Proceedings of the International Symposium on Engineering Lessons Learned fromthe 2011 Great East Japan Earthquake, March 1-4, 2012, Tokyo, JapanMICROTREMOR MEASUREMENTS OF THE
HIGH-RISE BUILDINGS LEANED BY THE 2011 OFF
THE PACIFIC COAST OF TOHOKU EARTHQUAKE
Kazuya MITSUJI1, Zhiwei LIU2, Susumu OHNO2 and Masato MOTOSAKA3
1

Associate Professor, Faculty of Education, Art and Science, Yamagata University,
Yamagata, Japan, mitu@e.yamagata-u.ac.jp
2
Graduate Student, Disaster Control Research Center, Tohoku University,
Sendai, Japan, lewiszhiwei@gmail.com
3
Associate Professor, Disaster Control Research Center, Tohoku University,
Sendai, Japan, ohnos@saigai.str.archi.tohoku.ac.jp
4
Professor, Disaster Control Research Center, Tohoku University,
Sendai, Japan, motosaka@archi.tohoku.ac.jp

ABSTRACT: High-rise buildings supported by pile foundations were damaged by themain shock of 2011/03/11, and one of them leaned to the south as a result of the damage
due to strong shaking. Damage investigation and microtremor measurements werecarried out in the damaged buildings. Results made it clear that the first natural
frequencies of the buildings appear around 1Hz and dynamic characteristics of thebuildings are different in TR and LN directions.
Key Words: The 2011 off the pacific coast of Tohoku earthquake, microtremor
measurement, damage investigation, high-rise building, dynamic
characteristics

INTRODUCTION
In the 2011 off the Pacific coast of Tohoku earthquake, lots of buildings have been suffered by thedamage caused by strong shaking. As one of the severe damage of buildings, several leaned buildings
have been observed. Some of them are supposed to have been caused by strong shaking of main shock,and others are by liquefaction where the site condition is soft soil ground.
The authors are in charge of damage investigation of the leaned buildings in Sendai city. One ofthem is the housing complex consisting of two high-rise buildings (building A and B) located in theeastern part of Sendai city. The damaged site is called by “STM” in the followings. Each building is
14-storey SRC structure and supported by pile groups. The building A is considered to be leaned bydamage of pile foundation and shows distinct inclination from the outside appearance.
The buildings had been also damaged by the 1978 Miyagi earthquake, then two sets of grouppiles were heavily damaged. According to the damage investigation report of the 1978 Miyagi
earthquake, concrete body of pile were crushed, some steel bars indicated buckling and shear cracks
were observed at the pile head in some pile groups (Shiga 1980, Sugimura and Oh-oka 1981). Those1180

buildings were restored after the 1978 Miyagi earthquake, and residents have kept living in thosebuildings until the earthquake of March 11.
Microtremor measurements were carried out as preliminary investigation for those buildings. The
authors also investigated damage survey of super-structure, foundation and surface ground, andinclination measurement of super-structure and foundation. In this paper, the state of the damage isreported and the results of microtremor measurements are presented.

LOCATION OF THE SITE AND OUTLINE OF THE LEANED BUILDING
Shown in the left figure of Fig.1, Sendai city is located on the east coast of the Pacific Ocean, about
300km north from Tokyo. There is about 130km between Sendai city and the epicenter of the main
shock of 2011/03/11. Location of the damaged site STM is shown in the right figure of Fig.1.
Earthquakes have been observed at the site of MYG013 (K-NET), and enormous number ofearthquakes after the main shock were also recorded at this site. The damaged site of STM is locatednear MYG013 of which distance is about 3km as shown in the right figure of Fig.1.

K-NET
MYG013

Sendai

STM
3km

Epicenter2011/03/11
Downtown
Sendai

Pacific
Ocean

Fig. 1 Location of the site
Left: Sendai city and the epicenter of the main shock of 2011/03/11, Right: K-NET MYG013
observation site and the damaged site of STM(Left: Image@2012 TerraMetrics, Map Data@2012 ZENRIN, Right: Image@2012 Ones Spot Image,
Digital Earth Technology, Digital Globe, GeoEye, Map Data@2012 ZENRIN
Velocity response spectra (damping coefficient h=5%) of the main shock of 2011/03/11 andaftershock of 2011/04/07 at MYG013 are shown in Fig. 2. The earthquake of 2011/04/07 is recognized
as the largest one among the aftershocks continuously occurred. While response in NS direction is
predominant in the main shock, response in EW direction becomes much larger than NS direction in
the aftershock. It is also distinctive that response spectra of the aftershock indicate two peaks.

