Proceedings of the Sixth International Conference on Seismic Zonation (6ISCZ)
EERI, November 12-15, 2000/ Palm Springs. California

LOCAL SITE EFFECT OF KOBE BASED ON MICROTREMOR
MEASUREMENT
Yutaka Nakamura, Tsutomu Sato, and Masayuki Nishinaga

ABSTRACTThe Hyogo-Ken-Nanbu Earthquake caused heavy damage in Kobe city and its
surrounding area. To clear the relationship between damage and local site effect, amonth after the earthquake, microtremor measurement was performed for about a year.
According to the measurement results in the damaged zone, the amplification factor (A)ranges between 2 and 3 which is not so high. However the predominant frequency (F)ranges between 1.5 and 2 Hz which corresponds to that of strong motion. Distributionof vulnerability index Kg value for ground confirmed the damage belt.
Introduction
The 1995 Hyogo-Ken-Nanbu Earthquake caused heavy damage to Kobe city and its
surrounding area. Especially, a part of area where extensive damage concentrated,spreads from east to west in the region. With its appearance the area called as Damage
Belt. Many studies have been done for investigating the reason why did the damage
occurred in such a belt shape.
In this paper, to clear the relationship between damage and local site effect,
microtremor measurement was made in damaged and surrounding area. Dynamic
characteristics of surface ground (predominant frequency F, amplification factor A) areestimated. Furthermore, vulnerability
index Kg value (Nakamura,1996) is also
calculated, and compared with real
observed damage information. As aresult, 1: Predominant frequencies (F)have a trend to gradually small value
from mountain side to sea side. 2:
Amplification
factor(A)andvulnerability index Kg value have atrend to the high value in damage belt
area. 3: As the measurement points
come nearer to a sea side, Kg value
becomehigherthesepoints
corresponded to liquefaction area.
Therefore, it is suggested that thedistribution of damage is caused by local
Fig.1 Measurement points in Kobe city
site effect.
46

42

Sea of Japan38

Pacific Ocean

T okyoKobe3434.74

RO1-0 1SU2-0 1

RO2-0 130

Kobe City

34.72

128

132

136

140144SU1-0 1NA2 -01148

NA1 -01

SMG-01

34.7

SU2-2 1

SMA-01

NAG-5 0

SMB-1 6MO-0 1NA1 -15

RO2-1 4
RO1-1 5

SU1-0 9NA2 -16

KO-0 1

NAG-2 3

SMB-0 1

34.68

Rokko
IslandSMA-13SMG-25

KO-0 6PortIsland

NAG-1 0

NAG-1 1

Osaka Bay

34.66

NAG-3 7NAG-5 2NAG-5 3NAG-3 6●: Measurement Points

34.64

135.12135.14135.16135.18135.2

135.22135.24135.26135.28

Measurement
The investigated area was in Kobe city and showed in Fig.1. The area is surrounded by
Mt. Rokko in north side, Osaka bay in south side, and after the earthquake this site was

System and Data Research Co., Ltd., 3-25-3 Fujimidai, Kunitachi-shi, Tokyo, 186-0003, Japan

Proceedings of the Sixth International Conference on Seismic Zonation (6ISCZ)
EERI, November 12-15, 2000/ Palm Springs. California

