1

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

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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

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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

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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

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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.

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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

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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

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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

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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.

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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.

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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

EW NS Name Depth to the Judea Gr.

Elevation

m TD

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

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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

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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)]:

( ) ( )( )fS

fSfA

V

NSNS = ( ) ( )

( )fSfS

fAV

EWEW = (5)

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:

( )( )( )

( )( ) ⎥

⎥

⎦

⎤

⎢⎢

⎣

⎡∑=

∑=

+=n

i

n

i f iS

f iS

f iS

f iS

nfA

V

EW

V

NS

1 121 (6)

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.

19

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.

1 102 50.50.20.01

0.1

0.03

0.3

T134

1 102 50.50.2

1

2

5

0.5

T134

f0

1 102 50.5

T33

1 102 50.5

T149

1 102 50.5

T149

f0

1 102 50.50.2

T79

f0

1 102 50.50.2

T33

f0

1 102 50.5

T79

Spe

ctra

l am

plitu

de, µ

m/s

*sSp

ectra

l rat

io

1 102 50.50.20.01

0.1

0.03

0.3

T134

1 102 50.50.2

1

2

5

0.5

T134

f0

1 102 50.5

T33

1 102 50.5

T149

1 102 50.5

T149

f0

1 102 50.50.2

T79

f0

1 102 50.50.2

T33

f0

1 102 50.5

T79

Spe

ctra

l am

plitu

de, µ

m/s

*sSp

ectra

l rat

io

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

1 102 50.50.2

1

2

5

0.5

T63

f0 f1

1 102 50.50.01

0.1

1

0.03

0.3

T63

1 102 50.50.20.01

0.1

0.03

0.3

T166

1 102 50.50.2

1

2

5

0.5

T166

f0 f1

1 102 50.50.20.01

0.1

0.03

0.3

T122

1 102 50.50.2

1

2

5

0.5

T122

f0 f1

1 102 50.50.20.01

0.1

0.03

0.3

T9

1 102 50.50.2

1

2

5

0.5

T9

f0 f1

Spec

tral a

mpl

itude

, µm

/s*s

Spec

tral r

atio

1 102 50.50.2

1

2

5

0.5

T63

f0 f1

1 102 50.50.01

0.1

1

0.03

0.3

T63

1 102 50.50.20.01

0.1

0.03

0.3

T166

1 102 50.50.2

1

2

5

0.5

T166

f0 f1

1 102 50.50.20.01

0.1

0.03

0.3

T122

1 102 50.50.2

1

2

5

0.5

T122

f0 f1

1 102 50.50.20.01

0.1

0.03

0.3

T9

1 102 50.50.2

1

2

5

0.5

T9

f0 f1

Spec

tral a

mpl

itude

, µm

/s*s

Spec

tral r

atio

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.

0.1 1 100.2 0.5 2 50.01

0.1

1

T146

0.1 1 100.2 0.5 2 5

1

2

5

0.5

f f0

T146

0.1 1 100.2 0.5 2 5 20

1

10

2

5

0.5

f f0

T35

0.1 1 100.2 0.5 2 5 200.01

0.1

1

T35

0.1 1 100.2 0.5 2 50.01

0.1

1

T62

0.1 1 100.2 0.5 2 5

1

2

5

0.5

f f0

T62

Spe

ctra

l am

plitu

de, µ

m/s

*sSp

ectra

l rat

io

0.1 1 100.2 0.5 2 50.01

0.1

1

T146

0.1 1 100.2 0.5 2 5

1

2

5

0.5

f f0

T146

0.1 1 100.2 0.5 2 5 20

1

10

2

5

0.5

f f0

T35

0.1 1 100.2 0.5 2 5 200.01

0.1

1

T35

0.1 1 100.2 0.5 2 50.01

0.1

1

T62

0.1 1 100.2 0.5 2 5

1

2

5

0.5

f f0

T62

Spe

ctra

l am

plitu

de, µ

m/s

*sSp

ectra

l rat

io

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.

