ratios relative to a bedrock reference station widely used to analyze microtremor records with purpose to study site effects. This approach can be valid when on reference and investigated sites microtremor originated from the same source. Kagami et al. (1982, 1986) concluded that microtremor generated by distant oceanic disturbances can be used as a measure of ground motion amplification. Really, law frequency microtremor over relatively short distances should have similar source and path effects. The applicability high frequency microtremor in site effect studies has been investigated by several studies (Field et al., 1990; Rovelli et al., 1991; Lermo and Chávez-García, 1994; Gaull et al., 1995; Zaslavsky et al., 1995; Ojeda and Escallon, 2000; Horike et al., 2001 and other). They consider it possible to estimate resonance frequency and amplification of sediments from high frequency microtremor by SSR method. However, in urban and suburban areas, microtremor is generated by human activities and intensity of microtremor source may essential change from place to place. Therefore, this method should be used within limited areas (a distance from reference station some hundred meters). Nakamura (1989) hypothesized that site response could be estimated from the spectral ratio of the horizontal versus vertical component of microtremor (HVSRN) observed at same site. The technique HVSRN widely using for estimating site amplification factors of S wave due to earthquake. Lermo and Chávez García (1994), Chávez García and Cuenca (1998) show that result obtained from H/V ratios agree with SSR method of the S-wave of weak and strong motion. Seekins et al., (1996) reported comparison of earthquake and microtremor using traditional station-par SSR method in order to clarify the applicability of microtremor data to ground motion prediction. They pointed out that microtremor station pair method may be use only for identifying the frequency of the fundamental resonance of soil site while Nakamura's method resulting are similar to those that derived from S-wave station-pair ratio. Zhao et al. (2000) also concluded that the H/V ratios of microtremor almost coincide with those of seismic motion. The experiment of Enomoto et al. (2000) based on 11 simultaneous recording microtremor at basement and surface using borehole at two sites. From these results H/V ratio is much more coincided to the theoretical transfer function due to SH wave propagation in the surface soil layer than the observed transfer function used simultaneous microtremor measurements between the basement and surface. Toshinawa et al. (1997) show that weak motion amplification factors for peak ground acceleration with respect to rock site correlated well with peak values of HVSRN. Konno and Ohmachi (1998), support HVSRN ratios because for high frequency microtremor their results agree with theoretical site amplification factors. Teves Costa and Senos (2000) presented predominant frequencies and respective amplification factor for the Lower Tagus Valley obtained with microtremor analysis. They concluded that the distribution of predominant frequency is in good correlation with the surface geology; however authors do not make comments on distribution of amplification factor. To the contrary, Horike et al. (2001) showed based on array measurements of microtremor, that amplification factor can be inferred from horizontal component ratio to a reference site, but HVSRN do not agree with site amplification factors. During recent years, more field studies suggest that the amplification determined from HVSRN exhibit clear peak that is well correlated with the resonance frequency but amplitude of this peak different from S-wave amplification determined by SSR (Teves Costa et al., 1996; Lachet et al., 1996; Bonilla et al., 1997; Satoh et al., 2001). Nevertheless, Malagnini et al. (1996) pointed out that H/V spectral ratio failed not only in identification amplification level but also of fundamental resonance frequency. It is necessary to remind that many parameters of recording influence data quality and its reliability: instruments, weather and ground conditions, soil-structure interaction, traffic, different almost-harmonic motion generated by the several of machinery operating in urban areas and others. Mucciarelli (1998) showed that under proper measurements conditions Nakamura's technique provides stable and reliable results, but great care has to be devoted to avoid factors that may significantly and adversely affect the results. Recently a European project SESAME (see detailed results on the webpage http://sesame-fp5.obs.ujf-grenoble.fr/) was initiated aiming to study the site effects assessment techniques using microtremor. Guidelines prepared within the framework of this project included the experimental factors affecting the H/V ratio results, the standardized processing software and the recommendations with regard to the experimental evaluation (Atakan et al., 2004), will be helpful for site 12 effect assessment. In a resent comprehensive study of Nakamura's method (Bard and SESAME participants, 2004) it has been concluded that the microtremor HVSRN does allow to identify the site fundamental frequency. However, they pointed that the microtremor H/V peak amplitude is smaller than actual spectral amplification factor measured by standard spectral ratio. It is farther concluded that the H/V amplitude at that fundamental frequency may serve as a lower bound of the expected amplification level. Lermo and Gháves-Garsía (1994) attempted to apply the Nakamura's method to study the intense shear wave part of strong or weak earthquake recording in different cities in Mexico. They found that in all cases the results give a robust estimate of the frequency and amplitude of the first resonant mode. Early, this technique has been applied to studies of the Earth's interior from teleseismic P-waves (Langston, 1979) and usually refers as receiver function. Many studies s report that the frequency dependence of site response can be obtained from measurements made at only one station at the analyzed site. For example, Yamazaki and Ansary (1997) used horizontal-to-vertical Fourier spectrum ratio (receiver function) of accelerogram consist 2166 three component sets from 387 events. The stability of the spectrum ratio they explained by the transfer function between the transfer function between the ground surface and soft-soil outcrop due to S-wave propagation. Zaslavsky et al. (2000) and Moya et al. (2000) demonstrated that sediment-to-bedrock spectral ratio and receiver function are remarkable similar for fundamental frequency and amplification factors of a site. Mucciarelli et al. (2004) compared stability horizontal- to-vertical spectral ratios, composed of 674 triggered noise record and 132 earthquakes and showed that resonance peaks obtained with two different data sets are very similar as in frequency as amplitude. The present report shows that ambient vibration measurements can be used to obtain reliable information related to seismic behavior of sediment layers (in linear conditions) with thickness from 20 to 600m; and appropriate ensembles of carefully selected windows of ambient vibration time segments and careful analysis of the horizontal and the vertical amplitude spectra and their ratios, provide estimates of the site response similar to those obtained from the H/V spectral ratio of seismic events.

3. H/V SPECTRAL RATIO TECHNIQUE

The Nakamura's technique has been applied to the microtremor measurements carried out in the presented study. Very simplified geological section for the town of Bet Shean is presented in Figure 1. On this factual example, we shall give simple explanations of the Nakamura's technique. Main assumptions of Nakamura's approach are: • Horizontal soil layering over a hard bedrock (half-space); • Ambient noise consists of different types of waves; • Vertical component of ambient noise displays the characteristics of local noise sources and is relatively uninfluenced by the soft sediment layers overlying a half-space; • Components of ground motion are equal in all directions at the basement.

Figure 1. Very simplified geological section as an illustration of Nakamura's technique assumptions.

If we follow these assumptions and for a geological situation shown in Figure 1, site amplification can be defined as expression

SE(ω) = Hs(ω) / HB(ω) where Hs(ω) and HB(ω) are the horizontal amplitude spectrum and the ground surface and at the bedrock. In the absence of amplification in the vertical component, microtremor source spectrum can be expressed as ratio of vertical Fourier spectrum at the surface and bedrock, i.e.

As(ω) =Vs(ω) /VB(ω) After normalization by spectrum allowing removing the unknown source effects from the soil amplification we obtain transfer function of the soil layer 14

SM (ω) = SE(ω) / AS(ω) = [HS(ω) /VS(ω)]/[HB(ω) /VB(ω)] Since according to Nakamura's assumption

HB(ω) /VB(ω) = 1 Transfer function may be expressed as following:

SM (ω) = Hs(ω) /VS(ω) Or, by the other words, the vertical component of ambient microtremor on the surface retains the characteristics of horizontal component of the bedrock.

4. GEOLOGICAL SETTING

Figure 2 presents the geological map of Israel to a scale of 1:200,000 (Sneh et. al., 1998) in the Lower Galilee, where the studied areas of Afula and Bet Shean are located. The Lower Galilee is characterized by tilting blocks bounded by listric faults. There is no clear structural and morphological expression of the Syrian Arc fold belt in this region. The principal fault structures were established in the Miocene, in relation to the tectonic activity of the Dead Sea Fault, and were active mainly in Plio- Pleistocene times, when the Dead Sea Rift was formed. The Miocene faults are mainly right lateral strike-slips, whereas the later are characterized more by vertical displacements of up to a few hundred meters (Garfunkel, 1981). The NW-SE trending tectonic depression of the Yizreel and Herod valleys meets the Jordan Rift in the Bet- Shean Valley area. The region is underlies by a volcano-sedimentary sequence up to 800 m thick of Miocene to Pleistocene age. The Bet Shean valley is separated from the Jordan Valley by a morphological escarpment 30-50 mw high, which is the surface expression of the western marginal fault of the Dead Sea Rift. The town of Afula is situated in the central part of the Yizreel Valley, while the Bet Shean town is built above the western marginal fault of the Dead Sea Rift which is an active fault (Zilberman et al., 2004). The investigated areas include threemorphotectonic units: lower Afula (Yizreel Valley) unit (+60m); the Givat Hamore unit (+517m); the town of Bet Shean (-120m in the west and -200m in the east).

Figure 2. A fragment of the Geological map of Israel (1:200 000) in Lower Galilee with the towns of Afula and Bet Shean. 16

4.1. BET SHEAN

Bet Shean is located around the junction of the aligned Harod-Bet Shean valleys with the Dead Sea Rift. It is bounded in the north by Ramot Yissakhar. The area in underlies by Neogene-Quaternary sequence, which consists of a fluviatile- lacustrine sediments interbedded with and volcanic and pyroclastic rocks, included in the Tiberias Group. This sequence, which is 500-800 m thick (Gardosh and Bruner, 1998) unconformably overlies Cretaceous- Tertiary rocks. The hard basalt units, which are interbedded in the Pliocene sequence, can be assumed as potential reflectors and therefore they were the subject of our geological analysis is this area. The geological map presented in Figure 2 is based on studies of Hatzor (2000), Rozenbaum et al., (2004) and also our own field investigations. Tiberias Group. The Tiberias Group includes the Hordos Fm. and the Lower Basalt unit. The typical section of the Hordos Fm. consists of a thick fluvial-lacustrine sequence, which interfingers with several basalt flows which are the northernmost extensions of the Lower Basalt (Schulman and Rosenthal, 1968). According to Shaliv et al. (1991) the Hordos Fm. is subdivided into two members: Lower Conglomerate up to 20m thick consisting of poorly sorted and poorly rounded pebbles cemented by carbonate; and Clastic-Carbonate Member sequence thickening eastwards from 30m up to 160m. The second member consists of alternations of thinly bedded red calcilutite, sand, dolomite and chalk. The age of the Hordos Fm. ranges from Early Miocene to Lower Late Miocene. The Lower Basalt reaches the thickness of 630 m in Ramat Yissakhar and extends as far as the Gilboa in the south (Schulman, 1962). The Lower basalt is of the alkali-olivine type, finely crystalline, with partially altered olivine, pyroxene, and plagioclase, olivine, pyroxene, and ore minerals, in a trachytic texture. The age of the Lower Basalt in the Bet Shean and Harod valleys ranges from 17.5 to 10.3 Ma (Shaliv et al., 1991). 17

714000 714000 R AMOT Kb YISSA B' KHAR βc βc Kba al Tr al R H 713000 erod RL-2 ql 713000 Valley qt βc βc

E qt A 712000 712000

A' D RL-1 qt

E Ir A R-0016 SG

R T6 711000 I 711000 F al A T

710000 710000 BE B T SH EAN VA al LLE Y qt

709000 709000 245000 246000 247000 248000 249000 250000

Alluvium, colluvium, and Fault according to Hatzor (2000) al anthropogenic sediments (Holocene)

Dip trend qt Bet Shean Travertine (Pleistocene)

A Cross section ql Lisan FM. (Pleistocene) Measurment point βc Cover Basalt (Pliocene)

RL Refraction line Morphot ectonic steps according to Rozenbaum (2004) Borehole Fault according to Rozenbaum (2004) Boundary of the investigated area Fault according to Zilberman (2002)

Nahal Herod

Figure 3. Geological map of Bet Shean (1:50,000) showing distribution of measurement points. 18

The lower basalt is penetrated by Sede Nahum-2 well at a depth of 217 m 2 km away from the study area near Ramat Yissakhar. Lithological section of Sede-Nahum-2 borehole is shown in Figure 4.

Sade Nahum-2 Ain Ashtadri T/6 0 0 Cover basalt Pliocene Alternation of 57 travertine and conglomerate

Pleistocene Marl,clay,gypsum Pliocene

217 65

Lower basalt Miocene 309

Figure 4. Lithological section of Sade Nahun-2 and Ain Ashtadri T/6 wells.