damaged like a belt spread from east to west. Kobe city which had huge damage is anarrow area about 5km from northern mountain side to southern sea side, has grown on
the typical irregular ground. This was the reason, that the damage occurred in a belt of1-2km width to east-west direction which attracted many researchers to concern thisphenomenon.
In this paper, our main purpose is to prove that the cause of damage belt was becauseof the influence of local site effect. A month after the earthquake microtremor
measurement was made from February 1995 to February 1996 for this purpose.
Measurement points showed in Fig.1. Each profile has 15-20 points, total 25 profiles
spreaded north-south through mountain side, damage area, and sea side. The distance
between each points was 100 to 300m, and each profiles length was 1-2km. Total
measurement points were about 400. In this paper analyzed 15 profiles will be discussed,
to be concentrate on a central part in Kobe city.
Analysis
An instrument named Portable Intelligent Collector (PIC) was used for microtremor
measurements. A sensor is set on the asphalt or the soil, are measured at the 2 horizontalcomponents (NS and EW direction) and a vertical component same time. Samplinginterval is 1/100 sec and the length of each record is 40.96sec. Measurements wasrepeated three times at each observation points. After measurements, Fourier spectrum
for each components are calculated. One frequency spectrum of one component wasestimated by averaging the three Fourier spectra. Then, from a spectral ratio ofhorizontal to vertical components QTS spectrums (Quasi-Transfer Spectrum) arecalculated, Nakamura(1989). Predominant frequency F and amplification factor A
which represents dynamic characteristics of the ground are found from this analysis andVulnerability index Kg are calculated as explained below. Details of the methodology
can be found in Nakamura(1989, 1996).
Vulnerability Index Kg values for Ground
For the vulnerability index Kg of surface ground, shear strain γ is considered(Nakamura, 1996). According to the Ishihara (1982) ground soil becomes plastic state atabout γ=1000 x10-6 and for γ>10000 x10-6 landslide or collapse of foundation occurs.
Fig. 2. Shows the simplified shear deformations of the surface ground.
Ag ×dH

VfBasement Groundvb

Figure 2. Shear deformation of surface ground.
Average shear strain γ can be estimated as γ=Ad/H, where A is amplification factor ofsurface layer, H is thickness of surface layer, and d is seismic displacement of basement

Proceedings of the Sixth International Conference on Seismic Zonation (6ISCZ)
EERI, November 12-15, 2000/ Palm Springs. Californialayer. Details about formulation can be find in Nakamura(1996). Without going into adetails, we are going to write shear strain as follows (Nakamura, 1996);
A2aγ =⋅ 2F π ⋅ vb
2In this equation, A /F is called as Vulnerability index, Kg value for surface ground. a isthe acceleration in the basement. vb is the S wave velocity of the basement.
Results and Discussion
Relationship between the damage and Predominant frequency (F), Amplification factor(A), and Vulnerability index (Kg) value calculated from microtremor measurement isdiscussed.
Distribution of Predominant frequency F
Fig.3 shows distribution of predominant frequency (F) estimated from QTS in eachmeasurement points. The value of F divided into 4 ranges (F<1.5Hz, 1.5<F<2.5Hz,2.5<F<3.5Hz, 3.5<Hz) to easily catch the whole tendency. This area is Mt. Rokko innorthern part, and sea in southern part. Distribution of F in the Fig.3 presents large Fvalue on northern part overall, and small F value on southern part. However, the area ofseismic intensity of 7 (on the Japanese scale) does not show the same phenomenon as
F.
RO1-01SU2-01
RO2-0134.73

SU1-01

Kobe City
NA2-01NA1-01

34.71

SMG-01

Intensity 7SMA-01

NAG-50

RO1-15MO-01

SU1-09NA1-15

KO-01

34.69

SU2-21
RO2-14NA2-16NAG-23

Rokko
IslandSMA-13SMG-25
KO-06

34.67

PortIsland

NAG-10
NAG-11F<1.5
1.5<F<2.5
2.5<F<3.5
3.5<F

NAG-3734.65
NAG-5334.63
135.12135.14NAG-52

Osaka Bay

NAG-36135.16135.18135.2

135.22135.24135.26135.28

Fig.3 Distribution of Predominant Frequency F

More detailed graph of F in the damage belt is shown in Fig.4, F values are little lessthen 2Hz in general. This frequency is predominant frequency of strong motion records
in 1995 Hyogo-Ken-Nanbu Earthquake recorded in this area, this was guessed to be areason of resonance of strong motion in this area.
Distribution of Amplification factor AFig.5 shows distribution of amplification factor (A) in each points. This distribution
again divided into 4 ranges as we did in F, intervals are A<2.0, 2.0<A<2.5, 2.5<A<3.0,Proceedings of the Sixth International Conference on Seismic Zonation (6ISCZ)
EERI, November 12-15, 2000/ Palm Springs. California