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

0.1 1 100.2 0.5 2 5

1

2

5

0.5

f f0

T70

0.1 1 100.2 0.5 2 5

1

2

5

0.5

f0

T115

0.1 1 100.2 0.5 2 5

1

2

5

0.5

f f0

T10

0.1 1 100.2 0.5 2 5

1

2

5

0.5

f0

T50

0.1 1 100.2 0.5 2 5

1

2

5

0.5

T156 13.08.08TVR 06.02.07

0.1 1 100.2 0.5 2 5

1

2

5

0.5

T13 17.06.08T13a 16.07.08

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 No. Recording site Date Time ML Lat.(N) Long.(E) Distance

(km) Epicentre

region Poriya Hospital 117

Tiberias 122 1 Tiberias Hotel

04/02/11 08:15 5.1 31.70 35.56 120

Dead Sea

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

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);

b

a

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 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).

a

b

27

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).

28

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

(a) (b)

(c)

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)).

247000 248000 249000 250000 251000 252000

741000

742000

743000

744000

745000

746000

Profile 2

Prof

ile 1

Fault detected by shift in H/V frequency

Nasr ed D

inR

abbi Aqiva

Mizpa

Tiberias

Ein et Tina

Herodes

Fuliya

Poriya

Fault according to Schulman, 1966

0.8

1

1.5

2

3

5

8

Rock

f, Hz

0.7

Fault according to Sneh, 2008

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

30

247000 248000 249000 250000 251000 252000

741000

742000

743000

744000

745000

746000

3

4

5

7

2

Amplification

Rock

Prof

ile 1

Profile 2

Fault detected by shift in H/V amplitude

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.

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

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

Geophysical data TB-3 Analytical model Site Layer

No. Soil Thickness,

m Vs,

m/sec Thickness,

m Vs,

m/sec Density,

g/cm3 Damping,

% 1 6-7 180 5 190 1.6 5 2

Alluvium 10 340 10 350 1.7 4

3 Talus? ? 470 50 470 1.8 3 4 Hordos 80 1100 1.9 1

52

5 Dolomite (Judea Gr.) - 1900 2.4

1 6-7 180 3 200 1.6 5 2

Alluvium 10 340 10 390 1.7 4

3 Talus? ? 470 30 470 1.8 3 4 Hordos 65 1280 1.9 1

18

5 Dolomite (Judea Gr.) - 1900 2.4

1 102 50.5

1

2

5

0.5

T18

1 102 50.5

1

2

5

0.5

T52

1 102 50.5

1

2

5

0.5

T18

1 102 50.5

1

2

5

0.5

T52

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.

1 102 50.5

1

2

5

0.5

T8

1 102 50.5

1

2

5

0.5

T49

1 102 50.5

1

2

5

0.5

T162

1 102 50.5

1

2

5

0.5

T163

1 102 50.5

1

2

5

0.5

T8

1 102 50.5

1

2

5

0.5

T49

1 102 50.5

1

2

5

0.5

T162

1 102 50.5

1

2

5

0.5

T163

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.

35

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

Geophysical data Analytical model Site Layer

No. Thickness,

m Soil Vs,

m/secThickness,

m Vs,

m/secDensity,

g/cm3 Damping%

1 20 Alluvium 330 20 390 1.7 4

2 ? Talus 460 30 460 1.8 3

3 Conglomerate (Hordos Fm.) 140 800 1.9 1

8

4 Dolomite (Judea Gr.) Half-

space 1900 2.4

1 20 Alluvium 330 23 400 1.7 4

2 ? Talus 460 15 470 1.8 3

3 Conglomerate (Hordos Fm.) 180 850 1.9 1

162

4 Dolomite (Judea Gr.) Half-

space 1900 2.4

1 20 Alluvium 330 15 400 1.7 4 2 ? Talus 460 15 470 1.8 3

3 Conglomerate (Hordos Fm.) 115 810 1.9 1 163

4 Dolomite (Judea Gr.) Half-

space 1900 2.4

1 20 Alluvium 330 22 380 1.7 4 2 ? Talus 460 32 450 1.8 3

3 Conglomerate (Hordos Fm.) 100 890 1.9 2 49

4 Dolomite (Judea Gr.) Half-

space 1900 2.4

36

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

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.