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300

Vel. Resp. Spec. (cm/s)

Vel. Resp. Spec. (cm/s)

400

2011/03/11 14:46
NS
EW
UD

200
100
0
0

1

2

3

4

5

6

7

8

9

10

400
300

2011/04/07 23:32
NS
EW
UD

200
100
0
0

1

2

Period (s)

3

4

5

6

7

8

9

10

Period (s)Fig. 2 Velocity response spectra (h=5%) at MYG013
Left: 2011/03/11 main shock, Right: 2011/04/07 aftershock

SITE CONDITION AND OUTLINE OF THE LEANED BUILDING
Standard penetration tests had been conducted at the four places in the site of STM. One of the
N-value distributions along depth is shown in Fig.3. For comparison, the N-value distribution of
MYG013 is also illustrated in the right figure of Fig. 3.
From GL-25m up to the surface, alluvial sand and silt layers are reciprocally accumulated at thesite of STM. On the other hand, the thickness of the alluvial deposit is only 15m at MYG013. Basedon the N-value distribution, shear wave velocity distribution along the depth of the site of STM isestimated by use of the empirical relationship (Ohta and Goto 1976). Shear wave velocity distributionof MYG013 is open to the public at the K-NET web site. Shear wave velocity distributions of bothsites are shown in Fig. 4. It should be noted that depth that the solid gravel or rock layer appears isdifferent in both sites.
Amplitude characteristics of both sites are compared in Fig. 5. Amplitude ratio is estimated bytheoretical transfer function obtained from linear one-dimensional multi-reflection analysis. Peakaround 5Hz meets good agreement with in both sites. The effect of the soft surface layer above from
GL-15m seems to appear in the peak around 2Hz at the site of STM.

Clay
Silt
5

5

Silt

10

GravelDepth (m)15

Fine Sand

20

Silt

25

15

Rock

20
25

GravelSTM

Depth (m)Sand
10K-NET MYG013
30

30
0

0

10 20 30 40 50

10 20 30 40 50

N-valueN-value

Fig. 3 N-value distribution of the site of STM and MYG013 (K-NET)

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5

10

10

Depth (m)Depth (m)

5

15
20
25

15
20
25
K-NET MYG013

STM
30

30
0

100 200 300 400

0100 200 300 400

Est. Vs(m/s)

Est. Vs(m/s)

Fig. 4 Estimated Shear wave velocity distribution of the site of STM and MYG013 (K-NET)

Amplitude Ratio

5
STM
K-NET:MYG013

4
3
2
1
0
0

1

2

3

4

5

6

7

8

9

10

Frequency (Hz)Fig. 5 Theoretical amplitude characteristics of the site of STM and MYG013 (K-NET)

Build. A

Build. B
N

Build. B

Damaged piles
in 1978

39,500: Sensor

1,100

Build. A

4,400

Fig. 6 Schematic plan and façade of the housing complex of STM

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Schematic plan and façade of the housing complex of STM are illustrated in Fig. 6. Photos of theexterior appearance of the buildings are shown in Photo 1. Housing complex STM consists of the twobuildings. Both of them are SRC 14-storey buildings supported by pile foundations. Height of bothbuildings is about 40m. Construction of the buildings have completed in 1976. After the completion,
the buildings suffered severe shaking and were heavily damaged by 1978 Miyagi off shore earthquake(Mj=7.4).
The buildings had been also damaged by the 1978 Miyagi off shore earthquake, then some grouppiles that are marked by red circles in the left figure of Fig. 6 were heavily damaged (Shiga 1980,
Sugimura and Oh-oka 1981). According to the damage investigation report of the 1978 Miyagi off
shore earthquake, concrete body of pile were crushed, some steel bars indicated buckling and shear
cracks were observed at the pile head in some group piles. Those buildings were restored after the
1978 Miyagi earthquake, and residents have kept living in those buildings before the earthquake ofMarch 11 occurred.

