Proceedings of the International Symposium on Engineering Lessons Learned fromthe 2011 Great East Japan Earthquake, March 1-4, 2012, Tokyo, Japan

SITE RESPONSE CHARACTERISTICS
IN SENDAI CITY
ESTIMATED FROM ACCELERATION RECORDS
OF THE 2011 TOHOKU EARTHQUAKE, JAPANSusumu OHNO1, Masato MOTOSAKA2, Kazuya MITSUJI3 and Akihiro SHIBAYAMA4
1

Associate Professor, Disaster Control Research Center, Tohoku University,
Sendai, Japan, ohnos@archi.tohoku.ac.jp
2
Professor, Disaster Control Research Center, Tohoku University,
Sendai, Japan, motosaka@archi.tohoku.ac.jp
3
Associate Professor, Faculty of Education, Art and Science, Yamagata University,
Yamagata, Japan, mitu@e.yamagata-u.ac.jp
4
Assistant Professor, Disaster Control Research Center, Tohoku University,
Sendai, Japan, ashiba@archi.tohoku.ac.jp

ABSTRACT: Site amplifications and distribution of strong-motion spectra in Sendai, thelargest city in the damaged area, are estimated for the 2011 Tohoku earthquake
sequences by using the DCRC strong-motion network records and subsurface structure
models in Sendai. Short-period (less than 1s) amplifications are predominant at
northwestern area, while not only short but also long-period (around 3s) amplifications
are predominant at southern area of Sendai.
Key Words: Great East Japan earthquake, strong motions, site amplification, Sendai

INTRODUCTION
During the M9.0 Tohoku earthquake, 3/11/2011, large accelerations were observed over the wide area
along Pacific coast of Tohoku district, Japan. In the damaged area, Sendai is the largest city and had
experienced the 1978 Off-Miyagi earthquake, M7.4. We have been conducting strong-motion
observation in Sendai since 2004, and we obtained earthquake records during the fore, main andaftershocks of the 2011 Tohoku earthquake sequences. In this paper, we investigate the site response
characteristics and estimate spatial distribution of strong-motion spectra in Sendai using thisstrong-motion network records.

395

STRONG MOTION OBSETVATION IN SENDAI
Fig.1 shows the location of observation stations in Sendai. In Sendai, there are two major strong
ground motion networks: Small Titan by Tohoku Institute of Technology (Kamiyama et al., 2001;
Kamiyama, 2011) and DCRC network by Disaster Control Research Center of Tohoku University(Ohno et al., 2004). Almost all DCRC stations are located on the 1st floor of low-rise building(simultaneous observation with top floor at some places).
Sendai has a complicated subsurface structure. Fig. 2 shows the depth distributions of engineering
bedrock (S-wave velocity of 0.7km/s) and seismic bedrock (S-wave velocity of 3.0km/s) of subsurface
structure model used for earthquake damage estimation in Sendai (Sendai City, 2002).
Nagamachi-Rifu fault, a reverse-type active fault dipping to NW direction, crosses the central part ofcity area in NE-SW direction. West side of the fault is terrace and east side is lowland (alluvial
deposits). Thickness of surface soil shallower than engineering bedrock at the ease side of the fault is
up to 80m and deeper than that at the west side. On the other hand, thickness of the deep part ofsubsurface structure (from ground surface to seismic bedrock) becomes deeper from east to west
(Watanabe and Motosaka, 2002).

Fig. 1 Location of DCRC strong-motion stations in Sendai

Fig. 2 Depths of engineering bedrock and seismic bedrock in Sendai (Sendai City, 2002)

396

Table 1 shows 1-D subsurface structure models at several DCRC stations. The models at No.8, 23,
27, and deep part (deeper than engineering bedrock) at No.25 are adopted from the 250m-mesh modelof Sendai City (2002). The shallow part at No.25 is adopted from the optimized soil model for station
NAGA, nearly locating at No.25, in Sendai vertical array by Satoh et al. (1994).
Table 1 1-D subsurface structure models(a) No. 8(c) No. 23

idth
(m)

density(t/m )

Vs Soil(m/s) Type

5.0

1.5

100 Ac2

40.0

2.1

500

Dg

1300

Mi

15.0

13.3

28.8

82.1

0

3

2.0

2.0
2.3
2.4

2.6

450 Ag2
700

2000

3000

Pl
P

Width

density(m)(t/m )