1 102 50.5

1

2

5

0.5

T115

1 102 50.5

1

2

5

0.5

T166

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.

Geophysical data (1995) Geophysical data (2008) Layer No.

Soil Depth, m Vs, m/sec

Thickness m

Vs, m/sec

1 Clay From 0 to 5-10 200 From 0 to 5 300

2 Marly clay (Bira Fm.) From 5-10 to 10-20 440 From 5 to 30-33 400

3 Marl (Bira Fm.) Below 20 630 Below 30-33 690 4 Conglomerate (Hordos Fm.) 5 Dolomite (Judea Gr.) ≈ 180

37

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

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.

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

Soil-column model Site Layer

No. Thickness, m Vs, m/sec Density,

g/cm3 Damping, %

1 5 300 1.6 5 2 34 350 1.7 4 3 17 690 1.8 2 4 150 1280 1.9 1

115

5 - 2.4 1 6 300 1.6 5 2 35 320 1.8 3 3 25 690 1.9 2 4 150 1130 2.1 1

166

5 - 2.4

1 102 50.5Frequency, Hz

1

2

5

0.5

H/V

ratio

T121

38

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

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.

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).

Well Kineret 6 Derived soil column model

Lithology Depth

interval m

Thickness m

Vs m/sec

Density gr/cm3

Damping %

Loam, sandy loam, clay 0-15 15 250 1.6 5

Sandy marl 15-55 35 470 1.7 4 Conglomerates 55-155 100 810 1.9 2

Limestone chalky (Senonian?) 155-167 12 1000 2.0 1

Dolomite (Judea Gr.) 167 and below - 1900 2.3

1 102 5 200.5Frequency, Hz

1

2

5

0.5

H/V

ratio

T125

39

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

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.

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

Well Kineret 10-B Derived soil column model

Lithology Depth interval m

Thickness m

Vs m/sec

Density gr/cm3

Damping %

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 (Turonian) 84-210 150 1050 2.0 1

Dolomite (Judea Gr.) 210 and below - 1900 2.3

Lithology Vs, m/sec Alluvium 180-350

Talus 400-470 Weathered Cover Basalt (Pliocene) and

limestone (Gesher Fm.) 960-1200

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

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 ],[ 21 ww we look for thickness ( ih ) and S-velocity ( iv ) that minimize the misfit function

∑=

−=N

kkk fgF

1

2))()(( ωω ,

where kω are points from the frequency interval ],[ 21 ww , )(ωg is 1-D theoretical transform

function calculated by SHAKE program; and )(f ω is H/V spectral ratio. Velocity and thickness

are limited:

1,1,21 +=≤≤ MiVvV iii and MiHhH iii ,1,21 =≤≤

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

16571

Tibe

rias

lake

113157

795961636456151001889

2290

135134133

?

0 1000 2000 3000 4000 5000-600

-500

-400

-300

-200

-100

0

100

200

-600

-500

-400

-300

-200

-100

0

100

200

Scale: Vert./Horiz.= 1 / 2

South North

BSL (m)

BSL (m)