Dead Sea Group The Dead Sea Group, Zak (1967) overlies the Tiberias Group. In the study area it consists of the Umm Sabune Conglomerate, Bira Fm., Intermediate Basalt, Gesher Fm., Cover Basalt, Wadi Malih Conglomerate, Lisan Fm., and Bet Shean Travertine (tufa). The Cover Basalt builds the top of the south-west tilting block of Ramot Yissakhar and it is also exposed in the northeastern part of Nahal Harod stream-channel (See Figure 2). The Cover Basalt is well known from boreholes in Harod Valley, where it reaches a thickness of a few tens of meters. "The basalt flowing from Ramot Yissakhar towards the northern Gilboa area, was blocked by the already-existing Gilboa escarpment" (Rosenthal, 1972). The radiogenic age of this basalt in the Nurit area is 5.2 Ma and in Nahal Avinadav is 4.9 Ma, Pliocene (Shaliv et al., 1991). The outcrops of the Basalt units and the tufa were mapped more accurately in the study area (see Fig.3). 19

The Bet Shean travertine (tufa), is rather local unit extending along a belt about 10 km long in north to south direction and 4-6 km in wide extending from the Gilboa margins eastwards. It was first described by Picard (1943). Shaliv et al. (1991) subdivided it into the following two members: the travertine Member of 60m thick found, for example, in Ain Ashtori T/6 well (see Figure 4) and the marl Member, which overlies the travertine Member and is about 30m thick. Rozenbaum, A. G. et al., (2004) divide tufa into two main facies: first is Phytoherm framestone and phytoclast tufa facies, and second is intraclast tufa and Cyanolith "oncoidal" tufa. "The travertine units are the stratigraphic markers permitting an identification of young tectonic displacement in the Bet Shean area." The Bet Shean travertine is exposed in the study area along the creek of the Harod River and along the NS oriented morphological escarpments, which separates the Bet Shean valley from the Jordan Valley. The contours of the travertine shown in the Geological map (Figure 3) are copied mainly from the Preliminary map of tufa outcrops in the area of Bet Shean Valley (Rozenbaum et al., 2004). Holocene sediments are represented by colluvium, alluvium and anthropogenic sediments. These sediments are exposed in the southwestern part of the study area. They consist of few meters of topsoil and in the eastern part contain scattered tufa fragments in a gray silty matrix up to 4-5m thick. These sediments overlay tufa. In the north Industrial Zone colluvium consists of clay with debris basalt of 0-15m thick overlying Cover Basalt, according to the data from well R, Mifal “Rada”. All available wells located within the study area are given in Table 1. Tectonics features Picard (1943) suggested that "the subsidence of the Dead Sea Rift (DSR) in the Jordan Valley area and the uplift of the surrounding mountains occurred in the Neogene and that these differential movements continue, with less intensity, through the Quaternary". This tectonic history framework is still accepted today. The Bet Shean Valley is a triangular depression (Belitzky, 2002) bounded to the west by the Mount Gilboa block and to the east by the western marginal fault of DSR. According to Rozenbaum et al., (2004), two main groups of morphological linear elements were observed in the area (brown line in Fig. 3). The first one is a set of N-S oriented morphological escarpments, which separates the Bet Shean Valley. 20

Table 1. Brief description of wells located in the town of Bet Shean

Short Depth Full name EW NS Description name (m) 0-65m Alteration travertine and Aih Ashtadri T6 248560 711160 65 conglomerate 0-7m Silty clay; 7-30m marly clay with Iriya-1 Ir 247100 711377 30 travertine loose Mifal Kabel* Kb 247090 713860 6 0-6m Clay with debris basalt Mifal Kabel* Kba 247070 713600 6 0-6m Clay with debris basalt School 0-2m Silty clay; 2-12m marly clay with SG 247256 711311 12 Gilboa* travertine loose Mifal R 247040 713245 4-15 0-15m Clay with debris basalt "RADA"* Mifal 0-5.5m Clay with debris basalt and Tr 246830 713450 3-5.5 "Trufot"* limestone

(* - information on the wells was kindly provided by the company “Eng. David David Ltd.”) These escarpments represent the trace of the western marginal fault of the DSR. The second one is a set of NW-SE oriented lineaments that cross the Bet Shean and Jordan valleys assumed to represent deformation related to another fault system. A paleoseismic study of Bet Shean Valley (Zilberman, 2004) carried out along a segment of the marginal fault of the DSR north of Tel Rehov provided evidence of continuous tectonic activity since the latest Pleistocene. The total vertical displacement along the marginal fault in the last 20,000-30,000 years is estimated by Zilberman (2004) as 40-50 m.

4.2. AFULA

Geological analysis of the Afula area was based on the researches of Dicker (1969), Shaliv (1991) as well as the geological map to a scale of 1: 50000 by Dicker (1969) shown in Figure 5. The study area was divided into three structural zones: • The Yizreel basin of Neogen age including the Lower Afula and the settlements of Merhaviya and Sulam. This zone is a flat plain overlain by the Quaternary deposits of 70 meters thick maximum. In the Merhaviya kibbutz crop out clay with conglomerate of the Bira Fm. (Pliocene). 21

728000 Kf. Gidon13 Dovrat1 p β L al A' 727000 E 1 Balfouriyya K Afula Illit β L β L Balfouriyya al Balfouriyya KA cm 726000 C β L E 2 Balfouriyya11 al E 1 Sarid-1 al Givat Hamore 725000 Af-2

A Af-1 λ E 2 B' GIDON-5 E Merhaviya 1 Afula Alef A 724000 Afula R Afula Gan Tapukhim Afula3 Sulam Lower Afula Merhaviya1 p cm Afula32 E 2 723000 Merhaviya Y Afula Mifalei Sukar i Afula34 al z β L r Af-3 C' S e Tamra 722000 C e Legend: l al Alluvium (Holocene) E 1 Limestone, chalk, Timrat Fm. (E.Eocene) t Marginal conglomerate (Quaternary) Chalk, Mt.Scopus Gr. (Senonian) V al cm S a β c Cover Basalt (Pliocene) t Limestone, Bina Fm. (Turonian) 721000 l B Nari - Covered basalt (Neogene) C Limestone, Sakhnin Fm. (Cenomanian) l p Marl-clay, Bira Fm. (Pliocene) Faults of Top Judea Gr. Cross section β c e λ Dolerite (Miocene) Faults of surfase along profiles y β Lower Basalt (Miocene) L Wells Refraction profile E 2 Limestone, Bar Kokhba Fm. (M.Eocene) Investigated area Measuring point β L 720000 224000 225000 226000 227000 228000 229000 230000 231000 232000 233000 234000 235000 236000

Figure 5. Geological map of the Afula area 22

• Balfouriyya – Afula Illit Lower basalt ridge. A low, 3.5 km long basalt ridge extends in a western direction as a continuation of the Givat Hamore block, from Afula Illit in the east to Balfouriyya in the west. The Lower Basalt (partly weathered) in this zone crops out in the central part of the ridge. To the north it is overlain by the recent sediments of ten meters thick. • Givat Hamore Mount block. This zone is an uplifted block, structurally controlled by normal marginal faults trending NW- SE. These were active at the beginning of the Neogene and were rejuvenated thereafter. It consists of the Limestone complex of Eocene age and peak is igneous intrusive basic bodies of Miocene age. Igneous intrusions connected with the earliest activity of the WNW system. The Lower basalt flowed from Givat Hamore, where it is discordantly resting on the Limestone complex. In the northern part of block crops out marginal conglomerate overlying basalt. Stratigraphy and lithology Sediments of Jurassic-Cretaceous period Jurassic sediments penetrated in Sarid-1well at a depth of 1640m (see Figure 6) are represented by continuous sequence of limestone, dolomite and minor shale (Politi, 1983).

Gan Tapukhim Gidon-5 Sarid-1 0 0 Clay with conglomerate Pleistocene 0 Clay with conglomerate Pleistocene Pleistocene 29 Marl,clay Pleistocene 40 Clay 43 Conglomerate Pleistocene 22 113 Cover basalt Pleistocene 70 Conglomerate Pleistocene 120 Cover basalt Pliocene 37 96 Cover basalt Sandstone with marl Pliocene 112 Pliocene 200 121 Sandstone with marl Pliocene Marl,clay Pliocene 143 243 Marl,clay Pliocene 215 Marl,clayClay Pliocene Limestone Pleistocene 96 Marl,clayClay Pliocene 500

Lower basalt Miocene

700 Shale with limestone Cretateous 780 468 Marl,clay Pliocene Limestone with shale Cretateous 192 960 Lower basalt Miocene

Limestone Clay Pliocene Cretateous

276 760 Basalt conlomerate with clay Miocene 808 1450

Lower basalt Miocene Lower basalt Miocene Extrusive Cretateous 340 1640 968 Basalt conlomerate with clay Miocene 1023 Limestone Jurassic Lower basalt Miocene 1100

2013 Figure 6. Lithostratigraphyc section for Gan Tapukhim, Gidon-5 and Sarid-1 wells

The Cretaceous sediments, consisting mainly of dolomite with limestone, some marls, is also penetrated in Sarid-1 well at a depth of 700m and crop out in the SE part of the map in the Khirbet Qara tilted blocks. At this place is also exposed the Chalk complex of the Mt Scopus Gr. (Senonian age) of 120 m thick. Eocene Limestone Complex This complex is exposed at the Givat Hamore Mount block and discordantly beds on the Chalk complex of the Mt. Scopus Gr. The thickness of Eocene sediments in the western part of Givat Hamore is approximately 300m (Dicker, 1969). This complex consists of Limestone with chalk and flint of the Timrat Fm. (Early Eocene) of 210m thick (unit E1 in the Geological map) and fine-grained limestone of the Bar

Kokhba Fm. (Middle Eocene) of 95m thick maximum (unit E2). Miocene igneous intrusive bodies are exposed at Givat Hamore. The petrography of these rocks is micromeltegeite and alkaline dolerites (Oppenheim, 1962). The country rock belongs mostly to the Eocene Limestone complex. The intrusive bodies are dykes, stocks and single volcanic vent (unit λ).

Lower Basalt (unit βL) exposes along Balfouriyya – Afula Illit basalt ridge in the western direction as a continuation of the Givat Hamore block. The rock is olivine basalt. The basalt is intensely calcite jointed. Contact basalt-limestone on the western slopes of Givat Hamore is often altered to a soft, calcareous material, on which a secondary nari crust develops (Coordinates: 230800/725870). In Afula Illit (Coordinates: 230100/725100) basalt sometimes is weathered with sporadic patches of clay-alteration products of basalt. The basalt flowed from Givat Hamore, where it is discordantly resting on the erosion surface of the Eocene Limestone complex. The two basalt bodies located in Givat Hamore (Coordinates: 233370/725100 and 234500/725870) asserted that their provenance was from a central eruption centre in the Givat Hamore. In wells drilled in the Yizreel basin, Lower Basalt was found at elevations that comply with the general southwestern plunge observed on the surface. Within the study area following wells penetrated the Lower Basalt: Merhaviya well at a depth of 44m; Merhaviya-1 well at a depth of 84m; Afula Gan Tapukhim well at 276m and Gidon-5 well at 468m (see Figure 6). Dead Sea Group. Sediments of the Dead Sea Group overly the Lower Basalt. Their thickness increases from Givat Hamore to the southwest reaching 500 m (well Sarid-1). On the 24 basis of the borehole data the following six lithology-stratigraphic sequences can be selected, from bottom to top: 1.”Clay series” deposited unconformably on the Lower Basalt and consists of the calcareous brown soft clay. Its maximum thickness penetrated in the well Gidon-5 is 253m and minimum thickness in the well Merhaviya 1 is 19m. 2. Bira Fm. deposited conformably on the Clay series and consists of the calcareous clay to marl white to grey, interbedded chalk and silts. Its maximum thickness is 96 m (Afula Gan Tapukhim well) and it is outcropping in northeastern part of the area (kibbutz Merhaviya). 3. Gesher Fm. Consists of a series of freshwater limnic, mainly oolitic limestones unconformably overlying the Bira Fm. Its facies change gradually into calcareous sandstone, green marl, chalky limestone, occasional hard limestone beds and oncolitic gray limestone (Shaliv, 1991). Thickness of the formation according to the borehole data ranges between 20 and 70m. 4. Fragments of Cover Basalt layer (Pliocene age) is well known from boreholes in Yizreel Valley, where it reaches a thickness of 16m maximum (Gidon-5 well). Its basalt flowing from Gilboa area towards the NW exists in the Yizreel Valley and unconformable overlay Gesher Fm. According to the well data thickness of layer basalt unsteady, sometimes exist debris of basalt (wells Afula Alef, Afula Mifalei Sukar). The basalt is mostly hard, barely weathered, sometimes interbedding with tuff (well Afula Gan Tapukhim). 5. Conglomerate (Pleistocene age) has thickness varying from 70m to few meters with average of 20 meters. This conglomerate may be compared with the Wadi Malih Conglomerate. It is resting on the Cover Basalt and consists of pebbles limestone ranging from the L. Cretaceous to the Eocene and basalt, the matrix is silt. Nari- covered conglomerates and slope-breccia (unit cm) are found at the northern and southern margins of the Givat Hamore block, resting on basalt and abuts against a fault scarp in the Eocene Limestone complex. 6. Silty clay with conglomerate (Quaternary age). It is overlain by conglomerate in Yizreel basin, has thickness from 75m to few meters (average 25m). Northern Afula Illit recent sediments overlying the Lower Basalt have thicknesses up to ten meters. 25

5. THE PLANNING OF MICROTREMOR MEASUREMENTS AND DATA ACQUISITION

Microtremor measurements were carried out during the period from May to September 2005 in the town of Bet Shean (W-245850; E-249510; S-709840 and N- 713900) and from September to December, 2005, in the town of Afula including Merhavia, Balforyya and Sulam settlements (W-223450; E-235400; S-720080 and N- 727455). The work areas are approximately 20 km2 and 39 km2 for Bet Shean and Afula, respectively. The distributions of measurement points over Bet Shean and Afula are shown in Figures 3 and 5. In the beginning we designed a large spacing between measurement points (500 m grid) but through lateral variation of the results, density the grid point spacing, down to 250 m and for specific sites spacing was 150 m. An important issue that was raised before and during the investigations is the question of how dense should the grid of measured points be? In retrospective, after many projects in order to estimate site response functions in different towns of Israel, we may state that we gained reliability to the obtained results only because we had a dense grid of measured sites. Again, it has to do with the application of the Nakamura technique. Furthermore, in most of Israel there is very limited availability of densely distributed geotechnical information such as S-wave velocities and densities of the materials, especially at depth. We could compensate for the need of a dense grid of measured points of microtremor by drilling new borehole, conduct many geophysical surveys and monitor strong enough earthquakes at points across the area. These alternatives are by far more expensive, time consuming and may not always provide the necessary information. Reliability and applicability of fundamental frequency and its amplitude obtained from Nakamura's technique may be influenced from different factors during microtremor records. So far as this approaches to microzonation usually is used in urban areas such factors as anthropic noise, underground piping and construction, soil-structure interaction may seriously to change results. Therefore, during data collection we not only visual checking of quality of time series of microtremor at each site, but twice (in the beginning and at the end set of record) computed Fourier spectra and horizontal-to-vertical spectral ratios for two-three time windows. Ground motion (velocity time history) was recorded using the multi-channel digital seismic data acquisition system designed for site response field investigations 26

(see Shapira and Avirav, 1995). The system includes: a multi-channel amplifier with band pass filters 0.2-25 Hz, GPS (for timing) and a laptop computer with analog-to- digital (A/D) conversion card. The seismometers (L4C) used were sensitive velocity transducers with a natural frequency of 1.0 Hz and damping at 70% of critical. The microtremor motions were digitized at the ratio of 100 samples per second by a 16-bit A/D converter. The duration of each microtremor recording also is very important parameters. For many authors (Rovelli et al., 1991; Mucciarelli and Monachesi, 1998; Lebrun et al., 2004) duration of each noise recording was 10-20 min. Our experiment saw that for high stability and high fidelity of H/V ratio estimation preferable to perform at 50-70 min observation at each point. Therefore, at each site, the microtremor was recorded continuously for 60 minutes, creating data files of 3 minutes each of microtremor data. In Figures 7 and 8 we present examples of the locations of the seismic stations during the site investigation in the towns of Bet Shean and Afula. Prior to performing measurements we checked and determined the transfer function of the instrumentations in order to facilitate transformation of the record signals into true particle velocity system. The individual seismometer constant (free- frequency, damping and motor constant) were determined from sinus and step calibration signal. The instrument characteristics of the stations are given in Table 2.

