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Seismic Microzonation and Damage Assessment of Bam City, SoutheasternIranRamin Motamed a; Abbas Ghalandarzadeh b; Ikuo Tawhata a;S. H. Tabatabaei c

a Department of Civil Engineering, The University of Tokyo, Tokyo, Japan b Department of CivilEngineering, University of Tehran, Tehran, Iran c Geotechnical Department, Building and HousingResearch Center, Tehran, Iran

To cite this Article Motamed, Ramin , Ghalandarzadeh, Abbas , Tawhata, Ikuo andTabatabaei, S. H.(2007) 'SeismicMicrozonation and Damage Assessment of Bam City, Southeastern Iran', Journal of Earthquake Engineering, 11: 1, 110— 132To link to this Article: DOI: 10.1080/13632460601123164URL: http://dx.doi.org/10.1080/13632460601123164

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Journal of Earthquake Engineering, 11:110–132, 2007Copyright © A.S. Elnashai & N.N. AmbraseysISSN: 1363-2469 print / 1559-808X onlineDOI: 10.1080/13632460601123164

UEQE1363-24691559-808XJournal of Earthquake Engineering, Vol. 11, No. 1, December 2006: pp. 1–29Journal of Earthquake Engineering

Seismic Microzonation and Damage Assessment of Bam City, Southeastern Iran

Seismic Micronization, Damage Assessment of Bam, IranR. Motamed et al. RAMIN MOTAMED

Department of Civil Engineering, The University of Tokyo, Tokyo, Japan

ABBAS GHALANDARZADEH

Department of Civil Engineering, University of Tehran, Tehran, Iran

IKUO TAWHATA

Department of Civil Engineering, The University of Tokyo, Tokyo, Japan

S. H. TABATABAEI

Geotechnical Department, Building and Housing Research Center, Tehran, Iran

This article aims to study the dynamic characteristics of soil in Bam, southeastern Iran. Fundamen-tal frequency and amplification factor of soil sediment were estimated by microtremor measure-ment. This procedure was performed at 49 sites in the city. Two 120-second data were recorded ateach site. Segmental cross spectra were applied to calculate spectrum, Nakamura’s method (H/V)was used for analyzing data and fundamental frequency and amplification factor values werederived. Iso-frequency and iso-amplification maps of the city were prepared. Results show that soiltype in Bam city is mainly stiff, although amplification factor is relatively large. There is a shortperiod zone in northwest to southeast direction. Sediment depth was estimated using a correlationbetween fundamental frequency and sediment thickness. Results obtained from microtremors werecompared to the geotechnical boreholes and it was shown that microtremors can be used for arough estimation of sediment thickness. Damage distribution map of Bam due to the Bam earth-quake on December 26, 2003 was prepared by both field and aerial investigation. Quality of build-ings in Bam was evaluated, as a result a crude zoning map on the building quality was prepared.Finally, it was concluded that several factors have contributed to damage intensity variation; earth-quake characteristics, local site effect, and buildings’ quality. All mentioned parameters affected thedamage variation.

Keywords Microtremors; Fundamental Frequency; Amplification; Damage Variation; BamCity

Received 17 May 2005; accepted 29 June 2006.Address correspondence to Ramin Motamed, Geotechnical Engineering Laboratory, Department of

Civil Engineering. The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan.E-mail: [email protected]

Downloaded At: 23:47 2 May 2010

Seismic Micronization, Damage Assessment of Bam, Iran 111

1. Introduction

Safety against earthquake hazards has two aspects: firstly, structural safety againstpotentially destructive dynamic force and secondly the safety of a site itself related withgeotechnical phenomena such as amplification, landsliding, and liquefaction. In order tomitigate the risk from earthquakes, the first is accomplished by considering dynamic load-ing in the design process and the second is considered by using zoning maps based on geo-technical investigations. Several attempts have been made to identify and appraisegeotechnical hazards and to represent them in the form of maps. Generally, the outcome ofa hazard assessment is presented on a zoning map in which locations or zones with differ-ent levels of hazard potential are identified. According to the TC4 manual [TC4, 1997], thezoning map may be used in land use planning or implementation of mitigative measures.Three grades of approach for zoning are described as follows based on the level of resolu-tion. The first level of zonation (Grade-1) is the crudest and lowest-cost approach, while thethird level (Grade-3) is the most expensive and requires detailed site-specific information.Microtremor measurement is classified in Grade-2 and can be utilized to obtain informationon subsurface soil or amplification characteristics of ground motion [TC4, 1997].