3(b) No. 27
Soil

Width

density(m/s) Type(m)

(t/m )

Vs

10.0

1.90

280

D

288.1

306.2

2.00

700

Pl

409.5

273.0

2.30

1300

Mi

227.3

357.9

2.40

2000

P

0

0

2.60

3000

3

Vs Soil(a) No.Type8(m/s)(c) No. 23
Widthdensity(m)(t/m )

2.00
700
Pl
5.0
Width density
Vs Soil2.30 1300 3 Mi
15.0(m)
(t/m ) (m/s) Type2.40 2000
P
40.0
10.0
1.90
280
D
2.60 3000
213.3
306.2
2.00
700
Pl
128.8
273.0
2.30 1300 Mi
282.1
357.9
2.40 2000
P
002.60 3000

3

Vs Soil(b) No.
27
(m/s)
Type

8.2

1.75

290

Dg

2.10

600

Dg

24.5

1.95

530

Pl

242.4

2.00

700

Pl

Engineering

335.4

2.30

1300

Mi

Bedrock

226.0

2.40

2000

P

0

2.60

3000

Width

density(m)(t/m )

1.5
100 Ac2
0.8
Width density
Vs Soil
2.0
450 Ag2
4.0
3(m)(t/m ) (m/s) Type
2.1
500 Dg
4.5
288.1
2.00
700
Pl
2.0
700 Pl10.9
409.5
2.30 1300 Mi
2.3 1300 Mi
8.2
227.3
2.40 2000
P
2.4 2000
P28.1
02.60 3000
2.6 3000
24.5(d) No. 25
D: backfill
density
Soil sand, Ag*: Holocene gravel;
Ac*:Width
Holoceneclay, As*: Vs
Holocene3) (m/s) Type(m)(t/m
Dg: Pleistocene
Gravel
D: backfill
0.8
105
Ac2
PI: Pliocene,
Mi:1.65
Miocene,
P: Paleozoic
Ac*: Holocene clay, As*: Holocene sand, Ag*: Holocene gravel;
4.0
1.65
105 As1
Dg: Pleistocene Gravel
4.5
1.95
290 Ag1
PI: Pliocene, Mi: Miocene, P: Paleozoic
10.9
1.70
170 Ac3
28.1(d) No. 25

242.4
335.4
226.0

Seismic
Bedrock

OBSERVED RECORDS
Table 2 shows a list of observation records by the DCRC network for the 3/9 foreshock, 3/11
mainshock, 4/7 and 4/11 aftershocks. During the 3/11 mainshock, records at 14 of 21 stations wereobtained. PGA and PGV range 318-840 Gal and 30-88 cm/s, respectively. The largest acceleration(822Gal) and seismic intensity (6.2) in the DCRC network was observed at No.9. At the otherorganizations, 1517 Gal was observed at K-NET 013, where boil sand and acceleration spikes
probably due to soil liquefaction were observed. In Sendai, the 4/7 aftershock also caused severe
damage, as did the mainshock. PGA and PGV of this aftershock range 167-767 Gal and 14-76 cm/s,
respectively.
Fig. 3 shows the mainshock velocity waveforms in observation components near NS direction at all
DCRC stations, with JMA E06, K-NET MYG013, and ground station of Izumi electric power building(IZU), Tohoku Electric Power Co. Velocity waveforms in Fig. 3 are calculated from acceleration
records with low-cut frequency of 0.02Hz. Two major wave groups (hereafter Part A and Part B of themainshock) can be commonly identified. There are gaps in waveforms at some QDR stations in Table
2. This gap is due to limitation of QDR that the record length of one file is up to 100s.
Fig. 4 compares pseudo velocity response spectra (5% damping) at stations locating east and west
sides of Nagamachi-Rifu fault. The spectrum at No.27, locating near Sendai railway station, iscommonly plotted in each side as a reference, because this station locates on the engineering bedrockas shown in Table 1. Spectra at west sides are equal to or relatively larger at short period (less than 1
second) than the No.27 spectrum, while the spectra at east sides are significantly larger than the No.27
spectrum, especially around 1 and 3 seconds. This is clearly due to the difference of subsurface
structures described before.

