Site Effect and Seismic Hazard Microzonation Across the Town of Tiberias

SITE EFFECT AND SEISMIC HAZARD MICROZONATION ACROSS THE TOWN OF TIBERIAS

February, 2009 Report No 502/416/09

Principal Investigator: Dr. Y. Zaslavsky

Collaborators:

M. Gorstein, M. Kalmanovich, I. Dan, N. Perelman, D. Giller, G. Ataev, T. Aksinenko, V. Giller and A. Shvartsburg

Prepared for Geological Survey of Israel

CONTENT LIST OF FIGURES ...... 3 LIST OF TABLES ...... 5 ABSTRACT ...... 6 GEOLOGICAL AND TECTONIC CONTEXT ...... 8 Stratigraphy and lithology ...... 10 BRIEF REVIEW OF SEVERAL EXPERIMENTAL METHODS FOR SITE EFFECT ASSESSMENT ...... 13 MICROTREMOR RECORDING AND PROCESSING ...... 15 Site response in Tiberias estimated by H/V spectral ratio from microtremor ...... 20 Comparison of H/V spectral ratios from microtremor and seismic events...... 24 DISTRIBUTION OF THE RESONANCE FREQUENCY AND ITS ASSOCIATED H/V AMPLITUDE OVER THE STUDY AREA...... 28 ESTIMATION OF SHEAR-WAVE VELOCITY MODELS AND RECONSTRUCTION OF SUBSURFACE STRUCTURE...... 32 SEISMIC HAZARD MICROZONATION...... 47 CONCLUSIONS...... 57 ACKNOWLEDGEMENTS...... 58 REFERENCES...... 59 3

LIST OF FIGURES Figure 1. Tiberias - the New and Old together...... 7 Figure 2. Geological map of the study area compiled from Schulman (1966) and Sneh (2008) with locations of the refraction profiles TB-1, TB-2 and TB-3 (Ezersky, 2008); R-1 and R-2 (Shtivelman, 1995) and profiles 1 and 2 for constructing cross sections...... 9 Figure 3. Location of the measurement sites in the study area. Numbers indicate the sites used as examples. TB-1, TB-2 and TB-3 - refraction survey profiles (Ezersky, 2008); R-1 and R-2 – refraction survey profiles (Schtivelman, 1995); TVR, TVR2 and POR – accelerometer locations; Profile1 and Profile2 – profiles for reconstructing subsurface structure...... 17 Figure 4. Examples of seismometers locations during various sets of the site investigations in different geological conditions...... 18 Figure 5. Examples of seismic station locations in Tiberias...... 19 Figure 6. Examples of average Fourier spectra (top) and H/V spectral ratios (bottom), which yield a single peak. The black line indicates a vertical spectral component; the grey line indicates the average of NS and EW horizontal components of motion...... 20 Figure 7. Examples of average Fourier spectra (top) and H/V spectral ratios (bottom), which yield two resonance peaks. The black line indicates a vertical spectral component; the grey line indicates the average of NS and EW horizontal components of motion...... 21 Figure 8. Fourier spectra (top) and H/V spectral ratios (bottom) obtained at sites located on the exposure of the Cover Basalt. f0 indicates the fundamental frequency of the measurement site; f is an artificial frequency. H/V ratios at site T62 obtained by processing and reprocessing of the measurement data are shown by dashed and solid lines respectively...... 22 Figure 9. H/V spectral ratio obtained at sites located on the outcropped Bira Fm. (T70 and T115) and alluvium (T10 and T50)...... 23 Figure 10. Comparison between the H/V spectral ratios obtained near the Tiberias Town Hall (T156 and TVR) and at site T13 at different times...... 23 Figure 11. Accelerograms of the earthquakes recorded at site Tiberias Hotel (TVR2): (a) earthquake occurred in the Dead Sea (2004 02-11, 08:15, ML=5.1, R=120 km) and (b) earthquake occurred in the Dead Sea fault (2004 07-07, 14:35, ML=4.7, R=89 km);...... 25 Figure 12. Accelerograms of the earthquake occurred in the Dead Sea (2004 02-11, 08:15,

ML=5.1, R=120 km) and recorded at site Tiberias Town Hall (TVR)...... 26 4

Figure 13. Accelerograms of the earthquakes recorded at site Poriya Hospital: (a) earthquake that occurred in the Dead Sea basin (2004 02-11, 08:15, ML=5.1, R=120 km) and (b) earthquake that occurred in the Dead Sea fault (2004 07-07, 14:35, ML=4.7, R=89 km)...... 26 Figure 14. Comparison of different estimates of site amplification based on H/V spectral ratio techniques applied to earthquakes and microtremor recordings at sites TVR2 (Hotel) – (a); TVR (Town Hall) – (b) and Poriya Hospital– (c)...... 28 Figure 15. Distribution of the fundamental resonance frequency over Tiberias...... 29 Figure 16. Distribution of amplitude associated with the fundamental frequency...... 30 Figure 17. Comparison between the analytical transfer function (grey line) and experimental H/V spectral ratio (black line) obtained at two sites along TB-3 refraction profile...... 33 Figure 18. Comparison between the analytical transfer function (grey line) and experimental H/V spectral ratio (black line) obtained at four sites along TB-1 refraction profile...... 34 Figure 19. Comparison between the H/V spectral ratios obtained from microtremor measurements in 1995 (grey line) and 2008 (black line) near refraction profile TB-2...... 36 Figure 20. Comparison between the analytical transfer function (grey line) and experimental H/V spectral ratio (black line) obtained at two sites along refraction profile TB-2...... 36 Figure 21. Comparison between the analytical transfer function (grey line) and experimental H/V spectral ratio (black line) obtained well Kineret 6 (site 121)...... 37 Figure 22. Comparison between the analytical transfer function (grey line) and experimental H/V spectral ratio (black line) obtained well Kineret 10-B (site 125)...... 38 Figure 23. Schematic geological NS cross section beneath profile 1 ...... 42 Figure 24. H/V spectral ratio (black line) and analytical transfer function (grey line) for representative sites of profile 1 ...... 43 Figure 25. Schematic geological EW cross section along profile 2...... 45 Figure 26. H/V spectral ratio (black line) and analytical transfer function (grey line) for representative sites of profile 2...... 46 Figure 27. Seismic microzoning map of Tiberias presenting zones of common site effect characteristics...... 49 Figure 28. Examples showing influence of thin upper soft layers on spectral accelerations computed for two sites located on the Cover Basalt...... 50

LIST OF TABLES Table 1. Stratigraphic table of the geological map of Tiberias (Sneh, 2008) ...... 11 Table 2. Brief description of wells located in the Tiberias region ...... 12 Table 3. Parameters of earthquakes recorded by accelerometer stations used in this study. Distance is to the surface projection of the rupture...... 25 Table 4. Geophysical and analytical models for calculating transfer functions at points located along TB-3 refraction profile ...... 33 Table 5. Geophysical and analytical models for calculating transfer function at sites located along refraction profile TB-1...... 35 Table 6. Geotechnical data obtained from refraction surveys carried out in 1995 and 2008...... 36 Table 7. Soil-column model for sites along refraction profile TB2 ...... 37 Table 8. Geotechnical data and soil-column for well Kineret-6...... 38 Table 9. Geotechnical data and soil-column for well Kineret-10B...... 39 Table 10. Ranges of S-wave velocities for litho-stratigraphycal units represented in the study area and used in calculating site response...... 39 Table 11. Soil column models for representative sites of zones, their transfer functions and spectral accelerations...... 51 6

ABSTRACT

To quantify the seismic hazard across the town of Tiberias we used a methodology in which horizontal-to-vertical spectral ratio from microtremor (the Nakamura’s technique) obtained on a dense measurement grid is utilized to assess the site-specific uniform acceleration spectra. This process of hazard assessment involves: a detailed mapping of the fundamental and other natural frequencies and amplitudes of H/V spectral ratios; compiling geological, geophysical and borehole data and integrating it with H/V observations to develop models of the subsurface at many sites across the study area. The subsurface model serves as an input for computing the expected Uniform Hazard Site-Specific Acceleration Response Spectra at the investigated sites. The final stage is generalizing the hazard by mapping zones that feature similar seismic hazard functions. Microtremor measurements were carried out at 175 sites, which are characterized by amplification from 2 up to 8 in the frequency range 0.7-8 Hz. The receiver function, which is horizontal-to-vertical spectral ratio obtained from earthquakes (shear wave) confirms the results obtained from microtremor records at three acceleration locations. H/V ratios, geological data and information from S-velocity refraction profiles enables construction of geological cross sections. Certain sharp differences in the H/V ratios have been interpreted as being associated with a subsurface discontinuity, i.e. fault. By comparison of the Uniform Hazard Acceleration Spectra calculated for probability of exceedance of 10% during an exposure time of 50 years and a damping ratio of 5% at more the 50 sites and in consideration of the constructed subsurface models, we subjectively divided the study area into eleven zones. The linear spectra for eight zones significantly exceed the design spectra required in the same area by the current Israel Standard 413 (IS-413) in the period range 0.1-0.5 sec. 7

INTRODUCTION

Figure 1. Tiberias - the New and Old together.

Tiberias, famed as a city in the region where Jesus preached, as the capital of Herod Antipas, the seat of the Sanhedrin, and the place where the Jerusalem Talmud was written, is so rich in antiquities that archaeologists in Israel call it “the City of Treasures.”

