thThe 14 World Conference on Earthquake Engineering
October 12-17, 2008, Beijing, China

THE CHANGE OF THE DYNAMIC CHARACTERISTICS
USING MICROTREMOR
1

Tsutomu Sato , Yutaka Nakamura

1, 2, Jun Saita

1

1

2

System and Data Research Co., Ltd., Tokyo, Japan
Visiting Professor, Dept. of Built Environment, Tokyo Institute of Technology, Japan
Email:tsato@sdr.co.jp, yutaka@sdr.co.jp, Jun@sdr.co.jp

ABSTRACT :Considering the geotectonic environment and the historical importance, it is necessary to investigate thedynamic characteristics of the historical constructions in the city for rational and effective countermeasuresagainst earthquake disasters. Since autumn 1998, microtremor measurements had been started to investigate thedynamic characteristics of several structures in Istanbul. In 17th August 1999 the Kocaeli Earthquake (Mw7.6,
maximum PGA for rock site in Istanbul was 41 Gal) occurred and caused severe damage in the epicentral area.
Microtremor measurements were conducted for few structures including historical monuments such as
Suleymaniye mosque, Hagia Sophia museum, Sehzade mosque, and a newly constructed 14 stories officebuilding, before the earthquake. In order to assess the earthquake’s aftermath on the measured structures in
Istanbul, microtremor measurement was again conducted. The result of the measurement before and after theearthquake shows the change of natural frequencies decreasing after the earthquake up to about 8.9% for Hagia
Sophia Museum.
The avKb value, averaged Kb value was proposed by Nakamura et al. (2000) as vulnerability index for buildings.
Because the averaged drift angle of the buildings can be estimated by multiplying the avKb value and the
maximum input earthquake acceleration, the avKb value was proposed to assess the possibility of building
damage. The avKb values before the Kocaeli earthquake are corresponds to the change of natural frequencies.
Thus, it is expected that the danger of a structure can be assessed by the avKb in advance.
KEYWORDS:
microtremor, dynamic characteristic, vulnerability index, historical structure, damage
1. INTRODUCTION
During its long history, Istanbul has been damaged by many earthquakes. Considering the geotectonic
environment and historical importance of Istanbul, it is necessary to investigate the dynamic characteristics ofthe historical structures in the city and to work out rational and effective countermeasures to earthquake
disasters. Since the autumn of 1998, microtremor measurements have been carried out to investigate thedynamic characteristics of several structures in Istanbul.
In 17th August 1999 the Kocaeli Earthquake (Mw=7.6) occurred and caused severe damage in the epicentral
area. Although Istanbul is approximately 100km away from the epicenter and did not suffered serious damage,this earthquake might have affected some old structures. With this concern, the structures investigated before
the earthquake was re-investigated in September 1999. The impact of the Kocaeli earthquake was analyzed,
focusing on the shift of the natural frequency of structures. In Istanbul, although only 41Gal in PGA wasobserved at YPK station (Yapi Kredi Plaza strong motion station of Kandilli observatory), comparison of theresults of an investigation before and after the earthquake has shown that the natural frequency shifted lower by
several percent for both old and new structures. This frequency shift corresponds to the vulnerability index
against the earthquake disaster for structures proposed by Nakamura et al.(2000).