D i s t a n c e , m

Profile 2

Siltstone (Hordus Fm.)Vs=400 m/s

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

Dolomite (Judea Gr.)Vs= 1900-2000 m/s

Fault detected by measurments

Top seismic reflector

AlluviumVs=180-350 m/s

TalusVs=400-470 m/s

Weathered Cover Basalt and limestone(Gesher Fm.) Vs= 960-1200 m/s

Marly clay (Bira Fm.)Vs=400 m/s

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

Figure 23. Schematic geological NS cross section beneath profile 1

43

0.1 1 100.2 0.5 2 5

T71

f0

0.1 1 100.2 0.5 2 5

T64

f0

0.1 1 100.2 0.5 2 5

1

2

5

0.5

T165

f0

0.1 1 100.2 0.5 2 5

T157

f0

0.1 1 100.2 0.5 2 5

T79

f0

0.1 1 100.2 0.5 2 5

T61

f0

0.1 1 100.2 0.5 2 5

1

2

5

0.5

T63

f0

1 102 50.50.2

1

2

5

0.5

T89

f0

0.1 1 100.2 0.5 2 5

T22

f0

0.1 1 100.2 0.5 2 5

T133

f0

0.1 1 100.2 0.5 2 5

T134

f0

1 102 50.50.2

T15

f0

f1

f1

f1

f1

f

0.1 1 100.2 0.5 2 5

T71

f0

0.1 1 100.2 0.5 2 5

T64

f0

0.1 1 100.2 0.5 2 5

1

2

5

0.5

T165

f0

0.1 1 100.2 0.5 2 5

T157

f0

0.1 1 100.2 0.5 2 5

T79

f0

0.1 1 100.2 0.5 2 5

T61

f0

0.1 1 100.2 0.5 2 5

1

2

5

0.5

T63

f0

1 102 50.50.2

1

2

5

0.5

T89

f0

0.1 1 100.2 0.5 2 5

T22

f0

0.1 1 100.2 0.5 2 5

T133

f0

0.1 1 100.2 0.5 2 5

T134

f0

1 102 50.50.2

T15

f0

1 102 50.50.2

T15

f0f0

f1

f1

f1

f1

f1

f

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

1 102 50.50.2

1

2

5

0.5

T69

f0 f1

1 102 50.50.2

T1

f0 f1

1 102 50.50.2

T9

f0 f1

1 102 50.50.2

T11

f0f1

1 102 50.50.2

1

2

5

0.5

T15

f0 f1

1 102 50.50.2

T21

f0

1 102 50.50.2

T29

f0

1 102 50.50.2

1

2

5

0.5

T39

f0f1

1 102 5 200.50.2

T35

f0f1

1 102 50.50.2

T33

f0

1 102 50.50.2

T112

f0

1 102 50.50.2

T25

f0

1 102 50.50.2

1

2

5

0.5

T69

f0 f1

1 102 50.50.2

T1

f0 f1

1 102 50.50.2

T9

f0 f1

1 102 50.50.2

T11

f0f1

1 102 50.50.2

1

2

5

0.5

T15

f0 f1

1 102 50.50.2

T21

f0

1 102 50.50.2

T29

f0

1 102 50.50.2

1

2

5

0.5

T39

f0f1

1 102 5 200.50.2

T35

f0f1

1 102 50.50.2

T33

f0

1 102 50.50.2

T112

f0

1 102 50.50.2

T25

f0

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.

246000 247000 248000 249000 250000 251000 252000 253000 254000739000

740000

741000

742000

743000

744000

745000

746000

747000

I

II

III

IV

IV

V

VIa

VIb

VII

VIIIIX

f=4.0-7.0 HzAmpl. 5-7

II

f=1.1-1.3 HzAmpl. 2.5-3I

V

VIa

VIb

VII

VIII

IX

f=1.2-2.0 HzAmpl. 3-5

f=3.5-4 HzAmpl. 5-7

f=2.5-3 HzAmpl. 3-4f=1.4-1.7 HzAmpl. 3-5

f=1.1-1.3 HzAmpl. 2-2.5

f=2.5-4.5 HzAmpl. 3-4

IV f=0.85-1.0 HzAmpl. 2.5-3 Rock. No site effect

f=0.75-0.85HzAmpl. 2-2.5III

X

f=0.65-0.75 HzAmpl. 2-3X

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

50

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

0.1 1 100.2 0.5 2 5 20Frequency, Hz

1

2

5

0.5

Am

plifi

catio

n

0 0.4 0.8 1.2 1.6 2Period, sec

0.0

0.4

0.8

1.2

1.6

Spec

tral a

ccel

erat

ion,

g

0.1 1 100.2 0.5 2 5 20Frequency, Hz

1

2

5

0.5

Am

plifi

catio

n

0 0.4 0.8 1.2 1.6 2Period, sec

0.0

0.4

0.8

1.2

1.6

Spec

tral a

ccel

erat

ion,

g

Site T130

Site T43

(a) (b)