A destructive earthquake hit Bam, southeastern Iran on December 26, 2003 (Figure 1).Earthquake casualties are estimated around 40,000 people, while the population of Bam

FIGURE 1 Recent seismicity map of Iran (USGS website).


112 R. Motamed et al.

was about 100,000 at the time of the earthquake in 2003. The earthquake occurred at05:26:26 local time when most of the inhabitants were sleeping. This is one of the causesof the great loss of life. The city of Bam is well known for the 2,000-year old historicalcastle of Arg-e-Bam, which was destroyed by the 2003 earthquake.

In order to understand ground response characteristics in Bam and its relation todamage distribution, microtremor measurement was utilized. Microtremor is an ambientweak vibration of the ground caused by natural or artificial disturbances such as wind, seawaves, traffic, and industrial machinery. The amplitude level of the microtremor istypically less than several microns. Seismometers of high sensitivity are used formicrotremor measurements [TC4, 1997]. The application of microtremors to determinedynamic characteristics (predominant period and amplification factor) of soil layer waspioneered by Kanai and Tanaka [1954, 1961]. Presently, microtremor measurements arefrequently practiced in site characterization due to their simplicity, low cost, and minimaldisturbance to other activities.

A technique using horizontal to vertical spectral ratios (H/V) of the microtremors,which was first employed by Nogoshi and Igarashi [1970, 1971] and popularized byNakamura [1989], has been widely used to estimate the site effects. Several recent appli-cations of this technique have proved to be effective in estimating predominant period[Field and Jacob, 1993; Ohmachi et al., 1994] as well as amplification factor [Lermo andChavez-Garcia, 1994; Konno and Ohmachi, 1995].

Several methods have been proposed for spectral calculation of ground motionsincluding microtremors. Fourier spectrum is the most convenient one that is used widely.Some research has showed that different methods provide similar results [Dimitriu et al.,1998]; however, some researchers declared that a suitable spectral method allows for morereliable results [Ghayamghamian and Kawakami, 1997]. Ghayamghamian and Kawakami[1997] investigated the aspects of spectral analysis of microtremors. They introduced seg-mental cross-spectrum, as an effective tool for canceling unknown effects like sourceeffects and noise in input and output measurements. They evaluated the performance ofsegmental cross spectrum in contrast with conventional methods, i.e., Fourier or powerspectra through mathematical modeling and numerical simulation. Results of their studiesindicated that this method gives more reliable results for both amplification factor andpredominant resonance frequency of the site than the conventional methods[Ghayamghamian and Kawakami, 1997].

2. Bam Earthquake

The magnitude of the Bam earthquake was reported 6.5 Mw (IIEES website) and the epi-center was located in the city of Bam (location: 29.004°N, 58.337°E). It is difficult toexactly determine the fault responsible for the 2003 earthquake since there was no directsurface faulting associated with this event. Some small-scale fissuring was observed in thevicinity of the Bam fault by authors (Figure 2); however, elaborated researches have beencarried out to indicate causative fault [Funning et al., 2005; Fielding et al., 2005]. Funninget al.’s 2005 paper explains the deformation pattern by the Envisat advanced syntheticaperture radar (ASAR) data. They have constructed a three-dimensional displacementfield of the deformation due to the earthquake. As a result, stated elastic dislocation mod-eling shows that the observed deformation pattern can be best explained by slip on twosubparallel faults (Figure 3). They believe 85% of moment release occurred on a previ-ously unknown strike-slip fault running into the center of Bam, with peak slip of over 2 moccurring at the depth of 5 km and the remainder occurred as a combination of strike-slipand thrusting motion on a southward extension of the previously mapped Bam fault ∼5 km



to the east [Funning et al., 2005]. The strong motion in this event was recorded at 24 sta-tions of the national Iranian strong motion network (according to Building and HousingResearch Center, BHRC). The record observed in Bam station (government office) showsthe PGA of 0.78 g for horizontal and 0.99 g for vertical components (all corrected values