Tiberias now is a relatively small town (about 40,000 inhabitants), situated on the western shore of the Sea of Galilee on the seismically active Dead Sea Fault system, capable of generating earthquakes with magnitude as high as 7.5. The long documented history of destructive earthquakes in Israel shows that the whole area, where modern Tiberias is now located, is subject to strong earthquakes, which have in the past caused considerable damage and many casualties. In present millennium several worth mentioning earthquakes occurred: for example the 1033 in the Jordan Valley (massive destruction at Tiberias), 1759 (walls of Tiberias collapsed, seiche on the Sea of Galilee), 1837 the "Safed earthquake" (28% of the population of Tiberias were killed and city walls destroyed) and 1927 (Tiberia suffered damage) according to

Amiran, D.H.K (1961). In order to mitigate earthquake risk and assess the site specific seismic hazard in urban areas, we must estimate the possible consequences of strong earthquakes, i.e., implement our accumulated experience of past earthquakes to present a scenario of an eventual earthquake. It is known that local ground conditions played an important part in the amount of damage suffered at any particular locality. Most examples from several destructive earthquakes during the two past decades, for example, in Mexico-City, 1985 (Singh et al., 1988; Reinoso and 8

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. The greater damage in Tiberias was, at least in part, due to the fact that it was founded on unconsolidated alluvium, which produced an exaggerated response. A better assessment of the expected ground motions inside the town is thus a key element for urban and civil protection planning.

In the present study we 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, we performed microtremor measurements on a dense spatial grid and H/V spectral ratios, from which we obtained a spatial distribution of the frequencies at which amplification is likely to occur and the expected level of amplification at those frequencies. H/V spectral ratios of S-waves, often known as receiver functions, generated by earthquakes and recorded at three accelerometer locations are considered in the analysis. At the second step, all available geological information, geophysical and well data are collected and incorporated as an aid to construct subsurface models for different sites within the investigated area. Finally, one-dimensional analytical models are 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.

GEOLOGICAL AND TECTONIC CONTEXT

Figure 2 presents the geological map of Tiberias at a scale of 1:50,000 compiled from Sneh (2008), and Bogoch and Sneh (2008) with an overlay of faults after Schulman (1966). The town of Tiberias is located on the western shore of the Tiberias lake at the foot of the structural high of Poriya tilted block. The present Tiberias lake is a remnant lake that evolved from the ancient water bodies filling the Tiberias basin (the northern part of the Jordan Valley during the Pleistocene–Holocene periods). There is continuous exposure of the Cover Basalt from the 9 elevation of -210 m at the south-eastern corner of the town to the Tel Ma’on hill in the west, at +250m.

Figure 2. Geological map of the study area compiled from Sneh (2008), and Bogoch and Sneh (2008) with an overlay of faults according to Schulman (1966), and with locations of the refraction profiles TB-1, TB-2 and TB-3 (Ezersky, 2008); R-1 and R-2 (Shtivelman, 1995) and profiles 1 and 2 for constructing cross sections.

With the exception of the Upper Cretaceous rocks exposed in the structural highs of Poriya and Fuliya blocks, all the formations on the geological map are part of the Neogene. From 10 bottom to top these are: the Miocene Hordos Fm. and the Lower Basalt; the Neogene Bira Fm., Gesher Fm. and the Cover Basalt. The investigated area is dissected by two normal fault systems: the WSW-ENE transversal system with the down throw to the north, and the SE-NW system of step-faults with the down throw to the northeast. The two transversal faults in the south are of a Neogene pre-Cover basalt age. They were rejuvenated in the Pleistocene. The NW trending step- faults are of Pleistocene post-Cover Basalt age. Along the greater part of their traces they bring basalt against basalt. Only at the southeastern termination of two of them, where they abut against a transversal fault, Neogene sediments rise to the surface. Here the throw of the two step- faults is the greatest. A fourth step-fault is inferred within the lake and parallel to its shore. A significant feature is the considerable vertical displacement at the NE corner of the titled block, a result of the cumulative effect of the two fault systems. In the Upper Pliocene, the site of the town and its lakeshore were structurally higher than Tel Maon in the west (Schulman, 1966). Schulman (1966) proposed Ron et al. (1984) supported that the middle to upper Miocene sediments and basalts underwent intensive deformation by horizontal shear in a compressive stress field which operated during the end of the Miocene and early Pliocene times.

Stratigraphy and lithology The stratigraphic units are given in explanatory table to the geological map of Tiberias compiled and edited by Sneh (see Table 1). Upper Cretaceous sediments. The upper part of the Sakhnin, Bina and Menuha fms. crop out in two areas of the lake shore: at Tel Raqat (Hirbet Fuliya) and foothills at Mt. Hordos (Berenice).

According to Golani (1961), these formations are about 160 m, 30-70 m and 20-60 m thick accordingly. They consist of grey hard dolomite and lithographic limestone and chalk respectively. Upper Cretaceous sediments are penetrated by several wells situated at Tel Raqat, Mt. Hordos and Hamei Teveria. Information on depth of the Judea Gr. available from the wells is shown in Table 2. Eocene Chalky-Limestone Complex represented by the Avedat Gr. is exposed to the north of the study area, at Mt. Arbel. The Neogene deposits are divided into three formations Hordos, Bira and Gesher crop out along the Tiberias lakeshore. Fluviatile-lacustrine sediments of the Hordos Fm. including also the Hugog Cgl. comprise alternating red mudstone, sandstone, limestone and conglomerate. 11

Table 1. Stratigraphic table of the geological map of Tiberias (Sneh, 2008)

Thickness of the Hordos Fm. according to Shaliv (1991) reaches 750 m in the Poryya escarpment. Six basalt flows are intercalated within the Hordos Fm. They thicken southward and form a continuous basalt section – the Lower Basalt. Based on data from HZORM-1 well one can presume an increasing thickness of the Lower Basalt to the southwest as well. The rock is olivine basalt, usually porphiric. The basalt is intensely jointed with calcite-filled cracks. 12

The Bira and Gesher Fms. overly with slight unconformity the red beds of the Hordos Fm. in the mountain scarp along the shore of the lake. The Bira Fm consists of marly clay, siltstone or calcarenite and has a thickness of about 55-70 m. The Gesher Fm. up to 80-100 m thick consists of chalky limestone. These sediments are overlain by the thick Cover basalt (Michelson, 1987). The basalt flow is discordantly resting on the erosion surface of the Gesher beds (Heimann, 1993) or the Bira Fm. It consists normally of hard olivine basalt. Basalt on the surface is weathered with sporadic patches of clay-alteration products of basalt. The thickness of the Cover Basalt increases from the east to the west from 30–50 m in the easternmost block of Tiberias and 80-100 m up to 175 m thick west of it. Quaternary sediments are distributed along the lakeshore, the western part of the town of Tiberias, the Poriya escarpment and southwestern part of the study area. They are represented by anthropogenic (archeological) deposits along the lakeshore and in the old part of town, silty clay, conglomerate, mostly basaltic components, poorly cemented with layers of clay. Such a composition is typical for landslides found in the old part of town and Poriya escarpment.

Table 2. Brief description of wells located in the Tiberias region

Elevation Depth to the EW NS Name TD Judea Gr. m 249660 745820 D-907 0 -190 ? 251776 741419 D-965 >60 -189 60 251800 741440 D-966 >35 -198 >35 252000 741210 D-967 >69 -194 69? 251820 741472 D-968 >31 -203 31 251410 742050 D-969 26-39 -194 >39 251720 741510 D-974 4? -200 ? 251730 741400 D-984 90 -174 ? 251830 741513 D-990/971 >97 -205 97 251890 741440 H.TVRIA-1 >62 -206.7 62 246975 740275 HZORM-1 622 -28 570 250985 742600 J-1 18? -173 ? 250930 742650 J-2 20 -160 ? 249700 745800 K.1 15 -200 157.5 251600 741700 K.2 15 -200 107.6 249753 745723 K.5 55/84 -208 200 249753 745723 K10b 84 -208 ? 249943 745571 K.6 167 -208 603 13

BRIEF REVIEW OF SEVERAL EXPERIMENTAL METHODS FOR SITE EFFECT ASSESSMENT Various empirical techniques have been used to detect locations where site effects are likely to occur. - S-Wave spectral ratio with respect to reference site The most common technique for estimating site response is the standard (classic) spectral ratio procedure first introduced by Borcherdt (1970). This approach considers the ratio between the Fourier spectra of a seismogram recorded in the site of interest and the spectrum of a seismogram recorded at a reference site, which is usually the rock outcrop. This ratio can be considered as the transfer function between the bedrock and the surface assuming that the two recordings correspond to the same source, the same path effect and that the reference site has a negligible site effect. It is very difficult to implement all these assumptions in real conditions. First, in many cases we do not have a nearby bedrock site and therefore the condition that the path of the propagating seismic waves is the same is not fulfilled; second, it is known (e.g., Steidl et al., 1996, Zaslavsky et al., 2002) that weathered and cracked bedrock site exhibits a significant site effect, associated with frequency-selective ground motion amplification; third, there are many cases in Israel, when nearby bedrock outcrop is not the same rock at the base of the soil layer which is responsible for amplifying seismic waves amplitudes. It should also be noted that performing simultaneous measurements at two sites is often relatively costly. Nevertheless, when all the conditions are observed, this method maybe considered the most reliable estimate of the empirical transfer function of site. Many investigators used this method and evaluated site response functions from moderate to weak motion recording of earthquakes (Tucker and King, 1984; McGarr et al., 1991; Field et al., 1992; Liu et al., 1992; Carver and Hartzell, 1996; Hartzell et al., 1996; Steidl et al., 1996; Zaslavsky et al., 2000 and others). - Horizontal-to-vertical S-wave spectral ratio (Receiver Function) In this technique applied by Lermo and Chávez-García (1993) the receiver function can be obtained from ratio between horizontal and vertical amplitude spectra computed at the same investigated site from S-waves, respectively. Receiver function was introduced by Langston (1979) to determine the velocity structure of the crust and upper mantle from P-waves of teleseisms. Langston made the assumption that the vertical component of motion is not influenced by the local structure, whereas the horizontal 14 components, owing to the geological layering, contain the P to S conversion. In the spectral domain this corresponds to a simple division of the horizontal spectrum by the vertical. Many studies report that the frequency dependence of site response can thus be obtained from measurements made at only one station at the analysed site (Lermo and Chavez-Garcia 1994; Malagnini et al., 1996; Seekins, et al., 1996; Theodulidis et al., 1996; Castro et al. 1997; Yamazaki and Ansary, 1997; and others). Their results confirm the validity of the method to estimate S-wave site response. We obtained similar conclusion in our investigations (Zaslavsky et al., 2000). Nevertheless, the implementation of this approach still requires a rather frequent occurrence of earthquakes. This requirement becomes an obstacle in regions of low seismicity. - Microtremor spectral ratio with respect to reference site Kagami et al. (1982) proposed that the ratio of the spectra of the horizontal ground motions of the microtremor at the investigated site to those of a reference site can be used as a measure of the site response function. This method can be successfully applied for long period microtremors with period ranging from 1.0 to 10 sec. When higher frequencies are of interest, the distance between the measured sites should not exceed few hundred meters. The reliability of this method depends on whether or not the simultaneously measured motions at each site are from the same source and propagation path. This technique is widely used for site response estimates (Lermo et al., 1988; Field et al., 1990, 1995; Rovelli et al., 1991; Dravinski et al., 1995, 2003; Gaull et al., 1995). However, experimental study of site effect by sediment-to-bedrock spectral ratio in urban and suburban regions can be successful only under particular circumstances, because microtremor would be influenced by local artificial sources generated by human activities which essentially change from place to place. - Horizontal-to-vertical microtremor spectral ratio Nakamura (1989) proposed the hypothesis that site response function under low strain can be determined as the spectral ratio of the horizontal versus the vertical component (H/V) of motion observed at the same site. He hypothesized that the vertical component of microtremor is relatively unaffected by the unconsolidated near-surface layers. Hence, the site response is the spectral ratio between the horizontal component of microseisms and vertical component of microseisms recorded at the same location. Many authors, among them Lermo and Chávez-García (1994), Seekins et al. (1996), Toshinawa et al. (1997), Chávez-García and Cuenca (1998), Enomoto et al. (2000), Shapira et al. 15