Table 2. The instrument characteristics of the measurement stations

Generator constant Damping Sensor Frequency Code at 1 Hz % Number Hz V/m/sec 3406 V406 88.5 1.00 65 3209 H290 103.0 1.00 70 3210 H210 106.8 1.00 70 3408 V408 13.1 1.00 65 3401 H401 87.8 1.00 70 3400 H400 87.4 1.00 70 3404 H404 91.8 1.00 65 3396 H396 96.7 1.00 70 3395 H395 91.6 1.00 65

In addition, all seismometers were placed at the same locations and in the same orientation to record the same waves (Figure 9). These measurements allow assessing identity different channels of the entire monitoring system i. e. transducer, amplifier, filter, and analog-to-digital conversion. Figure 10 a,b present, as example, the seismograms (volts) and corresponding Fourier spectra of microtremor of 6 horizontal and 6 vertical seismometers. We can see that traces and its Fourier amplitude spectra show very good identity. The procedure "instrument response removal" used in seismology de- convolves output of seismometer channel to estimate real ground motion. Figures 11a, b shows horizontal components of ground velocity of microtremor (micron/sec) and Fourier velocity spectra after remove responses instruments. From visual inspection we can see that both time series and spectra are not identical for different channels and there are essential distinctions in identity of channels in frequency range 0.2-0.8 Hz. (Figure11b). Therefore, if the fundamental frequency of site effect is less than 1.0 Hz we do not recommend "remove instrument" in data processing. Figures 12 and 13 demonstrate influence of "remove instrument" on the vertical seismometers channels with analogous conclusions. However, influence of the "remove instrument" procedure to the vertical seismometers channels is somewhat less.

Figure 7. Examples of of seismic station location in Bet Shean 29

Figure 8. Examples of seismic station location in Afula

a b

Figure 9. Setup of (a) horizontal and (b) vertical transducers for identity test of channels

Figure 10. Ambient noise recordings for the setup of (a) horizontal transducers and (b) their spectral amplitudes without application of "Instrument response removal" procedure

Figure 11. Ambient noise recordings for the setup of (a) horizontal transducers and (b) their spectral amplitudes with application of "Instrument response removal". 32

Figure 12. Ambient noise recordings for the setup of (a) vertical transducers and (b) their spectral amplitudes without application of "Instrument response removal" procedure.

Figure 13. Ambient noise recordings for the setup of (a) horizontal transducers and (b) their spectral amplitudes with application of "Instrument response removal" procedure. 34

6. DATA ANALYSIS

The reliability and applicability range of the Nakamura's technique is strongly depended on the different stages of data processing and requires special research, knowledge, experience and intuition. In the spectral analysis of microtremor data we have to answers several questions, for example: • Which technique has to be adapted to select time windows: manual or automatic? • What must be the minimum time window duration? • What is the "carefully" selected time window? • What is the effect of window shape on smoothing? • How can be diminished the influence of anthropic microtremor (complex or almost harmonic motions)? • What is the required duration of microtremor record to obtain sufficient number of time windows for good reliability? • How to understand when "good" microtremor sample has been collected and when "not good"? To study the spectral character of the microtremor within the Bet Shean and Afula towns, we computed spectra and spectral ratios using two different time windows, consisting of 30 sec records for sites with resonance frequencies above 1 Hz and with resonance frequencies of 60 sec records for sites with resonance frequencies less than 1 Hz. The selected time windows were Fourier transformed, using cosine- tapering (1 sec at each end) before transformation and then smoothed with a triangular moving Hanning window. The H/V spectral ratio was 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 microtremor, we used several time windows (60-70) that yielded a number of spectral ratios that, in turn, were averaged.

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). If the shapes of SNS/V and SEW/V are similar then the average of the two horizontal-to-vertical ratios is defined. 35

⎡ n f n f ⎤ 1 S NS ( ) S EW ( ) A()f = ⎢∑∑i + i ⎥ (3) 2n ⎢ i==11()f i ()f ⎥ ⎣ SV i SV i ⎦

We have consistently observed that averaging the spectral ratio arithmetically or geometrically does not significantly change the results. It is worth noting the importance of this averaging procedure, the main problem related to the selection of time windows. Routine analysis and processing of data were carried out using the software SEISPECT developed in the Seismology Division of GII (Perelman and Zaskavsky, 2001). SEISPECT is a MATLAB application for spectral analysis and processing of ground motion, including seismograms recorded by short-period and broad-band seismic stations, as well as strong motion accelerometers. The main modules realized in the program are visualizing and editing of the input data; selecting time window and computing FFT and H/V spectral ratios; saving and displaying results. SEISPECT was distributed and successfully used in the countries-participants of the RELEMR program.

7. DISTRIBUTION OF THE FUNDAMENTAL FREQUENCY AND ITS ASSOCIATED H/V SPECTRAL RATIO LEVEL

The fundamental resonance frequencies and maximum values of H/V spectral ratios maps were constructed to integrate all results from the microtremor measurements and puts constraints on the 1-D subsurface model to be developed using geological and geophysical information. Also assuming that the increased intensity of the damage during earthquakes is, to a great extent, correlated with resonance effects, mapping the predominant frequency and maximum relative amplification of ground motion become key elements for seismic hazard scenarios (Shapira et al., 2001).

7.1. BET SHEAN

The maps for the town of Bet Shean shown in Figures 14 a,b were constructed on the basis of 210 H/V spectral ratios from microtremor. One glance at these maps 36 tells that the prominent feature of the both maps is sub-meridian strike of isolines that coincide with young tectonic displacements in the Bet Shean area. As seen from the fundamental frequency map presented in Figure 14a, resonance frequency in the town of Bet Shean varies within the wide range 0.9-14 Hz. The highest frequency values are attained in the areas surrounding the outcrops of basalts and near the north edge of the town. Moving from the north to the south and southeastern directions we observe general decrease of the resonance frequency that correlates with the depth of the basalt occurrence. Two zones, where we did not detect site effects, are distinguished in the map. Since the first zone is located at the exposed basalt, it was not surprising that H/V ratios in the first zone contain no evidence of ground motion amplification. The lack of resonance frequency at points distributed in adjacent the outcrop areas may be explained by too thin sediments overlying reflector. The absence of site effects in the second, southeastern zone, attached to the western marginal fault system of the DSR may be connected to the ruptured zone along the faults. Distribution of the maximum H/V amplitude within Bet Shean is depicted in Figure 14b. Reflecting the variation of the impedance contrast between the bedrock and overlying sediments, the highest amplification values (up to factor 6) are attained at sites in the north and eastern edge of the study area where alluvial deposits are lying directly over the Cover basalt Pliocene age. Moderate site response (range from 2.5 to 4) is observed in areas where the lithological section consists of alluvium of various thickness and travertine overlying basalt. With the exception of the areas with no site effects, around the N-S faults of the DSR, whose lithological section is represented by the relatively high velocity travertine overlying basalt, yield low amplifications (less than factor 2.5). Within this field of low amplifications there we observe an islet of higher values connecting probably with variation of S-velocity for the upper layer.

12 0 10 714000 5 7 5 B B 4 4 3 3 2 0 2.5 1.5 713000 1 2 RL2 RL2 0.9

No No site effect site effect

0 A 712000 A

RL1

RL-0016 RL-0016 RL1 I1 SG

0 711000 A A

B B 710000 0

246000 247000 248000 249000 246000 247000 248000 249000

Figure 14. Distribution of H/V resonance frequency (a) and its associated amplitude level (b) within the town of Bet Shean.

7.2. AFULA Typical examples of H/V ratios from microtremor obtained in the town of Afula are shown in Figure 15. In these examples two clear resonance peaks appear at different frequencies. Therefore, to characterize distribution of the site effect parameters in Afula we considered frequency and amplitude for two peaks.

Figure 15. Typical examples of the H/V ratios obtained in the town of Afula

Map of the resonance frequency and amplitude of the first H/V ratio peak

H/V spectral ratio exhibits the first resonance peak at frequencies 0.3 -12 Hz with amplitude changing from 2 up to 8 (see Figures 16 and 17). According to the geological data, this peak is associated with the Lower Basalt of Miocene age and its morphology is reflected in the measurement results. The general trend from 0.3 up to 3 Hz toward the east and northeast is observed. In the Lower Afula are distinguished four areas divided by deep fault systems. The northwestern area is the deepest and is characterized by the frequencies of 0.3-0.6 Hz. According to the Gidon-5 well data, the Lower Basalt is found at a depth of 476 meters. In the northeastern area the resonance frequency is distributed in the range 0.6-0.8 Hz and disappears on the contact with the Lower Basalt Ridge that allows suggesting continuation of the southern marginal fault system to the west, exposed only in the Giv'at Hamore (Dicker, 1969). This area is limited in the south by fault, which is identified by the sharp shift of the resonance frequency from 0.7 Hz up to 1 Hz. To the southeast from this fault the increase of the resonance frequency up to 3 Hz is detected, confirmed by the data from Kfar Merhavia well, according to which the lower basalt is found at a depth of 40 meters. In the south area of the Lower Afula we obtained the resonance frequencies from 0.4 Hz up to 0.8 Hz. In the Lower Afula the impedance contrast 39

0 72800

' A 727000

C 726000

Af2 725000 Af1 ' B

A 10 724000 A 4

C 2 ' 723000 1.5 1 Af3 0.8 722000 0.6

0.5 B 721000 0.4 0.370.28 No site effect

720000 223000 224000 225000 226000 227000 228000 229000 230000 231000 232000 233000 234000 235000 236000

Figure 16. Distribution of resonance frequency of the first H/V peak in Afula 40

728000

727000 A '

726000 C Af2 725000 B' Af1

A 724000

A 8

723000 6 C' Af3 4 722000 3 B

721000 2 No site effect

720000 223000 224000 225000 226000 227000 228000 229000 230000 231000 232000 233000 234000 235000 236000

Figure 17. Distribution of amplitude level at first resonance frequency in Afula 41

728000

727000 A'

726000 C Af2 725000 Af1 B'

A 724000 A 8

723000 5 C' 3 722000 Af3 2

B 1.5 721000 1 No site effect

720000 223000 224000 225000 226000 227000 228000 229000 230000 231000 232000 233000 234000 235000 236000

Figure 18. Distribution of resonance frequency of the second H/V peak in Afula 42

728000

727000 A'

726000 C

Af2 725000 Af1 B ' '

A 724000 A

723000 C' 7

5 Af3 722000 4 B 3 721000 2 No site effect

720000 223000 224000 225000 226000 227000 228000 229000 230000 231000 232000 233000 234000 235000 236000

Figure 19. Distribution of amplitude level at the second resonance frequency in Afula 43 determining amplitude of the main H/V peak is formed by Pliocene-Pleistocene sediments overlying the lower basalt. Therefore, the amplitude values that we observed are 2-4. Only in the southeastern part of the Lower Afula the amplitudes reach value of 5. There, according to the Kfar Merhavia well data, the Pleistocene conglomerates overly directly the Lower Basalt. Flat H/V spectral ratios with no resonance frequency are attained at sites located at the Balfouriyya-Afula Illit ridge (outcrop of the Lower Basalt) and Giv'at Hamore block (exposure of the Eocene deposits). Within the limits of area with no site effect, some anomalies characterized by the frequencies 4-12 Hz and amplitudes of 3-9 are detected that is probably connected with weathered basalt, alluvium and marginal conglomerates of Quaternary age covering the Lower Basalt.

Resonance frequency and amplitude level of the second H/V ratio peak

The resonance frequency and its amplitude level for the second H/V ratio peaks are depicted in Figures 18 and 19. This peak is related to the shallower reflector represented in the greater part of the study area by rocks of Gesher Fm. together with the Cover Basalt (Pliocene age) and only in the southeastern part of the Afula depression, where the Gesher Fm. and Cover Basalt are wedging out, the Quaternary conglomerate is reflector. Like the first resonance frequency the changes in the second frequency reflect the relief of the shallow reflector. The highest resonance frequencies as well as the highest amplitude value are revealed in the eastern part of the Afula depression. It is explained by the small thickness of alluvial sediments (silty clay) over the uplifted Cover basalt. The area without site effects is extended in comparison with that for the first frequency at the expense of sites in the southeastern part of the Afula depression, where alluvium layer is too small to produce the resonance frequency.