FIGURE 2 Surface cracks along Bam fault.



reported by BHRC) (Figure 4). Large vertical component exhibits a vertical directivityeffect caused by a near fault effect. Acceleration response spectra are calculated to find thepeak acceleration response frequency (Figure 4). Results are detailed in Table 1. Theacceleration response spectra depict that the strong band is mainly located in short periods.Predominant frequency for the horizontal records is around 5 Hz and for vertical record is10 Hz. These results illustrate that the main energy of the Bam earthquake is located in theshort periods.

3. Microtremor Measurements

Microtremor measurements were carried out at 49 sites in Bam. In Figure 5, the distribu-tion of the measured sites is shown. The equipment used to record the microtremors was aseismometer with 0.5 Hz natural frequency by Geospace, Italy (Figure 6). The seismometer

FIGURE 3 Location of Bam fault zone compared to Bam and Baravat (modified satelliteimage of Iranian Remote Sensing Center based on Funning et al., 2005).



comprises three components: two horizontal motions (in longitudinal and transverse direc-tions) and one vertical motion. Data were recorded for 120 sec with the sampling fre-quency of 100 Hz. This process was repeated two times in each site. During observation,special emphasis was given to minimize artificial noises due to traffic and human activi-ties by informing local people of the measurement procedure. Figure 7 shows a typicalrecord at site MT12.

To process microtremor data, a computer program was developed in MATLAB, usingthe Signal Processing Toolbox. In order to find the best processing method, the authorsconsidered the processing technique of microtremor and the findings show that a timewindow length of 20 sec, with an overlapping value of 10% and a Hamming window, arethe best choices in the processing of microtremor data. In this case, the obtained predomi-nant period and amplification factor are more consistent [Motamed et al., 2002].

FIGURE 4 Acceleration records and response spectra of data in Bam station.

15 20 25 30 35-1

0

1

T Component

V Component

Time (sec)

)g( LccA

L Component

-1

0

1

)g( VccA

-1

0

1)g( Tcc

A

0 1 2 3 4

0

2

4

L Component

Period (sec)

)g( noi tare le cc A e snop seR

0

2

4

V Component

)g ( noi tar elec cA es nopse

R

0

2

4

)g( noitarel eccA esn op se

R

T Component

TABLE 1 Characteristics of acceleration data recorded in Bam station

ComponentPeak

Acc. (g)Peak

Vel. (cm/s)Peak

Disp. (cm)Peak ResponseAcc. (g) (5%)

Predominant Frequency (Hz)

T 0.636 59.68 20.80 2.47 4.54V 0.999 39.63 8.62 3.65 10L 0.781 123.17 34.50 2.79 5



4. Seismic Microzonation Maps

Recorded data at each site were analyzed using H/V technique and segmental cross spec-trum was applied for spectral calculation. This technique can detect and recover the signal(here the site response) buried in extraneous noise. In this method, two-sided cross-spectralfunctions of two different segments of the recorded microtremors are used. The basic phys-ical concept behind this definition is that in microtremors the unknown effects (differencein source and/or noise) vary in the whole length of the record. Consequently, segmentalcross correlation of two different segments of the records compensates for the randomnoise and source effects for accurate evaluation of the site response by microtremors.

In Figure 8, for example, some H/V spectral ratio curves are shown. Two horizontalH/V curves, N-S and E-W directions, are averaged and final results are determined fromthe mean curves. Fundamental frequency and amplification factor values at each site areestimated, iso-frequency and iso-amplification maps of Bam city are prepared. These twomicrozonation maps are presented in Figures 9 and 10.

Fundamental frequency ranges from 1.36 to 10.53 Hz, and amplification factorranges between 1.33 and 4.77. These findings indicate that soil type in Bam city ismainly stiff, and amplification factor is relatively large. There is a high frequency zonefrom the north and northwest to the southeast of Bam where amplification factor trendsto large values. However, in west and southwest of the city, fundamental frequency islow and amplification factor exhibits smaller values compared to other parts of the city.