Build. A

Build. B

Build. A
Photo 1 Exterior appearance of the damaged housing complex
After 3/11
After 4/7

14

Gap

12

Floor

10
8
6
4
2
0

200 400 600 800 1000

Build. B

Gap(mm)Build. A

Fig. 7 Measured gap between the building A and B
After the main shock of 2011/03/11, the buildings have been heavily damaged by strong shaking
again. Building A and B were connected by expansion joint that were demolished by dynamic
behavior of the buildings. Building A and B forms L-shape plan shown in Fig. 6. Dynamic behavior ofeach building is completely different, because strong axes cross each other. Most characteristic
damage is that building A indicates remarkable inclination in NS direction as shown in the photo in1184

Fig. 7. Inclination of the building A creates the large gap between the building A and B. Inclination of
the building was not observed in the damage by the 1978 earthquake even though severe damage wasobserved at the pile head in some pile groups. Gap had been measured by the municipal office of
Sendai city after the main shock. The authors also measured gap between the buildings at all the floors
in October 2011. Measured gap at each floor between the buildings are shown in the left figure of Fig.
7. The figure clearly indicated gap at each floor increased after the aftershock of 04/07. Gap between
two buildings was about 1,000mm at the roof floor and 200mm at the first floor. Angle of theinclination of the building A was estimated about 1/60 to 1/40. Outstanding inclination as building A
was not recognized in the building B.
DAMAGE INVESTIGATION AFTER THE MARCH 11TH EARTHQUAKEThe state of the damaged buildings is shown in Photo 2 and 3. The left photo of Photo 2 is the gap
between the building A and B at the 14th floor that is the largest gap of over 1000mm. Large width
shear cracks are observed in almost all the non-structural walls facing corridors and balconies asshown in the right photo of Photo 2. This phenomenon was also observed in the damage by the 1978
earthquake.

Photo 2 Gap between the buildings at the 14th floor and damaged non-structural wall

Photo 3 Damage around the foundation and damaged non-structural wall at the first floor
The left photo of Photo 3 shows damage around the foundation of the north side of the building A.
After the main shock of 2011/03/11, building A leans to the south. Difference in the level shown in theright photo of Photo 3 is considered to have been generated by the uplifting behavior induced by the
inclination of the building A. This difference in the level is about 14cm, and settlement around thebuilding is about 20 to 25cm. The left photo of Photo 3 shows damage of non-structural wall of the1185south facade at the first floor of the building A. Severe shear crack is observed as well as the upper
floors. Large gap running on the ground is recognized the right photo of Photo 3. This large gap is also
considered to have induced by the uplifting behavior of the building A to the south.
As mentioned above, the building A leans to the south. The inclination is considered to be theeffect of the uplifting behavior caused by heavy damage at the pile head of some pile groups. Damagearound the foundation of the building A indicates the inclination of the building. Moreover, the angleof the inclination of each housing unit at all the floors was measured by the electrical spirit level.
Results show that the angle of the inclination of the housing unit floor is about 1/40 to 1/60
corresponding to the estimated angle of the gap between the buildings.

MICROTREMOR MEAURMENTS
As mentioned before, it is expected that pile heads are destructed in some pile groups of the building A.
Damage at the pile head may be considered to make the building lean to the south. Detailed
investigation under the foundation such as excavation of the ground will be conducted in the near
future. As preliminary investigation before, microtremor measurements were conducted in the building
A and B. The information obtained from microtremor measurements is expected to understand thedamage mechanism and help the demolition work.
Microtremor measurements were carried on 2011/10/07. The weather condition was clear andpartly cloudy with strong wind in mainly NS direction. The objective of the microtremor measurement
is to obtaining the vibration characteristics of the leaned building. Measutrment between 14F and 1F
was preferable. However, length of the cables was limited. Therefore, the measurements were split
into the two stages of 7F/1F and 14F/7F. In each measurement, two observation points such as 7F/1F
and 14F/7F were distributed in the building. Position of each sensor is shown in Fig. 6 by star marks.
Each observation point has two sensors for horizontal direction (EW and NS), one sensor for verticaldirection (UD). Sensor is the electromagnetic velocity-meter (VSE-15D, Tokyo Sokushin Co.Ltd.).
The duration of one measurement is 10 minutes and sampling frequency is 100Hz. The time windowof which length is 40.96 (s) is applied to the observed velocity record for the data processing. The time
window is moving toward the end of the record overlapping with the previous time window by half
length of 20.48(s). The Vibration characteristics of the buildings are estimated by taking the average ofthe results from the all the trials in the followings. The number of trial is 27.
Amplitude ratios between 7F/1F and 14F/7F of the building A are shown in Fig. 8. In case of thebuilding A, transverse direction of the building corresponds to NS direction, longitudinal direction to
EW direction as indicated in Fig. 6. The peak considered the first natural frequency of the building
appears around 1Hz in both of TR and LN direction in the case of 7F/1F. LN direction has the second
peak around 4Hz that is distinctive of the direction. The range of the peak around the first natural
frequency tends to be wide in the two horizontal directions. The day when the microtremor
measurements conducted was quite windy. Especially, the wind in NS direction corresponding to TR
direction of the building was severely strong. Therefore, the results suggest that the effect of the windy
condition might appear in the vibration characteristics of the building. In the results of 14F/7F, peaks
can be observed around 2Hz. While TR direction has three sharp peaks between 2Hz and 3Hz, thepeak around 2Hz in LN direction looks ambiguous.
Fig. 9 shows phase difference of 7F/1F and 14F/7F. If the peak around 1Hz of 7F/1F is considered
the first natural frequency, behavior of phase difference looks odd. The behavior of phase difference
around 1Hz in 7F/1F may indicates the effect of strong wind. Strange behavior around 1Hz such as
7F/1F cannot be observed in the result of 14F/7F.