(2001), Mucciarelli and Gallipoli (2004), Murphy and Eaton (2005), Maresca, (2006), show that the H/V spectral ratio technique can be a useful tool for the assessment of ground motion characteristics on soft sediments. However, other authors (for example, Bonilla et al., 1997; Horike et al., 2001; Satoh et al., 2001) conclude that whereas the predominant peak of H/V ratio is well correlated with the fundamental resonance frequency, the amplitude of this peak is not necessarily the amplification level as obtained from sediment-to-bedrock spectral ratio of earthquake records.

MICROTREMOR RECORDING AND PROCESSING Microtremor measurements were carried out during the period from June to September 2008 at 175 sites in an area of about 13 km2. Measurements are conducted using portable instruments (Shapira and Avirav, 1995) consisting of a multi channel amplifier, Global Positioning System (GPS) for timing and a laptop computer with 16-bit analogue-to-digital conversion card to digitize and store the data. In our experimental set-up, each seismograph station consists of three (one vertical and two horizontal) L4C velocity transducers (Mark Products) with a natural frequency of 1.0 Hz and damping ratio 70% of critical. The recorded signals are sampled at 100 samples per second and band-pass filtered between 0.2 Hz and 25 Hz. All the equipment: sensors, power supply, amplifiers, personal computer and connectors are carried in a vehicle, which also serves as a recording centre. The seismometers are fixed on levelled metal plate placed directly on the ground. To study the characteristics of spectra of the microtremor signals, we compute Fourier spectra and spectral ratios. The record length (time window) used for spectral calculations depends on the fundamental frequency. The basic criterion is to choose the minimal time window which yields spectra that practically do not change when increasing the record length. We have concluded that at sites with fundamental frequencies of 1 Hz (or more) we should use a record of at least 30 sec. At sites with lower frequencies, the time window should be increased to 60 sec. The selected time windows are Fourier transformed, using cosine-tapering (1 sec at each end) before transformation and then smoothed with a triangular moving Hanning window. More precisely, we apply “window closing” procedure (see Jenkins and Watts, 1968) for smart smoothing of spectral estimates so that any significant spectral peaks are not distorted. 16

The H/V spectral ratios are obtained by dividing the individual spectrum of each of the horizontal components [SNS(f) and SEW(f)] by the spectrum of the vertical component [SV(f)]:

S NS ( f ) SEW ( f ) ANS ()f = AEW ()f = (5) SV ()f SV ()f The average spectral ratio for each of two horizontal components is computed. If the curves of average spectral ratios of the two components are similar then the average of the two horizontal-to-vertical ratios is defined as:

1 ⎡ n S ()f n S ()f ⎤ A()f = ⎢ ∑ NS i + ∑ EW i ⎥ (6) 2n ⎢ S ()f S ()f ⎥ ⎣i = 11V i i = V i ⎦ The measurement sites in Tiberias were designed with variable grid spacing. Different surface sedimentary deposits, thickness of sediments and shear wave velocity contrast between sediments and bedrock were considered in the design stage. In the process of accumulating the data and understanding the general picture of site effect distribution, we made operative decisions as regards changing the grid to gain reliability of the results obtained. Sharp changes in frequency over a short distance, disagreement with geological data and equivocal measurement results are the reasons for additional points and a denser grid. The densest network was deployed inside the old part of town with remains of historical buildings and walls. Unlike the area of the old town that is covered mainly by soft sediments, the greater part of Tiberias is covered by Pliocene basalt. Therefore, the spatial density of the measuring sites was decreased to a grid spacing of 500 meters. Distribution of the 175 measurement sites within the study area is shown in Figure 3. The local topography ranging from -200 m above sea level to +250 m and inaccessibility of some sites led to changing the spatial density of the measurements planned in the design stage. Figures 4 and 5 present examples of the seismic station locations in the different geological and urban conditions. 17

Figure 3. Location of the measurement sites in the study area. Numbers indicate the sites used as examples. TB-1, TB-2 and TB-3 - refraction survey profiles (Ezersky, 2008); R-1 and R-2 – refraction survey profiles (Schtivelman, 1995); TVR, TVR2 and POR – accelerometer locations; Profile1 and Profile2 – profiles for reconstructing subsurface structure. 18

Figure 4. Examples of seismometers locations during various sets of the site investigations in different geological conditions.

Figure 5. Examples of seismic station locations in Tiberias. 20

RESULTS

Site response in Tiberias estimated by H/V spectral ratio from microtremor Empirical estimation of site effects in Tiberias is carried out by implementing H/V spectral ratio from microtremor method. Figure 6 displays examples of the average amplitude spectra of one of the horizontal, the vertical components of motion and spectral ratios obtained at several sites in Tiberias (for location see Figure 3). A common feature of the presented examples is the appearance of a single peak in the H/V spectral function which also coincides with a peak in the amplitude spectrum of the horizontal motion. The vertical spectral component is almost flat. Figure 7 presents cases where the Fourier spectra show two frequency bands of site effect, manifested on the H/V curves by two resonance peaks. The second peak is most likely caused by an intermediate hard layer in the subsurface. While the first resonance frequency is related to the hard rock at depth, the position of the second resonance peak depends mainly on the thickness of the intermediate hard layer. Amplitude level of both peaks is determined by the S wave velocity in the soft sediments.

0.3 T33

0.1

0.03

T134 T79 T149 0.01

Spectral amplitude, µm/s*s amplitude, Spectral 0.2 0.5 11025 0.5 11025 0.5 11025 0.5 11025

5 T134 T33 T79 T149

1 f f f 0 0 0 f0 Spectral ratio Spectral 0.5

0.2 0.5 11025 0.2 0.5 11025 0.2 0.5 11025 0.5 11025

Figure 6. Examples of average Fourier spectra (top) and H/V spectral ratios (bottom), which yield a single peak. The black line indicates a vertical spectral component; the grey line indicates the average of NS and EW horizontal components of motion. 21

1 0.3 0.3 0.3 0.3 0.1 0.1 0.1 0.1

0.03 0.03 0.03 0.03 T63 T122 T166 T9 0.01 0.01 0.01 0.01

Spectral amplitude, µm/s*s amplitude, Spectral 0.5 11025 0.2 0.5 11025 0.2 0.5 11025 0.2 0.5 11025 5 5 5 T63 T122 5 T9 T166

2 2 2 2 1 f f 1 f f f f f f 1 0 1 0 1 0 1 1 0 1 Spectral ratio 0.5

0.5 0.5 0.5 0.2 0.5 11025 0.2 0.5 11025 0.2 0.5 11025 0.2 0.5 11025

Figure 7. Examples of average Fourier spectra (top) and H/V spectral ratios (bottom), which yield two resonance peaks. The black line indicates a vertical spectral component; the grey line indicates the average of NS and EW horizontal components of motion.