FIGURE 5 Microtremor measurement sites in Bam.



As explained in Sec. 2, recorded ground motion in the earthquake is a short period;therefore, coincidence of the frequency between shaking and soil is one reason for thehigh damage. As a result, this high frequency zone was more vulnerable to the 2003earthquake.

5. Sediment Thickness Related with H/V Ratio

Recently, several studies [Yamanaka et al., 1994; Ibs-von Seht and Wohlenberg, 1999;Delgado et al., 2000; Parolai et al., 2002] showed that microtremor measurements can beused to map the thickness of sediment. Quantitative relationships between this thicknessand the fundamental frequency of the sediment, as determined from the maximum of theH/V spectral ratio of microtremors [Nakamura, 1989], were calculated for both the SeguraRiver valley (Spain) [Delgado et al., 2000], and Lower Rhine Embayment (Germany)[Ibs-von Seht and Wohlenberg, 1999].

Ibs-von Seht and Wohlenberg [1999] showed that the frequency of resonance (fr) of asoil layer is closely related to its thickness (h) through Eq. (5.1).

Parolai et al. [2002] derived Eq. (5.2) based on the microtremor measurements whichwere carried out in the Cologne area in Germany.

where h is in meters and fr is in Hz.

FIGURE 6 Microtremor measurement in Bam.

h afrb= (5.1)

h fr= −108 1 551. (5.2)



Based on a research project carried out by first two authors of this paper in Urmia city(Northwestern Iran), a new equation was obtained. The databases used to develop newcorrelation were 200 sites microtremor measurements, 4 geotechnical boreholes, and 25geophysical investigation sites (refraction method). Using the estimation of the fr [Motamedand Ghalandarzadeh, 2004] and the thickness of the sediment from boreholes and geophys-ical data, a nonlinear regression analysis was performed and the following equation wasoffered for Urmia city:

where h is in meters and fr is in Hz. Figure 11 shows a comparison between this equationand Eq. (5.2) in which the definition of sediment is the soil layer with shear wave velocitysmaller than 750 m/s. Sediment thickness in Bam city is estimated using Eq. (5.3) and isshown in Figure 12. Soil thickness increases in the west and southwest, while is less than7.54 m in most areas. During the investigation, authors observed rock outcrop in the north-east where Arg-e-Bam exists (Figure 13), and iso-depth map sediment coincides well withthis evidence. Reliability of Eq. (5.3) is investigated in the next section in which threeborehole data are presented.

FIGURE 7 Microtremor record at site MT12.

0 20 40 60 80 100 120-0.00005-0.00004-0.00003-0.00002-0.000010.000000.000010.000020.000030.00004 E-W Component

Time (sec)

-0.00008

-0.00006-0.00004

-0.000020.00000

0.00002

0.00004 N-S Component

-0.00008-0.00006-0.00004-0.000020.000000.000020.000040.00006

Velo

city

(cm

/s)

Velo

city

(cm

/s)

Velo

city

(cm

/s) Vertical Component

h fr= −135 19 1 9791. . (5.3)



FIGURE 8 H/V spectral ratio curves.

0.1 1 10

0.1 1 10

1

MT46H

/V S

pect

ral R

atio

H/V

Spe

ctra

l Rat

io

H/V

Spe

ctra

l Rat

io

H/V

Spe

ctra

l Rat

io

Frequency (Hz)0.1 1 10

0.1 1 10

1

2

3

4

56

MT28

Frequency (Hz)

1

2

3

4

MT23

Frequency (Hz)

1

2

3

4

MT15

Frequency (Hz)

FIGURE 9 Iso-frequency map of Bam.



FIGURE 10 Iso-amplification map of Bam.

FIGURE 11 Sediment (Vs < 750 m/s) thickness vs. resonance frequency by microtremors.



FIGURE 12 Sediment (Vs < 750 m/s) depth in Bam calculated from microtremor mea-surements.

FIGURE 13 Rock outcrop in Arg-e-Bam, northeastern Bam.



