table of contentsjoyner (1991) and other. in brief, the seeh process starts by monte carlo...

1

TABLE OF CONTENTS

LIST OF FIGURES..........................................................................................................................2

LIST OF TABLES ...........................................................................................................................3

ABSTRACT.....................................................................................................................................4

1. INTRODUCTION........................................................................................................................5

2. GEOLOGICAL FRAMEWORK .................................................................................................7

3. OBSERVATIONS AND DATA PROCESSING ........................................................................7

4. SITE RESPONSE ESTIMATIONS...........................................................................................14

4.1. The first stage ......................................................................................................................14

4.2. The second stage .................................................................................................................19

5. MODELLING ............................................................................................................................23

6. DISCUSSION AND CONCLUSIONS......................................................................................31

7. ACNOWLEDGMENTS.............................................................................................................34

8. REFERENCES...........................................................................................................................35

2

LIST OF FIGURES

Figure 1. Topographic map showing the study area and location of the observation points. The

depth to basement is taken from Fleisher and Gafsou (2003). Insert shows the area of

interest relative to the Tira town. .........................................................................................8

Figure 2. Lithological section of the Ramat HaKovesh –D (RH-D) borehole...............................10

Figure 3. Examples of seismic stations location in Ramat HaKovesh area ...................................12

Figure 4. (a) The average Fourier spectra and (b) individual and average H/V spectral ratios from

microtremor observed at Stations 1, 1*, 2 and 3. ...............................................................15

Figure 5. (a) The average Fourier spectra and (b) individual and average H/V spectral ratios from

microtremor observed at Stations 4, 5 and 6. .....................................................................16

Figure 6. The average H/V spectral ratio observed at stations located around the experimental

well. ....................................................................................................................................17

Figure 7. (a) The average Fourier spectra and (b) individual and average H/V spectral from

microtremor observed at Stations 7, 8 and 9. .....................................................................18

Figure 8. (a) The average Fourier spectra and (b) individual and average H/V spectral from

microtremor observed at Station 10. ..................................................................................19

Figure 9. (a) Location of seismic stations relatively to experimental borehole and (b) The average

Fourier spectra and individual and average H/V spectral ratios from microtremor observed

at Station 11........................................................................................................................20

Figure 10. Average microtremor H/V ratios for four stations deployed close to experimental

borehole..............................................................................................................................21

Figure 11. Average Fourier spectra and H/V spectral ratios from microtremor observed at

Stations 13, 14, 15, 16 and 17. ...........................................................................................22

Figure 12. Comparison of average H/V spectral ratios obtained from measurements carried out in

May 10, 2006 (red line) and in March 26, 2007 (green line). Station locations see in

Figure1. ..............................................................................................................................23

Figure 13. Velocity-depth section for P-and S-waves along refraction lines.................................24

Figure 14. Comparison between the average H/V spectral ratio obtained from microtremor

records at Station 11 located close to the experimental borehole (solid black line) and the

site response functions computed analytically according to the Structural Model-1 (green

line), Structural Model-2 (blue line) and Structural Model-3(red line). ............................26

3

Figure 15. (a) Lithological cross section of Petah Tikva borehole and (b) comparison between

average H/V spectral ratios obtained at borehole location (red line) and analytical transfer

function calculated using survey data (green line).............................................................28

Figure 16. Frequency response functions of Structural Model-4 (red line) and Model 5 (blue

line): (a) Amplification factor and (b) Phase factor. ..........................................................30

Figure 17. Average H/V spectral ratio obtained from microtremor records at Station 11 located

close to experimental borehole (black line) compared with transfer function calculated

using Structural Model-4 (red dashed line); Structural Model-5 (blue dashed line) and

Optimal Model obtained by summation of those models (green dashed line)...................30

Figure 18. Uniform hazard site-specific acceleration spectra for Ramat HaKovesh site calculated

on the base of different soil-column models: Structural Model-1 (green line), Structural

Model-2 (blue line), Structural Model-3 (red line) and Optimal Model obtained by

summation of Structural Model-4 and Structural Model-5 1D (black line).......................32

LIST OF TABLES

Table 1. The locations of seismic stations........................................................................................9

Table 2. Station characteristics.......................................................................................................11

Table 3. Structural Model-1 ...........................................................................................................25



Table 6. Structural Model-4 ..........................................................................................................29


4

ABSTRACT

In order to determine the stability of horizontal-to-vertical (H/V) spectral ratio from

microtremor obtained in different months and to verify the applicability and validity of this

method for prediction of fundamental frequency and its associated amplification of the weak

motion site response, we compared assessment based on H/V spectral ratio made at locations

where the high resolution refraction profile was carried out and shear velocity, thickness of

sediments up to reflector and shear velocity of reflector are known.

This report presents the results of two-stage study. The first stage was preliminary

analysis of site effect in Ramat HaKovesh area before drilling experimental borehole down to

depth of 60-70 m. For this purpose microtremor measurements were carried out at ten points

along a profile over supposed borehole location on May 10, 2006. The second stage aimed to

estimate site effect near the drilled borehole and to check stability of preliminary measurements

at some locations from the first stage. This experiment was carried out on March 26, 2007. The

measurement conducted in different months yield strong similar results.

The shear-wave velocity of shallow subsurface has been documented using surface-

wave and borehole methods. In addition during drilling experimental borehole was compiled

description of the lithology and soil samples of the major lithological units were collected for

geotechnical investigations.

Observed transfer functions were modeled using relative amplification function from

computer program SHAKE. In according with experiment conditions (blind prediction) we could

use surface-wave refraction survey results only. Therefore we started with direct modeling and

used shear-wave velocity and layer thickness from seismic refraction data. The density and specific

attenuation in different lithological units were chosen on the base of different literature sources.

Our results suggest that calculated transfer function based on geotechnical

measurements may not accurately predict empirical weak motion transfer function. A better fit to

the amplitude of resonance peak was obtained by integrating data from extensive microtremor

measurements, information about local geology and S-wave velocity profile derived from

refraction line.