Spectral analysis of microtremor measurements revealed another common feature characterizing both Fourier spectrum and spectral ratio obtained at a great part of measuring sites throughout the study area. This is a trough in amplitude of the vertical component of Fourier spectra in the frequency range 0.3-0.5 Hz, whose origin is not clear. When the fundamental frequency of site is significantly higher than 0.3-0.5 Hz, it looks like as a single trough in the vertical component (site 146 in Figure 8) and clearly visible looking peak is distinguished in the spectral ratio curve at frequency 0.35 Hz, while the fundamental frequency is 5 Hz. Some spikes in the frequency range 2-8 Hz are generated by the various types of machinery operating nearby. However, we obtained at many sites the fundamental frequency in the range 0.8-1.1 Hz. In this case, the typical picture is a wide-bottomed common trough like at site T35 or T62 in Figure 8. We note that if at site 35 the fundamental frequency albeit not plainly but can be seen at the vertical spectral component, at site T62 it is impossible divide two frequencies in the Fourier spectra. Only reprocessing with careful selection of microtremor time windows allowed separating these peaks. It is of interest to understand a possible nature of deviation of the horizontal and vertical spectral component in the low frequency range and appearance of peak at part of the 22

measurement sites. First of all, we tried to correlate presence of this peak with the surface geology. Three sites from example in Figure 8 are located at the outcropped Cover Basalt. However, there are a lot of sites in the similar geological conditions whose spectral ratios yield no peak at frequencies 0.3-0.4 Hz. Together with this, we do revealed this peak at sites located on expose of the Bira Fm. in the central part of the study area (site 70 in Figure 9) and do not reveal it at Bira outcropped in the northeastern part (site 115 in Figure 9). Similarly, we observe the peak in question at only one of sites T10 and T50 located on alluvium (Figure 9). Generally speaking, distribution of H/V spectral ratios yielding low frequency peak superimposed on the geological map does not show apparent correlation with lithological units in the study area. 1 1 1

0.1 0.1 0.1

T146 T35 T62 0.01 0.01 0.01 Spectral amplitude, µm/s*s amplitude, Spectral 0.10.2 0.5 1 2 5 10 0.10.2 0.5 1 2 5 10 20 0.10.2 0.5 1 2 5 10 10 T62 5 T146 T35 5 5

2 2 2 f f f f0 Spectral ratio Spectral f 0 f 0 1 1 1

0.5 0.5 0.5 0.10.2 0.5 1 2 5 10 0.10.2 0.5 1 2 5 10 0.10.2 0.5 1 2 5 10 20 Figure 8. Fourier spectra (top) and H/V spectral ratios (bottom) obtained at sites located on the exposure of the Cover Basalt. f0 indicates the fundamental frequency of the measurement site; f is an artificial frequency. H/V ratios at site T62 obtained by processing and reprocessing of the measurement data are shown by dashed and solid lines respectively.

It was interesting to look at H/V spectral ratios obtained from microtremor measurements carried out near the Town Hall in February 2007 (site TVR) and during the current measurement campaign in August 2008 (site T156). H/V spectral ratios shown in Figure 10 demonstrate similarity in both frequency and amplitude of the fundamental and second peaks. However, a 23 peak at frequency 0.4 Hz is clearly seen on the H/V curve at site T156 while it is absent at site TVR. Another example shows the results of two microtremor recordings performed in June 2008 and July 2008 at the same site T13. Spectral ratios (T13 and T13a in Figure 10) again demonstrate resemblance and all the difference is the presence of peak at frequency 0.5 Hz for one of measurements.

5 T70 5 T115

2 2

f f0 f0 1 1

0.5 0.5 0.10.2 0.5 1 2 5 10 0.10.2 0.5 1 2 5 10

T50 5 T10 5

2 2

f f f0 0 1 1

0.5 0.5 0.10.2 0.5 1 2 5 10 0.10.2 0.5 1 2 5 10

Figure 9. H/V spectral ratio obtained at sites located on the outcropped Bira Fm. (T70 and T115) and alluvium (T10 and T50).

T156 13.08.08 T13 17.06.08 5 5 TVR 06.02.07 T13a 16.07.08

2 2

1 1

0.5 0.5 0.10.2 0.5 1 2 5 10 0.10.2 0.5 1 2 5 10

Figure 10. Comparison between the H/V spectral ratios obtained near the Tiberias Town Hall (T156 and TVR) and at site T13 at different times.

This brief research aimed to find out whether the peak controlled by trough in the vertical spectra at low frequencies relates to the geological structure. Since we failed to find such a 24 correlation, we concluded that this peak should not be considered while developing analytical model of the subsurface.

Comparison of H/V spectral ratios from microtremor and seismic events The site response estimated from microtremor measurements we compared with that obtained from two local earthquakes occurred in 2004 and recorded by accelerometers. Locations of the strong motion stations used in this study are shown in Figure 3. Figure 11 shows two horizontal and vertical components of accelerograms from two seismic events given in Table 3 and recorded at site Tiberias Hotel (TVR2). The accelerograms demonstrate the considerable differences in amplitude and duration that characterize horizontal and vertical components. In terms of peak acceleration, amplitudes recorded at horizontal components are more than twice larger than at vertical components. The quasi-monochromatic nature of the motion of horizontal components strongly suggests sediment resonance. Horizontal- to-vertical spectral ratios (NS and EW components) for earthquake occurred on 11.02.04 indicate amplification about 2 near 0.8 Hz significantly and higher effect in the frequency range 2-5 Hz. Analysis of the earthquake occurred in the North Dead Sea reveals significant difference in amplification ground motions between EW and NS components in the frequency range 2-5 Hz and the low frequency peak. We note that the low frequency peak, which is clearly seen on H/V ratio from February earthquake is missing in the H/V ratio from July Earthquake. Figure 12 depicts acceleragram recorded at site Tiberias (TVR) from the Dead Sea earthquake (February 2004). Horizontal motions at this site are about three times higher than the vertical ones. While receiver function of NS component shows a clear peak characterized by amplitude of 3 at frequency 0.85 Hz, EW component is different and reveals two peaks at frequencies 0.55 Hz and 0.85 Hz. Figure 13 displays the three components of accelerograms from two earthquakes recorded at site Poriyya Hospital. One can see that in both cases the shear waves on the horizontal components exhibit larger amplitude than the vertical components. However, the difference is significant noticeable on EW components. The amplification in the time domain is comparable to that seen in the frequency domain and the common feature of H/V curves from both seismic events is a clear peak at frequency 0.5 Hz. 25

Table 3. Parameters of earthquakes recorded by accelerometer stations used in this study. Distance is to the surface projection of the rupture.

Geographic coordinates Distance Epicentre No. Recording site Date Time M L Lat.(N) Long.(E) (km) region Poriya Hospital 117 Dead Sea 1 Tiberias 04/02/11 08:15 5.1 31.70 35.56 122

Tiberias Hotel 120 Poriya Hospital 85 North 2 04/07/07 14:35 4.7 31.97 35.55 Tiberias Hotel 89 Dead Sea

Figure 11. Accelerograms of the earthquakes recorded at site Tiberias Hotel (TVR2): (a) earthquake occurred in the Dead Sea (2004 02-11, 08:15, ML=5.1, R=120 km) and (b) earthquake occurred in the Dead Sea fault (2004 07-07, 14:35, ML=4.7, R=89 km); 26

Figure 12. Accelerograms of the earthquake occurred in the Dead Sea (2004 02-11, 08:15, ML=5.1, R=120 km) and recorded at site Tiberias Town Hall (TVR).

Figure 14 presents a comparison between the average H/V spectral ratios from accelerograms recorded at strong motion stations (Receiver Functions) and spectral ratios obtained from microtremor measurements recorded at the same sites in different years. Figure 14a shows the spectral ratio of EW component of accelerogram from the earthquake occurred in February, 2004, and recorded at strong motion station Tiberias Hospital (TVR2). The fundamental peaks of the earthquake and microtremor spectral ratios at frequency 0.85 Hz look surprisingly similar. However, the amplification range observed in the receiver function from 2 Hz up to 4 Hz has shifted toward higher frequencies in the spectral ratio of microtremor. It is noteworthy that the peak at 2 Hz is not a resonance frequency but a result of soil-structure interaction. It is confirmed by conducting of ambient vibration test on the roof and at the basement of building where the strong motion station was installed. Peak at frequency of 2 Hz is interpreted as the fundamental frequency of the building so it is practically not visible in the spectral ratio from microtremor obtained at site situated 100 meters from the accelerometer location. In the case of Tiberias (TVR) strong motion station shown in Figure 14b both the fundamental frequency and amplification factors determined from the February earthquake and microtremor recorded in 2007 and 2008 concur well. The curves of spectral ratio obtained from microtremor have an additional peak at 1.2 Hz with amplitude about 2. This peak is rather relates to topography. The horizontal-to-vertical spectral ratio from microtremor recorded at site Poriya Hospital shows fundamental peak identical to that identified by the receiver function considering both the resonance frequencies and amplitude (Figure 14c).

(a) (b)

(c)

Figure 14. Comparison of different estimates of site amplification based on H/V spectral ratio techniques applied to earthquakes and microtremor recordings at sites TVR2 (Hotel) – (a); TVR (Town Hall) – (b) and Poriya Hospital– (c).

DISTRIBUTION OF THE RESONANCE FREQUENCY AND ITS ASSOCIATED H/V AMPLITUDE OVER THE STUDY AREA

The increased intensity of the damage during earthquakes is, to a great extent, correlated with resonance effects, therefore mapping of resonance frequencies and their associated H/V amplitudes is very useful for at least a qualitative assessment of the seismic hazard. Figure 15 presents maps of the contoured fundamental resonance frequency (f0) and the associated H/V amplitude. The data exhibit peaks changing from 2 to 8, occurring at frequencies 0.7 -7 Hz. The western part of the study area is mostly characterized by H/V spectral ratios with a single peak at the fundamental frequencies 1.0-1.3 Hz and amplitude less than 3 and is associated probably with dolomite of the Judea Gr. Irregularly appearing second resonance peak is related to the soft 29 alluvial layer and has relatively high amplitude. Such high amplitude values (5-7) but associated with the fundamental peaks at frequencies 4-7 Hz are observed in the northwestern part of the study area. We suppose a change of the fundamental reflector. It may be the limestone of the Gesher Fm., whose local outcrop is marked on the geological map, while the Cover Basalt is eroded in this part. These two areas are separated by a sublatitudinal fault mapped also by the geological data (transversal fault of Mizpa according to Shulman (1966)).

