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

EARTHQUAKE BEHAVIOR OF THE SURFACELAYER AT TOKYO BAY AREA DURING THE 2011 OFF
THE PACIFIC COAST OF TOHOKU EARTHQUAKEUSING THE DOWNHOLE ARRAY OBSERVATION
RECORDS
Takaaki IKEDA1, Kazuo KONAGAI2 and Toshihiko KATAGIRI3
1

Division chief, Research Institute of Technology, Tobishima Corporation,
Tokyo, Japan, takaaki_ikeda@tobishima.co.jp
2
Professor, Institute of Industrial Science, The University of Tokyo,
Tokyo, Japan, konagai@iis.u-tokyo.ac.jp
3
Technical Assistant, Institute of Industrial Science, The University of Tokyo,
Tokyo, Japan, toshi@iis.u-tokyo.ac.jp

ABSTRACT: Underground motions within a hard diluvial stratum were obtained in the
March 11th the 2011 off the pacific coast of Tohoku earthquake at a downhole in the Tokyo
Bay area, which has suffered serious sand-liquefaction over its long 42km2 stretch. It is
one of the sites of the seismic network carried out along the subway of the Tokyo Bay
area since 1976. The seismometers comprised of the accelerometer of 3 components
were installed in 3 depths. The seismometer of the deepest depth was installed in the
diluvial sand deposit equivalent to the base layer. The motions in the diluvial sanddeposit are characterized by their (1) long duration times with 6 to 8s components
surpassing others, (2) moderate amplitude of 51.1cm/s2 in PGA, which is estimated tohave resulted in moderate average strains of 8 to 9 × 10-4 and 5 to 7 ×10-4 over shallow -2
to -30m and deep -30 to -77m soil deposits, respectively. The peak strain reached in theinterior of soil was estimated to be about 2× 10-3.
Key Words: Downhole array observation, Tokyo bay area, Non-linear, ground motion,
the 2011 off the pacific coast of Tohoku earthquake

INTRODUCTION
On 11th March 2011, the 2011 off the pacific coast of Tohoku earthquake (hereafter Great East Japan
Earthquake) was happened in the subduction zone of the Pacific offing of the Tohoku region, Japan. The
area of the source area is regarded as around 500km × 200km or more over. The characteristic groundmotion that has a long duration time due to large source area was observed at many sites around Japan
including near source area (Furumura et al. 2011 and Kamae and Kawabe 2011).
The characteristic ground motion was also observed in Tokyo which is a little far from source area.
Maximum seismic intensity of 5+ was observed many places in Tokyo (JMA 2011). This is the second

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earthquake that seismic intensity 5+ was observed in Tokyo except the island after 1926. The house damagein Tokyo was serious, such as 13 complete destruction, half destruction 190 and partially destruction 4,016(FDMA 2011). And a lot of liquefaction occurred in the reclaimed land along the Tokyo bay and caused
heavy damage to embedded lifelines and dwelling houses. These damages are regarded as the typicaldamage of urban area in our country where an urban area developed on the soft ground such as the alluvion
ground. It is important to clarify ground behavior and nonlinear characteristic at Tokyo bay area during theGreat East Japan Earthquake for the earthquake disaster mitigation of the urban area in Japan against futureearthquake.
Konagai Laboratory, IIS, the University of Tokyo has performed downhole array observation in 2 sites
on the reclaimed land in Tokyo bay area. In the Shinkiba site, seismometers were installed in the 3 depths
including an engineering base. Furthermore, soil investigation was carried out at this site, and soil
properties including non-linear property become clear. Fig.1 shows location of Shinkiba site with otherobservation sites (K-NET of the National Research Institute for Earth Science and Disaster Prevention and
Bureau of Port and Harbor, Tokyo Metropolitan Government).
In this paper, firstly we introduce a summary of the Shinkiba downhole array observation including anobservation system and soil profile. Next, we report earthquake behavior of the surface layer at Tokyo bay
area during earthquake from observation records and earthquake response analysis.

EpicenterFig. 1 Location of seismic observation sites in Tokyo bay area.
A red filled triangle indicate Shinkiba site.