397

0

3

Vs
(c) N
(m/s)

1.65
105
Width densit
1.65
105
(m)
(t/m
1.95
290
5.0
1.
1.70
170
15.0
2.
1.75
290
40.0
2.
2.10
600
213.3
2.
1.95
530
128.8
2.
2.00
700
282.1
2.
2.30 1300
02.
2.40 2000
2.60

3000

Part APart B

Fig. 3 Velocity waveforms of the DCRC records for the 2011 Tohoku earthquake
10

100

(cm)

10

1000
100
(Gal)

100
10

10
1
2011.03.11
27
9
E06

1

0.1

1
Period (s)

5
10
IZU

8
21

(cm)

East of
Nagamachi-Rifu Fault

h=5%
Pseudo Velocity Response (cm/s)

Pseudo Velocity Response (cm/s)

West of Nagamachi-Rifu Fault

100

h=5%

100
(Gal)

100
10

10
2011.03.11
27
3
18
20
23
25
MYG013

1

0.1

10

1000

1
Period (s)

1
4
22

10

Fig. 4 Pseudo velocity response spectra of the DCRC records for the 2011 Tohoku earthquake

398

Table 2 List of earthquake records by DCRC strong-motion network, Tohoku University
Origin Time (JST)Area
Mw, DepthType

2011/4/11
Eastern Fukushima Pref.
Mw 6.6, Depth 6km
Shallow Inland
PGA

No Sensor Station(cm/s2)

2
3
4
5
8
9
10
12
14
16
18

ETNA

Rokugo Elementary School

54

ETNA

Furujiro Elementary School

48

ETNA

Higashi Rokugo Elementary School

QDR

Daiichi Jr. High School

QDR

Shogen-Chuoh Elementary School

QDR
QDR

Matsumori Elementary School
Miyagi Prefecture Library 1F

QDR

Seiryo SecondarySchool 1F

QDR

Tsurugaya Elementary School 1F

QDR

Nakano Jr. High School 1F

QDR

Okino Elementary School 1F

20
21
22
23
24

QDR

Minami Koizumi Elementary
School

QDR

Nishitaga Jr. High School

QDR

Tomizawa Jr. High School

QDR

East Water Supply Center

QDR

25
26
27

QDR

Ryutaku-Ji
Nagamachi Minami Community
Center

QDR

Aoba Ward Office

SSA-1 Sumitomo Seimei Bldg.

PGV*(cm/s)
5.5

2011/4/7
Off Miyagi Pref.
Mw 7.1, Depth 66km
IntraSlabPGA
JMAInt. (cm/s2)
3.9
311

PGV*
(cm/s)JMA
Int.

Missing

30.6
Missing

5.7

360

31.8

5.6

512

77.6

6.2

220
186
232
472

25.7
16.4
21.1
37.3
Removed

5.3
5.0
5.2
5.8

381
400
416
613

63.0
45.1
54.6
75.4
Missing

5.6
5.5
5.7
6.1

19
23
29
30

2.4
3.0
3.2
2.6
Missing

3.1
3.4
3.4
3.3

Missing
264
29.5
5.5
494
69.3
Missing
43
6.0
3.7
318
21.9
5.2
31
3.9
3.5
167
14.0
4.9
318
29.2
* cut-off period of 10s, *** 50s
cut-off period of 10s, ** 50s

6.0

59
24
15

6.0
3.2
2.2

4.0
3.3
3.1

34
54
53
95

6.6
6.4
8.8
6.0
Removed

3.6
3.9
3.9
4.1

230
534
767
279

22.4
5.1
Removed
19.3
5.1
25.3
5.5
75.5
6.2
18.0
5.0
Missing

PGA
JMA
Int. (cm/s2)