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The 14 World Conference on Earthquake Engineering
October 12-17, 2008, Beijing, China
2. OUTLINE OF MEASURED STRUCTURES
Microtremor measurement was conducted for historical structures as Suleymaniye mosque, Hagia Sophia
museum, Sehzade mosque, and newly constructed office building. Figure 1 shows the overview of thesestructures. Figures 2 to 5 show the floor plan and elevation with measurement points, and Table 1 shows thedetail of the structures such as height, structural type, established year and the date of measurement.
Measured historical structures have a main dome supported by four arches on four big piers. In case of
Suleymaniye mosque and Hagia Sophia museum, a pair of opposite arches are reinforced by two semi-domes.
In case of Hagia Sophia museum with a main dome of diameter of 31 m, the spaces beneath the arches are filled
as a wall and the dome appears as though it is supported by the walls instead of arches. The internal space is
30 m width x 80 m length. The main dome has been partially affected by earthquakes several times and wasrestored each time. On the whole, the structure has deteriorated considerably and a certain countermeasure has
been required.
The four arches of Sehzade mosque supporting main dome are reinforced by four semi domes. Sehzademosque is an etude of an architect Sinan (1490-1588), and he adopted the structure for the main dome to besupported by four semi domes. And also the Suleymaniye mosque is one of the highest masterpieces of his
maturity.
The basement grounds for historical structures mentioned above are firm and are in good condition. An Office
Building made by Reinforced Concrete (RC) structure is a 14-story building with 5 underground levels and will
be used for shopping centers and business offices. Each floor has the 57 m x 16 m rectangular section, and in
the central part of the south side, facilities of utility such as elevators, stairs and etc. are equipped. Thisbuilding was constructed on the firm ground, and is situated near the YPK station, which recorded maximum
acceleration 41Gal at the time of the Kocaeli earthquake.

(a) Suleymaniye Mosque(b) Hagia Sophia Museum(c) Sehzade Mosque(d) An Office Building

Figure 1. Overview of Measured Structuresth

The 14 World Conference on Earthquake Engineering
October 12-17, 2008, Beijing, China

EHT

WHT

■

●

●

■

■

●

●

■

■

●

●

●

N

●

○

● ○
■

■
■

●
○■

N NE ESE ○S ■○W SW ○
●■
●
○ NW

○

HT
HL
○

Dome

■ 2nd Floor●●●

1st Floor or Ground●Figure 2. The elevation and floor planwith measurement points in Suleymaniyemosque. (○,■,●:Measurement points)

Figure 3. The elevation and floor planwith measurement points in Hagia Sophia
museum. (○,●,▲,■:Measurement points)

NT or ST
14F
11F
8F

57(m)

5F
2F

1F

HL

NE
E
N
NW SE
WSW S

HT

HL

N

5F-14FW

HT

N●

1st Floor or Ground○

Name of building
HeightDimensions of a plane
Diameter of main dome
Structural type
Completion year
Date of measurement
before the earthquake
Date of measurement
after the earthquake
Measured floors

S

E
(2F)

(1F)

16(m)

55(m)

Dome

Figure 4. The elevation and floor plan withmeasurement points in Sehzade mosque.
(○,●:Measurement points)
Table 1.

N

Figure 5. The elevation and floor plan withmeasurement points in a newly constructed
office building. (●:Measurement points)

Dimensions, Completion year and etc. of Measured Structures

Suleymaniye Mosque Hagia Sophia Museum

53 m
68 m × 63 m
27 m
Stone
1550AD
19th and 21st
October, 1998
7th September, 1999
1F, 2F, DM

56 m
95 m × 70 m
31 m
Stone and Brick
512AD
8th and 9th June,1999
3rd and 4th
September, 1999
1F, 2F, 3F, DM

Sehzade Mosque
37 m
50 m × 43 m
18 m
Stone
1543ADan Office Building
57 m
57 m × 16 m
RC(14F+5BF)
1999AD