5

1. INTRODUCTION

The Holy Land has a long documented history of destructive earthquakes (e.g. Amiran,

et al., 1994). This information, descriptive in its nature, is very useful for learning about the

earthquake history of the region. However, questionable reliability of many historical "facts", the

dramatic change in the demographic conditions, the changing engineering characteristics of the

buildings and changes in geographical locations of towns and villages present great difficulties in

using the macroseismic information to reliably assess the earthquake hazards.

In a series of studies we successfully applied the procedure developed by Shapira and

van Eck (1993) to assess the site specific uniform hazard acceleration response. That procedure,

which we term SEEH (Stochastic Estimation of the Earthquake Hazard), is based on the

stochastic method developed and used by Boore (1983), Boore and Atkinson (1987), Boore and

Joyner (1991) and other. In brief, the SEEH process starts by Monte Carlo simulations of the

expected seismic activity in seismogenic zones that may affect the study area/site. It follows by

using the stochastic method to synthesize ground motions at the investigated site location,

assuming hard rock conditions which are then propagating from the base-rock to the sites surface,

given the properties and structure of the subsurface at the analysed site. The synthetic free surface

motions are used to compute the acceleration response spectra for a 5% damping ratio. In the

final stage of SEEH all generated response spectra are used to estimate the spectral acceleration

levels, which correspond to a prescribed probability of exceedance level and yield the uniform

hazard, site specific acceleration response spectrum. The uncertainties associated with assigning

values to different parameters in the computations are considered by performing Monte Carlo

simulations throughout the SEEH process. Implementation of the SEEH process for assessing the

earthquake hazard throughout a region practically requires the same input data except for the

parameters that characterize the subsurface. These parameters, which determine the expected site

response to seismic waves, may vary significantly over very small distances.

Various empirical techniques for site response estimating were summarized and

discussed by Field and Jacob (1995). It is likely that the best evaluation of site effect will be

based on dense strong motion observations using spectral ratio of seismic records from

sedimentary sites with respect to bedrock reference sites. Such observations also include effects

of the nonlinear behaviour of the materials. Empirical response functions obtained from

measurements of different earthquakes may still vary significantly due to nonlinear effects as

borehole as other effects such as directivity, differences in the propagation path and properties of

6

the earthquake source functions (see also Boore, 2004). This empirical approach, however, is

impractical in regions where seismicity is moderate as in Israel. Furthermore, attempts to derive

site response estimations from simultaneous recordings on sediments and on hard rock in an

urban area may not be possible. Nakamura (1989, 2000) hypothesized that the site response could

be estimated from the spectral ratio of the horizontal versus vertical components of ambient noise

observed at same site. Many authors show (Lermo and Chávez-García, 1994; Seekins et al.,

1996; Toshinawa et al., 1997; Chávez-García and Cuenca, 1998; Enomoto et al., 2000;

Mucciarelli and Gallipoli, 2004, Shapira et al., 2001; Zaslavsky at al., 2000, 2003; and others)

that the H/V spectral ratio technique can be a useful tool for the assessment of ground motion

characteristics in alluvia. However, the other authors (for example, Bonilla et al., 1997; Horike et

al., 2001, Satoh et al., 2001) conclude that predominant peak of H/V ratio is well correlated with

fundamental resonant frequency, but amplitude of this peak is not similar to those obtained from

sediment-to-bedrock spectral ratio of earthquake records. Studies of Zaslavsky et al. (2000,

2005a,b, and 2006a,b) as well as many other investigators (Toshinawa et al., 1997; Chávez-

García and Cuenca, 1998; Mucciarelli et al., 2003) showed that horizontal-to-vertical spectral

ratio obtained from ambient noise can be used to obtain reliable information related to linear

seismic behaviours of sedimentary layers.

In 2006 the Steering Committee for National Earthquake Preparedness and Mitigation

established a working group to make a quantitative comparison fundamental frequency and

associated amplification of the weak motion site response obtained from microtremor and

analytical model inferred only from geotechnical measurements. The working group established

sediment-filled valley near town of Tira (Ramat HaKovesh) for comparison of different site-

response estimation techniques. The Ramat HaKovesh site was selected for deployment of the

seismometers owing to almost flat topography over a sufficiently broad area, its location about

500 m away from main road with intensity traffic, depth to reflector of 80-60 m only, simple soil

profile and sharp velocity contrast of shear wave between rock and soil.

In borehole drilled at the selected site, specific acquisition techniques involving special

horizontally oriented sources and receivers (borehole and surface-wave method) were

implemented to determine the subsurface structure down to the bedrock and shallow velocities. In

addition to the detailed lithological description, soil samples were collected for geotechnical

investigations. According to the conditions of the experiment (blind prediction), we could use

only refraction survey results (Ezersky, 2007).

7

The modelling has been simplified by assuming S waves that are vertically incident and

that the site response to S waves is one-dimensional, linear and viscoelastic. The reflector (half

space) is also assumed to be plane layered. Furthermore, since we assume that the first peak of

the observed H/V spectral ratio from microtremor is the first mode of relative site response, it

may be modelled by the SHAKE site amplification function (Schnabel, 1972).

2. GEOLOGICAL FRAMEWORK

The investigated area is situated close to the Tira town, 3 km west of the Shomron

Mountains slope represented by carbonates of Sakhnin Fm. of the Judea Gr. (Cenomanian age).

For location see Figure 1. The bedrock (the Judea Gr.) dips westward with less than 5o slope in

the study area. The plain is filled by sediments of the Rehovot Fm. (Kurkar Gr.). Available

borehole and refraction data provide us with depth to the carbonate bedrock from 70 to 100 m

and also indicate inhomogeneity of filling deposits. Locations of two boreholes and refraction

lines are shown in Figure 1. The lithological section of Ramat HaKovesh borehole is presented in

Figure 2.

3. OBSERVATIONS AND DATA PROCESSING

Microtremor measurements were carried out during on May 10, 2006 and on March 26,

2007. Figure 1 shows the locations of 17 temporary recording sites. Coordinates of the

measurement points are summarized in Table 1. Microtremor measurements are conducted using

portable instruments (Shapira and Avirav, 1995) consisting of a multi channel amplifier, Global

Positioning System (GPS) and a laptop computer with 16-bit analog-to-digital conversion card.