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20
Build. A(7F/1F)

15

Amplitude Ratio

Amplitude Ratio

20

TR(NS)
EW(LN)
UD

10
5

0

1

2

3

4

5

6

7

8

9

10

Build. A(14F/7F)
15

TR(NS)
LN(EW)
UD

10
5

0

1

2

3

Frequency (Hz)

4

5

6

7

8

9

10

Frequency (Hz)Fig. 8 Amplitude ratios between 7F /1F and 14F/7F of the building A
180

120

Phase Diff. (deg.)

Phase Diff. (deg.)

180
Build. A(7F/1F)

60
0
-60

TR(NS)
LN(EW)
UD

-120
-180

120

BUild. A(14F/7F)

60
0
-60

TR(NS)
LN(EW)
UD

-120
-180

0

1

2

3

4

5

6

7

8

9

10

0

Frequency (Hz)

1

2

3

4

5

6

7

8

9

10

Frequency (Hz)Fig. 9 Phase differences between 7F /1F and 14F/7F of the building A
Amplitude ratios between 7F/1F and 14F/7F of the building B are shown in Fig. 10. In case of thebuilding B, It should be noted that transverse direction of the building corresponds to EW direction,
longitudinal direction to NS direction as indicated in Fig. 6. Those are different from the building A.
The peak considered the first natural frequency of the building appears clearly around 1Hz in both of
TR and LN direction in the case of 7F/1F. In LN direction, the second peak can be clearly foundbetween 3Hz and 4Hz. Amplitude ratios of 14F/7F indicate that the first peaks appear around 2Hz inhorizontal directions as well as the building A.
Phase difference around 2Hz in the case of 7F/1F may indicate the effect of strong wind similarly
as the building A. The effect of strong wind seems not to be observed in phase difference of 14F/7F.
Through Fig. 8 to Fig. 11, the first natural frequency of the building seems to be between 1Hz and2Hz. The second peak can be observed between 3Hz and 4Hz only in LN direction. Second peakcannot be recognized in TR direction. Based on this result, it can be assumed that dynamic behavior in
LN direction tends elastic on the other hand, rigid behavior in TR direction. Phase difference aroundthe first natural frequency in 7F/1F indicates the effect of strong wind. The reason that the effect ofstrong wind cannot be recognized in 14F/7F can be due to the assumption that the transfer function
between 14F and 7F is isolated from the effect of the dynamic behavior of the ground.

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20
Build. B(7F/1F)

15

Amplitude Ratio

Amplitude Ratio

20
TR(EW)LN(NS)
UD

10
5

0

1

2

3

4

5

6

7

8

9

Build. B(14F/7F)
15

TR(EW)LN(NS)
UD

10
5

10

0

1

2

3

Frequency (Hz)

4

5

6

7

8

9

10

Frequency (Hz)Fig. 10 Amplitude ratios between 7F /1F and 14F/7F of the building B
180
Build. B(7F/1F)

120

Phase Diff. (deg.)

Phase Diff. (deg.)

180

60
0
-60

TR(EW)LN(NS)
UD

-120
-180

Build. B(14F/7F)

120
60
0
-60

TR(EW)LN(NS)
UD

-120
-180

0

1

2

3

4

5

6

7

8

9

10

0

1

2

3

Frequency (Hz)

4

5

6

7

8

9

10

Frequency (Hz)Fig. 11 Phase differences between 7F /1F and 14F/7F of the building B
20
Buid. A
15

Amplitude Ratio

Amplitude Ratio

20

TR(NS)
LN(EW)UD

10
50

Build. B
15

TR(EW)LN(NS)
UD

10
5
0

0

1

2

3

4

5

6

7

8

9

10

0

1

2

3

Frequency (Hz)

4

5

6

7

8

9

10

Frequency (Hz)Fig. 12 Amplitude ratios between 14F/1F of the building A and B
180

Phase Diff. (deg.)