746000 a y li u F

f, Hz Profile 1 745000 Rock T ib er ia 8 s 5 zpa R Mi 744000 a 3 N b a b s i r A e q 2 d iv D a in 1.5

743000 1

ina 0.8 et T P Ein o r 0.7 iy Profile 2 a es od 742000 er H

Fault detected by shift in H/V frequency Fault according to Sneh, 2008 Fault according to Schulman, 1966 741000

247000 248000 249000 250000 251000 252000

Figure 15. Distribution of the fundamental resonance frequency over Tiberias. 30

746000 Amplification

Rock Profile 1

745000 7

744000 4

3 743000

Profile 2 742000

Fault detected by shift in H/V amplitude 741000

247000 248000 249000 250000 251000 252000

Figure 16. Distribution of amplitude associated with the fundamental frequency.

Decrease in the fundamental frequency is observed in the central part of the study area, which is in turn subdivided by the Mizpa fault into northern and southern parts with characteristic resonance frequency 0.75 Hz and 0.85-0.95 Hz, respectively. This down-dip block is located between the NW trending faults with the downthrow to the northeast (Shulman, 1966). The eastern one (the Nasr ed Din fault) is mapped also by Sneh (2008). The fault of Rabbi Aqiva, however, is not identified by the microtremor measurements. A wedge-shaped structural block is distinguished in the study area owing to higher fundamental frequency values (2.5-4 Hz) within the field of 0.7-0.8 Hz. This block is limited by the faults. The eastern one, NW-SE directed, coincides with the southeastern segment of the Tiberias fault. Its continuation to the northwest is 31 not detected by shift in the H/V fundamental frequency. The fault delineating this uplifted structural block from the southwest is the most likely extension to the northeast of the Poriya fault. This interpretation is supported in the geological map of Tiberias edited by A. Sneh (see Figure 2). The fault of Poriya which is traced by N. Shulman (1966), Sneh (2008) and B. Medvedev (2008) is also clearly detected by the H/V analysis in the segment along the Judea outcrop; however near the southeastern edge of the study area we could not trace it accurately due to the sparse measurement network. We obtained almost flat H/V ratios with no resonance frequency on the Upper Cretaceous rocks exposed in the structural high of the Poriya block. The fundamental frequency in the range of 2-5 Hz characterizes sites adjacent to the Judea outcrop from the south. An area located at the lakeshore eastern of the Poriya fault is characterized by the fundamental frequency in the range of 1.1-1.7 Hz and the close second resonance frequency (2.2- 2.5 Hz) produced by the talus. Two faults of SW-NE direction are identified by shift in the H/V fundamental frequency. One of them is a segment of the fault of Herods (Schulman, 1966) and limits the Judea outcrop from the northwest. Another fault is detected at a distance of 500 m to the north by changing frequency from 1.5-1.7 Hz to 2.5-3 Hz. The northeastern part of the study area is characterized by gradual increase of the fundamental frequency from 0.85 Hz up to 2-2.5 Hz. This increase in the frequency is explained by the thinning sediment layers above the Judea Gr. due to the sharp topography of the erosion surface of the lakeshore. The main feature of the H/V ratios obtained at sites located on the slope is two inseparable resonance peaks. The first one is associated with the Judea Gr. and the second one is caused by impedance contrast between clayey marl of the Bira Fm. and underlying layers. The second resonance frequency varies from 2.5 Hz up to 5 Hz. Distribution of maximum amplitude associated with fundamental H/V peak (Figure 16) retains the general trends characterizing the frequency map, however a correlation with only part of the faults is revealed. These faults were taken into account in the map constructing. The amplitude varies from 2 to 3 at a great part of sites in the study area. The higher (up to 7) values are attained in those areas, where the higher fundamental frequency values are observed. In these areas there is the thick alluvial layer and there is no the Cover Basalt. The amplitudes up to 5 are also attained at sites located in the northeastern part of the study area, where the Bira marl outcrops at the lakeshore. The variations of amplitude associated with the second resonance peak are connected with local variations of Vs in the upper soft part of the geological section. 32

ESTIMATION OF SHEAR-WAVE VELOCITY MODELS AND RECONSTRUCTION OF SUBSURFACE STRUCTURE

A prerequisite of a reliable analytical model for site response estimation by using computer codes such as SHAKE (Schnabel et al., 1972) is the knowledge of the local geology, including spatial distribution of softer materials above the hard bedrock with corresponding S- wave velocity of each layer. Densities and specific attenuation in different lithological units were selected from of literature sources (Borcherdt et al., 1989; McGarr et al., 1991; Theodulidis et al., 1996; Reinozo and Ordas, 1999; Pergalani et al., 2000 and many others). Recently, Pratt and Brocher (2006) used spectral decay in the shear-wave spectral ratio with respect to reference site amplification curves and estimated Q-values for shallow sedimentary deposits. They concluded that the range of Q values is 10-40. These values agree well with those used in our studies. Data collected from a few seismic refraction profiles provide information on the S-wave velocities and thickness of shallow sediments within the accuracy and resolution of the geophysical technique. Refraction profiles are normally designed to obtain maximum information on Vs of the lithological units represented in the study area and in the vicinity of boreholes. However, in the area of special interest in terms of both the geological conditions capable of producing site effects and the location of historic Tiberias archaeological remains, considering that part of ancient town is still un-excavated, we found only two locations appropriate for deploying the refraction survey equipment (see TB1 and TB3 profiles in Figure 3). Refraction profiles TB-1 and TB-3 provide us P- and S-wave velocities on upper 30 meters represented by the alluvial deposits (Ezersky, 2008). According to the geological data, in this part of the study area the Quaternary sediments are underlain by the Miocene Hordos Fm. consisting of conglomerates and limestone over the dolomite of the Judea Gr., which is the fundamental reflector. Two upper layers determined by the refraction survey and characterized by S-velocities 180 m/se and 340 m/sec are alluvium. The third layer with Vs=470 m/sec is probably talus. Lacking direct Vs measurements of the dolomite in the well, we adhere to Vs assigned to Vs=1900-2000 m/sec for dolomite of the Judea Gr. suggested by a refraction survey in the Parsa area located in the Dead Sea area (Zaslavsky et al., 2000) and in the town of Dimona (Zaslavsky et al., 2008) and used everywhere in the previous studies. Thickness and velocity of the Hordos Fm. are fitted. Figure 17 shows the analytical transfer functions matching the experimental 33 spectral ratio for two sites located along TB-3 profile. Strictly speaking, site 18 is located less than one hundred meters south of the refraction profile, therefore we slightly adopted also thickness of the upper layers known from the refraction survey. Table 4 presents the geophysical and optimal models for sites 52 and 18.

5 5 T52 T18

2 2

1 1

0.5 0.5 0.5 11025 0.5 11025

Figure 17. Comparison between the analytical transfer function (grey line) and experimental H/V spectral ratio (black line) obtained at two sites along TB-3 refraction profile.

Table 4. Geophysical and analytical models for calculating transfer functions at points located along TB-3 refraction profile

Geophysical data TB-3 Analytical model Site Layer Soil Thickness, Vs, Thickness, Vs, Density, Damping, No. m m/sec m m/sec g/cm3 % 1 6-7 180 5 190 1.6 5 Alluvium 2 10 340 10 350 1.7 4 3 Talus? ? 470 50 470 1.8 3 52 4 Hordos 80 1100 1.9 1 Dolomite 5 - 1900 2.4 (Judea Gr.) 1 6-7 180 3 200 1.6 5 Alluvium 2 10 340 10 390 1.7 4 3 Talus? ? 470 30 470 1.8 3 18 4 Hordos 65 1280 1.9 1 Dolomite 5 - 1900 2.4 (Judea Gr.)

Similarly to refraction profile TB-3, TB-1 provides us very scarce information on velocity and thickness of layers, characterizing the geological section in the area of historic Tiberias. Two layers, presumably alluvium and talus or movement material, are detected in the S-wave velocity- depth section (Ezersky, 2008). In this case, the procedure of adjustment of the analytical model to 34 the H/V ratio at four sites located along TB-1 refraction profile yielded slightly lower values for Vs of the Hordos Fm. in comparison with TB-3 profile. Figure 18 shows the analytical transfer functions superimposed on the experimental functions at sites from the south to the north 8, 162, 163 and 49. One can see that the fundamental frequency varies from 1 Hz (sites 8 and 162) up to 1.4 Hz (sites 163 and 49). Thicknesses of layers vary correspondingly. The refraction survey data and the optimal model are given in Table 5.

5 T8 5 T162 5 T163 5 T49

2 2 2 2

1 1 1 1

0.5 0.5 0.5 0.5

0.5 11025 0.5 11025 0.5 11025 0.5 11025

Figure 18. Comparison between the analytical transfer function (grey line) and experimental H/V spectral ratio (black line) obtained at four sites along TB-1 refraction profile.

Location of refraction profile TB-2 was chosen close to the refraction and reflection surveys carried out in 1995 (Shtivelman, 1995) to obtain the geotechnical parameters used for assessing the seismic hazard for designing road 438 on the northern shore of Tiberias (Zaslavsky and Shapira, 1995). Taking into consideration the sharp topography in this area, we used the opportunity to test on the one hand variability of the geotechnical parameters and on the other hand reproducibility of the microtremor measurement results. It should be noted that we found surprising similarity between H/V spectral ratio curves obtained in 1995 and now (Figure 19). The results of both seismic surveys are given in Table 6. Table 7 presents the optimal soil- column model which provides the best fit with the experimental functions for sites 166 and 115 located along TB-2 refraction profile (see Figure 20). It is important that these refraction surveys provide us velocity range for marl of the Bira Fm. Thickness and Vs for conglomerates of the Hordos Fm, underlying the marl is fitted. We note that the results of the reflection survey (Shtivelman, 1995) show that depth of the deepest reflector which is associated with dolomite of Cenomanian age is about 180 meters at the southern edge of the profile. It also indicates that this reflector dips toward the southeast. This information supports our estimation of the fundamental reflector depth.

Table 5. Geophysical and analytical models for calculating transfer function at sites located along refraction profile TB-1.