6. Surface Geology in Bam City

Surface deposits in the area of Bam generally consist of alluvial deposits from major sea-sonal flooding over time, and the thickness ranges from less than a few meters to about 100 mdepending on the location. In the northeast of the city a rock outcrop is visible whereArg-e-Bam is located (Figure 13). The underlying layers consist of recent quaternaryalluvium, late quaternary sandstone, sedimentary and volcanic rocks. The seasonal river-bed (Posht-Rood River), mostly dry, is in the northern part of the city. Due to heavy use ofdeep water wells, the groundwater table is low (at depths in excess of 30 m) [EERI, 2004].

In this section, soil condition in Bam City is investigated based on the three geotech-nical boreholes which were logged in the city. In Figure 5, their positions are shown. Theboring machine was D750 and the depth of each borehole was around 30 m. Both dis-turbed and undisturbed samples were examined to determine soil characteristics at thegeotechnical engineering laboratory, Building and Housing Research Center, Iran. SPTwas performed every 1.5 m depth in every borehole according to the ASTM D1586, andsoil profiles and SPT blow counts are shown in Figure 14 and Table 2, respectively.

6.1 Borehole 1

This borehole is located at the government office of Bam where the accelerometerrecorded high values (Figure 4). Depth of the borehole was 31.5 m and no undergroundwater table was observed. Soil type was organic by 2 m depth and then silty sand wasobserved including brown gravel by 3 m. Then soil type was almost SP/SM with graycolor by the lowest depth. Eight disturbed samples were taken.

6.2 Borehole 2

This borehole is situated in the southern part of the city. Depth of the borehole is 30 mwhich by 0.3 m depth includes organic soil. Soil layers consist silty sand with gray tobrown color by depth 3 m and then changed to sand, silty sand, gravel and silty gravel by25 m depth. The borehole reaches coarse grain at the end.

6.3 Borehole 3

This borehole is located in the municipal office of Bam with 30 m depth. Soil was agricul-tural by 3 m depth. Between 3 to 8 m, soil type was clay and then gravel and silty gravelby 11 m. Below 11 m soil was silty sand.

6.4 Comparison of Geotechnical Boreholes with Microtremor Results

In order to compare the results estimated from geotechnical boreholes to microtremors,the following procedures are followed. Sediment thickness is defined as soil layer withshear wave velocity smaller than 750 m/s. Therefore, it is necessary to calculate the depthof Vs = 750 m/s in the boreholes. Shear wave velocity in boreholes were determined fromEq. 6.1. Then, by using a linear correlation, depth of Vs = 750 m/s was calculated. Shearwave velocity in boreholes were calculated using the relationship between shear wavevelocity and SPT values. In this study, Baziar’s relation (Baziar et al., 1998) that is basedon Iranian data, was used to calculate shear wave velocity:

V N DS = 134 0 2 0 4. . (6.1)



where Vs is the shear wave velocity in m/s, N is the SPT blow count, and D is depth inmeters. In Table 3 and Figure 15, results are shown. In two boreholes, BH1 and BH3,microtremor method has estimated sediment thickness well, while in case BH2, overesti-mated. Based on the Figure 15, it is concluded that microtremor measurement can be uti-lized for a rough estimation of sediment depth.

FIGURE 14 Soil profiles in boreholes.


124

TA

BL

E 2

SPT

blo

w c

ount

s in

bor

ehol

es

BH

1B

H2

BH

3

Blo

ws

Blo

ws

Blo

ws

Dep

th (

m)

15 c

m15

cm

15 c

mD

epth

(m

)15

cm

15 c

m15

cm

Dep

th (

m)

15 c

m15

cm

15 c

m

1.5

44

71.

518

3543

1.5

79

133

67

113

3550

/12

35

78

4.5

629

244.

550

/3–

4.5

2431

50/1

46

3425

396

4550

/56

2238

327.

521

3350

/13

7.5

950

/87.