8. VALIDATION OF H/V RATIO USING SEISMIC REFRACTION MEASUREMENTS

Many different techniques exist for mapping site effects based on various kinds of experimental data (strong or week earthquake recordings, explosion recording, and microtremor measurements), most frequent empirical method (e.g. sediment-to-bedrock spectral ratio, horizontal-to-vertical Fourier spectrum of 44 microtremor – Nakamura's method or using this method for S-wave from earthquake ground motion - receiver function) or many different empirical correlation between some geological and geotechnical information and some ground motion parameters. Their cost varies a lot from one to another. The Nakamura's method is much cheaper than other empirical techniques and, in addition, devoid of limitations hampering the site investigations in urban environments. But it is crucial to use low cost tools for site response investigation, from an economical as well as qualitative point of view. We must be fully confident that this low cost method is reliable. We should emphasize that during more than ten years of intensive site effect investigations in the different areas of Israel we have met quite a few cases when the Nakamura technique failed to yield conclusive results (Zaslavsky et al., 2001 (in Hebrew), 2004b, 2005). The key problem is that this method has been developed empirically and now we have some controversial theoretical investigations performed to clarify its underground physics (Lachet and Bard, 1994; Nakamura, 2000; Zhao at al., 2000; Fäh at al., 2001 and others). Thus, in order to evaluate the ability on Nakamura's method to predict resonance frequency and amplification level, we must have an independent estimate of site effects. The best validity of Nakamura's method is comparison with site response inferred directly from horizontal-to-horizontal spectral ratio for S-wave of weak or strong motion records obtained on the soft-soil site with respect to nearby hard-rock reference site. Unfortunately, to record earthquake with sufficient signal to noise ratio we sometimes have to wait very long time (some years). Instead, in the town of Bet Shean and Afula we compared assessment based on H/V spectral ratios from microtremor made at locations where a geophysical survey (seismic refraction) was conducted with theoretical one-dimensional transfer functions for a single or multi layers over a half-space subject to a vertically incident shear wave. Absolute value of the function, assuming a vertically incident shear wave, is computed using SHAKE program (Shnabel, 1972). The sediments and bedrock density and damping values needed for the 1-D model calculation were assigned in accordance with determined for similar rocks from the different places of Israel.

8.1. BET SHEAN

Data collected from the seismic refraction lines carried out in the area investigated provided us with S-wave velocities in the bedrock and shallow sediments. Locations of the refraction lines are shown in Figure 3. Velocity depth section along the line RL-1 is presented in Figure 20. Based on the analysis of the seismic survey results, geological and borehole information in the study area we could correlate the first layer with S-velocity 280 m/sec to alluvial sediments; the second layer corresponds to travertine, whose S-wave velocity is 1050 m/sec. The third layer (Vs=2030 m/sec) is associated with the Cover Basalt being the most likely reflector.

Vs=280 m/s

Vs=1050 m/s

Vs=2030 m/s

Figure 20. Velocity depth section along refraction line RL-1.

Four microtremor measurements at points 162, 201, 81 and 202 were carried out along RL-1 line. H/V spectral ratios obtained at these points are presented in Figure 21. Both fundamental frequency and its corresponding amplitude for all four points are similar and that is in accordance with the general picture of velocity-depth section of RL-1 line, where we observe only slight alterations in relief of the reflector. The typical feature of H/V spectral ratios curves is presence of the second peak, which broadens very considerably the frequency range of motion amplification. The absolute values of one-dimensional theoretical transfer functions, named further for simplicity the theoretical transfer functions, were computed with direct use of thicknesses and S-velocities from the seismic survey and compared with H/V ratios in

Figure 21. One can see a fair agreement between them. Two frequencies f0 and f1 are 46 indicated in the figure. The geophysical data and theoretical models for points along RL-1 line are given in Table 3.

5 162 202 4

Spectral ratio Spectral f0 f1 f0 f1

1 11024681102468 Frequency, Hz Frequency, Hz

5 81 201 4 3

2 Spectral ratio Spectral f f f0 81 0 1 f1 1 0.9 0.8 11024681102468 Frequency, Hz Frequency, Hz Figure 21. Average H/V spectral ratios obtained at point 162, 202, 81 and 201 along RL-1 refraction line compared with the theoretical transfer functions.

Table 3. S-wave velocity models obtained from refraction survey along RL1 line and soil column models

S wave velocity model Soil column model Layer Thickness, Vs, Thickness, Vs, Density, Damping, Point No. m m/sec m m/sec g/cm3 % 1 10 280 12 300 1.6 4 162 2 50 1050 50 1100 1.8 1 3 - 2030 - 2000 2.3 1 15 280 15 310 1.6 4 201 2 45 1050 45 1000 1.8 1 3 2030 2000 2.3 1 13 280 13 280 1.6 4 202 2 50 1050 45 1000 1.8 1 3 2030 - 2000 2.3 1 12 280 12 280 1.6 4 81 2 58 1050 56 1150 1.8 1 3 2030 2000 2.3

Refraction line RL-2 is located in the northern part of Bet Shean in the vicinity of the outcropped Cover Basalt. In the velocity depth section along line RL-2 shown in Figure 22 three layers are differentiated: low velocity upper layer of 10-15 m thick (290 m/sec) and two high velocity layers (1590 m/sec and 2050 m/sec) both correlating with the Cover basalt. Figure 23 shows the theoretical transfer functions superimposed with the H/V spectral ratios for points 199 and 200. These functions are calculated for model comprising two layers over the bedrock with Vs=2100 m/sec, as determined in the seismic survey. As is evident from this figure, direct use of S- velocities and thicknesses from refraction survey for calculating transfer function yields a good fit. Theoretical functions for both sites calculated assuming one-layer structure (in this case the upper basalt layer with Vs=1650 m/sec is the reflector) are shown in Figure 23 as well. As seen, difference between two theoretical functions is negligible. We suppose that this layer provides the resonance effect and the deeper basalt layer differentiated by geophysical data does not really influence the fundamental frequency and its amplitude. Therefore, the H/V spectral ratios have only one peak. In such a case, thickness of sediments overlying reflector and, consequently, drilling depth can be almost four times less than that predicted by geophysical data. In spite of aforesaid, we give in Table 4 the soil column model calculated for deep reflector in order to keep the general geological structure and velocity model in this area.

Figure 22. Velocity depth section along refraction line RL-2.

6 199 5 200 5 4 4 3 3

2 2

p Spectral ratio 1 1 0.9 0.9 0.8 0.8 0.7 0.7 2 3 4 5 6 7 8 910 2 3 4 5 6 7 8 910 Frequency, Hz Frequency, Hz

Figure 23. Average H/V spectral ratio (the red line) for points 199 and 200 superimposed with two theoretical transfer functions calculated assuming different basalt layers as possible reflector. Black and blue lines correspond to reflector represented by upper basalt layer and lower basalt respectively.

Table 4. S-wave velocity model obtained from refraction survey along RL-2

S-velocity model Soil column model Point Layer Thickness, Vs, Thickness, Vs, Density, Damping, No. m m/sec m m/sec g/cm3 % 1 10 290 10 300 1.6 4 199 2 30 1590 30 1600 1.8 1 3 - 2050 - 2100 2.3 1 12 290 12 330 1.6 4 200 2 48 1590 48 1600 1.8 1 4 - 2100 2.3 refraction line and soil column models

8.2 AFULA

Transfer functions calculated on the basis of refraction and boreholes data (see Figure 5 for locations), compiled with information on velocities from the previous investigations, were used to validate H/V ratios at corresponding locations and determine velocity structure. Lithological sections of Gidon 5, Afula Gan Tapukhim and Merhavia wells located at different parts of the study area and penetrating the Lower basalt at different depths (Figure 6), characterize generally the lithological structure of the Afula depression and provide us information on layer thicknesses. Velocity models of 49 the upper layers of the section we derived from the results of the refraction survey along profile Af-1 carried out close to Afula Gan Tapukchim well. The H/V ratio and analytical transfer function for point 115 located at Gan Tapukhim well are shown in Figure 24. The velocity depth section along profile Af-1 is shown in Figure 25. All available geotechnical data for point 115 are concentrated in the left part of Table 5. Since this refraction survey provides Vs for 30 upper meters only, rest of the required information on Vs (down to a depth of 275 meters) was supplemented from other sources. Vs of the deep reflector, the Lower Basalt, we assumed equal to 2200 m/sec by extrapolation of the available velocities down to a depth of 300-500 meters. Results of refraction survey along profile Af-2 near the Afula Hospital site (Ezersky, 2003), presented together with the H/V ratio and analytical function for point 295 in Figures 26a,b, identify alluvium layer (Vs=320 m/sec) overlying the weathered Lower Basalt layer (Vs=600 m/sec) and underlain by the hard Lower Basalt with Vs=1750 m/sec at depth of 13-25 m. These velocities are used for modeling at Balfouriyya-Afula Illit area. We utilized S-velocities for marl and clay layers derived from the investigations in the Coastal Plain and Hashefela regions (Zaslavsky et al, 2003b). Analytical transfer function, showing the pretty good match with the H/V spectral ratio at point 25 located at Gidon 5 well (see Figure 28a), was calculated by directly use of thicknesses from the borehole data and S-velocities described above. Soil column model for this point is given in Table 6.

115 5 3 2

ratio Spectral 0.5

0.3 0.10.2 0.3 0.5 1 2 3 5 10 Frequency, Hz Figure 24. H/V spectral ratio and analytical transfer function for point 115 (Gan Tapukhim well)

S-velocity for the conglomerate layer was inferred through modeling Merhaviya well penetrating the Lower Basalt at a depth of 44 meters, Lithological section of this well 50 includes 30-meters conglomerate layer.. For the upper alluvium layer we used Vs=160 m/sec, presence of which in the study area is indicated by the refraction data along line Af-3 (see Figure 27). The H/V ratio and analytical function for point 209 (Merhaviya well) is shown in Figure 28b.

Figure 25. (a) Velocity-depth section along refraction profile Af-1.

Table 5. Initial geotechnical data and soil column model for point 115 (Gan Tapukhim well)

Vs from refraction Borehole data Soil column model data Thickness, Thickness, Vs, Lithology Vs, m/sec m m m/sec

Clay (Alluvium) 22 300 22 220 Basalt, clay, tuff 15 1800 (Cover Basalt) 75 1700 Limestone, clay 60 ? (Gesher Fm.) Marl (Bira Fm.) 100 ? 100 750 Clay (Clay Series) 80 ? 85 650 Below 276 Basalt (Lower Basalt) ? -- 2200 m

51 a b

295 5

Spectral ratio 1

11023 5 20 Frequency, Hz

Figure 26. (a) Velocity-depth section along refraction profile Af-2 (Afula Hospital); (b) H/V spectral ratio and analytical transfer function for point 295 located at refraction line Af-2

Figure 27. Velocity-depth section along refraction profile Af-3

209 25 6 5 5 4 3 3

2 2

Spectral ratio Spectral 1 0.9 0.8 0.7 0.5 0.6 0.5 0.3 0.4 0.60.8 1102468 Frequency, Hz 0.10.2 0.3 0.5 1 2 3 5 Frequency, Hz

Figure 28. Analytical transfer function and H/V spectral ratio for points (a) 209 (Merhaviya well) and 25 (Gidon 5 well)

Table 6. Geotechnical data for calculation of model for point 25 (Gidon 5 well).

Borehole data Soil-column model Thickness, Lithology Thickness Vs, Density, Damping, m , m m/sec g/cm3 % Clay, conglomerates, 70 70 350 1.7 4 marl (Alluvium) Conglomerate 26 26 600 1.8 3 Basalt, clay, tuff 16 (Cover Basalt) 48 1700 2.1 Limestone, clay 32 (Gesher Fm.) Marl (Bira Fm.) 72 72 750 1.9 2 150 650 1.9 2 Clay (Clay Series) 253 100 700 1.9 2

Basalt (Lower 2200 2.3 Basalt)

H/V spectral ratios validated at borehole and refraction survey locations were utilized, by velocities extrapolation, to study other sites without geophysical and borehole information.

9. ESTIMATION OF SUBSURFACE STRUCTURE USING H/V SPECTRAL RATIOS FROM MICROTREMOR

The program based on the stochastic optimization algorithm (Storn, 1995) was applied in order to obtain a better fit of theoretical transfer function to spectral ratio, considering the dominant frequency, its level and the shape of the H/V curve. Within the chosen frequency interval [w1 , w2 ] we look for thickness ( hi ) and S-velocity ( vi ) that minimize the misfit function

N 2 F = ∑( g(ωk ) − f (ωk )) , k=1 where ωk are points from the frequency interval [w1 , w2 ], g(ω) is 1-D theoretical transform function calculated by SHAKE program; and f (ω ) is H/V spectral ratio. Velocity and thickness are limited:

V1i ≤ vi ≤ V2i ,i = 1, M +1 and H1i ≤ hi ≤ H 2i ,i =1, M where M is number of layers in 1-D model. Since we apply the stochastic optimization method practically not depending on number of parameters in question, an exhaustive search of the model is computationally quite reasonable.

9.1. BET SHEAN

Reconstruction of the subsurface structure in the town of Bet Shean is demonstrated on two cross sections A-A' and B-B' directed east west and north south correspondingly (locations in Figures 3). S-velocity characteristics for litho- stratigraphycal units in the study are summarized in Table 7. These velocities are used to find the thicknesses of layers providing the best fit of the calculated transfer functions and H/V ratios.