Geophysical data Analytical model Site Layer Thickness, Soil Vs, Thickness, Vs, Density, Damping% No. m m/sec m m/sec g/cm3 1 20 Alluvium 330 20 390 1.7 4 2 ? Talus 460 30 460 1.8 3 8 Conglomerate 3 140 800 1.9 1 (Hordos Fm.) Dolomite Half- 4 1900 2.4 (Judea Gr.) space 1 20 Alluvium 330 23 400 1.7 4 2 ? Talus 460 15 470 1.8 3 162 Conglomerate 3 180 850 1.9 1 (Hordos Fm.) Dolomite Half- 4 1900 2.4 (Judea Gr.) space 1 20 Alluvium 330 15 400 1.7 4 2 ? Talus 460 15 470 1.8 3 Conglomerate 163 3 115 810 1.9 1 (Hordos Fm.) Dolomite Half- 4 1900 2.4 (Judea Gr.) space 1 20 Alluvium 330 22 380 1.7 4 2 ? Talus 460 32 450 1.8 3 Conglomerate 49 3 100 890 1.9 2 (Hordos Fm.) Dolomite Half- 4 1900 2.4 (Judea Gr.) space

Table 6. Geotechnical data obtained from refraction surveys carried out in 1995 and 2008.

Geophysical data (1995) Geophysical data (2008) Layer Soil Depth, m Vs, Thickness Vs, No. m/sec m m/sec From 0 to 1 Clay 200 From 0 to 5 300 5-10 From 5-10 2 Marly clay (Bira Fm.) 440 From 5 to 30-33 400 to 10-20 3 Marl (Bira Fm.) Below 20 630 Below 30-33 690 4 Conglomerate (Hordos Fm.) 5 Dolomite (Judea Gr.) ≈ 180

Figure 19. Comparison between the H/V spectral ratios obtained from microtremor measurements in 1995 (grey line) and 2008 (black line) near refraction profile TB-2.

5 5 T115 T166

2 2

1 1

0.5 0.5

0.5 11025 0.5 11025

Figure 20. Comparison between the analytical transfer function (grey line) and experimental H/V spectral ratio (black line) obtained at two sites along refraction profile TB-2. 37

Table 7. Soil-column model for sites along refraction profile TB2

Soil-column model Site Layer Thickness, m Vs, m/sec Density, Damping, % No. g/cm3 115 1 5 300 1.6 5 2 34 350 1.7 4 3 17 690 1.8 2 4 150 1280 1.9 1 5 - 2.4 166 1 6 300 1.6 5 2 35 320 1.8 3 3 25 690 1.9 2 4 150 1130 2.1 1 5 - 2.4

Microtremor measurements were also carried out at Kineret-6 well, located approximately 1.5 km to the north of TB-2 refraction profile. Columnar section of this well contains 15-meters of Quaternary sediments and 35 meters of Neogene sandy and loamy marl. Vs for these layers are estimated by combining refraction survey data. S-velocity for thick conglomerate layer represented by limestone and brown chert with calcareous sandstone was adjusted. The analytical and experimental functions are plotted in Figure 21. For geotechnical data and model parameters see Table 8.

5 T121

H/V ratioH/V 1

0.5

0.5 11025 Frequency,Hz

Figure 21. Comparison between the analytical transfer function (grey line) and experimental H/V spectral ratio (black line) obtained well Kineret 6 (site 121). 38

Table 8. Geotechnical data and soil-column for well Kineret-6.

Well Kineret 6 Derived soil column model Depth Thickness Vs Density Damping Lithology interval m m/sec gr/cm3 % m Loam, sandy loam, 0-15 15 250 1.6 5 clay Sandy marl 15-55 35 470 1.7 4 Conglomerates 55-155 100 810 1.9 2 Limestone chalky 155-167 12 1000 2.0 1 (Senonian?) 167 and Dolomite (Judea Gr.) - 1900 2.3 below

Site response function calculated by modeling Kineret 10-B well approximates the spectral ratio from microtremor only assuming top Cenomanian dolomite (210 m) as the fundamental reflector depth, while the top limestone (84 m) is an intermediate interface. This is in agreement with correlation between Kineret 6 and 10-B wells (Weinberger, 1995). Comparison of the analytical function with H/V spectral ratio is shown in Figure 22. Table 9 contains well data and soil column model for Kineret 10-B well.

5 T125

atio r

H/V 1

0.5 0.5 11025 20 Frequency, Hz

Figure 22. Comparison between the analytical transfer function (grey line) and experimental H/V spectral ratio (black line) obtained well Kineret 10-B (site 125). 39

Table 9. Geotechnical data and soil-column for well Kineret-10B.

Well Kineret 10-B Derived soil column model Depth interval Thickness Vs Density Damping Lithology m m m/sec gr/cm3 % Alluvium 0-3 6 280 1.6 5 Marl 3-9 6 700 1.8 2 Limestone? 9-48 25 1080 2.0 1 Conglomerates 48-84 35 810 1.9 1 Limestone chalky 84-210 150 1050 2.0 1 (Turonian) Dolomite (Judea Gr.) 210 and below - 1900 2.3

S-wave velocities of the Neogene Lower Basalt and Pliocene Cover Basalt were taken from microzoning studies in the different places of Israel. Vs=2200 m/sec for the bedrock was obtained in the refraction survey along carried out at the Lower Basalt ridge in the town of Afula (Zaslavsky et al., 2005). The range 960-1200 m/sec for outcropped Cover Basalt is taken from the refraction survey in Qiryat Shemona (Ezersky and Schtivelman, 1999). Table 10 summarizes our investigations concerning ranges of S-velocities for lithological units in the study area.

Table 10. Ranges of S-wave velocities for litho-stratigraphycal units represented in the study area and used in calculating site response.

Lithology Vs, m/sec Alluvium 180-350 Talus 400-470 Weathered Cover Basalt (Pliocene) and 960-1200 limestone (Gesher Fm.) Marly clay (Bira Fm. in the northeastern part) 350-450 Marl (Bira Fm.) 670-690 Conglomerate (Hordos Fm.) 800-1300 Lower Basalt (Miocene) 2200 Chalk, chalky limestone (Senonnian-Turonian) 1200-1400 Dolomite (Judea Gr., Cenomanian) 1900-2000

Starting at sites close to refraction profiles and boreholes, we then propagate by means of extrapolation to neighbouring sites, using H/V spectral observations and information about the regional geology mainly to put constraints on thickness and the Vs values used in the models and to 40 maintain consistency across the investigated area. In the case of two H/V resonance peaks, when considering both frequencies, the layer thickness may be estimated quite accurately, using the second resonance peak as additional constrain in selecting a plausible value. The program, which is based on the stochastic optimization algorithm (Storn, 1995), is 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. Cross-sections over Tiberias, whose positions are indicated in Figures 3, 15 and 16, illustrate the results of H/V analysis. Profile 1 shown in Figure 23 has N-S direction and crosses a number of faults. Our investigation has allowed to specify faults defined earlier and to identify some new faults. We note that with the exception of two wells (K-6 and K-10) in the northeastern part and the regional structural map (Fleischer and Gafsou, 2003) which could not be used for quantitative estimations within the study area there is no reliable data on depth of the Top Judea Gr. For this reason we do not show on the schematic cross section the geological version of the top Judea Gr. Figure 24 depicts H/V spectral ratios for representative sites located along profile 1 together with the corresponding analytical transfer functions that were computed for the suggested 1D model beneath each site. We note that there are some sites where H/V spectral ratio shows two resonance peaks. While the first fundamental peak is associated with the dolomite of the Judea Gr., the second resonance peak has different origin and its amplitude depends mainly 41 on intermediate impedance contrast. For example, H/V ratios at sites T165 and T157 yield two resonance peaks. As mentioned above, the first one is related to the Judea Gr. and the second one is produced by impedance contrast between the weathered marly clay (Bira Fm.) and marl (Bira Fm.) together with conglomerates (Hordos Fm.). The position of the second resonance peak depends mainly on the thickness of the intermediate hard layer. Different situation is observed at sites T61 and T63, where the second resonance peak is produced by impedance contrast between alluvial sediments and Cover basalt. In this case, frequency of the second resonance peak is strongly correlated with thickness of the soft layer. At sites located on the outcrops of the Cover basalt (T79, T134 and T133) the second resonance peak does not exist and we observe the fundamental peak only. H/V spectral ratios at sites T71 and T64 show single resonance peak as well. Soil-column model for these sites is represented by the thin low velocity alluvial layer and the intermediate Hordos conglomerates over the Judea Gr. The analytical function calculated on the base such a model is characterized by single high amplitude peak. Thick layer of talus together with hard conglomerates of the Hordos Fm. overlaying the Judea Gr. produces H/V ratio with two merged low amplitude peaks (Site T15). Such spectral ratio represents the structural block from site T56 to site 18. In this case the Hordos Fm. is an intermediate hard layer and position of the second peak depends mainly on its thickness. Sharp shift in the fundamental frequency, change in H/V ratio shape and position of the second resonance peak between neighboring points is identified several times at profile 1. The H/V spectral ratios of sites T157 and T79 are different in all the three characteristics, i.e. fundamental frequency, amplitude and shape. Such occurrences are probably associated with a change of the velocity model. Measuring site T157 is located on a transition between the Cover Basalt and the Bira Fm. outcropped along the lakeshore in accordance with the geological map (Sneh, 2008). As we know from the refraction survey data, upper weathered part of the Bira Fm. of 30-35 m thick has Vs=350-400 m/sec. This layer determines shape of the H/V ratio showing two merged peaks. The fundamental peak is observed at frequency 1.5 Hz and has amplitude 5; the second one of amplitude 4 is at the frequency of 2 Hz. H/V ratio of site T79 located at the Cover Basalt yields single peak at frequency 1 Hz with amplitude less than 3. We suppose a decrease in the fundamental frequency from 1.3 Hz (site T157) down to 1.0 Hz (site T79) is due to sharp relief and not associated with vertical displacement. 42