6th June, 1999

11th June, 1999

10th September,
1999
1F, DM

2nd September,
1999
1F, 2F, 5F, 8F, 11F, 14F

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The 14 World Conference on Earthquake Engineering
October 12-17, 2008, Beijing, China
3. OUTLINE OF MEASUREMENT AND ANALYSIS
Microtremor measurement was simultaneously conducted at one or two points of whole points on each floor
shown in Table 1. Measurement points were set to grasp the dynamic behavior of these structures.
About minaret, two minarets of both Suleymaniye and Sehzade mosque were measured. Suleymaniye mosque
has two high minarets and two low minarets. High minarets (WHT and EHT) were measured at the highest
and lowest balcony of three balconies. Every balcony has four measurement points and a pair of them wasmeasured simultaneously. On the EHT minaret, however, only s-side point of highest balcony was measured
because of the failure of instrument at the time of before the earthquake.
Sehzade mosque has two minarets with two balconies each other. Measurement was conducted at fourmeasurement points only on the upper balcony. Moreover, for the base ground, measurement points weresituated around the structure.
A velocity sensor for microtremor measurement is a geophone GS-11D of GeoSpace. Although the natural
frequency of the sensor is 4.5 Hz originally, it is 1 second in this time because of astatization. A recorder is
PIC91 of SDR. This instrument has a 16bit A/D converter, and it can connect 2 sets of three components ofthe modified GS-11D. The characteristic of the sensor is adjusted so that three axes may be assembled, and thespectrum ratio can be used effectively in the frequency range between 0.3 Hz and 30 Hz.
At every measurement point, microtremor was recorded three times with 1/100 second sampling rate and 40.96
second data length. After measurements, Fourier spectrum for each component is calculated from the
waveform of whole length and three records are averaged. Fourier spectrums are smoothed by Hanningspectral window repeated twenty times as bandwidth to be approximately 0.2Hz before averaging.
For representative spectra of the historical structures, these calculated spectra related to the four supportingcolumns for each floor and these calculated spectra for eight points on the corridor of main dome (DM) for
every component are averaged. And these spectra of minarets are averaged for representative spectra. In
case of an office building, every three floor has four measurement points at the center of the each side of thefloor, and these spectra are averaged for representative spectra.
The transfer function of structures and minarets shall be presumed by the ratio of a representation spectrum
between the top floor and the ground floor. In addition, the ground floor is set to first floor (1F).

An

It is considered that the vulnerability of structures against
earthquake disasters can be estimated by the drift angle, related
to the input earthquake acceleration a in cm/s2. Here, a is aportion which affects this structure among whole earthquake
motion a, namely,

Ai+1

α = e× a(4.1)

where e shows the efficiency of earthquake motion working
for this structure.
A deformation performance and the degree of earthquake
motion amplification can be estimated from the dynamic
characteristic of structures.
Here, the primary natural

δnn-th Floor

4. VULNERABILITY INDEX AGAINST EARTHQUAKEDISASTERS FOR STRUCTURES (Nakamura et al., 2000)(i+1)-th
i-th

Ai

1st

A1

a

H

δi+1
δi

hi

δ1

Figure 6. Schematic model of n-th floor
structures and its mode shape. δi is thehorizontal displacement, hi is the height, Ai is
amplification factor of i-th story column, H isthe height of the n-th floor structure, and a isthe horizontal acceleration of foundation
ground.

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The 14 World Conference on Earthquake Engineering
October 12-17, 2008, Beijing, China
frequency of the structure that seems to have influence on earthquake damage is considered. Displacement δiof i-th floor is estimated from this primary natural frequency F and amplitude Ai of i-th floor as followings (Seefigure 6).

δi = Ai × α /( 2πF ) 2(4.2)

So, the drift angle γi of i-th floor is shown as,

γ i = (δ i +1 − δ i ) / hi
= ∆ Ai × α /( 2πF ) 2 / hi
= e × K bi × a

(4.3)
(4.4)
(4.5)

where,

K bi = ∆Ai /( 2πF ) 2 / hi × 10000(4.6)∆Ai : difference of amplification of the i-th floor, (=Ai+1-Ai ), andhi : the height of i-th floor in m.
Thus, the drift angle γi for each floor is estimate from vulnerability index Kbi multiplied by the maximum
acceleration on the surface ground a in cm/s2 and the efficiency e of earthquake motion.
Here, avKb value is derived as averaged Kbi for each structure for the discussion followings.
av

K b = A /( 2πF ) 2 / H × 10000

where,
A : amplitude of the top floor, andH : height of the structure in m.
In addition, when avKb is substituted for Kbi of formula (4.5), averaged drift angle γav will be calculated.
-6
avKb are expressed in unit of 10 , 10000 in Eqs. (4.6) or (4.7) is multiplied for adjustment.