Table 7. Ranges of S-wave velocities for litho-stratigraphycal units represented in the study area taken from refraction survey

Lithology Vs, m/sec Alluvium of Quaternary age 230-350 Upper Travertine 550-660 Lower Travertine of Quaternary age 1000-1200 1500-1600 Cover Basalt of Pliocene age 2000-2100

9.1.1. Profile A-A-A'

A simplified sketch of the geological cross section beneath the profile A-A-A' constructed using microtremor measurements is depicted in Figure 29. Microtremor measurement at points 162, 202, 81 and 201 indicated on the cross section by the blue triangles owing to their location at refraction line Rl-1, were used for calibration of H/V spectral ratios (see Figure 21 and Table 3). H/V spectral ratios for these points are characterized by two close resonance frequencies about 4 Hz and 6-7 Hz and our soil column model explains this by influence of two reflectors: travertine and basalt.

W B-B'

12 80 79 82 162 202 81 201 4 6 7 27 9 -100 8 164 10 11 E 107 -150 108 -150 191 192 -200 -200 el el m v -250 -250

e sea le -300 v -300

-350 -350

ation abo ation v alluvium, Cover Basalt, ele Vs=250-350m/sec Vs=2000m/sec -400 -400 upper travertine, zone without site effect- measurement point Vs=650-780m/sec Fault zone ? -450 -450 travertine, measurement point faults detected Vs=1000-1200m/sec along refraction lines by measurements -500 -500 0 500 1000 1500 2000 2500 Distance m

Figure 29. Characteristic 2-D cross section along profile A-A' (Bet Shean)

This feature is distinctive also for H/V ratios obtained at points 79, 82, 80 situated to the west from RL-1 line (see Figure 30). Therefore, for calculation of the transfer functions at these points we applied analogous velocity models. One can see in Figure 30 that the theoretical transfer functions match fairly well to the corresponding H/V spectral ratios.

5 79 80 82 4

Spectral ratio Spectral f0 f1 f0 f1 f0 f1 1 0.9 0.8 1102468 Frequency, Hz 11024681102468

Figure 30. H/V spectral ratios compared with the theoretical transfer functions for points 79, 80 and 82.

Figure 31 presents H/V spectral ratios and theoretical transfer functions for points 1 and 2 located at the eastern edge of the profile. Comparison with previous point shows that while shape and H/V amplitudes are remained practically the same, sharp decrease of the fundamental frequency from 4.5 Hz down to 2.1 Hz at point 2 is observed. Such a variation of the fundamental frequency may be the most likely explained by the presence of fault in the basalts. After fitting the theoretical transfer function to H/V ratio we estimated vertical shift as about 60 meters. Point 1 next to point 2 exhibits the similar fundamental frequency and, therefore, reflector depth was not changed. H/V ratios for points 4, 7, 27 and 9, shown in Figure 32, exhibit one clear resonance peak with amplitude of about factor 2-2.5 and they are completely different from all points observed earlier. 6 2 6 1 5 5

4 4

3 3

2 2

Spectral ratio Spectral f 0 f1 f0 f1 1 1 0.9 0.9 0.8 0.8 1102468 1102468

Figure 31. H/V spectral ratio for points 2 and 1 compared with the theoretical transfer functions 56

5 4 5 7 4 4

3 3

2 2 Spectral ratio Spectral Spectral ratio

1 1 0.9 0.9 0.8 0.8 1102468 1102468 Frequency, Hz Frequency, Hz 5 27 5 9 4 4

3 3

2 2

ratio Spectral Spectral ratio Spectral

1 0.9 1 0.8 0.9 1102468 0.8 Frequency, Hz 0.8 1102468 Frequency, Hz Figure 32. Comparison of the average H/V spectral ratios with the theoretical transfer function for points 4, 7, 27 and 9.

For all numerated points and also for point 6 located hereabout, H/V ratio for which is shown separately in Figure 33, we failed in attempts to fit the theoretical transfer function to H/V spectral ratio keeping S-wave velocity model accepted for the western part of the profile A-A'. The S-velocity model needed to calculate theoretical transfer functions was obtained from seismic refraction survey and downhole measurements on BH-1 well (Ezersky and Shtivelman, 1999). The velocity models are in Table 8. BH-1 well is located 10 meters from the refraction line was drilled to a depth of 30 meters, penetrated alluvium layer, upper and lower travertine layers and did not reach the basalt. S-velocity for the basalt was taken from another refraction survey along RL-1 line. Once S-wave velocities are determined, it become possible to adjust thickness of the lower travertine layer to get the best fit between analytical transfer function and H/V ratio at point 6 located at refraction profile (see Figure 33). The S-wave velocity model from refraction survey and downhole measurements are given in Table 6 together with the optimal soil column model. The theoretical transfer function for point 6 is shown together with H/V spectral ratio.

5 6 4 3

ratioSpectral 1 0.9 0.8 1102468 Frequency, Hz

Figure 33. Comparison of the average H/V spectral ratio obtained at point 6 located at refraction line R-0016 with the theoretical transfer function.

The theoretical transfer functions for points 4, 7 and 27 shown by the black dashed lines in Figure 32, are calculated using velocity model for point 6. Variations of velocity for the upper travertine layers obtained in the fitting process (see Table 9) exceed significantly the accuracy of frequency determination and clearly indicate the tendency Vs to be increased in the east direction. Such tendency is reflected in the model distribution in the study area. Velocities for Lower travertine layer turned out practically the same. Different thickness of the travertine layers for points 6 and 7 was interpreted as a fault. Figure 34 demonstrates individual H/V ratios at point 164 with no resonance frequency. It should be noted that we observed the whole area with analogous pictures of the. This area is contoured in the frequency map (Figure 14) and we mark it on our cross section as well.

Table 8. S-wave velocity models obtained from refraction survey and downhole measurements and soil column models for point 6.

Soil column Layer Refraction data Downhole measurements model Thickness Vp Vs Thickness Vp Vs Thickness Vs

m m/sec m/sec m m/sec m/sec m m/sec 1 4 520 300 6 510 5 300 6 1260 625 2 12 920 550 6 1260 660 30 650 6 1550 570 3 15 1560 6 1550 640 4 2240 1000 60 1000 5 2000

Table 9. Vs values for the of upper travertine layers derived from fitting the theoretical transfer functions to H/V ratios for points from 4 to 9 along the profile

Thickness S-wave velocity of Upper Site m travertine layer, m/sec 4 30 650 6 30 650 7 65 720 27 70 750 8 65 750 9 80 780

164

Figure 34. H/V individual spectral ratio for site 164 without site effect

The H/V spectral ratios for points 10, 11 107, 108 and 191 are shown in Figure 35. All this ratios are characterized by very low amplitude level for the reason that impedance contrast in this geological situation is formed by thick travertine layer outcropped in this part of the study area and basalt. We observe an increase of resonance frequency at point 108 in comparison with point 107 (1.8 Hz vs. 1.0 Hz), which we connected with the fault. The modeling procedure allowed estimating the relative vertical displacement roughly as 50-70 meters; however, absolute values of reflector depth may be hardly determined due to low impedance contrast between travertine and basalt. Therefore, in this part of the profile the top of basalt is shown by dashed line, underlining uncertainty of the depth estimation.

10 11 5 5 4 4

3 3

2 2

Spectral ratio Spectral ratio Spectral

1 1 0.9 0.9 0.8 0.8 0.8 1102468 0.6 0.8 1102468 Frequency, Hz Frequency, Hz

5 5 108 109 4 4 3 3

2 2

Spectral ratio Spectral ratio Spectral

1 1 0.9 0.9 0.8 0.8 0.8 1102468 Frequency, Hz 0.8 1102468 Frequency, Hz

Figure 35. H/V spectral ratios and analytical transfer functions for points 10, 11, 108 and 191

9.1.2. Profile B-B'

The geological cross section along profile B-B' passed through the town of Bet Shean in the south-north direction is presented in Figure 36. The spectral ratios obtained at points distributed along the profile demonstrate very broad range of resonance frequency. There are sites at basalt outcropped where no resonance frequency was detected; and there are sites where resonance frequency reaches 13 Hz. H/V amplitude level varies in the range from factor 2 up to 7. Figure 37 shows H/V spectral ratios for points 120, 112 and 109 located in the southern part of the profile. They are characterized by main clear peak at frequency near 1.7 Hz and minor peak at 4 Hz. Bearing in mind distribution of the fundamental frequency and its corresponding H/V level within the study area (maps in Figures 14 and 15), we applied for calculation of the theoretical transfer functions the velocity model used for points 6, 4, 7, 8 and 9 at the east-west profile A-A' (for details see Figures 32, 33 and Table 8). Modeling showed that difference between soil column models for points on profiles A-A-A' and B-B' is redistribution thicknesses of layers. 60

In particular, the thicknesses in the soil column model characterizing the southern part of the B-B' profile (points 120, 119, 112 and 109) are distributed in the following way: alluvium layer is of 10-15 m thick, upper travertine layer is of about 50-60 m thick and lower travertine of 60-70 m thick overlay basement represented by the Cover Basalt. The theoretical transfer functions are shown together with spectral ratios at Figure 37. In the middle part, profile B-B' intersects with east-west profile A-A' near points 3, 162, 82 and 79. These points were already analyzed (see Figures 21 and 30). Surrounding points in this section (91, 87, 83 and 76) are depicted in Figure 38 and show shape of the curves very similar to mentioned above with a typical broad resonance frequency range formed by influence of two reflectors. At a border between southern and central sections (points 109 in Figure 35 and 91 in Figure 38) the fault is detected. It is fixed, first of all, by sharp shift in the fundamental frequency (from 1.5 Hz up to 3. 5), that corresponds to the vertical displacement of about 100 meters. What is no less important that the H/V amplitude level and shape of the curves changed and show the same broad resonance frequency range typical for two reflectors. We have already seen that such variations are a sign of model change. The soil column model for the middle part of the profile comprises alluvium and lower travertine layers overlying the cover basalt. As we are moving to the north from point 91 to point 76 the gradual increase from 3.5 Hz up to 7 Hz is detected. Correspondingly, the total sediment thickness decreases from 70 meters down to 40 meters. This reduce of total thickness is distributed evenly between all three layers in agreement with H/V amplitude level for points in this section, which is about factor 4 for all points. The part of the cross section with points 161, 51 and 44 cuts through a sector where the basement (the Cover basalt) crops out. The H/V method presents flat spectral ratios for this zone (see Figure 39).

' 61

A-A' S N 177 120 119 112 109 91 87 3 162 82 79 176 -100 83 44 46 -100 76 161 51 -150 -150

-200 -200

-250 -250

-300 -300 elevation above sea level, m 0 500 1000 1500 2000 2500 3000 3500 Distance m alluvium, Cover Basalt, Vs=250-350m/sec Vs=2000m/c measurement point upper travertine, fault detected aling refraction lines Vs=650-780m/sec by measurements

travertine, measurement point Vs=1000-1200m/sec

Figure 36. Characteristic cross section beneath profile B-B' (Bet Shean)

120 112 109 4 4 4

3 3 3

2 2 2

Spectral ratioSpectral Spectral ratio Spectral ratio Spectral

1 1 1 11024680.8 11024680.8 1102468 Frequency, Hz Frequency, Hz Frequency, Hz Figure 37. Comparison between average H/V spectral ratios with calculated transfer functions for points, located in the souther part of profile B-B'

4 91 5 86

4 3

2 2

Spectral ratio Spectral Spectral ratio Spectral

1 1 1102468 1124680 5 83 5 76 4 4

3 3

2 2

Spectral ratio Spectral ratio Spectral 1 1 0.9 0.9 0.8 0.8 0.8 1102468 0.8 1102468 Frequency, Hz Frequency, Hz Figure 38. Comparison between average H/V spectral ratios and analytical transfer functions for points located in the central part of profile B-B'

161 51 45

Figure 39. Individual H/V spectral ratios for points located on outcrop of basalt.

The fault is mapped near this outcrop and the microtremor measurements yield total change of the subsurface model that is confirmed by the refraction survey data along line RL-2 located not exactly but close to our profile. As seen from Figure 40, the measurements at point 46, 176, and 177 situated in the northern part of profile B-B' yield the high amplitudes peak (up to factor 10) at extremely high frequencies (8-14 Hz). On the basis of the geological and geophysical observations we constructed soil column models consistent with our measurements. They are shown together with H/V ratios. The lithological section for this zone may be represented by a few meters of alluvium overlying basalt.

10 46 10 176 10 177

5 5 5 3 3 3 2 2 2

p p

Spectral ratio Spectral 1 1 1

0.5 0.5 0.5

2 4 6 8 10 20 2 4 6 8 10 20 Frequency, Hz 2 4 6 8 10 20 Frequency, Hz Frequency, Hz

Figure 40. Comparison between average H/V spectral ratios and analytical transfer function for 46, 176 and 177 located in the northern part of profile B-B'.