200South North200 135 100 133 134 100

0 90 0 22 BSL Profile 2 BSL (m)-100 -100(m) 89 18 100 15 61 59 79 165 -200 56 71 64 63 157 -200

113 Tiberias lake -300 -300

-400 ? -400

-500 -500 Scale: Vert./Horiz.= 1 / 2 -600 -600 0 1000 2000 3000 4000 5000 D i s t a n c e , m

Alluvium Siltstone (Hordus Fm.) Vs=180-350 m/s Vs=400 m/s

Talus Conglomerate (Hordus Fm.) with alternating Lower Vs=400-470 m/s Basalt, chalk,chalky limestone Vs=800-1300 m/s

Weathered Cover Basalt and limestone Dolomite (Judea Gr.) (Gesher Fm.) Vs= 960-1200 m/s Vs= 1900-2000 m/s

Marly clay (Bira Fm.) Fault detected by measurments Vs=400 m/s

Marl (Bira Fm.) Top seismic reflector Vs=670-690 m/s

Figure 23. Schematic geological NS cross section beneath profile 1 43

5 T165 T157 T79 T61

1 f f f0 f f0 f f 0 1 0 1 0.5

0.1 0.2 0.511110 2 5 0 0.1 0.2 0.5 112500.1 0.2 0.511 2 5 0 0.1 0.2 0.5 11110250

5 T63 T71 T64 T15

1 f f f f f f 0 1 0 0 0 1 0.5

0.1 0.2 0.511110 2 5 0 0.1 0.2 0.5 112500.1 0.2 0.511 2 5 0 0.2 0.5 11250

5 T89 T22 T134 T133

1 f f 0 0 f0 f0 0.5

0.2 0.5 11025 0.1 0.2 0.5 112500.1 0.2 0.511 2 5 0 0.1 0.2 0.5 11250

Figure 24. H/V spectral ratio (black line) and analytical transfer function (grey line) for representative sites of profile 1

Unlike this case, the observed difference between sites T63 and T71 may be explained only by fault running between them. Increases in both fundamental frequency from 0.7 Hz and amplitude from 2 (T63) up to 4.5 Hz and amplitude of 7.5 (T71) are associated with vertical displacement of about 300 m. One side of this fault has columnar section alluvium-Cover Basalt- Bira Fm.-Hordos Fm. over the Judea Gr., while the other uplifted side has alluvium-Hordos Fm. overlying the Judea Gr. Sharp decrease in both amplitude and frequency at site T89 in comparison with H/V ratio at site T18, which is identical to that at site T15 (see Figure 24) and further their increase at site T22 imply two faults with vertical displacements of 110 m and 150 (100) m respectively. These two faults are marked on the geological map by Sneh (2008). 44

Starting at site 90 and southward there are no more measurements on the grid. A few measurements were carried out along this profile to estimate possible subsurface models to the accelerometer location at Poriya Hospital site (T133). Subsurface models are derived by fitting of the analytical functions to empirical ones obtained at sites T134 and T133. This estimation suggests a depth to the main reflector of 415 and 450 meters. An important issue that is raised in the process of geological interpretation of the measurement results is the fundamental reflector in this segment of profile. The first interpretation assumes dolomites of the Judea Gr. as a main seismic reflector. Despite the fact that this version seems more plausible, we also suggest another subsurface model which could be developed on the base of the columnar section of the Tiberias Gr. at Poriya escarpment (Shaliv, 1991). This subsurface model implies that part of the Hordos Fm., which contains three layers of the Lower Basalt intercalated with conglomerates and limestones and has total thickness of about 150 meters as a fundamental reflector or in terms of model - half-space. In such a subsurface structure we cannot judge the depth of the Judea Gr. We note that this assumption does not alter significantly the fundamental frequency and corresponding amplitude of the analytical response function; therefore only geological reasoning may be a criterion for correctness of the interpretation. The schematic cross section illustrating reconstruction of the subsurface beneath profile 2 oriented W-E is shown in Figure 25. Figure 26 presents the analytical transfer function superimposed on H/V ratios for representative sites along the profile. We start the description of this cross section on the east side (Tiberias lakeshore) towards the west. H/V ratio for site T69 representing the eastern edge of the profile exhibits a fundamental peak at 0.65 Hz and the second one at 2 Hz. The analytical model for this and surrounding sites suggests thick upper layer represented by talus, debris and alluvium. Beneath this layer the Bira marl and Hordos conglomerates overlay the Judea Gr. at a depth of about 350 m. Both geological source Schulman (1966) and Sneh (2008) provide information on presence a fault in close proximity. H/V ratio at site T1 exhibits two merged resonance peaks. The fundamental one is at frequency 1.8 Hz. The vertical cross section shows that sites T69 and T1 are located at the two sides of a fault which is detected by sharp change in the fundamental frequency from 0.65 Hz at site T69 to 1.8 Hz at site T1 corresponding to a vertical displacement of about 200 meters. 45

Figure 25. Schematic geological EW cross section along profile 2. 46

5 T69 T1 T9 T11

1 f f f f f f f f 0 1 0 1 0 1 0 1 0.5

0.2 0.5 11250 0.2 0.5 11250 0.2 0.5 11250 0.0.22 0.5 11250

5 T15 T21 T25 T29

1 f f 0 1 f 0 f0 f 0.5 0

0.0.22 0.5 11250 0.0.22 0.5 11250 0.2 0.5 11250 0.0.22 0.5 11250

5 T39 T35 T33 T112

1 f f f f f0 1 f0 1 0 0 0.5

0.0.22 0.5 11250 0.2 0.5 11250 200.0.22 0.5 11250 0.2 0.5 11025 Figure 26. H/V spectral ratio (black line) and analytical transfer function (grey line) for representative sites of profile 2.

Changes in the shape of H/V curve and amplitude level of the peaks indicate that there is a change in the velocity model. We suppose that in this uplifted block the Bira Fm is absent. From site T1 to site T17 there is a gradual increase from 1.8 Hz to 3 Hz, which corresponds to the decrease of the total sediment thickness from 140 meters to 100 meters. Since the H/V ratio retains generally its shape, we can conclude that there is no change in S-wave velocities. Increase in the fundamental frequency from 1.65 Hz at site T9 up to 2.3 Hz at site T11 is associated with vertical displacement of 90 m. Single H/V peak at frequency 1.2 Hz with lower amplitude is observed at site T21 located on the Hordos Fm. outcrop. Sites T17 and T21 are situated at two sides of fault, which is mapped by the geological data (Sneh, 2008). From site T25 up to the western edge profile 2 runs through the Cover Basalt outcrop. The segment between sites T25 and T29 is 47 characterized by the fundamental peak at in the frequency range 0.85-0.9 Hz that corresponds to depth of the Judea Gr. 220-260 m. The thickness of the Cover Basalt according to our calculations does not exceed 50 meters. The last segment in the profile 2 from T39 to T112 retains in general the characteristics of H/V ratios from the previous segment; however the fundamental frequency increases to 1.1-1.3 Hz and the corresponding amplitude is less than 3. We suggest a fault with the vertical displacement of about 50 meters between sites T29 and T39. This fault is mapped by the geological data between sites T37 and T39 that is, in fact, a negligible difference. The second resonance peak appearing at sites T35 and T39 is related to thin alluvial layers. It should again be mentioned that we have developed the subsurface models in terms of soft sediments and reflector; therefore the question whether the fundamental reflector inferred from the H/V analysis is the Judea Gr. requires at least structural map more detailed than the regional one. We note that according to the geological interpretation suggested by Dr. M. Abelson and Dr. A. Sneh (personal communication) the western segment of profile 2 is uplifted block of the Judea Gr.

SEISMIC HAZARD MICROZONATION

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. Neither do we have good reasons to assume that empirical attenuation functions of spectral accelerations that have been developed from observations in other parts of the world are applicable in Israel, let alone in areas where we expect geological site effects. Consequently, we prefer to resort to the use of synthetic data where local and regional characteristics of the geology and the seismicity are incorporated into the modeling. The SEEH procedure (Stochastic Estimation of the Earthquake Hazard) developed by Shapira and van Eck (1993) and briefly described above, generates synthetic site specific acceleration response functions while considering; possible scatter of the attenuation parameters of S waves propagating the region, estimations of seismic 48 moments from local magnitudes, possible stress drop values that are likely to be associated with earthquakes in the region etc. All mentioned uncertainties are incorporated in the process by using Monte Carlo statistics. The latter is also used to incorporate the uncertainty in estimating the seismic activity in the regional seismogenic zones located within 200 km of the investigated site. The response function of the soil column of the site is calculated by using the program SHAKE. The seismic hazard function, i.e., the uniform hazard site-specific acceleration response spectrum is computed for 10% probability of exceedance in an exposure time of 50 years and a damping ratio of 5%. By comparison to the Uniform Hazard Acceleration Spectra calculated for 55 selected sites and in consideration of the constructed subsurface models across the investigated area, we subjectively divided the area into 11 zones (see Figure 27). The grouping of the subsurface models is done manually taking into consideration the fundamental frequency, amplitude and the shape of H/V spectral functions. Each zone is characterized by a generalized seismic hazard function representative of the sites within that zone. The characteristic acceleration response spectrum for each zone is shown in Table 11. For comparison, we plot also the design spectra required in the same area by the current Israel Standard 413 (IS-413) and for ground conditions that meet the BSSC (1997) soil classification scheme. The shape of the hazard functions differ from those prescribed by the IS-413 code in all zones. Thus, in zones II, Via, VIb and X the Israel code underestimates the acceleration up to three times in the period range 0.1-0.4 sec. For zones I, III and IV the Israel the acceleration exceeds the design spectra in the period range from 0.5 to 1.8 sec. It should be noted that for zones I, II and IV located on the cover basalt, the generalized analytical models for calculating characteristic acceleration spectra do not take into account local thin (up to 10-12 meters) soft layers found at many measuring sites. H/V peaks produced by these layers are revealed in the spectral ratios at frequencies 8-13 Hz. We show in Figure 28 two examples of the spectral acceleration computed on the base of analytical transfer functions which take into account upper soft layer. Sites T130 and T43 are situated in zones I and IV, respectively. One can see that amplitude of spectral acceleration peaks observed at period about 0.2 sec corresponding to the second resonance peak in the transfer functions reaches 1.5 g. We recommend considering these results in design of structures. For the rest of the zones (V, VIb, VII, 49 VIII and IX) the standard underestimates the spectral acceleration in the period range 0.1- 1 sec.