Phase Diff. (deg.)

180
120
60
0
Build. A-60

TR(NS)
LN(EW)
UD

-120
-180
0

1

2

3

4

5

6

7

8

9

120
60
0
Build. B-60

TR(EW)LN(NS)
UD

-120
-180

10

0

Frequency (Hz)

1

2

3

4

5

6

7

Frequency (Hz)Fig. 13 Phase differences between 14F/1F of the building A and B

1188

8

9

10

Combining transfer function of 7F/1F and 14/7F, transfer function between 14F/1F is estimated.
Estimated amplitude ratios and phase differences between 14F/1F are shown in Fig. 10 and Fig. 11,
respectively. Left figures are the results of the building A, and right figures are those of the building B.
Amplitude ratios of the building A show the peaks around 1Hz having the relatively wide range.
Especially, the peak in LN direction split into two peaks around 1Hz. LN direction has also the second
peak between 3Hz and 4Hz. Amplitude ratios of the building B have clear peaks around 1Hz in bothhorizontal directions. LN direction has the second peak between 3Hz and 4Hz as well as the building
A. Namely, TR direction has one peak around 1Hz, on the other hand, LN direction has two peaks
around 1Hz and between 3Hz and 4Hz. This result can be interpreted as the fact that dynamic behaviorof TR direction is close to that of rigid medium with rocking motion at the bottom. Phase difference in
UD direction of the building A fluctuates around 1Hz. Broaden wide range of the first natural
frequency and phase difference behavior around 1Hz in the building A can be considered to be
relevant to the effect of strong wind.

5

25

Amplitude Ratio

H/V Spectral Ratio

30

20
15
10
5

1F/FF

4

NS
EW
UD

3
2
1

0
0

1

2

3

4

5

6

7

8

9

10

0

Frequency (Hz)

1

2

3

4

5

6

7

8

9

10

Frequency (Hz)Fig. 14 H/V spectrum of the ground and amplitude ratios between 1F and the free field

Measurement to obtain the dynamic characteristics of the ground was also carried out. Results ofthe ground are expressed by “FF (Free field)”. H/V spectrum and amplitude ratios between 1F and FF
are shown in Fig. 14. The peak of H/V spectrum exists around 1Hz. This result doesn’t meet agreewith the results from the theoretical transfer function using estimated shear wave velocity data shownin Fig. 5. Peaks are also observed around the natural frequency of the buildings in amplitude ratios of
1F/FF.

CONCLUSIONSIn this paper, microtremor measurements of the leaned building and the relevant investigation were
presented as the preliminary investigation of the damaged buildings by the 2011 off the Pacific coastof Tohoku earthquake. The results are summarized as follows.
1) Uplifting behavior around the foundation of the leaned building was observed. Broaden shear
cracks on non-structural walls were observed in almost all the floor. This damage is the sameas the case of 1978 earthquake.
2) Gap between the building A and B generated by the main shock of 2011/03/11 indicates over
1,000mm at the roof floor. Gaps at all the floor increased after 2011/04/07 aftershock.
3) The first natural frequency of the building is estimated around 1Hz for both buildings. The
peak in TR direction of the building A indicates to distribute in wide range showing the effectof strong wind.

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4) While TR direction has only one peak in amplitude ratio, LN direction has two peaks. This
result leads to the interpretation that dynamic behavior in TR direction is close to rigid
medium with rocking motion at the bottom.
5) H/V spectrum shows that the natural frequency of the ground may exists 1Hz.
Detailed investigations such as excavation under the foundation and the inspection at the pile tops
are expected in the near future.
ACKNOWLEDGMENTS
The authors are deeply grateful to Mr. Shuji TAKE of Sendai city, Mr. Tatsuo SASAKI, technicalofficer of Tohoku University, and students of Prof. Motosaka’s Lab. of Tohoku Univ. Earthquake
records to obtain velocity response spectra were provided by K-NET.

REFERENCES
Shiga, T. (1980). “Earthquake damage of concrete piles.” Concrete Journal, Vol. 18, No. 8, 14-21.
Sugimura, Y. and Oh-oka, H. (1981). “Report on the damage of precast prestressed concrete piles
during the 1978 Off-Miyagi prefecture earthquake.” Kenchiku Kenkyu Shiryo BRI, No. 31.]
Ohta, Y. and Goto, N. (1976). “Estimation of S-wave velocity in terms of characteristics indices ofsoil”, Buturi-Tanko, Vol.29, No.4, 34-41

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