Each seismograph station consists of three L4C (Mark Product) velocity transducers (one vertical

and two horizontal oriented in the north-south and east-west directions) with a natural frequency

of 1.0 Hz and critical damping ratio 0.7. 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 were installed on a vehicle, which also

served as a recording centre.

8

Figure 1. Topographic map showing the study area and location of the observation points. The depth to basement is taken from Fleisher and Gafsou (2003). Insert shows the area of interest relative to the Tira town.

Study area

CitiesMeasuring pointsAlluvium (Holocene)Red sand & loam (hamra) (Quaternary)Limestone, dolomite (Cenonian-Turonian)

Well

Site instrumented during:

October 20, 2002

May 20, 2006

March 26, 2007

P-wave refraction line

S-wave refraction line

80

80

70

195200 195400 195600 195800 196000 196200 196400

680300

680500

680700

240

1

2

3

4

56

78

910

12

14

17

Ramat Hakovesh /D

Exp.well

1* 11

13 1516

RK-1SRK-1P

qh

q

c3

t

c2

TIRE

EYAL 02

195000 196000 197000 198000 199000

6780

0067

9000

6800

0068

1000

6820

0068

3000

9

Table 1. The locations of seismic stations

Israel Grid Station Number East North

1 195501 680464

1* 195546 680496

2 195424 680268

3 195215 680510

4 195522 680706

5 195612 680450

6 195766 680418

7 195928 680380

8 196091 680352

9 196259 680323

10 196392 680290

11 195757 680471

12 195792 680476

13 195760 680402

14 195961 680378

15 196088 680360

16 196247 680316

17 196342 680305

10

Figure 2. Lithological section of the Ramat HaKovesh –D (RH-D) borehole.

Prior to performing the measurements, the individual seismometer constant (free-

frequency, damping and motor constant) are determined using sine and step calibration signals,

and then the frequency response function of all channels are computed. The instrument

characteristics of the stations are given in Table 2. As a final test, all seismometers are placed at

the same location and in the same orientation to record the same waves. These measurements

provide relative calibration between the different channels of the entire monitoring system. In

Figure 3 we present examples of the locations of the seismic stations during the site investigation

in the Ramat HaKovesh area.

Chalk

Limestone

Chalky limestone

Shattered zone

Shale

Dolomitic limestone with debris,porous

Hamra

Sand and sandy loam

Clay andsandy clay

Kurkar,sand,clay,conglomerate

-200

-150

-100

-50

0

T.D. 217.1m

Debris of flint and carbonates

11

Table 2. Station characteristics

Sensor Number Code

Generator Const. at 1 Hz

V/m/sec

Frequency Hz

Damping

Component (Direction)

Site Number

3406 V406 88.5 1.00 0.65 Vertical

3209 H209 103.0 1.00 0.70 Horizontal (NS)

3210 H210 106.8 1.00 0.70 Horizontal (EW)

11,14, 3, 6, 9

3224 V224 130.1 1.00 0.65 Vertical

3232 H232 87.8 1.00 0.70 Horizontal (NS)

3231 H231 87.4 1.00 0.70 Horizontal (EW)

1, 1*, 5, 8, 1

1690 V690 124 0,97 0.70 Vertical

1564 H564 120 1.00 0.67 Horizontal (NS)

1565 H565 129 1.00 0.69 Horizontal (EW)

11, 12, 16,

3403 V403 91.6 1.00 0.70 Vertical

3393 H393 118.8 0.99 0.68 Horizontal (NS)

3394 H394 119.3 1.00 0.68 Horizontal (EW)

4, 7, 10

3407 V407 110.7 1.02 0.66 Vertical

1398 H398 110.3 1.01 0.68 Horizontal (NS)

1399 H399 111.7 1.01 0.65 Horizontal (EW)

11, 15, 2

2753 V753 105.3 1.00 0.65 Vertical

2748 H748 97.90 1.00 0.64 Horizontal (NS)

2747 H747 96.20 1.00 0.63 Horizontal (EW)

11, 13, 17

12

Figure 3. Examples of seismic stations location in Ramat HaKovesh area

Station 1

Station 3

Station 6

Station 10

Station 2

Station 4

Station 8

Station 13

13

As already observed by many researchers, there is a high scatter in the H/V spectra. The

source of the scatter is debated between the researchers; Mucciarelli (1998), for example, claims

that traffic is not a major reason, whereas Horice et al. (2001) used in their analysis recordings of

microtremor originated by passing traffic. Recently, Parolai and Galiana-Merino (2006) showed

that influence of transients on the H/V spectral ratio is insignificant. Our observations also

indicate that the effect of transients is almost unnoticeable. In order to reduce the scatter and

increase stability, our processing scheme involved a careful selection of the time windows from

which we obtained H/V functions. It followed the concept that at site with no site effects the

amplitude spectra of the H and V components of the ground motions are of the same level

throughout the spectrum. At sites with significant site effects, the spectral amplitudes of the two

components differ only within a certain limited frequency band, probably at the neighbourhood

of the resonance frequency. Time windows that exhibit those or similar conditions were selected.

The selection is made manually and yields an appreciated reduction in the H/V scatter.

Continuous record of ambient noise for 60-70 minutes enables us to select sufficient number of

suitable samples for analysis.

For each site we determined the average H/V spectral ratios and their corresponding

standard deviations by applying the following process: Time windows, each of 30 sec long,

provided sets of H and V ground motions that were Fourier transformed using cosine-tapering (1

sec at each end) before transformation. The spectra were then smoothed with a triangular moving

Hanning window. More precisely, we applied “window closing” procedure (see Jenkins and

Wats, 1968) for smart smoothing of spectral estimates so that any significant spectral peaks are

not distorted. For each site we compiled a set of up to 50 selected time windows, each window

providing an H/V spectral function.

Program "SEISPECT" developed in the Geophysical Institute of Israel (Perelman and

Zaslavsky, 2001) was used in routine data processing. SEISPECT is a MATLAB application for

spectral analysis and processing of ground motion including seismograms recorded by short-

period and broad-band seismic stations, as well as strong motion accelerometers. The main

modules realized in the program are: visualizing and editing of the input data; selecting time

window and computing FFT and H/V spectral ratios; saving and displaying results.