747000

746000

V 745000

III IV II 744000 VIa X IV I VIb 743000 VII

VIII IX 742000

f=1.1-1.3 Hz I f=1.2-2.0 Hz 741000 Ampl. 2.5-3 V Ampl. 3-5 VIII f=1.1-1.3 Hz f=4.0-7.0 Hz II f=3.5-4 Hz Ampl. 2-2.5 Ampl. 5-7 VIa Ampl. 5-7 f=2.5-4.5 Hz IX 740000 f=0.75-0.85Hz Ampl. 3-4 III VIb f=2.5-3 Hz Ampl. 2-2.5 f=0.65-0.75 Hz Ampl. 3-4 X Ampl. 2-3 f=0.85-1.0 Hz f=1.4-1.7 Hz IV VII Ampl. 2.5-3 Ampl. 3-5 Rock. No site effect 739000 246000 247000 248000 249000 250000 251000 252000 253000 254000

Figure 27. Seismic microzoning map of Tiberias presenting zones of common site effect characteristics. 50

(a) Site T130 (b)

5 1.6 n io

t 1.2 2

plifica 0.8 1 m A 0.4 0.5 Spectral acceleration, g acceleration, Spectral 0.0 0 0.4 0.8 1.2 1.6 2 0.10.2 0.5 1 2 5 10 20 Frequency, Hz Site T43 Period, sec

5 1.6 n

io t 1.2 2

plifica 0.8 1 m A 0.4 0.5 Spectral acceleration, g acceleration, Spectral 0.0 0 0.4 0.8 1.2 1.6 2 0.10.2 0.5 1 2 5 10 20 Frequency, Hz Period, sec

Figure 28. Examples showing influence of thin upper soft layers on spectral accelerations computed for two sites located on the Cover Basalt. 51

Table 11. Soil column models for representative sites of zones, their transfer functions and spectral accelerations.

Number Thickness, Density, Vs, Damping, Analytical transfer function and of zone m g/cm3 m/sec % spectral acceleration

5 37 2.0 1200 1

2 ication f 1 Ampli 0.5 100 1.8 680 2

0.10.2 0.5 1 2 5 10 Frequency, Hz I 0.8 94 2.0 1290 1

ion, g ion, 0.6 t a r 0.4 al accele r t 0.2 - 2.4 1900 Spec 0.0 0 0.4 0.8 1.2 1.6 2 Period, sec

5 5 1.6 180 5 2 ication f 1 Ampli 0.5 5 1.6 350 4

0.10.2 0.5 1 2 5 10 20 Frequency, Hz II 1.6 ion, g 9 1.7 535 3 t a

r 1.2

0.8 al accele r t 0.4

1.8 1200 Spec 0.0 0 0.4 0.8 1.2 1.6 2 Period, sec 52

60 2.0 1200 1 n io t 2

plifica 1 m A 0.5 100 1.8 700 2 0.10.2 0.5 1 2 5 10 Frequency, Hz III 0.8 170 2.0 1100 1 , g n

io 0.6 t a r 0.4 al accele r 2.4 1900 t 0.2 Spec 0.0 0 0.4 0.8 1.2 1.6 2 Period, sec

40 2.0 1130 1 n io t 2

plifica 1 m A 0.5 120 1.8 670 2

0.10.2 0.5 1 2 5 10 Frequency, Hz IV 0.8 , g

110 2.0 1100 1 n

io 0.6 t a r 0.4 al accele r t 0.2

2.4 1900 Spec 0.0 0 0.4 0.8 1.2 1.6 2 Period, sec

40 1.6 300 5 n io t 2

plifica 1 m A 0.5 40 1.8 690 2

0.10.2 0.5 1 2 5 10 V Frequency, Hz

1.2

110 2 1100 1 ion, g t a r 0.8 al accele r

t 0.4

2.4 1900 Spec 0.0 0 0.4 0.8 1.2 1.6 2 Period, sec

18 1.6 310 5 n io t 2

plifica 1 m A 0.5 31 2 1390 1

0.10.2 0.5 1 2 5 10 20 VIa Frequency, Hz 2.5

2.0 ion, g ion, t a r 1.5

2.4 1900 1.0 al accele r t 0.5 Spec 0.0 0 0.4 0.8 1.2 1.6 2 Period, sec

5 35 1.7 440 4 n io t 2

plifica 1 m A 0.5 30 1.8 860 2

0.10.2 0.5 1 2 5 10 VIb Frequency, Hz

1.2

25 2.0 1200 1 ion, g t a r 0.8 al accele r

t 0.4

2.4 1900 Spec 0.0 0 0.4 0.8 1.2 1.6 2 Period, sec \

45 1.6 350 5 n io t 2

plifica 1 m A 0.5 30 1.9 800 2

0.10.2 0.5 1 2 5 10 Frequency, Hz VII 1.2

105 2.0 1100 1 0.8

0.4

2.4 1900 Spectral acceleration, g 0.0 0 0.4 0.8 1.2 1.6 2 Period, sec

20 1.7 400 4 n io t 2

plifica 1 m A 0.5 15 1.7 460 3 0.10.2 0.5 1 2 5 10 VIII Frequency, Hz

1.2 ion, g 180 1.9 850 2 t a r 0.8 al accele r

t 0.4

2.4 1900 Spec 0.0 0 0.4 0.8 1.2 1.6 2 Period, sec

5 n

20 1.7 400 4 io t 2

plifica 1 m A 0.5 60 2.0 1000 1 0.10.2 0.5 1 2 5 10 IX Frequency, Hz 1.6 ion, g t a

r 1.2

2.4 1900 0.8 al accele r t 0.4 Spec 0.0 0 0.4 0.8 1.2 1.6 2 Period, sec

10 1.6 190 5 n io t 2

plifica 1 m A 0.5 10 1.7 480 4 0.10.2 0.5 1 2 5 10 Frequency, Hz X 160 1.9 690 2 2.0 1.6 ion, g t a r 200 2.1 1260 1 1.2 0.8 al accele r t 0.4 Spec 2.4 1900 0.0 0 0.4 0.8 1.2 1.6 2 Period, sec

CONCLUSIONS

In the town of Tiberias, H/V measurements performed on urban noise have been used to quantify soil responses for evaluation of the site specific seismic hazard. Our conclusions may be summarized as follows: • The stability and reproducibility of measurements are confirmed by data from continuous measurements during several months as well as repeated measurements in different months and years which yield almost identical shapes of average spectral ratios obtained at the same site under the same conditions of measurements. • Comparison between the average H/V spectral ratios obtained from accelerograms of horizontal components and from microtremor recorded at the same site shows that an appropriate ensemble of carefully selected windows of microtremor provides estimations of site response which are similar to those obtained from seismic events. • Experimental estimation of the site response over Tiberias yields variation in the fundamental frequency in the range 0.7-7 Hz and H/V amplitude from 2 up to 8. Maps of the spatial distribution of the fundamental frequency and their associated H/V amplitude delineate potentially vulnerable sites. This information is useful for land use considerations in urban planning and for identifying sites which require in depth site investigations to better evaluate the seismic hazard. • Limited data on S-wave velocities and sediment thickness of the upper layers obtained from seismic refraction surveys used to calibrate the H/V spectral ratio with an analytical site response derived from a 1D subsurface model. It is also used to justify further H/V ratios utilization, by velocities extrapolation, to study other sites, away from refraction profiles and boreholes. A stochastic optimization algorithm is applied to calculate the layer thickness, yielding transfer functions to match in the best way the observed H/V curves, considering all resonance peaks. Two cross-sections in Tiberias illustrate the results of H/V analysis. • The microtremor measurements enable identifying discontinuity in the subsurface and locate faults. These are associated with significant change in fundamental frequency, amplitude and shape corresponding to both vertical displacement and change in the 58

velocity profile. Some, but not all faults detected by H/V analysis are identified also by geological data. • Analytical models cross-checked with observed data were extrapolated over the study area and integrated into computations of the uniform site specific acceleration response spectra for a probability of exceedence of 10% during exposure time of 50 years and damping of 5%. The sites with common site effect characteristics were united into zones. In eight out of eleven selected zones the current Building Code IS-413 significantly underestimates the acceleration in the period range 0.1-0.6 sec. • Since 2000 when strong motion stations were installed in the Tiberias area, two local earthquakes that occurred in 2004 were recorded. Taking into account the complicated geology of the region, we strongly recommend deploying seismic stations for continuous recording weak earthquakes to validate and improve the subsurface models derived from microtremor analysis and contribute to seismic hazard assessment. • We should emphasize that calculated analytical transfer functions are associated with weak motions and linear behavior of soils. Non-linear characteristics of site in Tiberias are beyond the scope of this study. Nevertheless, based on the result presented above nonlinear site response can be determined by different mathematical models of soil nonlinearity, making use of the models developed for each zone. In that respect, the microzonation maps developed in this study are also relevant for the prediction of ground motions from earthquakes of high magnitudes.

ACKNOWLEDGEMENTS

This work was funded by the Steering Committee for National Earthquake Preparedness and the Geological Survey of Israel. A special thanks for Dr. M. Abelson and Dr. A. Sneh for valuable suggestions and useful comments which helped to improve our work. We appreciate very much the comments of Dr. A. Hofstetter. We thank Y. Menahem for assistance in preparing this report.

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