9.2. AFULA 9.2.1. Profile A-A'

West-east and further northeast direction of profile A-A' was chosen to demonstrate representative points in the different geological conditions and suggest an interpretation of contact between the Balfouriyya-Afula Illit ridge and Afula depression. The ranges of frequency 0.35-10 Hz with amplitude from 2 up to 8 units for the fist resonance peak; and 1-8 Hz with amplitude from 2 up to 8 units for the second peak imply very significant variations of the subsurface structure expressed in both sediment thickness and velocity profile (see maps in Figures 16-19). Profile A-A' is depicted in Figure 41. Among a number of H/V ratio curves derived from the records along the selected profile we will show in Figure 42 only points which are distinguished by the uncommon features, such as sharp changes in frequency or amplitude of peaks at a short distance. Point 25 located directly at Gidon 5 well was described in details previously (see Figure 28 and Table 6). H/V ratios obtained at points from 25 up to 88 64 show the stable first (0.35 Hz with amplitude up to 3) and second peaks (1.1 Hz with amplitude of 4). The thinning of the conglomerate layer is compensated by small increase in velocity of the upper layer at the expense of some gravel within the alluvium (data from Afula-A well). At point 89 we observe increase of the second resonance frequency up to 2 Hz. Such change according to our model corresponds to decrease of the sediment thickness above upper reflector (the Cover Basalt and Gesher Fm.) from 70 meters down to 40 meters. 40-meters displacement of the deep reflector (the Lower basalt) is reflected in the increase in the first frequency from 0.35 Hz up to 0.4 Hz. Further gradually increase of the first resonance frequency up to 0.65 Hz at point 115 (Afula Gan Tapukchim well) matching to variation of the reflector depth from 480 m to 280 m, is broken only once by fault located between points 104 and 115, the vertical shift of which is about 50 meters. Our observation at points located to the northeast from point 115 yield very close first resonance frequencies, which indicates only slight slope of the Lower Basalt. The second resonance frequency increases gradually up to 9 Hz (the alluvium layer 3m thick). Point 295 exhibits H/V ratio with two peaks at 6 Hz with amplitude 3.5 and 14 Hz with amplitude 5. This point is situated at the Balfouriyya-Afula Illit ridge in the completely different geological conditions. Shift in the frequencies of the first peak for neighboring points 116 and 295 (0.6 Hz Vs. 6 Hz) corresponds to the 230-meter shift in the depths of the Lower Basalt and must be followed by fault. This fault is traced by the geological data in the southeast, at the contact of the Afula depression and Givat Hamore. The shallow reflector associated with the second peak is marginal conglomerate, which is identified in the velocity depth section along refraction profile Af-2 (see Figure 26). Beginning from point 15 and to the end of the profile the H/V ratios is characterized by high frequency and high amplitude single peak (see point 19 in Figure 42 as an example. The second H/V peak disappears due to small thickness of conglomerates.

21 172 17 18718 19 20 512 24 22 150 Afula Gan 171 150 15 Gidon 5 Afula A Afula 3 Tapukhim 295 213 14 217 216 100 225 100 138 277 1800m/sec 104 115 511 89 134 81 159 25 164 168 29 86 84 88 50 50

0 0

-50 -50

-100 -100

-150 -150 Measuring point

-200 -200 Borehols Fault detected by measurements -250 -250 Alluvium (Holocene) Marl-clay,Bira Fm. (Pliocene) Vs=150-200m/sec Vs=750-800m/sec

-300 -300 Alluvium (Holocene) "Clay series"(Pliocene) Vs=300-400m/sec Vs=650-750m/sec

-350 -350 Conglomerate(Pleistocene) Lower Basalt (Miocene) Vs=500-600m/sec Vs=2200;1800m/sec

-400 Cover Basalt & Gesher Fm. Lower Basalt weathered -400 (Pliocene) (Miocene)Vs=600-630m/sec Vs=1650-1750m/sec 2200m/sec Distance m -450 -450 0 1000 2000 3000 4000 5000 6000 7000 8000

Figure 41. Geological cross section along profile A-A-A'(Afula). For location see Figures 16-19. 66

25 8 5

3 2

1 Spectral ratio

0.5

0.3 0.10.2 0.5 1 2 5 10 0.1 0.2 0.5110 2 5 Frequency, Hz Frequency, Hz 89 5 104 115

3 2

1 Spectral ratio Spectral

0.5

0.3 0.10.2 0.5 1 2 5 10 .2 0.5 11025 0.10.2 0.5 1 2 5 10 Frequency, Hz Frequency, Hz Frequency, Hz 216 295 19 5

3 2

ratio Spectral 0.5 0.3 0.2 0.5 11025 0.2 0.5 11025 11025 Frequency, Hz Frequency, Hz Frequency, Hz

Figure 42. H/V spectral ratios and analytical transfer functions for points located along profile A-A’

9.2.2. Profile B-B’

Profile B-B’, reconstructed on the basis of the microtremor measurement, is depicted in Figure 43. Characteristic examples of the H/V ratios and corresponding analytical transfer functions for representative points along profile are shown in Figure 44. Common feature for all the points located at the southwestern edge of the profile B-B' (46, 48 and 3) as well as surrounding points is H/V ratios with the low amplitude second peak that implies relative high velocity layers above the shallow reflector represented by the Cover Basalt and Gesher Fm. This layer is possibly Pleistocene conglomerate. After the analytical transfer function was adjusted to the 67 corresponding H/V spectral ratio we derived thickness of this layer of about 100 meters. The first resonance frequency changes between points 46 and 3 from 0.5 Hz up to 0.8 Hz. Depth of the deep reflector (the Lower Basalt) varies from 390 m up to 250 m correspondingly. Since in the southwestern part of Profile B-B' there is no borehole data on thicknesses of Marl-clay of the Bira Fm. and "clay series" and, mainly, owing to close S-velocities, we united these two layers into Pliocene marl- clay layer. H/V ratios of next some points along the profile show common feature significantly different from previous points, specifically the second peak has amplitude almost three times higher than at point 3, for example and higher than the their own first peak. This situation is already examined on profile A-A' and the model is similar to that for point 115 at Gan Tapukhim well (see Figure 24). Points 3 and 54 are divided by fault with vertical displacement of 70 meters. H/V spectral ratio with the second dominant peak is retained up to point 145(well Mifalei Sukar). We observed gradual changes in the depth of the Lower Basalt from 230 m down to 285 m. Considerable variations in thicknesses of the Cover Basalt & Gesher Fm. and united marl-clay layer on different sides of the fault, inferred from the models, may be connected with period of lifting and erosion, which preceded the forming of the Gesher Fm., as it is indicated by Shaliv, 1991. At point 140 we again came back to the model including the thick conglomerate layer owing to changing balance in the H/V amplitudes in favor of the first, main peak, like as in case of points 46-3. Fault between points 145 and 140 is traced by the geological data. The fault located below point 270 is mapped by the microtremor measurements only. It is interesting that H/V ratio for point 270 (see Figure 44) has more complicated shape of the second peak than the other ratios, which is probable connected with presence of fault. From point 271, located at the Bira Fm. outcropped and showing single H/V peak, up to points 180 and 198, located at exposure of the Timrat Fm., we observe general increase of the fundamental frequency with local shifts up and down (point 144). Points 309 and 209 reveal the second H/V peak related to alluvium layer a few meters thick overlying the conglomerate layer. The depth of the Lower Basalt at point 209 (Merhaviya well) is 45 meters. According to the borehole data the Lower Basalt layer is underlain by limestone and chalk of the Timrat Fm. Points 180 and 298 yield H/V ratios with no resonance frequency.

68 250

19 SW200 8 200 NE 180

150 150

Pr C-C ' Merhaviya 100 209 100 309 Afula Mifalei Sukar Merhaviya -1263 144 50 271 50 46 48 3 54 148 153 146 147 510 145 140 273270

0 0

-50 -50

-100 -100

-150 Borehole -150

Measuring point -200 -200 elevation above sea level

Alluvium (Holocene) Fault detected Vs=150-200m/sec by measurements -250 -250 Alluvium (Holocene) Marl-clay (Pliocene) Vs=300-400m/sec Vs=650-750m/sec -300 -300 Conglomerate Lower Basalt (Miocene) ( Pleistocene? ) Vs=2200m/sec Vs=500-600m/sec -350 -350 Cover Basalt & Gesher Fm. Limestone,chalk (Pliocene) Vs=1650-1750m/sec ( Eocene, senonian) -400 -400 0 1000 2000 3000 4000 5000 6000 7000 Distance m

Figure 43. Geological cross section along profile B-B' (Afula)

48 54 145 5

3 2

Spectral ratio 0.5 0.3 0.2 0.5 11025 .2 0.5 11025 .2 0.5 11025 Frequency, Hz Frequency, Hz Frequency, Hz 140 270 271

3 2

Spectral ratio 0.5

0.3 0.2 0.5 11025 .2 0.5 11025 .2 0.5 11025 Frequency, Hz Frequency, Hz Frequency, Hz 209 198 5

ratio Spectral

0.5 0.3 0.2 0.5 11025 Frequency, Hz

Figure 44. H/V spectral ratios and analytical transfer functions for points located along profile B-B' 70

Pr A-A' PrB-B' Balforiya 11 100 278 263 100 293 249 211 299 262 222 314 289 287 288 308 312 313

50 50

0 0

-50 -50

-100 -100

-150 Borehols -150

level sea above elevation Measuring point -200 -200

Fault detected by measurements -250 -250 Alluvium (Holocene) Marl-clay,Bira Fm. (Pliocene) Vs=150-200m/sec Vs=750-800m/sec -300 -300 0 1000 2000 3000 4000 5000 Distance m Alluvium (Holocene) "Clay series"(Pliocene) Vs=300-400m/sec Vs=650-750m/sec

Cover Basalt & Gesher Fm. Lower Basalt (Miocene) (Pliocene) Vs=1650-1750m/sec Vs=2200; Figure 45. Geological cross section along profile C-C' (Afula) 71

9.2.3. Profile C-C'

Profile C-C', directed NW-SE is shown in Figure 45. Characteristic H/V spectral ratios for measuring points along this profile one can see in Figure 46. Points from 313 to 211 show identical two-peak shape of H/V ratios; the second peak is higher than the first one. Similar pictures we observed at both profiles A-A' and B-B' and according to the refraction survey and borehole data columnar section of this part of the profile C-C' from the top to the bottom consists of alluvium, the Cover Basalt and Gesher Formation as shallow reflector, "Clay series" and Lower Basalt as deep reflector. The frequency of the first peak equal to 0.65 Hz for all these points indicates practically constant sediment thickness above the Lower Basalt. Moreover, slight changes in the second frequency and its amplitude are connected with variations in the velocity of the upper layer, while its thickness varies in the limits of the ten meters. Sharp shift in the first frequency from 0.75 Hz for point 299 and 1.1 Hz for point 262 supposes fault with the vertical displacement of 80 meters. Comparing the H/V ratios for point 262 with next point 263 we see significant decrease in the second amplitude from 8 units down to 5 units. According to the geological data further along the profile is found exposure of the marl-clay of the Bira Fm., S-velocity of which is 750-800 m/sec vs. 1700 m/sec for the Cover Basalt and the Gesher Fm. This fact may explain decrease in the second amplitude. Point 288 yields further increase in the frequency of the first peak and low amplitude of the second peak and, at last, point 312 shows H/V ratio with no resonance frequency being located at the Bira Fm. with very small thickness not far from the outcropped Basalt.

10 313 313 314314 249 249 5 3 2

Spectral ratio

0.5

0.3 0.2 0.5 11025 0.2 0.5 11025 0.2 0.5 11025 Frequency, Hz Frequency, Hz Frequency, Hz

10 299299 262262 263263

3 2

1 Spectral ratio 0.5 0.3 0.2 0.5 11025 0.2 0.5 11025 0.2 0.5 11025 Frequency, Hz Frequency, Hz Frequency, Hz 10 288 288 5 3 2

1 Spectral ratio 0.5

0.3 0.2 0.5 11025 Frequency, Hz

Figure 46. Characteristic H/V spectral ratios obtained at measuring points along profile C-C'

10. IDENTIFICATION OF FAULTS USING H/V SPECTRAL RATIO FROM MICROTREMOR

Despite the issue of fault identification was already arisen in the previous sections, here we once again analyze and classify all available in Bet Shean and Afula cases of sharp changes in the H/V ratio parameters, which may be interpreted as vertical displacement in the basement. Two basic faults systems were mapped in the Bet Shean area by geological data: NW-SE oriented lineaments that cross the Bet Shean and Jordan valleys and the western faults of the DSR. There are a lot of investigations in which the authors by different way trace the faults (Shulman, 1962; Shaliv, 1991; Hazor, 2000; Gardosh and Bruner, 1998; Zilberman, 2004, etc.). In Figure 47 we suggest our interpretation of fault identification and location based on H/V spectral ratio measurements. Some of our results correlate well with the one or another geological interpretation, but in some cases our interpretation is different. It is known that complicated geological conditions in the vicinity of faults 73 influence very significantly the original time domain recordings and, consequently, results of H/V curves from microtremor. Therefore, not every fault was mapped with

714000 R amot Y B issak har βc βc al al

H 21 42 713000 erod ql Valley qt 50 βc 74 161 βc

D 76 72

e 36

y 712000 qt a e

d l A l 80

a 81 S 2 qt

e 4

a n 87

91 R e 85 711000 204 i h 109 f

S 13 t

A t al

e al 99 147 qt

B B 710000 Faults according to: Microtremor Zone without site effects measurements - Fault zone? Rozenbaum,2004 Boundary ot the study area Zilberm an,2002 qt 21 Measuring point used Hatzor,2000 as examples 709000 245000 246000 247000 248000 249000 250000 Figure 47. Map showing different interpretations of faults location in the Bet Shean area equal confidence and reliability. In some cases only distribution of H/V frequency and amplitude level in the extensive areas allowed to make conclusion about faults presence. Below we demonstrate the examples illustrating the main criteria, on which we based in detecting faults. Type I. While the H/V fundamental frequencies for points located on the both sides of the fault are different, their amplitude and shape of curve are identical. In terms of subsurface models it means change of reflector depth and unaffected impedance contrast between sediments and reflector. For points 2 and 80 shown in Figure 48 the vertical displacement is more than 60 meters. It is important that we trace the fault 74 until this feature is true. The theoretical transfer functions are shown in the same figure. Type II. For this type is typically that all three characteristics of H/V spectral ratio, i.e. frequency, amplitude and shape are different. The first example (Figure 49) showing comparison of points 81 and 4 illustrates the situation which was already explained in the comments to the cross section A-A'. It was said that we interpreted variation in H/V characteristics as a change of velocity model. Soil column models for points 81 and 4 are represented by following sequences: alluvium-travertine over cover basalt and alluvium-upper travertine- lower travertine- over cover basalt respectively. Variation of the reflector depth is about 50 m.