432

30
48
81
71

251

PGA
JMA
PGV**
Int. (cm/s2) (cm/s)
5.7
Missing

2011/3/9
Far E Off Miyagi Pref.
Mw7.2, Depth 8km
Subduction

24
29
28
30
46
20
19
20
40
37

47
76

7.3
3.7
Removed
6.3
3.8
5.3
3.9
Missing
Missing
5.4
3.7
4.3
3.5
7.8
4.3
6.9
4.1

PGV*
(cm/s)
42.1

2011/3/11
2011 Tohoku Eq.
Mw 9.0, Depth 24km
Subduction

320
613
383
840
822
407

59.5
74.2
39.4
60.4
85.7
62.7
Missing
Missing
Missing

5.7
6.0
5.6
6.0
6.4
5.6

5.3

3.1
3.4
2.9
2.2
4.2
2.4
3.5
1.9
3.2
3.5

3.3
3.4
3.5
3.2
3.7
3.2
3.3
3.1
3.6
3.5

SITE AMPLIFICATION FACTORS
Fig. 5 compares pseudo velocity response spectra in NS component from the 3/9 foreshock to the
4/11 aftershock at No.8 (northwestern side), No. 27 (near Sendai station), No.23 (Oroshi-machi, eastside of the fault), No.25 (southeastern side) stations. It is found that the amplitudes of Part B are equal
to or larger than that of Part A of the mainshock, and the short-period amplitude of the 4/7 aftershockis almost the same as that of Part A of the mainshock at some stations. At No.25, two predominantperiods (around 3s and less than 1s) can be identified and the shorter predominant period clearlyshows dependency on the amplitude level. This may due to the nonlinearity of the surface soil.
In Fig. 5, soil amplification factors from seismic bedrock to ground surface are also plotted. These
amplification factors are calculated by equivalent linear analysis described in the later section with
subsurface structure in Table 1. Long predominant period amplification, affected by deep undergroundstructure from seismic bedrock, commonly appear around 2-3s at 4 stations. At periods shorter than 1s,
the predominant period and amplification factors are strongly affected by shallow subsurface structure
above engineering bedrock. Thus variations of the short-period amplifications at deep alluvial sites,
No.23 and 25, are larger than those of the other two stations due to the nonlinear soil amplification.
Fig. 6 compares empirical (response spectral ratio of NS components) and theoretical amplification
factors (1-D amplification shown in Fig. 5) at No.8, 23, 25 versus No.27. Although the empirical ratios
are scattered, the period characteristics are similar to the theoretical ratios at periods shorter than 1s.
At periods longer than 1s, almost no amplification is expected at No.25 by the 1-D theory, while about
twice amplifications around 3s are calculated by the observation. This difference is probably due to thesurface waves, as clearly shown in the later phases at southern area (No. 22 and 25) in Fig. 4

399

(a)

(b)

(c)

(d)

Fig. 5 Pseudo velocity response spectra and equivalent linear soil amplification factors of the DCRC
records for the 2011 Tohoku earthquake sequence

400

Observation

1-D Eq.-linear Theory(a) No.8(b)No.23(c) No.25

Fig. 6 Ratios of observed response spectra and theoretical amplification factors
versus those of No.27 station

DISTRIBUTION OF RESPONSE SEPCTRA
Using the method of Ohno and Shibayama (2010), we estimate distribution of response spectra at
ground surface in 250m-mesh over the city area of Sendai. Fig. 7 shows a flowchart of the method,which takes into account the effects of subsurface structure including nonlinear amplification of
surface soil and spatial correlation of the spectra. The procedure is composed of 3 steps: 1) estimate
response spectra at outcropped seismic bedrock at observation station location, by recursively
applying equivalent linear spectral modal analysis of 1-D subsurface structure (linear analysis for thepart deeper than engineering bedrock), 2) estimate spatial correlation of outcropped spectra andinterpolate in 250m-mesh by ordinary kriging method, 3) estimate ground surface spectrum in eachmesh by equivalent linear spectral modal analysis.
Observed Response Spectrum!"#$%&'()*

Eq. Linear Soil Response (Recursively)

Response Spectrum at Bedrock•Period-dependent spatial correlation+,-./.0#1

•Spatial Interpolation (kriging)Interpolated Response Spectrum at Bedrock+,-./.0#1

Eq. Linear Soil Response at each mesh

Response Spectrum at Ground surface

Fig.7 Flowchart of estimating distribution of response spectra in Sendai (Ohno and Shibayama, 2010)401