534

50/1

3–

946

50/1

2–

950

/14

–9

3744

4710

.525

50/1

2–

10.5

4250

/510

.550

/11

––

1230

50/1

2–

1240

/15

50/1

212

50/1

0–

–13

.538

4450

/14

13.5

50/1

1–

13.5

50/1

4–

–15

2650

/14

–15

50/1

5–

1550

/13

––

16.5

3746

5016

.550

/8–

16.5

50/9

––

1850

/10

––

1850

/9–

1850

/5–

–19

.550

/12

––

19.5

50/1

1–

19.5

50/6

––

2150

/521

50/1

1–

2150

/522

.532

50/1

2–

22.5

50/9

–22

.550

/8–

–24

2035

50/1

324

50/8

–24

50/5

––

24.7

50/8

25.5

50/9

–25

.75

50/7

––

25.5

50/1

0–

–27

50/7

–27

50/6

––

26.8

50/1

3–

–28

.5–

–28

.550

/6–

–27

50/5

––

30–

–30

50/4

––

28.5

50/6

SPT

> 5

0 is

Ref

used

3050

/5



6.5 Laboratory Tests

In Bam, soil is mainly sand and gravel. Grading test was performed according toASTM D422-87 and ASTM D422-63 for hydrometer tests. Based on the gradingcurves (Figure 16), most of the soil samples are poorly graded. Atterberg limit testwere performed to find liquid limit and plasticity index of soil layers. The results areshown in Table 4.

7. Damage Assessment of the Bam Earthquake

The authors visited the damaged area two times after the Bam earthquake of 2003. It wasobserved that damage intensity is low in some parts of the city, whereas some partssustained high damage. Figure 17 shows a low damaged zone in the west part of city andFigure 18 depicts a high damaged zone at the central part of Bam City. These sites areshown in Figure 19. The damage distribution map of the Bam earthquake was prepared byboth field and aerial investigation by helicopter and is shown in Figure 19.

To describe the variation in damage, several factors should be considered. In Sec. 2, itwas shown that predominant frequencies of the recorded acceleration in Bam city arearound 5 Hz and 10 Hz for horizontal and vertical components, respectively. Hence,recorded ground motion is of short period type. Zonation maps which were presented in

TABLE 3 Comparison of sediment depth from microtremors and boreholes

Geotechnical Borehole Microtremor

BoreholeSPT below

counts Corresponding VsSediment thickness

Sediment thickness

BH1 64 at 6 m 527 8.53 6BH2 78 at 1.5 m 361 3.11 15.0BH3 91 at 9 m 638 10.57 11.0

FIGURE 15 Comparison of sediment depth from microtremors and boreholes.

0 2 4 6 8 10 12 14 16 18 200

2

4

6

8

10

12

14

16

18

20

BH2

BH3

by M

icrot

rem

ors

(m)

by Geotechnical Boreholes (m)

Depth of Sediment(Vs<750 m/s)BH1



FIGURE 16 Grain size distribution curves.

0.01 0.1 1 10 1000

10

20

30

40

50

60

70

80

90

100

110

)thgiew( egatnecre

P gnissaP

Grain Size Distribution (mm)

Sample Depth 2 m 4 m 6 m 8 m 16 m 19 m 23 m

Borehole 1

0.1 1 10 100

0

10

20

30

40

50

60

70

80

90

100

110

)thgiew( egatnecre

P gnissaP


Sample Depth 3 m 7.5 m 8 m 18.5 m 26.5 m

Borehole 2

0.1 1 10 1000

10

20

30

40

50

60

70

80

90

100

110

)thgiew( egatn ecr eP gnis sa P


Sample Depth 3 m 8 m 11 m 19.5 m

Borehole 3



Sec. 4, depicted that fundamental frequency of soil is varied from 1.36 to 10.53 Hz, andthere is a short period zone which spreads from north and northwest to southeast. This areahas suffered dramatically for which one reason is the coincidence between fundamental fre-quency of shaking and soil. Sediment thickness was consequently estimated in Sec. 5 usingEq. 5.3. Comparing Figures 12 and 19, shows a rough correlation between sediment thick-ness and damage intensity, where sites located on thin soil suffered more; however, it is dif-ficult to correlate sediment thickness vs. damage, because sediment thickness is estimatedfrom fundamental frequency (Eq. 5.3). So, more geotechnical investigation is needed.