10.00

10.00

8.00

8.00

6.00

6.00

Ｆ and AＦ and A

3.0<A. For whole area, while damaged area of seismic intensity of 7 have larger A
value in measurement points, slightly damaged area have smaller A. Overall distribution
has the characteristic that A has large value in the highly damaged area.
Fig.4, shows more detailed graphs of A. A does not show clear and characteristic trend
as F. However, in lots of measurement points on high damage area, A increase and tends
to show value of 3. On
F(Hz)
F(Hz)
A(times)Damage Zone
A(times)
theotherhand,Damage Zone
distribution of A in
MO and KO lines, inthe slightly damaged(Sea side)(Mt. side)(Sea side)(Mt. side)
Measurement Points (NO.53-NO.10)
Measurement Points (NO.11-NO.52)
area (the place was cutF(Hz)F(Hz)
the damage belt.) A inA(times)A(times)Damage Zone
Damage Zone
almost all the points
in this area show
smaller value same as(Sea side)(Sea side)(Mt side)(Mt. side)
Measurement Points (NO.23-NO.36)
Measurement Points (NO.37-NO.50)
the points in mountainside .
10.00
F(HZ)
F(Hz)
A(times)A(times)
Briefly, we can say 8.00
6.00
Damage Zonethat
amplification 4.00
factor (A) has large 2.00
values in the area 0.00
01000 Distance (m)2000 (Mt. side)(Sea side)(Mt. side)(Sea side)
Measurement Points (KO)Measurement Points (SMG)
where heavy damage
F(Hz)
F(Hz)
occurred.
Ifwe
A(times)A(times)consider the behavior
Damage Zoneof F and A together we
can say that these(Sea side)(Mt. side)(Sea(Mt. side)characteristics caused
Measurement Points (SMA)
Measurement Points (MO)severe vibration of
Fig.4 Distribution F and A in each profile
strong motion comingfrom basement.
4.00

4.00

2.00

2.00

0.00

0.00

500

1000

1500

2000
2500
Distance （ｍ）

3000

3500

4000

01000

2000

3000

Distance （ｍ）10.00

10.00

8.00

8.00

6.00

6.00

F and AF and A

0

4.00
2.00

4.00
2.00

0.00

0.00

01000

2000

3000

4000

5000

6000

01000

2000

3000

Distance (m)Distance （ｍ）

10.00

6.00

F and AF and A

8.00

4.00

2.00

0.00

01000

2000

3000

10.00

10.00

8.00

8.00

6.00

6.00

F and AF and A

Distance (m)

4.00

4.00
2.00

2.00

0.00

0.00

01000

2000

Distance (m)

03000

1000

2000

Distance (m)RO1-01SU2-01

34.73

RO2-01SU1-01NA2-01

Kobe City
NA1-01

34.71
SMG-01

SU2-21

SMA-01
NAG-50

RO2-14
RO1-15MO-01

NA1-1534.69

SU1-09NA2-16KO-01
NAG-23

RokkoIsland

SMA-13SMG-25
KO-06

34.67

NAG-11

NAG-52

34.65

PortIsland

NAG-10NAG-533.0<A
2.5<A<3.0
2.0<A<2.5
A<2.0

NAG-37NAG-36

Osaka Bay

34.63
135.12135.14135.16135.18135.2

135.22135.24

Fig.5 Distribution of Amplification Factor A

135.26135.28

Proceedings of the Sixth International Conference on Seismic Zonation (6ISCZ)
EERI, November 12-15, 2000/ Palm Springs. California

Distribution of Vulnerability index Kg value, comparison between Kg value anddetail damage data
Fig.6 is a distribution of vulnerability index Kg value calculated from the relationshipof F and A. This figure also given for 4 ranges like F and A. Kg has larger value in thedamage area spreaded like a belt, and small value in the other areas. Kg value is related
to damage distribution. In measurement points of large Kg value near coastline (NA2and SU1 lines), although the area is out side of seismic intensity of 7, liquefaction wasobserved extensively.
RO1-01SU2-01