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:

14

( )( )( )

( )( ) ⎥

⎥

⎦

⎤

⎢⎢

⎣

⎡+= ∑ ∑

= =

n

i

n

i iV

EW

iV

NS

f

f if

f in

fASS

SS

1 121 (1)

where SNS(f)i and SEW(f)i are individual spectra of the horizontal components and SV(f)i is

individual spectrum of the vertical component.

4. SITE RESPONSE ESTIMATIONS

4.1. The first stage

Originally Station 1 was located inside of industrial premises built for pumping station.

However, complex motions generated by the water pump of the RH-D borehole did not allow

deploying Station 1 near the well and it was reinstalled about 60 m away of the well. Stations 2,

3, 4, and 5 were deployed around the RH well at distances ~ 200-220 m. It should be noted that

site effect at Station 1* (see Figure 1) was measured in October, 2002, in the framework of

microzoning project of Hashefela area (Zaslavsky et al., 2005).

Figures 4 and 5 display the average Fourier spectra for vertical and horizontal

components and individual and average H/V spectral ratio from microtremor recorded at Stations

1, 1*, 2, 3, 4, and 5. In these figures we notice that spectra of all components have maximum near

frequency 3 Hz. One can see this peak in all microtremor records obtained in urban areas and it

may be attributed to heavy traffic. Another common feature of the spectra is a wide frequency

band where horizontal and vertical components deviate. It is approximately 0.6-1.5 Hz and two

inseparable resonance peaks are revealed in the H/V ratios at frequencies near 1.5 Hz (main peak)

and near 1.8 Hz with amplification factor 6.5 – 7.5. A few points are worth noting. First, we can

see insignificant change of the fundamental frequency from 1.4 Hz at Station 4 to 1.6 Hz at

Station 6. At the second point, the variation of individual spectral ratios is small, and all curves

are similar in shape. Figure 6 shows average H/V spectral ratios obtained at Stations 1, 1*, 2, 3,

4, and 5. Comparing these ratios we note that average curves are very similar not only in terms of

the peak position, but also in the whole shape. It is very important because measurements at Site

1* were carried out 4.5 years prior to the current project.

15

a) b)

Figure 4. (a) The average Fourier spectra and (b) individual and average H/V spectral ratios from microtremor observed at Stations 1, 1*, 2 and 3.

Station 1

Station 1*

Station 2

Station 3

16

a) b)

Figure 5. (a) The average Fourier spectra and (b) individual and average H/V spectral ratios from microtremor observed at Stations 4, 5 and 6.

Station 4

Station 5

Station 6

17

Figure 6. The average H/V spectral ratio observed at stations located around the experimental well.

Examples of the average Fourier spectra and individual and average H/V spectral ratios

obtained for Stations 6, 7, 8 and 9 are displayed in Figure 7. These examples demonstrate the

high stability in the site response across the study area. An increase in the spectral levels of the

horizontal components is clear in the frequency range from 1.0 to 2.5 Hz. Therefore, spectral

ratios show a prominent peak at about 1.6 Hz with an amplification factor 6.0. In the shapes of

curves of spectral ratio amplification at frequency near 2.0 Hz is observed.

In Figure 8, we plotted the average Fourier spectra and individual and average spectra

ratios obtained at Station 10. These plots again demonstrate that scatter of the individual curves is

small and from average functions of the spectral ratio it may be concluded that there is

amplification up to 6.5 at frequency of about 1.9 Hz.

18

a) b)

Figure 7. (a) The average Fourier spectra and (b) individual and average H/V spectral from microtremor observed at Stations 7, 8 and 9.

Station 7

Station 8

Station 9

19

a) b)

Figure 8. (a) The average Fourier spectra and (b) individual and average H/V spectral from microtremor observed at Station 10.

4.2. The second stage

At this stage, microtremor measurements have been carried out at experimental

borehole location by four stations, which recorded microtremor simultaneously over period of

three hours. Figure 9a shows locations of recording stations relatively to experimental borehole.

In Figure 9b, we plotted examples of the average Fourier spectra for vertical and horizontal

components and individual and average H/V spectral ratio from microtremor recorded at Stations

11a and 11b. All the spectral ratios show a well-defined peak at about 1.6 Hz and supplementary

peak near 2.0 Hz. The observed variations of individual amplitudes are small and at resonance

frequency less than 20% of average. Average microtremor H/V ratios for four stations deployed

close to experimental borehole are shown in Figure 10. Now, it is very difficult to explain

deviation of curves in frequency range from 0.2 to 0.9 Hz. Nevertheless, in the frequency range

1-3 Hz of amplification these curves are absolutely identical, not only for Stations 11a, 12 and 13

(see Figure 1) where microtremor records carried out in the same day (March 26, 2007), but also

for Station 6 where analyzed data were recorded 9.5 months prior to the current measurements

(May 10, 2006).

Station 10

20

Figure 9. (a) Location of seismic stations relatively to experimental borehole and (b) The average Fourier spectra and individual and average H/V spectral ratios from microtremor observed at Station 11.

Station 11a

Station 11b

Experimental Borehole

St. 11d St. 11c

St. 11b

St. 11a

b

a

21

Figure 10. Average microtremor H/V ratios for four stations deployed close to experimental borehole.

We calculated the average spectra Fourier and spectral ratios for the NS and EW

components obtained from microtremors which were recorded at Stations 13, 14, 15, 16 and 17

(Figure 11). From the examination of this figure we can draw the following conclusions:

- amplitudes and horizontal spectra shapes of the stations are practically identical;

- the H/V spectral ratios for NS and EW components do not vary from one station to

other both in terms of maximum value and shape and exhibit site effect at frequency

near 1.6 Hz with amplification of about 7.0;

- the shape of average curves of spectral ratios for NS and EW component show

resemblance;

- the shape of the H/V ratios is influenced by the second inseparable peak near 2.0 Hz.