(4.7)

Kbi and

5. RESULT OF ANALYSIS
Figure 7 shows the velocity locus of microtremor at every measurement point on Sehzade mosque.

HL

It shows

UD

Figure 7. Normalized Loci of Microtremors in HL-UD Plane for 40.96 second at each Measurement
Points of Sehzade Mosque (non-simultaneous measurement). Dots in circle indicate the normalized
locus, and the number near the circle means maximum value of microtremor in unit of 10-8 m/s.

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The 14 World Conference on Earthquake Engineering
October 12-17, 2008, Beijing, China
the locus of microtremor for 40.96 seconds in the circle to the position corresponding to every measurement
point, and the number near the circle means maximum amplitude in 10-8 m/s. This figure shows that minarets
are vibrating more intensely than the structures. The state of the vibration for main dome can also be grasped
by this figure.
Figure 8 shows the transfer functions of four structures before and after the earthquake estimated from the resultof microtremor measurement. Figure 9 shows examples of the transfer functions for minarets.
In case of Suleymaniye mosque, the peak frequency near
0.8 Hz is significant on the corridor of the main dome (DM)
only before the earthquake. And in case of Sehzademosque, the peak amplification factor near 1.2 Hz
increased after the earthquake. These peak frequencies
may correspond to the peak frequencies of minarets.
Tables 2 to 5 shows the shift of the natural frequencies andamplification factor of the estimated transfer functions
before and after the earthquake for both horizontal
components and torsion component, except the peakfrequencies mentioned above. Measurement before theearthquake of a Suleymaniye mosque was performed in
two days on alternate days. The day WHT minaret wasmeasured was not windy and was a quiet day. However,the day DM and EHT minaret was measured was windy.
So the amplitude of EHT minaret was ten times or morecompared with that of WHT minaret. Therefore, neither
0.8 Hz vibration of Suleymaniye mosque nor 1.2Hz
vibration of Sehzade mosque was transmitted from thefoundation; it is speculated that minarets were excited by
the wind and then the vibration was transmitted to themain dome.

before the earthquake

1

0.1

1
Frequency in Hz

0.1

100

10

Spectral Ratio

Spectral Ratio

11Frequency in Hz

10

1000

Hagia Sophia EW(HL)

1

Hagia Sophia NS
(HT)

100

10

1

100

1
Frequency in Hz

10

Sehzade Mosque NE-SW
(HL)

10
1

0.1
100
Spectral Ratio

0.1

Spectral Ratio

10

10

1000

0.1

1
Frequency in Hz

10

Sehzade Mosque EN-WS
(HT)

10
1

0.1
0.1

1
Frequency in Hz

10

0.1

1
Frequency in Hz

10

1000

An Office Building EW
(HL)
100

10

Spectral Ratio

1000

Spectral Ratio

Suleymaniye Mosue
EN-WS
(HT)

0.1

0.1

Tables 2 to 5 shows the natural frequencies estimated fromthe transfer function of structures for horizontal andtorsional vibrations. From these table, the vibration thatis assumed to be affected by the vibration of minarets are
ignored.
Additionally, these tables include theamplification factor.

1

An Office Building NS
(HT)
100

10

1

0.1

1
Frequency in Hz

10

0.1

1
Frequency in Hz

10

Figure 8. Estimated transfer functions of thehighest floor of structures.
10000

10000

1000

1000
Spectral Ratio

Spectral Ratio

Because the data length of microtemor measurement is
40.96 seconds, the frequency resolution ∆F on Fourier
analysis become 0.025 (=1/40.96) Hz. In Tables 2 to 5,
the shift of natural frequency is also shown in a unit of
frequency resolution on analysis. According to Tables 2
to 5, there are some cases that the shift of frequencies are
smaller than the resolution. The natural frequency of apeak is estimated using the data of four samples aroundpeak. Therefore, the reading error should be less than ahalf of the resolution. Moreover, a spectrum is computedby averaging 12 times of measurement data(corresponding to 8 minutes approximately) or 24 times ofmeasurement data (corresponding to 16 minutes
approximately), and the reliability appear to be satisfactory.
Thus, if the shift of frequency is below the resolution, this