34.73

RO2-01SU1-01NA2-01

Kobe City
NA1-01

34.71

SU2-21SMG-01
RO2-14
RO1-15

SMA-01

NAG-50MO-01

SU1-09NA1-1534.69

NA2-16KO-01

NAG-23

Rokko
IslandSMA-13SMG-25
KO-06

34.67

PortIsland

NAG-10
NAG-11

NAG-5234.65

NAG-5334.63
135.12NAG-36135.144.0<K
3.0<K<4.0
2.0<K<3.0
K<2.0

NAG-37

Osaka Bay135.16135.18135.2

135.22135.24135.26135.286.0
4.0
2.00.0DamageＫg Value

DamageＫg ValueＫg Value

DamageNA1-01NA1-02NA1-03NA1-04NA1-05NA1-06NA1-07NA1-08NA1-10NA1-09NA1-11

6.0
4.0
2.00.0

Ssea side

Mountain side
10987654321010.0

Ｋg ValueDamage

8.0
6.0
4.0
2.0
0.0

RO2-1

RO2-2

RO2-3

RO2-4

Mesurement Points

RO2-5

RO2-6

RO2-7

RO2-8

RO2-9

RO2-10

Fig.7 Comparison between Kg and damage ratio

RO2-11

RO2-12

RO2-13

RO2-14

RO1-1

RO1-2

RO1-3

RO1-4

RO1-5

RO1-6

RO1-7

RO1-8

RO1-9

RO1-10

RO1-13

RO1-12

RO1-11

RO1-14

RO1-15NA2-01NA2-02MA2-03NA2-04NA2-05NA2-06NA2-07NA2-09NA2-08NA2-10NA2-11NA2-12NA2-13NA2-14NA2-16NA2-15109876543210

Ｋg Value
Damage8.0

DamageMO-01

MO-020.0

DamageKg Value

DamageＫg Value

NO.50

NO.47

MO-03

MO-04

MO-05

MO-06

2.0

Measurement Points
Mountain side

Measurement Points

MO-07

4.0

NA1-120.0

MO-08

6.0

NA1-132.0109876543210

Ｋg Value
DamageNA1-144.0

MO-09

Mountain side

8.0

SMG1

SMG4

SMG3

SMG7

SMG9

SMG11

SMG13

SMG15

SMG17

SMG19

SMG20

SMG22

6.0

MO-10

MO-11

MO-12

Sea side

10.0

NA1-150.0

SMA1

SMA2

SMA3SMA4

SMA5

SMA6

SMA7

SMA8

SMA9

SMA10

SMA11

SMA12

SMA13

1098765432
10

Ｋg Value
Damage

8.0

NO.48

0.0
MO-13

2.0

Sea side

NO.49

2.0
MO-14

4.0

10.0

NO.46

4.0

Measurement Points
Mountain side

1098765432
10

6.0

MO-15

1098765432
10

6.0

Measurement Points

NO.45

Mountain side

Ｋg Value
Damage8.0

Measurement Points
Mountain side

Ｋg Value
Damage8.0

NO.44

Sea side

10.0

KO-01

KO-02

KO-03

Sea side

NO.43

0.0

10.0

NO.42

NO.41

NO.40

2.0
KO-04

Ｋg Value

0.0
NO.39

4.0

SMG24

0.0

Ｋg Value

2.0
NO.38

6.0

KO-05

2.0

Measurement Points

109876543210

Ｋg Value
DamageKO-06

4.0

Sea side

4.0

Measurement Points
Mountain side

8.0

NO.23

NO.24

NO.25

NO.26

NO.27

NO.28

NO.29

NO.30

NO.31

NO.32

NO.33

NO.34

NO.36

NO.35

1098765432
10

6.0

10.0

6.0

NO.37

10.0

109876543210

Ｋg Value
DamageMeasurement Points
Mountain side

Ｋg Value
Damage8.0

Mountain side

8.0

NO.11

Sea sideMeasurement Points
Sea side

10.0

NO.12

0.0

NO.13

2.0

NO.14

4.0

NO.15

6.0

NO.16

109876543210

Ｋg Value
Damage

8.0

NO.17

10.0

Mountain side

Sea side

10.0

Measurement Points

Measurement Points
Sea side

NO.18

NO.19

NO.20

NO.21

NO.22

NO.10

NO.9

NO.8

NO.7

NO.6

NO.5

NO.4

NO.3

NO.2

NO.51

NO.1

NO.58

NO.57

NO.56

NO.55

NO.54

NO.53

NO.52

0.0

109876543210

Damage2.0

Mountain side

Kg Value
Damage8.0

DamageＫg Value

4.0

Sea side

10.0

DamageＫg ValueＫg Value

6.0109876543210Damage
Ｋg Value

Ｋ g Value
DamageＫg Value

Mountain side

8.0DamageSea side

10.0

DamageFig.6 Distribution of Vulnerability index Kg