It is a difficult task to prepare a zoning map for buildings’ quality after an earthquakeevent due to the complete collapse of more than 50% of the buildings; however, damageinvestigation was carried out by both field and aerial surveys two times and rough classifi-cation of buildings’ quality was performed. Buildings’ quality attributed to the construc-tion method, material type and age. Buildings were mainly adobe and masonry, andmaterials were brick and clay. Generally speaking, buildings’ quality was weak and brit-tle. As a result, buildings’ natural period was short that coincided with ground motion andsoil layer. In Sec. 7, it is concluded that structure quality is also an important factor whichcontributed to damage intensity. Zone 2, which represents the area of poor quality struc-tures, suffered drastically compared to Zone 1 where construction of structures followedsome engineering methods. It is to be noted that the presented quality zoning map is acrude classification.

8. Conclusions

Two seismic microzonation maps of iso-frequency and iso-amplification factor were pre-pared from this study. Regarding iso-frequency map, it is understood that soil type in Bamis mainly stiff, i.e. shear wave velocity reaches 750 m/s in the depth of 7.5 m in mostareas. However, amplification factor map presents relatively large values. Sediment depthwas estimated through microtremor results and compared to three boreholes that logged inthe city. Damage distribution map was prepared through both field and aerial surveys. Thefollowing conclusions are drawn from this study:

• Acceleration response spectra of the recorded data in Bam show that ground shakingis of short period type.

• Iso-frequency map shows that there is zone of short period from north and north-west to southeast, while predominant period increases gradually at west and south-west of city.

TABLE 4 Atterberg limits of Boreholes

Borehole Depth of Sampling (m) Percent passing sieve #200 LL PL PI

BH1 2 48.54 19.8 13.69 6.21BH1 4 17.4 21 11.65 9.35BH1 19 18.97 16.5 14.27 2.23BH1 23 14.2 18 14.96 3.04BH2 3-4 15.3 17 12.85 4.15BH2 18.5 36.87 12.7 10.95 1.75BH2 26.5 16.7 15.5 12.41 3.09BH3 3 70.71 27.4 13.09 14.31BH3 11 30.11 18.5 13.11 5.39



FIGURE 17 Low damaged zone of Bam.



FIGURE 18 High damaged zone of Bam.



• Sediment thickness map calculated from microtremors, illustrates that surface soillayer is thin in the northwest-southeast direction and is thick in west and southwestparts. The validity of the results was checked with the three geotechnical boreholes,and it was shown that microtremor can be a measure for crude estimation of sedi-ment thickness.

• Bam was classified roughly based on the buildings quality into two zones. Zone 1stands for newly constructed parts and zone 2 for old adobe structures. Buildingsmainly were brittle with a short natural period.

• Damage distribution map of city was prepared based on the field and aerialinvestigations.

• Several factors were considered to understand damage intensity variation; earth-quake characteristics, local site effect, and buildings quality. All of the mentionedparameters contributed to the damage variation.

The findings should be considered somewhat provisional at this stage, because limitedborehole data was available. Finally, it is concluded that microtremor measurement can beused as a fast, cheap, and preliminary tool for microzonation studies.

Acknowledgments

Site investigation and microtremor measurements were carried out as a part of the work ofthe reconnaissance team that visited the damaged areas from January 7–11, 2004. Theauthors greatly appreciate the funding by the University of Tehran which enabled thereconnaissance. They are also grateful for the provision of the equipment from SAHELConsultant Engineers and thank Mr. A. Sadrekarimi for his valuable cooperation on themicrotremor measurements, Mr. A. Radjabi for his help with the measuring devices, Mr.V. Joekar for helping to prepare maps in GIS software, and Mr. J. P. Richard for his will-ingness in revising paper in English respect. Satellite image of Bam city was provided bythe Iranian Remote Sensing center which is highly acknowledged. Travel expense of thethird author was paid by the Ministry of Education, Culture, Sports, Science and Technol-ogy of the Japanese Government. This support is deeply appreciated.

FIGURE 19 Damage distribution of Bam together with the zoning of buildings’ quality.



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