Suleymaniye Mosque
NW-SE
(HL)

10

after the earthquake
100
Spectral Ratio

Spectral Ratio

100

100

10

100

10

Suleymanie WHT
NE-SW (HT)Sehzade NT minaret
NE-SW (HT)

1

1
0.1

1
Frequency in Hz

10

0.1

1
Frequency in Hz

Figure 9. Examples of Estimated Transfer
Function of Minaret

10

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The 14 World Conference on Earthquake Engineering
October 12-17, 2008, Beijing, China

Table 2. The Dynamic Characteristics ofSuleymaniye mosque before and after the1999 Kocaeli earthquake
Main body
HL: NW-SE

Before After dF/F (%)
3.43
3.36
2.21
20.5
20.2
11.0
11.4

Table 3. The Dynamic Characteristics ofHagia Sophia museum
before and after
the 1999 Kocaeli earthquake

dF
3.11

Main body
HL: EW

4.04

HT: NS

After dF/F (%)
1.75
8.92
60.5
122

dF
7.00

2.01
59.6
90.5

6.82

6.04

Torsion
F(Hz)
6.69
6.45
3.59
9.83
Minalet WHT
Before After dF/F (%) dF
HL: NW-SE
F(Hz)0.843 0.819
2.86 0.987
HT: NE-SW
F(Hz)0.867 0.842
2.79 0.991
Torsion
F(Hz)
9.69
9.59
1.03
4.10
Minalet EWT
Before After dF/F (%) dF
HL: NW-SE
F(Hz)0.842 0.842-0.01 -0.003
HT: NE-SW
F(Hz)0.842 0.842
0.08 0.028
Torsion
F(Hz)
9.36
9.33
0.32
1.23
Note: dF is frequency shift in unit ∆F, ∆F = 1/40.96 Hz

Torsion
F(Hz)
3.32
3.10
6.63
Note: dF is frequency shift in unit ∆F, ∆F = 1/40.96 Hz

9.01

Table 4. The Dynamic Characteristics of
Sehzade mosquein before and after of the1999 Kocaeli earthquake

Table 5. The Dynamic Characteristics of An
Office BLDG
in before and after of the1999 Kocaeli earthquake

Main body
HL: NW-SE

HT: NE-SW

F(Hz)
A
-6
avKb (10 )
F(Hz)
A
-6
avKb (10 )

3.55
18.6
9.3

F(Hz)
A
-6
avKb (10 )Before
4.06
24.3
14.3

HT: NE-SW

F(Hz)
A
-6
avKb (10 )

4.26
23.0
12.3

Torsion

F(Hz)

3.46
17.5
9.3

2.78

After dF/F (%)
4.78
3.87
21.5
14.0

dF
7.96

Main body
HL: EW

4.07
20.2
11.9

4.56

7.96

HT: NS

5.25
5.00
4.76
Before After dF/F (%)
HL: NW-SE
F(Hz)
1.13
1.14
-0.12
1.97
HT: NE-SW
F(Hz)
1.33
1.31
Torsion
F(Hz)
11.8
11.6
1.69
NT
Before After dF/F (%)
HL: NW-SE
F(Hz)
1.15
1.10
4.26
HT: NE-SW
F(Hz)
1.27
1.12
11.5
Torsion
F(Hz)
11.4
11.0
3.51
Note: dF is frequency shift in unit ∆F, ∆F = 1/40.96 Hz

10.2
dF
-0.06
1.08
8.19
dF
2.00
6.00
16.4

ST

F(Hz)
A
-6
avKb (10 )Before
1.92
156
261

F(Hz)
A
-6
avKb (10 )

2.16
54.6
72.0

F(Hz)
A
-6
avKb (10 )Before
1.45
91.6
217

After dF/F (%)
1.33
8.38
147
414

F(Hz)
A
-6
avKb (10 )

0.720
118
1138

0.745
114
1029

dF
4.99

-3.38 -0.999

TorsionF(Hz) 1.037 0.964
7.05
Note: dF is frequency shift in unit ∆F, ∆F = 1/40.96 Hz