6 2 6 80 5 f = 2.2 Hz 5 f = 4.5 Hz 4 4 3 3

2 2

ratio Spectral ratio Spectral

1 1 0.9 0.9 0.8 0.8 1102468 1102468 Frequency, Hz Frequency, Hz

Figure 48. H/V spectral ratio and theoretical transfer function for point 2 vs. point 80 (Type I of fault identification).

81 4 5 5 4 4

o 3 3 i t

a l r

a 2 2 r

t c 6.5 Hz Spe

ratio Spectral =4.8 Hz = f =3.2 Hz 1 0 0 f f f1=6.5 Hz 1 1 0.9 0.9 0.8 0.8 1102468 1102468 Frequency, Hz Frequency, Hz Figure 49. H/V spectral ratio and theoretical transfer function for point 81 vs. point 4 (Type II of fault identification).

Next example in Figure 50a,b shows by pairs points located in the beginning and end of the fault in the southwest of the area. One can see that while the difference 75 in the amplitude level for opposite points (109 vs. 91 and 85 vs. 87) is not great, H/V curves are completely dissimilar. This dissimilarity is revealed in the location of the first and second peaks of H/V curves and their balance. This example characterizes transition between soil column consisting of alluvium-upper travertine-lower travertine over cover basalt into alluvium-lower travertine over cover basalt.

4 109 4 91

3 3 a

2 2

3.8 Hz = 5.5 Hz Spectral ratio Spectral 0 ratioSpectral f f0=1.8 Hz = 1 f1=4.0 Hz f

1 1 0.8 1102468 1102468 4 85 4 87

3 3

b 2 2

5.0 Hz =3.3 Hz = =2.2 Hz Spectral ratio Spectral ratio Spectral 1 0 5.2 Hz 0 f f

f = 1 f

1 1 2345678910 2345678910 Frequency, Hz Frequency, Hz

Figure 50. H/V spectral ratio and theoretical transfer function for points (a) 109 vs. point 91; and (b) 85 vs. 87.

One more example of fault detected by the criteria of Type 2 is shown in Figure 51. Again, the combination of the frequency shift and different H/V ratio shapes was reason to change velocity model and suppose fault between points 50 and 74. This fault divides two blocks with different models. And an example of reverse transition is from alluvium-cover basalt (point 21) to alluvium-travertine-cover basalt (point 42) is shown in Figure 52. 76

5 50 5 74 4

3 2 o t a 2 a 1 t

c e p Spectral ratio Spectral 5.0 Hz 7.5 Hz S f=7.5 Hz = = f 0.5 f 1 0.9 0.8 0.7 2 3 4 5 6 7 8 910 2 3 4 5 6 7 8 910

Figure 51. H/V spectral ratio and theoretical transfer function for point 50 vs. 74.

7 7 6 21 6 42 5 5 4 4 io t a 3 3 l r a

r 2 2 t Spec f=7.5 Hz ratio Spectral f=4.0 Hz 1 1 0.9 0.9 0.8 0.8 0.7 0.7 2 3 4 5 6 7 8 910 2 3 4 5 6 7 8 9 10 Frequency, Hz Frequency, Hz

Figure 52. H/V spectral ratio and theoretical transfer function for point 21 vs. 42.

In Figure 53 one can see additional examples of H/V ratios for opposite points (204 vs. 13 and 72 vs. 36) in the different places of the study area, which indicate presence of faults between them.

4 13 4 204

3 3

2 2

ratio Spectral

1 1 0.9 0.9 0.8 0.8 246810 2345678910 7 7 6 72 6 36 5 5 4 4 3 3

2 2

ratio Spectral 1 1 0.9 0.9 0.8 0.8 0.7 0.7 23 510 20 23 5 10 Frequency, Hz Frequency, Hz

Figure 53. H/V spectral ratio 13 vs. 204; and 72 vs. 36. 77

Type 3 is a pretty rare situation in the study area, in which one of two neighboring points is located on the outcrop of basalt and we observe flat H/V curve in contrast to the peak with amplitude of about 4 (point 10 vs. point 164 in Figure 54). The transition to the zone without site effects, which accompanies the western branch of the DSR (see Figures 13, 14), is reflected in the similar picture, as one can see in Figure 55 where point 99 is opposed to point 147.

5 76 10 161 4 5 3

2 2

1 Spectral ratio Spectral Spectral ratio Spectral

1 0.5 0.9 0.8 0.8 1102468 2 3 4 5 6 7 8 910 Frequency, Hz Frequency, Hz

Figure 54. H/V spectral ratio and theoretical transfer function for point 76 vs. point 161 (Type 3 of fault identification).

99 147 4 10

3 5

2 2

1 Spectral ratio Spectral ratio Spectral 0.5

1 2345678910 20 0.6 0.8 1102468 Frequency, Hz Frequency, Hz

Figure 55. H/V spectral ratio and theoretical transfer function for point 99 vs. point 147 (Type 3 of fault identification).

The same principles were assumed as a basis for fault identification and mapping in the Afula area. The geological map in Figure 56 depicts faults detected by microtremor measurements compared with those mapped by geological data. 78

728000 β L al p A'

727000 E 1 β β L L al cm 726000 C β L E 2 al E 1 al 725000 B'

λ E 2 A E 1 724000

A cm p E 2

723000

al β C' L S 722000 C Legend: t Faults detected by measurements

al Faults detected by measurements (inferred) Faults of Top Judea Gr. 721000 B Faults of surfase β c Investigated area

Wells β L 720000 224000 225000 226000 227000 228000 229000 230000 231000 232000 233000 234000 235000 236000

Figure 56. Geological map showing faults mapped by microtremor measurements in the Afula area 79

Summarizing all aforesaid about identification and tracing faults in the study area using H/V ratio method we could conclude as follows: • The results of microtremor analysis yielded that the Bet Shean area has the complicated basement morphology formed by blocks descending from the northwest to the southeast. This finding is in agreement with the conception of Bet Shean Valley as "a part of a NW-SE oriented system of extensional Miocene depressions" (Zilberman et al., 2004). The general subsurface structure is reflected in distribution of the H/V fundamental frequency. The Cover Basalt is assumed as reflector. • However, in spite of the general trend of the fundamental frequency, we should say that sharp topography in the north of the area and subsurface structure in blocks do not allow detecting and tracing faults based on the fundamental frequency only. The combination of H/V ratio analysis for sites distributed on the both sides of supposed fault with the geological and geophysical data provides information needed for model construction and comparison of the reflector depth. • A series of faults of sub-latitudinal strike divide the Bet Shean area into north uplifted and south down-faulted parts. The estimated vertical displacement of the reflector is 30-50 meters. • Faults having north-south strike are associated with younger tectonic activity of late Pleistocene age. • The zone without site effects attached to the DSR, as shown in Figure 47, is attributed to a rupture zone of western branch of DSR. This area almost coincides with zone of missing coherent reflections in Seismic line GP-5037, indicated as "Fault zone" (Zilberman, et al., 2004). • On the basis of microtremor measurements three main fault sets were mapped in the Afula area. The first set directed NW- SE separates Yizreel basin from Balfouriyya – Afula Illit Lower basalt ridge and Givat Hamore Mount block. Second and third sets of NE-SW and sub-meridian directions are found in the Yizreel basin.

11. PRELIMINARY SEISMIC ZONATION AND PREDICTION OF ACCELERATION RESPONSE SPECTRA FOR LINEAR AND NON-LINEAR BEHAVIOR OF SOIL SEDIMENTS

Theoretical subsurface models constructed on the basis of H/V measurements together with available geological and geophysical information, in turn, we used for estimating the expected site effects during earthquakes. In the engineering practice, the a-seismic building design and assessments of the earthquake risk refer to the site- specific acceleration (or displacement) spectrum. The design acceleration spectrum is essentially a representation of the maximum acceleration amplitudes for a prescribed probability of occurrence developed on a set of one degree of freedom oscillators with a given damping ratio. Since seismic activity in areas such as Israel is low, local acceleration data from strong earthquakes is insufficient to estimate directly the design acceleration spectrum; therefore, in areas covered by soft sediments, we must resort to the use of synthetic data. For this purpose Shapira and van Eck (1993) developed the SEEH method (Stochastic Estimation of the Earthquake Hazard). In brief; SEEH produces a number of synthetic earthquake catalogues that represent the possible future seismic activity within 200 km of the investigated site. These catalogues adhere to the available information about the seismogenic zones in the area and their associated seismicity. The Monte Carlo statistics are used to generate different catalogues which reflect the uncertainty associated with the spatial and temporal parameters of the seismicity. For each of the earthquakes in a catalogue, SEEH implements the stochastic simulation method (see e.g. Boore, 2000) to generate synthetic S waves accelerogram for the surface of the bedrock which then propagate through the soil column of the site (Shnabel, 1972) to the surface. The synthetic free surface accelerogram is used to calculate the acceleration response spectrum for a predefined damping. Here again, the Monte Carlo statistics are used to select the values of the parameters used in the ground motion simulations. For example; we assume a unified distribution for locating the hypocenter within a defined seismogenic zone and within a 5-20 km depth, we assume that the estimated seismic moment of the event (and thus the energy at the source) are log-normally distributed around the expected value with an uncertainty factor of 3 and so forth. The parameters used are based on studies done in the area and reflect our current knowledge (and uncertainty) about seismic activity and the main parameters that control the SPECTRA of expected ground motions at a 81 given site. The ensemble of these hundreds (sometimes thousands) of synthetic acceleration response spectra are statistically analyzed in order to assess the spectral amplitude level to be exceeded at least once in a certain exposure time (usually 50 years) and a certain probability (usually 10%) (for more details see Shapira and van Eck, 1993). Seismic zonation comprised two stages. At the first stage, H/V spectral ratios for 210 sites in Bet Shean and 100 sites in Afula were categorized into several characters of their shape, H/V frequency and amplitude values. In Afula, two H/V ratio peaks were considered. At the next stage, for 70 representatives of selected groups for Bet Shean and 60 for Afula the Uniform Hazard Acceleration Spectra were computed and again compared. In dependence on characteristics of the acceleration spectra and also spatial distribution of sites within the study area the selected groups were or united into greater zones or, vice versa, subdivided if obvious outsiders were revealed.

11.1. BET SHEAN

The final version of zones division for Bet Shean is presented in Figure 57. The generalized theoretical transfer functions and spectral accelerations together with soil column models are given in Table 12. One can see that maximum of the spectral acceleration varies from 0.6 g up to 1.6 g. We also plotted on the same graphs in Table 12 acceleration spectra required by the current building code IS-413 for ground types S1, S2 or S3 corresponding to the model with the design horizontal Peak Ground Acceleration (PGA) value of 0.247. With the exception of Zone I, the computed spectral acceleration exceeds to the different degree the accelerations prescribed by IS-413 in the period range from 0.1 sec to 0.4 sec.The calculated theoretical model can than be easily used to modify synthetic seismogram computed for rock and predict also nonlinear site specific ground motion during large earthquakes at sites where the ground motions have not been recorded (Shapira and van Eck, 1993). For many years, the geotechnical engineering and seismology communities have had different approaches to significance of non-linear soil behavior during ground shaking. The central question of the discussion is when soil amplification is amplitude dependent. For example, Gutierrez and Singh (1992) described character of strong motion on soft sites and assert that they did not found clear evidence of nonlinear behavior even then when peak horizontal acceleration exceeding 0.3 g. 82

714000 f=0.9-1.5 Hz f=3.5-6 Hz I A=2-2.5 IV A=4-6 II f=1.5-2.5 Hz f=3.5-5.5 Hz VI A=2.5-3.5 V A=2-3 III f=2.5-3.5 Hz f=6-13 Hz A=2.5-3.5 VI A=4-8 B 713000 IV No site effect

712000 IV III IV

711000 B I

710000

246000 247000 248000 249000

Figure 57. Map showing zone division in Bet Shean

In accordance with Su et al (1998) the difference between weak-and strong- motion site responses becomes significant at stations where peak acceleration was above 0.3 g. At lakebed sites in Mexico city, the ground motions were amplified as much as several tens times relative to hill zone during the 1985 Michoacan earthquake, however only at Station Central de Abastos (Reinoso and Ordas, 1999) little non-linear behavior where observed. The identification of nonlinearity in site response is challenging because classical spectral technique with reference station only partly relieves this problem. Perhaps, the reality of nonlinear soil response could be completely hypothetical a decade ago. The field observation of recent earthquakes by modern surface and downhole vertical array of seismometers indicated that soil non-linearity influenced on ground motion (Field et al., 1997; Trifunac and Todoravska, 1998; Pavlenko and Irikura, 2002; Frankel et al., 2002 and other) and its correct prediction have major 83 importance for seismic hazard assessment and building design requirements. In USA, the National Earthquake Hazard Reduction Program (NEHRP, 1994) soil classification scheme is used to consider the effect of different soil types. Site categories A, B, C and D for Bet Shean are based on the average shear-wave velocity in the top 30 m, VS30 as given in Table 12. Numerous methods and programs developed for calculating the ground response in strong motion in various conditions of stress-strain relations. Hartzell et al., (2004) compared different nonlinear soil models for wide range of site condition from rock to soft soil in terms the Fourier spectral ratio of ground motion at the surface with respect to the input motion at the base of velocity profile. They found that for site class B (VS30 = 760-1500 m/sec) no significant differences between different nonlinear soil models. For site class C (VS30 = 360-760 m/sec), differences is small at low input motion (0.1g to 0.2g), but become significant at higher input levels. Based on these comparisons, they recommended a nonlinear approach for sites class D (VS30 = 180-360 m/sec), E (VS30 < 180 m/sec) and for site class C for input motion greater than a few tenths of the acceleration of gravity. In this study we used an algorithm elaborated by Joyner and Chen (1975) and program of Joyner (1977) designed for calculating the nonlinear seismic response of a system of horizontal soil layers underlain medium. The physical properties of the soil layers specified following values: thickness density, shear wave velocity and dynamical shear strength. To estimate dynamical shear strength for different class of soils we used method of "trial and error" to find best agreement between our calculation and result obtained by Hartzell et al., (2004). Optimal values of dynamical shear strength are summarized in Table 10. Figure 58 shows amplification spectra for nonlinear formulation for different zones of Bet Shean. The peaks of input motion (rock) are 0.1 g, 0.2 g, and 0.3 g.