51

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

Thickness, m

Density, g/cm3

Vs, m/sec

Damping, %

Analytical transfer function and spectral acceleration

37 2.0 1200 1

100 1.8 680 2

0.1 1 100.2 0.5 2 5Frequency, Hz

1

2

5

0.5

Am

plifi

catio

n

94 2.0 1290 1

I

- 2.4 1900

0 0.4 0.8 1.2 1.6 2Period, sec

0.0

0.2

0.4

0.6

0.8

Spec

tral a

ccel

erat

ion,

g

5 1.6 180 5

5 1.6 350 4 0.1 1 100.2 0.5 2 5 20

Frequency, Hz

1

2

5

0.5

Am

plifi

catio

n

9 1.7 535 3

II

1.8 1200 0 0.4 0.8 1.2 1.6 2

Period, sec

0.0

0.4

0.8

1.2

1.6

Spec

tral a

ccel

erat

ion,

g

52

60 2.0 1200 1

100 1.8 700 2

0.1 1 100.2 0.5 2 5Frequency, Hz

1

2

5

0.5

Am

plifi

catio

n

170 2.0 1100 1

III

2.4 1900

0 0.4 0.8 1.2 1.6 2Period, sec

0.0

0.2

0.4

0.6

0.8

Spec

tral a

ccel

erat

ion,

g

40 2.0 1130 1

120 1.8 670 2

0.1 1 100.2 0.5 2 5Frequency, Hz

1

2

5

0.5

Am

plifi

catio

n

110 2.0 1100 1

IV

2.4 1900

0 0.4 0.8 1.2 1.6 2Period, sec

0.0

0.2

0.4

0.6

0.8

Spec

tral a

ccel

erat

ion,

g

53

40 1.6 300 5

40 1.8 690 2

0.1 1 100.2 0.5 2 5Frequency, Hz

1

2

5

0.5

Am

plifi

catio

n

110 2 1100 1

V

2.4 1900

0 0.4 0.8 1.2 1.6 2Period, sec

0.0

0.4

0.8

1.2

Spec

tral a

ccel

erat

ion,

g

18 1.6 310 5

31 2 1390 1

0.1 1 100.2 0.5 2 5 20Frequency, Hz

1

2

5

0.5

Am

plifi

catio

n

VIa

2.4 1900

0 0.4 0.8 1.2 1.6 2Period, sec

0.0

0.5

1.0

1.5

2.0

2.5

Spec

tral a

ccel

erat

ion,

g

54

35 1.7 440 4

30 1.8 860 2

0.1 1 100.2 0.5 2 5Frequency, Hz

1

2

5

0.5

Am

plifi

catio

n

25 2.0 1200 1

VIb

2.4 1900

0 0.4 0.8 1.2 1.6 2Period, sec

0.0

0.4

0.8

1.2

Spec

tral a

ccel

erat

ion,

g\

45 1.6 350 5

30 1.9 800 2

0.1 1 100.2 0.5 2 5Frequency, Hz

1

2

5

0.5

Am

plifi

catio

n

105 2.0 1100 1

VII

2.4 1900 0 0.4 0.8 1.2 1.6 2

Period, sec

0.0

0.4

0.8

1.2

Spec

tral a

ccel

erat

ion,

g

55

20 1.7 400 4

15 1.7 460 3

0.1 1 100.2 0.5 2 5Frequency, Hz

1

2

5

0.5

Am

plifi

catio

n

180 1.9 850 2

VIII

2.4 1900 0 0.4 0.8 1.2 1.6 2

Period, sec

0.0

0.4

0.8

1.2

Spec

tral a

ccel

erat

ion,

g

20 1.7 400 4

60 2.0 1000 1 0.1 1 100.2 0.5 2 5

Frequency, Hz

1

2

5

0.5

Am

plifi

catio

n

IX

2.4 1900

0 0.4 0.8 1.2 1.6 2Period, sec

0.0

0.4

0.8

1.2

1.6

Spec

tral a

ccel

erat

ion,

g

56

10 1.6 190 5

10 1.7 480 4

0.1 1 100.2 0.5 2 5Frequency, Hz

1

2

5

0.5

Am

plifi

catio

n

160 1.9 690 2

200 2.1 1260 1

X

2.4 1900 0 0.4 0.8 1.2 1.6 2

Period, sec

0.0

0.4

0.8

1.2

1.6

2.0

Spec

tral a

ccel

erat

ion,

g

57

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.

59

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