We used the subsurface structure model (including nonlinear soil properties of soil) in Sendai City
(2002) and used the records of DCRC, JMA, NIED, PARI, BRI, Tohoku Electric Power Company.
Fig. 8 and Fig. 9 show the estimation results at periods of 0.2, 1, and 3.2s for parts A and B of themainshock, and for the 4/7 and 4/11 aftershocks. The following tendencies are found:1) Strong-shaking areas are different by period. At 0.2s, amplitude at the northwestern area islarger than the other area. This is due to the shallow soil over engineering bedrock. An
exception is the 4/11 shallow inland aftershock, probably due to the lack of short-period
incident waves by strong attenuation in propagating the shallow crust.
At 1.0s, amplitudes at the eastern side of Nagamachi-Rifu fault are larger, due to the surface soil
amplification.
At 3.2s, amplitudes at the southern area are larger, due to the deep underground structure
including existence of surface waves as discussed before. The 4/11 aftershock again shows adifferent distribution. In this earthquake, large amplitude area expands from southern side tonorthwestern side. The similar distribution was found for the 6/14/2008 Iwate-Miyagi-Nairiku
shallow inland earthquake (Ohno and Shibayama, 2010).
2) The amplitudes of Part A are larger than those of Part B during the mainshock.
3) Amplitudes at NS direction are larger than those at EW direction during the mainshock (Parts A
and B), while EW direction is predominant at the 4/7 aftershock. At the 4/11 aftershock, NS
direction is significantly predominant at long periods.

CONCLUSIONS
Site amplifications and distribution of strong-motion spectra in Sendai are estimated for the 2011
Tohoku earthquake sequences by using the DCRC strong-motion network records and subsurface
structure models in Sendai.
Short-period (less than 1 second) amplifications are observed in the northern area, while not only
short but also long-period (around 3s) amplifications were observed in the southern area. Also,
significant amplitude-dependencies of predominant periods were shown at some alluvial sites. This
short period amplifications are mainly due to the shallower part above engineering bedrock. The
long-period amplifications are partly explained by 1-D response of subsurface structures above
seismic bedrock, while surface wave contribution cannot be ignored at some sites locating southernpart of Sendai.

ACKNOWLEDGMENTS
Strong motion records from National Research Institute for Earth Science and Disaster Prevention(NIED), Port and Airport Research Institute (PARI), Japan Meteorological Agency (JMA), Building
Research Institute (BRI), and Tohoku Electric Power Company are used.

402

(a) 20110311 (Part A)(b) 20110311 (Part B)
Fig. 8 Estimated distribution of response spectra at ground surface for the 2011 Tohoku earthquake403

(a) 20110407(b) 20110411
Fig. 9 Estimated distribution of response spectra for the aftershocks of the 2011 Tohoku earthquake

404

REFERENCES
Building Research Institute and Japan Association for Building Research Promotion (1999). “Sendai
high-density strong-motion array comprehensive report”. (in Japanese)
Kamiyama, M., Shoji, Y. , Matsukawa, T., Asada, A., and Nakai, N. (2001). “An On-Line Array
Observation System of Earthquake Motions: The Deployment and Records”, Journal of Structural
Mechanics and Earthquake Engineering, Vol. 698, 283-298. (in Japanese with English Abstract)
Kamiyama, M. (2011). http://www.st.hirosaki-u.ac.jp/~kataoka/Tohoku_EQ/Kamiyama__SmallTitan_01.pdf. (in Japanese)Ohno, S., Motosaka, M., Sato, T., and Yamamoto, Y. (2004). “Strong-motion Observation Network
for Seismic Capacity Evaluation of Buildings in Sendai, Japan”, Summaries of Technical Papers ofAnnual Meeting, Architectural Institute of Japan, Structures II, 1075-1076. (in Japanese)Ohno, S. and Shibayama, A. (2010). “Estimation of spatial distribution of response spectra consideringsoil amplification and spatial correlation of ground motions”, Proc. 13th Japan Earthquake
Engineering Symposium, GO1-Thu-PM-10. (in Japanese with English Abstract)
Satoh, T., Kawase, H. and Sato, T. (1994). Engineering bedrock waves obtained through theidentification analysis based on borehole records and their statistical envelope characteristics,
Journal of Structural and Construction Engineering, No. 461, 19-28. (in Japanese with English
Abstract)
Sendai City (2002). “Heisei 14 Fiscal Year Earthquake Damage Estimation Report”. (in Japanese)
Watanabe, T. and Motosaka, M (2002). “Estimation of Deep Underground Structure in Sendai Area
based on Microtremor Observations and Strong-motion Prediction”, Proc. AIJ Tohoku Chapter
Architectural Research Meeting, No.65, 133-136. (in Japanese)405