22

Figure 11. Average Fourier spectra and H/V spectral ratios from microtremor observed at Stations 13, 14, 15, 16 and 17.

Station 14

Station 15

Station 16

Station 17

Station 13

23

In order to further understand the stability of the amplification effects, we have anew

compared measurements at Sites 13, 14, 15, and 16 with those that were carried out in May,

2006. As Figure 12 demonstrates, there are no considerable variations in the H/V ratio at Stations

13, 14 and 15. Only at Station 17, that was installed about 60 m from Station 10, one can see a

difference in the resonance frequency for two sites that may be explained by variation in the

reflector depth.

Figure 12. Comparison of average H/V spectral ratios obtained from measurements carried out in May 10, 2006 (red line) and in March 26, 2007 (green line). Station locations see in Figure1.

5. MODELLING

P- and S-wave refraction lines (for locations see Fig. 1) were designed to obtain

maximum information on Vs and thickness of the lithological units represented in the study area.

The velocity depth section along these lines is shown in Figure 13 (Ezersky, 2007).

24

Figure 13. Velocity-depth section for P-and S-waves along refraction lines.

Four layers are distinguished in the section. According to the lithological section of

Ramat HaKovesh borehole (see Figure 2) they may be correlated with hamra (Vs=250 m/sec);

sand and sandy loam (Vs=380 m/sec); clay and sandy clay (Vs=500 m/sec) that overlies the

broken dolomitic limestone (VS=1400m/sec) respectively. In the first step we used for modelling

solely refraction data assuming the broken dolomitic limestone as a fundamental reflector.

Thickness and dynamic parameters characterizing the Structural Model-1 (SM-1) are summarised

in Table 3. Density and specific attenuation (quality factor, Q) in the different lithological units

were chosen on the base of literature sources (Borcherdt et al., 1989; McGarr et al., 1991;

Theodulidis et al., 1996; 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. Figure 14 displays response function SM-1 that was

calculated using 1-D method (SHAKE program) described in the Introduction of this report. We

can see that computed amplification underestimates the obtained value at Station 11 by a factor of

2. Therefore, in the second step, we constructed the structural model 2 (SM-2), in which we

retain the SM-1 down to a depth of 70 m; and below we add, in accordance with Ramat

HaKovesh borehole, the broken dolomitic limestone of 50m thick (Vs=1400 m/sec) as

intermediate layer; limestone of the Judea group with VS=2000m/sec is the fundamental reflector.

Table 4 shows geotechnical parameters used in SM-2.

25

Table 3. Structural Model-1


Comparing analytical transfer function calculated from SM-2 with observed average

H/V ratio (Figure 14), we can see that while the frequency of the highest peak is in good

agreement, still there is a remarkable difference in the amplifications.

How well the calculated transfer function matches the frequency and amplitude of

resonant peak of the observed H/V ratio is a measure of the goodness of fit the modelling. The

stochastic optimization algorithm (Storn and Price, 1995) is applied in order to fit an analytical

transfer function to an observed H/V spectral ratio, focusing mainly on the dominant frequency

and considering the shape H/V curve. Systematically variation in the SM-2: (a) Vs of the first

layer and thickness of the broken dolomite to better match observed predominant frequency and

(b) the damping of soils and S-velocity of reflector to better match observed resonance amplitude

lead to a Structural Model-3 (SM-3).

Layer Lithology Thickness

m

VS

m/sec

Density

gr/cm3

Quality

factor

1 Hamra 5 250 1.6 16

2 Sand and sandy loam

35 380 1.7 25

3 Clay and sandy clay

35 500 1.8 25

Half Space

Broken dolomite

50 - ∞ 1400 2.0 -

Layer Lithology Thickness

m

VS

m/sec

Density

gr/cm3

Quality

factor

1 Hamra 5 250 1.6 16

2 Sand and sand loam

35 380 1.7 25

3 Clay and sandy clay

35 500 1.8 25

4 Broken dolomite

50 1400 1.9 50

Half Space

Chalky limestone

∞ 2000 2.4

26


Figure 14. Comparison between the average H/V spectral ratio obtained from microtremor records at Station 11 located close to the experimental borehole (solid black line) and the site response functions computed analytically according to the Structural Model-1 (green line), Structural Model-2 (blue line) and Structural Model-3(red line).

Layer Lithology Thickness m

VS m/sec

Density gr/cm3

Quality factor

1 Hamra 5 200 1.6 25

2 Sand and sand loam 35 380 1.7 50

3 Clay and sandy clay 35 500 1.8 50

4 Broken dolomite 10 1390 2.0 100

Half Space

Chalky Limestone ∞ 2200 2.4

27

Geotechnical parameters of SM-3 are shown in Table 5. In comparison with SM-1 and

SM-2, Vs of the first layer is decreased by 20 percents; Vs of reflector is increased by 10

percents; thickness of broken dolomite and quality factor of layers are essentially changed to

reach a better fit with observed amplitudes of the main spectral peak.

However, as seen in Figure 14, despite a good agreement in the frequency and amplitude

of the main resonance peak, the 1-D Structural Model-3 does not yield the prominent feature of

the observed H/V ratio, i.e. essentially broader band of frequencies where amplification occurs.

We note that in the previous investigations of site effects (see Zaslavsky at al., 2006) we

had a lot of cases where H/V ratio with broad band of amplification could be accurately predicted

by 1-D model based on geotechnical data.

Figure 15a shows lithological column of Petah Tikva 25 borehole. VS=280m/sec for

loam and soil, VS=400 m/sec for gravel and sandstone and VS=1000 m/sec for limestone are

provided by the refraction survey (Ezersky, 2006). VS=1900 m/sec for dolomite of the Judea Gr.

was obtained in our investigations in Hashefela region. The analytical transfer function calculated

on the basis of this 1-D model and superimposed on the average spectral ratio from microtremors

recorded at this borehole is shown in Figure 15b. We note that H/V ratio is characterized by two

close peaks in the frequency range 1.5-2.5 Hz. In our measurement we repeatedly observed

broadening frequency band of site effect for sites where intermediate hard layer is represented by

chalk or chalky limestone (Zaslavsky et al., 2006). In these cases 1-D model approximates well

the experimental estimation.