Proceedings of the Sixth International Conference on Seismic Zonation (6ISCZ)
EERI, November 12-15, 2000/ Palm Springs. California

Kg value reflects local site effect and corresponded with the damage belt andliquefaction as well. It is possible to conclude that the damage of 1995 Hyogo-KenNanbu Earthquake was related to dynamic characteristics of the ground.
Fig.7 compares Kg value and damage ratio for small-scale structures. Damage ratio
divided into 5 ranges; 1: no damage, 2: 0-12.5%, 3: 12.5-25%, 4: 25-50%, 5: 50-100%.
In each figure, distribution form of Kg value and damage ratio has similar tendency.
Especially, in NAG, SMA, SMG area, distribution form of Kg, very well correspondswith damage ratio of structure from mountain side to sea side. The damage area
between mountain parts and sea parts shows larger Kg value and bigger damage ratio.
On the other hand, distribution of Kg in in almost all the points on MO and KO lines (inlightly damaged area), show smaller value. With these results Kg value which is defined
as vulnerability index for ground, found to be related also with damage ratio ofstructures.
To compare Kg value and damage ratio overall, in Kobe city, it has been found thatsome damage occurred for Kg>2. For acceleration value of 200-300gal and S-wave
velocity of 300m/s in the basement of this city, ground strain estimated as γ = 1 ~ 2 × 10 −3
when earthquake occurred in the area. This value corresponds to arise ground damage.
In Fig.7, Kg value does not agree with damage ratio in some parts. Points with large
Kg value in spite of small damage ratio are almost located in liquefaction area. To findout the reason for this, building characteristics in this area will be checked in detail infurther part of the study.
Conclusion
To explain dynamic characteristics of the ground in the damage of 1995-Hyogo-KenNanbu Earthquake, microtremor measurement was made in Kobe city. Amplification
factor A and vulnerability index Kg value are showed larger value in damage belt.
Vulnerability index Kg value in spite of for the ground agreed with damage ratio forstructure very well. Therefore, it can be conclude that damaged belt occurred after theearthquake had a unique character of local site effect in the area.
Acknowledgment
Microtremor measurement in Kobe city was preformed with Railway TechnicalResearch Institute (RTRI). Thank for their cooperation and help during measurements.
And we would like to thank you Dr. E. Dilek Gurler for her help and comments.
References
Nakamura, Y,. A Method for Dynamic Characteristics Estimation of Subsurface using
Microtremor on the Ground Surface, Quarterly Report of RTRI, Railway TechnicalResearch(RTRI), 1989, Vol.30, No.1.
Nakamura, Y,. Seismic Vulnerability Indices For Ground and Structures Using Microtremor,
World Congress on Railway Research in Florence, Italy, November, 1997.
Building Research Institute Ministry of Construction, The report of damage on the 1995
Hyogo-Ken-Nanbu Earthquake, 1996