2.994

would be the shift within the error range, and it would be proper to judge that the natural frequency did not shift
before and after the earthquake.
6. DISCUSSION
At the time of the Kocaeli Earthquake, maximum PGA in Istanbul for hard rock site was only 41 Gal at YPKstation. It is interesting to occur the shift of the natural frequency under such a weak seismic motion.
Before and after the earthquake, the transfer functions kept their shapes as shown in Figure 8, but the peakfrequency has slightly shifted to lower, except the lowest peak frequency seen in Suleymaniye and Sehzademosques.
For minarets, the shift rate of the frequency is smaller than that of the main structures except the NT minaret of
Sehzade mosque and WHT minaret of Suleymaniye mosque. This means that minarets are in better condition
than main structures.

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The 14 World Conference on Earthquake Engineering
October 12-17, 2008, Beijing, China
However, it is thought that the minarets themselves vibrated quite
intensely during the earthquake. Especially in the case of WHT
minaret of Suleymaniye mosque, damage as shown in Figure 10 has
appeared in incidental facilities such as a roof. That is, bucklingarise in the west side of a lower part of a conic roof, and theornament at the top of the conic roof is crooked on the east side.
The shift of the natural frequency for WHT minaret is larger thanothers. As a result, the shift rate of the natural frequency
correspond to the damage grade of incidental facilities.
Hereafter, the measurement result for every structure is examined.
6.1. Suleymaniye mosqueThis mosque was designed after the model of the Hagia Sophia
museum and has similar dimensions and structure. It is consideredas the masterpiece of an architect Sinan's maturity term. Although
500 years have passed, it dose not seem to be in good condition.
As shown in Table 2, a natural frequency of HL (NW-SE) Figure 10. Damage of WHT minaret
component of the main body was decreased to 3.36 Hz after the of Suleymaniye Mosque
earthquake from 3.43 Hz before the earthquake. And that of HT(NE-SW) component was decreased to 3.46 Hz from 3.55 Hz. These changes correspond to 3 – 4 ∆F and areconsidered a significant change. These rates of change are 2.2 – 2.8 %, and are equivalent to approximately 5% of rigidity lowering. It is presumed that this is the reason for slight damage.
The avKb value is also shown in Table 2. If effective seismic motion is assumed to be approximately 1/3 of the
maximum acceleration (approx. 15 Gals), averaged drift angle will be presumed to be approximately 1/6000,
and this drift angle is far from collapse.
6.2. Hagia Sophia museum
Hagia Sophia museum was constructed at a beginning of sixth century with four arches on the four huge main
piers, which the main dome is supported. A pair of opposite arches is reinforced with the wall, and the otherpair is reinforced with semi dome. Moreover, there are two layers of large scale corridor around the four main
piers.
As shown in Table 3, a natural frequency of HL (EW) component was decreased to 1.75 Hz after the earthquake
from 1.92 Hz before the earthquake. And that of HT (NS) component was decreased to 2.01 Hz from 2.16 Hz.
These frequency shifts correspond to 6 – 7 ∆F and are considered a significant change. These rates of shift are
6.8 % (HT component) to 8.9 % (HL component), and are equivalent to approximately 13 – 17 % of rigidity
lowering. As a result, this might have been affected not a little.
The avKb value is also shown in Table 3. If effective seismic motion is assumed to be approximately 1/3 of the
maximum acceleration (approx. 15 Gals), averaged drift angle would be presumed to be approximately 1/250.
Generally it is thought that a structure begins to collapse on 1/100 – 1/200 of drift angle. It is considered that it
stood against this earthquake motion without great margin. For this reason, it is thought that the significant
decrease of natural frequency occurred.
6.3. Sehzade mosque
Sehzade mosque is an etude of architect Sinan. Although 500 years have passed, it dose not seem to be ingood condition. According to the result of the measurement at the base of the four main piers and the dome
corridor equivalent to the upper part, natural frequencies shifted from 4.06 Hz and 4.26 Hz before theearthquake to 3.87 Hz and 4.07 Hz after the earthquake for HL (NW-SE) and HT (NE-SW) component,
respectively. These shifts correspond to 8 – 9 ∆F and are considered a significant change. These rates ofth