The current approaches to most site response problems among engineers are to use the

1D model for simplicity, regardless of how far the soil may extend horizontally. The 1D

approach is based on the assumption that response in soil deposits is caused by the upward

propagation of shear waves from the underlying rock. We know that finite lateral extent of

surface layers introduces additional effects that tend to increase the amplitude as well as the

duration of ground motion (Aki and Larner, 1970). Many authors have studied soils with lateral

irregularities considering the particular case of alluvial valleys where the focalisation and

resonance effects are associated with wedge shaped morphology of the dipping bedrock. Bard

and Gariel (1986) demonstrated that valley edges are shown to undergo both large amplification

and large differential motion. The results obtained by Bruno et al. (1999) demonstrated that even

when the angle of the bedrock dip is 5o-10o in case of velocity Vs contrast of 0.5 between the first

two layers 1-D and 2-D theoretical models shows entirely different results.

28

Our measurements show two fundamental frequencies in the wave field of the

investigated area. First of them (f0~1.6 Hz) is probably generated by layers overlaying the broken

dolomite with thickness of ~ 100m (Stations 1-6) and other (f0~1.8-2.0 Hz) is generated by layers

with thickness 60-70m (Station 7-10). Consequently, it is quite possible that the average H/V

ratio is a result of superposition of their vibrations.

Unfortunately, now we have no algorithm to calculate two-dimensional structures.

Therefore, in the first approximation, we consider two fundamental frequencies by the Structural

Model 5 (see Table 6) with fundamental frequencies f0=1.54 and Structural Model 6 with

fundamental frequency f0=1.8 Hz and amplification factor up to 7, that is typical for site effect at

Stations 16, 9, 17 and 10 (location stations see in Figure 1). Table 7 summarizes the geotechnical

parameters for Structural Model 6. We would like to emphasize, that Models 5 and 6 use in situ

measurements of thicknesses and corresponding S-velocity of upper layers obtained at refraction

lines.

Figure 15. (a) Lithological cross section of Petah Tikva borehole and (b) comparison between average H/V spectral ratios obtained at borehole location (red line) and analytical transfer function calculated using survey data (green line).

a b

-150

-100

-50

00 500 1000

VS

TD 170m

Red loam and soil

Gravel and sandstone yellow

Limestone, chalky limestone.

Dolomite

29



Figure 16 shows the frequency response function of both models. The summation of these

models enables us to predict the generation of two close fundamental frequencies at dipping

bedrock. The optimal model that is closer to the observed function was obtained by summation of

Structural Model-4 and Structural Model-5. It is important that SM-5 is weighted with coefficient

of 0.35, which is optimized via fitting of the observations results. Figure 17 shows a comparison

of average H/V spectral ratio of microtremor recorded near experimental borehole (Stations 11)

and theoretical transfer function obtained by summation of two 1-D models.


VS m/sec

Density gr/cm3

Quality factor

1 Hamra 5 250 1.6 17



4 dolomite 10 1400 1.9 50 Half

Space Chalky

limestone ∞ 2000 2.4


VS m/sec

Density gr/cm3

Quality factor

1 Hamra 5 250 1.6 17



4 dolomite 10 1400 1.9 50 Half

Space Chalky

limestone ∞ 2000 2.4

30

a

b

Figure 16. Frequency response functions of Structural Model-4 (red line) and Model 5 (blue line): (a) Amplification factor and (b) Phase factor.

Figure 17. Average H/V spectral ratio obtained from microtremor records at Station 11 located close to experimental borehole (black line) compared with transfer function calculated using Structural Model-4 (red dashed line); Structural Model-5 (blue dashed line) and Optimal Model obtained by summation of those models (green dashed line).

31

6. DISCUSSION AND CONCLUSIONS

There are a lot of various theoretical methods for site response assessment, each involving

different assumptions and approximations. It is very important to understand that the true site

response prediction from each calculation can only be as good as the input parameters. Cramer

and Real (1990) have concluded that ”… the accuracy of the geotechnical model used to

characterize the site is more important than the particular method used to calculated the

response”. As was already mentioned subsurface model in our investigations is developed by

means of integrating information from empirical H/V spectra, available geological data and

geophysical refraction data. It should again be stated that we have used the microtremor records

(Nakamura’s method) to add information and constraints for the development of a subsurface

model rather than directly assessing the site response function.

It is obvious that site response function determination is an important stage in the overall

process of seismic hazard assessment despite the fact that the function itself has no direct

engineering application. In order to estimate the ability of buildings at a certain site to withstand

seismic activity, we need to obtain the site-specific acceleration spectrum. This design

acceleration spectrum is essentially a representation of the maximum acceleration amplitudes, for

a prescribed probability of occurrence, developed on a set of one degree of freedom oscillators

with a given damping ratio.

Since seismic activity in areas such as Israel is low, local acceleration data from strong

earthquakes is insufficient to estimate directly the design acceleration spectrum; therefore, we

must resort to the use of synthetic data. For this purpose Shapira and van Eck (1993) developed

the SEEH method (Stochastic Estimation of the Earthquake Hazard), which is based on the

generation of synthetic strong ground motions by means of stochastic simulations (e.g. Boore,

2000) of events assembled in simulated earthquake lists. These simulations adopt local

seismological characteristics such as mechanism and strength of the event, epicenter location,

mechanism and dynamic characteristics and characteristics of the propagation paths. The

ensemble of thousands of synthetic acceleration response spectra are statistically analyzed in

order to assess the spectral amplitude level that should at least once in certain exposure time with

certain probability.

32

Figure 18. Uniform hazard site-specific acceleration spectra for Ramat HaKovesh site calculated on the base of different soil-column models: Structural Model-1 (green line), Structural Model-2 (blue line), Structural Model-3 (red line) and Optimal Model obtained by summation of Structural Model-4 and Structural Model-5 1D (black line).