Table 10. Site classification and optimal dynamic shear strength (DSS) used 2 5 Site Class VS30 (m/sec) DSS (dyne/cm )*10

B 760-1500 100 C 360-760 20 D 250-360 8

The SEEH approaches was used to simulate the propagation of the hypothetical earthquake with ML=5.5 at distances 15 km through the typical soil 84 column for Zones 1, 2, 3 and 4 (see Table 11). Figure 59a shows qualitative changes in amplitude, frequency and duration as the bedrock accelerogram is propagated through different typical soil columns for three zones. The peak acceleration at the bedrock was 0.1 g and the peaks of linear surface acceleration were 0.146 g, 0,179 g and 0.345 g for Zones 1, 2, and 3 respectively. The peaks acceleration of nonlinear models is roughly at 15 % smaller than for linear models. Nevertheless, we have not differences in acceleration Fourier spectra for linear and nonlinear models (Figure 59 b). Figure 60 shows the same comparison in the time and frequency domain as Figure 59 only for the peak acceleration at the bedrock 0.2 g. By comparison, the level spectra nonlinear accelerograms (Figure 60b) for Zones 1 and 2 below spectra linear accelerograms between approximately 1-5 Hz, and maximum nonlinear spectra are shifted to lower frequencies with respect to those in the linear spectra. For Zone 3 this difference is negligibly. The nonlinear Uniform hazard site specific acceleration spectra for the six zones of Beit Shean were computed. The results of our computation are shown in Table 11. We can see significant difference between linear and non linear approaches only for Zone 1. In the frequency range 2.5-5.0 Hz the difference reaches 50%. For the stiffer soil (Zone 2, Table 11) the linear response spectra is 15% more that nonlinear in the frequency range 1.0-10 Hz. It is interesting that nonlinear effect in first layer of softer soil (Class D) with thickness 10-12 m in soil-column models for Zones 5 and 6 reduces the maximum response spectra of linear model only by 15%. 85

Zone I Zone II

Zone III Zone IV

Zone V Zone VI

Figure 58. Spectral amplification curves for the nonlinear DSS calculated using Joyner program (1977) for different zones of Beit Shean for peak input motion from 0.1 to 0.3 g. 86

Zone 1: Class B

a b

Zone 2:ClassC

Zone 4:ClassD

Figure 59. Comparison linear and nonlinear soil response for peak input motion of 0.1 g for different Zones of Beit Shean: (a) time domain records on rock and surface; (b) spectra Fourier for rock, linear and nonlinear models

Zone 1 : Class B

a b

Zone 2:ClassC

Zone 4:ClassD

Figure 60. Comparison linear and nonlinear soil response for peak input motion of 0.2 g for different Zones of Beit Shean: (a) time domain records on rock and surface; (b) spectra Fourier for rock, linear and nonlinear models 88

Table 11. Soil column models typical for representative sites of zones, transfer functions and linear and nonlinear spectral accelerations (Bet Shean)

Transfer functions (a) and Thickness, Density, Vs, Damping, Zone response spectra (b): m g/cm3 m/sec % linear (red) and non-linear (blue) 5

3 ication f 210 2.0 1000 1 2 Ampli

1 0.4 0.60.8 1102468 Frequency, Hz I 1.6

1.2

0.8 S1 - 2.3 2000

0.4 Spectral Acceleration, g Acceleration, Spectral

0 0 0.4 0.8 1.2 1.6 2 Period, sec 5

50 1.8 600 3 3 ication f 2 Ampli

110 2.0 1100 1 1 0.4 0.60.8 1102468 Frequency, Hz II 1.6

1.2

0.8 - 2.3 2000 S2

0.4 Spectral Acceleration, g Acceleration, Spectral

0 0 0.4 0.8 1.2 1.6 2 Period, sec 89

Transfer functions (a) and Thickness, Density, Vs, Damping, Zone response spectra (b): m g/cm3 m/sec % linear (red) and non-linear (blue) 5

45 1.7 700 3 2 Amplification

1 0.4 0.60.8 1102468 Frequency, Hz 1.6 III 45 2.0 1100 1 1.2

0.8 S2

0.4 - 2.3 2000 Spectral Acceleration, g

0 0 0.4 0.8 1.2 1.6 2 Period, sec 5

15 1.7 300 4 3

2 Amplification

45 2.0 1000 1 1 0.4 0.60.8 1102468 Frequency, Hz IV 1.6

1.2

0.8 S3 - 2.3 2000

0.4 Spectral Acceleration, g

0 0 0.4 0.8 1.2 1.6 2 Period, sec

Transfer functions (a) and Thickness, Density, Vs, Damping, Zone response spectra (b): m g/cm3 m/sec % linear (red) and non-linear (blue) 5

35 1.8 550 3 2 Amplification

1 0.4 0.60.8 1102468 Frequency, Hz V 1.6

1.2

0.8 S2 - 2.2 1600

0.4 Spectral Acceleration, g

0 0 0.4 0.8 1.2 1.6 2 Period, sec 5

10 1.7 350 4 3

2 Amplification

40 2.0 1600 1 1 0.4 0.60.8110246820 Frequency, Hz VI 1.6

1.2 S3 0.8 - 2.3 2050

0.4 Spectral Acceleration, g

0 0 0.4 0.8 1.2 1.6 2 Period, sec

11.2. AFULA

The map of zones division for Afula is shown in Figure 61. The generalized theoretical transfer functions and spectral accelerations together with soil column models are given in Table 12. One can see that maximum of the spectral acceleration varies from 0.6 g up to 1.1 g. We also plotted on the same graphs in Table 12 acceleration spectra required by the current building code IS-413 for ground types S1, S2, S3 or S4 corresponding to the model with the design horizontal Peak Ground Acceleration (PGA) value of 0.197. One can see that only for Zone 2 spectral acceleration curve is covered by IS-413. For the rest of the zones the computed spectral acceleration exceeds to the different degree the accelerations prescribed by IS-413 within the different period ranges.

728000

727000

VI 726000

725000 IV III 724000 I V II 723000 III IV 722000 I II 721000

720000 223000 224000 225000 226000 227000 228000 229000 230000 231000 232000 233000 234000 235000 236000

Figure 61. Map showing zones division in Afula

Table 12. Soil column models typical for representative sites of zones, transfer functions and linear (red line) and nonlinear (blue line) spectral accelerations (Afula)

Thickness, Density, Vs, Damping, Transfer function (a) and Zone m g/cm3 m/sec % acceleration spectra (b)

60 1.7 350 4 3 io t 2 ral ra ral t 1 Spec 25 1.8 550 3 0.5 0.3 0.10.2 0.5 1 2 5 10 Frequency, Hz 1.2 I 50 2.2 1700 - 1.0

0.8

0.6

320 1.8 700 2 0.4 Spectral acceleration, g acceleration, Spectral 0.2

0.0 - 2.3 2200 0.00.40.81.21.62.0 Period, sec 5 100 1.8 600 2 3 2

1 Spectral ratio Spectral

0.5 40 2.2 1700 0.3 0.10.2 0.5 1 2 5 10 Frequency, Hz 1.2 II

1.0 170 1.8 700 2 0.8

0.6

0.4 Spectral acceleration, g 0.2 - 2.3 2200

0.0 0.00.40.81.21.62.0 Period, sec

Thickness, Density, Vs, Damping, Transfer function (a) and Zone m g/cm3 m/sec % acceleration spectra (b)

3 io t 2 ral ra ral t 25 1.7 250 4 1 Spec

0.5

0.3 0.10.2 0.5 1 2 5 10 Frequency, Hz III 1.2 70 2.2 1700 1.0

0.8

0.6

240 1.8 700 2 0.4 Spectral acceleration, g 0.2

0.0 - 2.3 2200 0.00.40.81.21.62.0 Period, sec 5

10 1.6 200 5 io t 2 ral ra ral t 1 Spec

0.5 80 2.2 1700 0.3 0.10.2 0.5 1 2 5 10 Frequency, Hz IV 1.2

1.0 230 1.8 700 2 0.8

0.6

0.4 Spectral acceleration, g 0.2 - 2.3 2200

0.0 0.00.40.81.21.62.0 Period, sec 94

Thickness, Density, Vs, Damping, Transfer function (a) and Zone m g/cm3 m/sec % acceleration spectra (b)

3 io t 2 ral ra ral t 1 8 1.7 250 4 Spec 0.5

0.3 0.10.2 0.5 1 2 5 10 Frequency, Hz V 1.2

1.0

0.8

20 1.8 800 2 0.6

0.4 Spectral acceleration, g 0.2

- 2.3 2200 0.0 0.00.40.81.21.62.0 Period, sec 5

3 2

8 1.6 250 4 1 Spectral ratio Spectral

0.5

0.3 0.10.2 0.5 1 2 5 10 Frequency, Hz VI 1.2

1.0 5 1.8 600 2 0.8

0.6

0.4 Spectral acceleration, g 0.2 - 2.1 1800

0.0 0.00.40.81.21.62.0 Period, sec

12. CONCLUSIONS

This report presents a seismic hazard microzonation study for the towns of Bet Shean and Afula. The following main tasks were performed within the framework of the project: - obtaining H/V spectral ratios from microtremor observations and mapping of fundamental frequency and its corresponding amplitude; - validation of H/V spectral ratios from microtremor by applying velocity models obtained from geophysical survey combined with borehole data at corresponding sites; - using the velocity models for construction of the subsurface structure along profiles in different directions; - extrapolating the derived theoretical models over the study area and integrating them into computing the uniform hazard site specific acceleration response spectra for 10% probability during an exposure time of 50 years and damping ratio of 5%. - dividing the Bet Shean and Afula areas into zones based on distribution of the acceleration spectra. The results of our investigation could be briefly summarized as follows: In the Bet Shean area fundamental frequency obtained from 210 microtremor measurements varies in the range 0.9 – 12 Hz. Two zones without site effect were revealed at the basalt exposure and along the western marginal fault system of the DSR. The last finding indicates rather the case when Nakamura's technique failed to yield conclusive results due to the low impedance contrast (less than 1.5-2) between rock and soil (Zaslavsky et al., 2005). Without counting the mentioned two zones with no site effects, H/V ratio amplitude varies from 2 up to factor 8. At a greater part of the measuring sites in the Afula area we obtained H/V ratios showing two resonance frequencies, the lower of which varies from 0.3 Hz up to 12 Hz and correlates, mainly, with relief of the Miocene Lower Basalt. H/V amplitude of the first peak within the Afula depression varies generally in the range of 2-4 units. Only at sites located on the islets of the weathered basalt, alluvium and conglomerates, surrounded by the outcropped Basalt on the Balfouriyya_Afula Illit ridge, the amplitude reaches 3-9 units. The Giv'at Hamore block does not revealed signs of site effect. The second resonance frequency, varying in the range 1-10 Hz, is 96 related to the shallow reflector. At sites where the shallow reflector represented by the Cover Basalt underlying the low-velocity alluvium we obtained the highest amplitude of the second H/V peak up to 7 units. Area with no site effects caused by the shallow reflector is observed at sites, where alluvium layer is too small to produce resonance frequency. Refraction velocity-depth sections in Bet Shean and Afula combined with data from boreholes, which penetrate the Lower Basalt (for Afula), provided us information on S-wave velocity models. Theoretical transfer functions calculated by 1-D SH wave propagation analysis using the SHAKE program agree well with H/V spectral ratios for sites at the refraction profiles. Thus, observed H/V ratios farther constraint the possible thicknesses of layers in the substrata that are consistent with the spatial distribution of fundamental frequency and its associated amplification at different location in the study area. The cross sections constructed on the basis of measurement data reflecting our concept of subsurface structure in the study area are generally systematic with the geological survey data. We diverge, however, in some cases in determination of faults locations. Thus, we plan to create structural map of the Lower Basalt in the Afula area and trace the strikes of the inferred faults. For this aim additional cross sections will be constructed. Uniform Hazard Site-Specific Acceleration Spectra for a probability of exceedence of 10% during an exposure time of 50 years and a damping ratio of 5% were computed using SEEH procedure by applying the evaluated subsurface models served as a basis for seismic zonation of the towns. For five of six selected zones in Bet Shean and for five zones in Afula the current Building Code IS-413 underestimates the acceleration in the period range 0.1-0.4 sec. A nonlinear site effect in Bet Shean will probably reduce amplification of ground motion during future earthquakes. However, the amplitude and frequency band of these effects are highly variable from site to site and depend on the physical properties of soils. Nonlinearity may be considerable for soft clays and sands and negligibly for stiffer materials. The problem of forecast nonlinear ground motion is very complex, since such components of the site response analysis as input motion, site characterization, soil model and numerical analysis influence significantly the calculated result. 97

ACKNOWLEDGMENT

This study was financed by the Ministry for Absorption and the Earth Sciences Research Administration of the Ministry for National Infrastructures. Special thanks are due to Dr. Ezra Zilberman of the Geological Survey of Israel for the fruitful discussion and constructive comments. 98

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