The 14 World Conference on Earthquake Engineering
October 12-17, 2008, Beijing, China
shift are approximately 5 %, and are equivalent to approximately 10 % of rigidity lowering. If effective
seismic motion is assumed to be approximately 1/3 of the maximum acceleration (approx. 15 Gals), the avKb
value in Table 4 shows that the averaged drift angle will be presumed to be approximately 1/5000, and this drift
angle is far from collapse.
Moreover, it is confirmed that natural frequency shifts were quite remarkable in NT minaret of Sehzade mosqueand that the damage from the earthquake was relatively larger than ST minaret. In addition, it is said that NT
minaret have been slightly damaged when the measurement before the earthquake was done.
6.4. An office building
In case of the office building, on the vibration of NS component, the shift of predominant frequency correspondto less than 1 ∆F and is not considered a significant change. Considering that the floor shape of this building isflat and extremely short in the NS component as shown in Figure 5, its rocking vibration excels in vibration of
the NS direction. And it is thought that the rigidity of a base is more strongly reflected rather than the rigidityof a building itself. The fact that the natural frequency of the NS component did not change before and after
the earthquake suggests that the earthquake did not influenced the base at all.
On the other hand, it is supposed that the vibration of EW component reflects the rigidity of the building itself.
The shift of natural frequency correspond to 5 ∆F and is considered a significant change. These rates of shift
are approximately 8.4 % and are equivalent to approximately 15 % of rigidity lowering.
10

Hagia Sophia: Stone and Brick

98

An Office Bldg: RC

dF/F in %7
6

Sehzade: Stone5
4

Suleymaniye: Stone

3
2
1
0
1

10

100
-6

1000

2

av K b in 10 /(cm/s )Figure 11. Relationship between avKb and dF/F for Measured Structures
Natural frequency shift can be considered as an expression of the degree of damage. Figure 11 shows therelationship between avKb value and the rate of natural frequency shift. The relation between avKb and thefrequency shift rate suggests that avKb can be given as damage degree before the earthquake.
Figure 11 shows large avKb correspond to larger damage in general. Both Suleymaniye and Sehzade mosquehave the same type of structure, and Hagia Sophia museum has a structure type of similar to them. On theother hand, the office building has different characteristics as RC structure.
In addition, the avKb value for an office building was examined using the EW direction considered to reflect therigidity of the building itself.

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The 14 World Conference on Earthquake Engineering
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7. CONCLUSIONAt the time of the Kocaeli earthquake, the seismic motion in Istanbul was relatively small because of the hardbase ground. For some structures measured before the earthquake, it was possible to measure after theearthquake again to investigate the shift of the natural frequency. As a result, the shifts of natural frequency
were 2.8 %for Suleymaniye mosque built 500 years ago, 8.9 % Hagia Sophia museum built 1500 years ago, 4.8% for Sehzade mosque built 500 years ago and 8.5% for an office building newly built. It seems that the
impact of the earthquake is small; thus the change of the natural frequency is also small. For the measured
structures, they were affected by the Kocaeli earthquake as 7 – 16 % of rigidity lowering. The change ofnatural frequency also corresponds well to the vulnerability index against earthquake disasters for structures.
This suggests that the proposed index avKb value is appropriate.
8. ACKNOWLEDGEMENT
Special thanks are due to Prof. Mustafa Erdik of Boazici University, who guided us in measuring the historicalbuildings such as Hagia Sophia museum as well as to Prof. Karadogan of Istanbul Technical University, whogave us opportunity to measure a newly completed office building. Thanks are also due to those many who
cooperated with us during our field investigation. We are thankful to Dr. E. Dilek Gurler, a former researcherof SDR, who provided us research assistance, and Ms. Sawako Nakayama of SDR, who helped us with theanalysis and arrangement of data. The opinions of reviewers were very useful to the qualitative improvementof our research. We would like to express to them our sincere gratitude.
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