The seismic hazard functions, i.e., the uniform site-specific acceleration response

spectra for different structural models (Figure 18) are computed for 10% probability of

exceedance in an exposure time of 50 years and damping ratio 5%. The shape of the hazard

functions calculated by using Structural Models 1 and 2 developed using in situ measurements of

thickness and S-velocity differs significantly from Structural Model 3, in which we varied the

initial parameters (thickness, S-wave and Q) to find best fit with observed H/V spectral ratio. We

can also see a small difference between the seismic hazard functions from Structural Model-3

(best fit) and Optimal Model obtained by summation of two one-dimensional models. In

particular, our results prove that if 1-D models are used in the correct context, they can provide

satisfactory results.

Based on the analysis described in the previous sections we can conclude as follows:

• The characteristics of the H/V spectral ratios are very stable. We recorded

microtremors on different days, month and years and obtained the same

dominant frequency and practically the same amplitudes.

33

• The ground motions that generate the spectral ratios observed in this study are

small. Non-linear soil effects, especially those associated with damping and

shear modulus degradation of sand and clay, are not covered by this report.

• A one dimensional flat layer model for investigated area cannot explain broad

band frequencies where amplification is observed. Furthermore, it is possible

that because of diffraction and focalization phenomena near 2-D structures, 1-D

algorithm (e.g. SHAKE) can correctly evaluate dynamic amplification factor

only at a safe distances of several hundred meters from 2-D structure.

• Predictions of ground motion based on models inferred only from geological and

geophysical information may differ significantly from empirical estimates owing

to the geological complexity of the site and the significant uncertainty associated

with evaluating model parameters.

• Reliable estimations of the site response are obtained by combining different

empirical approaches supplemented with analytical computations where the

empirical observations, geophysical data and geological information constrain

the model parameters.

• In areas of low to moderate seismicity, microtremor measurements is the most

practical approaches for assessing the site response functions to be implemented

in earthquake seismic hazard and delineation of future locations of severe

damage.

34

7. ACNOWLEDGMENTS

This project was financed by the Steering Committee for National Earthquake

Preparedness. Installation and measurements would not have been possible without the dedicated

work of V. Giller, I. Livshits and A. Shvartsburg. We also thank Y. Menahem for their help in

preparing this report.

35

8. REFERENCES Aki, K., and Larner, K. L., 1970: Surface motion a layered medium having an irregular interface

due to incident plane SH waves. Journal of Geophysical research, 75, 933-954. Amiran, D.H.K., Arieh, E., Turcotte, T.; 1994: Earthquakes in Israel and Adjacent Areas:

Macroseismic Observations since 100 B.C.E. Israel Exploration Journal, 2, 261-305. Bard, P.-Y., and Gariel, J.-C.; 1986: The seismic response of two-dimensional sedimentary

deposits with large vertical velocity gradients. Bull. Seism. Soc. Am., 76, 343-366. Bonilla, L.F., Steidl, J.H., Lindley, G.T., Tumarkin, A.G. and Archuleta, J., 1997. Site

amplification in the San Fernando Valley, California: variability of site effect estimation using S-wave, coda and H/V method, Bull. Seism. Soc. Am., 87: 710-730.

Boore, D. M.; 1983: Stohastic simulation of high-frequency ground motions, Bull. Seism. Soc.

Am., 73: 1865-1894. Boore, D. and. Atkinson G.; 1987: Stochastic prediction of ground motionand spectral response parameters at hard- rock site in eastern North America, Bull. Seism. Soc., 77, 440-467. Boore, D. M., Joyner, W.B.; 1991: Estimation of ground motion at deep-soil sites in the eastern North America, Bull, Seism. Soc., 81, 2167-2185. Boor, D. M., 2004: Can site response be predicted? Journal of Earthquake Engineering, Vol.8,

Spesial Issue 1, 1-41. Borcherdt, R., Glassmoyer, G., Andrews, M. and Cranswick, E.; 1989: Effect of site conditions

on ground motion and damage, Earthquake spectra, Special supplement, Armenia earthquake reconnaissance report, 23-42.

Bruno, P. P. G., Di Fiore, V., Rapolla, A., and Roberti, N.; 1999: Influence of geometrical and

geophysical parameters on the seismic site amplification factor, European Journal of Environmental and Engineering geophysics, 4:51-70.

Chávez-García, F. J., and Cuenca, J.; 1998: Site effects and microzonation in Acapulco.

Earthquake Spectra, 14, 75-93. Cramer, c. H., and Real, C. R.; 1990: Turkey Flat, USA site effects test area, report 5, Weak-

motion test: statistical analysis of submitted predictions and comparison to observation, Ca. Dept. of Conservation/Div. of Mines and Geol., Tech. Report No. 91-1.

Enomoto T., Kuriyama, T., Abeki, N., Iwatate, T., Navarro, M., and Nagumo, M.; 2000: Study on

microtremor characteristics based on simultaneous measurements between basement and surface using borehole. In: Proc. of 12th World Conf. of Eart. Eng., CD-Rom, Auckland, January, 2000.

36

Field, E. H., and Jacob, K. H.; 1995: A comparison and test of various site-response estimation techniques, including three that are not reference-site dependent, Bull. Seism. Soc. Am., 85: 1127-1143.

Horike, M., Zhao, B., and Kawase, H.; 2001: Comparison of site response characteristics

inferred from microtremors and earthquake shear wave, Bull. Seism. Soc. Am., 91; 1526-1536.

Jenkins, M. G., and Watts, D. G.; 1969: Spectral analysis and its applications. Holden-Day, San Francisco, 1969, 471pp

Lermo, J. and Chávez-García, F. J.; 1994: Are microtremors useful in site response evaluation?

Bull. Seism. Soc. Am., 84, 1350-1364. McGarr, A., Celebi, M., Sembera, E., Noce, T. and Mueller, C.; 1991: Ground motion at the San

Francisco international airport from the Loma Prieta earthquake, sequence, Bull. Seism. Soc. Am., 81, 1923-1944.

Mucciarelli M., 1998. Reliability and applicability of Nakamura’s technique using microtremors:

an experimental approach, Journal of Earthquake Engineering, 4: 625-638. Mucciarelli M., and Gallipoli, M. R.; 2004: The HVSR technique from microtremor to strong

motion: empirical and statistical considerations. In: Proc: of 13th World Conference of Earthquake Engineering, CD-Rom Vancouver, August, 2004.

Mucciarelli, M., Gallipoli, M. R., and Arcieri, M.; 2003: The stability of the horizontal-to-

vertical spectral ratio of triggered noise and earthquake recording. Bull. Seism. Soc. Am., 93, 1407-1412.

Nakamura, Y., 1989. A method for dynamic characteristics estimation of subsurface using

microtremor on the ground surface. Quarterly Report of Railway Technical Research 30, 25-33.

Nakamura, Y.; 2000: Clear identification of fundamental idea of Nakamura’s technique and its

applications. In: Proc. of 12th World Conf. of Eart. Eng., CD-Rom, Auckland, January, 2000.

Parolai, S., and Galiano-Merino, J.J.; 2006: Effect of transient seismic noise on estimates on H/V spectral ratios, Bull. Seism. Soc. Am., 2006, 228-236.

Perelman, N. and Zaslavsky Y.; 2001: Analysis of seismic signals in the spectral domain

(SEISPECT), Technical Report GII, 569/345/01, The Geophysical Institute of Israel, 41pp. Pergalani, F., Pomeo, R., Luzi, L., Petrini, V., Pugliese, A., and Sano, T., 2000. Criteria for

seismic microzoning of a large area in central Italy. In: Proc. of 12th World Conf. of Eart. Eng., CD-Rom, Auckland, January, 2000.

Pratt, T. L., and BrocherT. M.; 2006: Site response and attenuation in the Puget Lowland,

Washington state, Bull. Seism. Soc. Am., 96, 536-552.

37

Satoh, T., Kawase, H., Matsushima, S.; 2001: Difference between site characteristics obtained from microtremors, S-wave, P-waves, and codas, Bull. Seism. Soc. Am., 91: 313-334.

Schnabel, P.B., Lysmer, J. and Seed, H.B.; 1972: SHAKE – A computer program for response

analysis of horizontally layered sites, Report No. EERC 72-12, Univ. of California at Berkeley.

Seekins, L. C., Wennerberg, L., Margheriti, L. and Liu, Hsi-Ping; 1996: Site amplification at fife

locations in San Francisco, California: a comparison of S waves, codas and microtremors, Bull. Seism. Soc. Am., 86: 627-635.

Shapira A., Feldman, L., Zaslavsky, Y., and Malitzky, A.; 2001: Application of a stochastic

method for the development of earthquake damage scenarios: Eilat, Israel test case. The Problems of Lithosphere Dynamics and Seismicity, Computational Seismology, 32, 58-73.

Shapira, A. and Avirav, V.; 1995: PS-SDA Operation Manual. Technical Report IPRG, The

Institute for Petroleum Research and Geophysics, Z1/567/79, 24pp. Shapira, A. and van Eck, T.; 1993: Synthetic uniform hazard site specific response spectrum,

Natural Hazard, 8: 201-205. Storn R., and Price, K., 1995: Differential evolution: A simple and efficient adaptive scheme for

global optimization over continuous spaces. Technical Report TR-95-012, International Computer Science Institute, Berkeley.

Theodulidis, N., Bard, P.Y., Archuleta, R. and Bouchon, M.; 1996: Horizontal-to-vertical

spectral ratio and geological conditions: the case of Garner valley downhole in Southern California, Bull. Seism. Soc. Am., 68: 767-779.

Toshinawa, T., Taber J. J., and Berrill, J. B; 1997: Distribution of ground motion intensity

inferred from questionnaire survey, earthquake recordings, and microtremor measurements- a case study in Christchurch, New Zealand, during 1994 Arthurs pass earthquake. Bull. Seism. Soc. Am., 87, 356-369.

Zaslavsky, Y., Shapira, A., Arzi, A.A.; 2000: Amplification effects from earthquakes and ambient

noise in Dead Sea Fault (Israel). Soil Dynamics and Earthquake Engineering, 20, 187-207.

Zaslavsky, Y., Shapira, A., and Leonov, J.; 2003: Empirical evaluation of site effects by means of

H/V spectral ratios at the locations of strong motion accelerometers in Israel. J. of Earth. Eng., 7, 655-677.

Zaslavsky, Y., Shapira, A., Gorstein, M., Kalmanovich, M., Giller, V., Perelman, N., Livshits, I.,

Giller D., and Dan, I., 2005a. Site response from ambient vibrations in the towns Lod and Ramle (Israel) and earthquake hazard assessment, Bulletin of earthquake engineering, Volume 3 No. 3, 355-381.

38

Zaslavsky, Y. Shapira, A. Gorstein, M. Aksinenko, T. Kalmanovich, M. Ataev, G. Giller, V. Perelman, N. Livshits, I. Giller, D. and Dan, H.; 2005b: Expected site amplifications in the Coastal Plane of Israel. In: Proc. of Inter. Conf. of Earth. Eng. in the 21st Century (EE-21C), Skopje, CD-Rom, August, 2005.

Zaslavsky, Y., Ataev, G., Gorstein, M., Hofstetter, A., Kalmanovich, M., Giller, V., Dan, H.,

Giller, D., Perelman, N., Aksinenko, T., Livshits, I. and Shvartsburg, A.; 2006a: Site effect and seismic hazard assessment for Petah Tiqwa, Hod Hashraon, and Haayin towns: Continuation of measurements in the Hashfela area, Report No.569/237/06.

Zaslavsky, Y., Gorstein, M., Aksinenko, T., Kalmanovich, M., Ataev, G., Giller, V., Dan, H.,

Giller, D., Perelman, N., Livshits, I and Shvartsburg, A.; 2006b: Interpretation of mocrotremor H/V ratio in multilayered media: a study at Haifa bay, Israel. In: Proc. of First European Conf. on Earthquake Engineer. and Seism., CD-Rom.

.274\251\07י "ח מג"דו. כבר סבה, ייסמית באזור רמת הכובשסקר רפרקציה ס. 2007, .מ, יזרסקי

table of contentsjoyner (1991) and other. in brief, the seeh process starts by monte carlo...

Documents