SHINKIBA DOWNHOLE ARRAY OBSERVATIONThe Shinkiba site is one of the seismometry sites carried out along the subway of the Tokyo bay area since1976. Fig.1 shows location of Shinkiba site. The ground around the Shinkiba site was filled up with thedredging sand from 1961 to 1970 (Endoh 2004 and Tokyo Geotechnical Construction Association
2004).
Fig.2 shows soil profile based on boring investigation. Soil deposits of G.L. to G.L.-7m is reclaimed
soil, therefore the depth of the sea bottom before reclamation is considered as G.L.-7m. G.L.-7m to-37.2m is Yurakucho layer of alluvium stratum. The layer deeper than G.L.-37.2m is diluvium stratum.
G.L.-37.2m to -56.0m is No.7 layer. Lower No.7 layer at G.L.-53.5m to -56.0m is a humus silt layer,
but the SPT-N value is relatively high (SPT-N value ≈ 30). Because the upper part of To layer iscomposed of sand deposit, an SPT-N value is high. The Ed layer is hard gravel deposit, and the SPT-N
value over 50. Therefore, Ed layer is regarded as the engineering base layer. Fig.3 shows geological384

cross-section at Tokyo bay area around Shinkiba site.
The Great East Japan Earthquake has caused sand-liquefaction over a long streach of reclaimed
lands along the coast of Tokyo bay. According to the survey by Konagai Laboratory, intenseliquefaction was found around Shinkiba station of Tokyo Metropolitan subway that is 350m awayfrom Shinkiba site, but was not found Shinkiba site (Konagai et al. 2011). Fig.4 shows liquefaction
findings around Shinkiba site by MLIT and JGS (Kanto Regional Bureau, MLIT and JGS 2011). In
these findings, the liquefaction is not found around Sinkiba site. We can understand that a liquefaction
area and a non-liquefaction area are separated with clarity in this area. The cause is regarded as thedifference of the material for reclamation, detailed study and survey will be necessary in future.
The SA-355CT (Type A) of Tokyo Sokushin Co., Ltd, is used for the seismometer of Shinkiba
downhole array observation, it is installed in three depth of G.L.-2m (surface layer), G.L.-30m (middle
layer) and G.L.-77m (base layer). Each seismometer also can observe 3 components acceleration at thesame time. The seismometer is installed to agree with a magnet direction. However, seismometers of
G.L.-30m and G.L.-77m have orientation error (Ikeda et al. 2011). Table 1 shows orientation error forthe seismometer of G.L.-2m. There may be an orientation error in a seismometer of G.L.-2m, but does
not correct it. Therefore, N-S component shows Y direction and E-W component shows X direction
respectively.

Fig. 2 Soil profile at Shinkiba site and installation depth of the seismometers.

Table 1 Orientation error of seismometer for the G.L.-2m
Depth
Orientation error
G.L.-30m
2
G.L.-77m
31
Sing of orientation error: + Counter clockwise direction- Clock wise direction

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Shinkiba downhole array observation site

Holocene
Reclaimed layer
Yuraku cho Layer(Sandy)
Yuraku cho Layer(Cohesive)
Yuraku cho Layer(Sandy)
Yuraku cho Layer(Cohesive)

Pleistocene
No.7 Layer(Sandy)
No.7 Layer(Cohesive)
Buried terrace sediment(Sandy gravel))
Edo gawa layer(Sandy)
Edo gawa layer(Cohesive)
Edo gawa layer(Sandy gravel)

Fig. 3 Geological cross-section around downhole array observation site

Shinkiba downhole
array observation site
N35.645681 E139.824562

Sand boiling or Water gushes out were observed
Sand boiling or Water gushes out were not observed
Liquefaction area
Non-Liquefaction area

Fig. 4 Liquefaction investigation results around Shinkiba downhole array observation site
after the 2011 off pacific of Tohoku earthquake(Revised Kanto regional bureau, MLIT and JGS 2011)

386

OBSERVATION RECORDS AT THE SHINKIBA SITE
DURING THE GREAT EAST JAPAN EARTHQUAKE
Fig.5 shows the waveforms of horizontal ground motions recorded at Shinkiba site during the Great
East Japan earthquake. The origin of X-axis is 14:47:03 on March 11th, 2011 (JST). The velocity data
and the displacement data were calculated from the acceleration data with bandpass-filtered(0.1-10.0Hz) by Fourier integration method. The acceleration amplitude becomes large at around 60
seconds and relatively-large amplitude continues until 140 seconds. The maximum amplitude occurs
from 120 to 130 seconds. On the other hand, the displacement waveform shows different form fromthe acceleration waveform. The large amplitude of the displacement waveform continues until about
340 seconds.
Table 2 shows maximum amplitude of the observation records. The vertical component of
G.L.-30m hasn’t recorded for the instrumental trouble. PGA of each seismometers are 126.8cm/s2 of
G.L.-2m, 96.9cm/s2 of G.L.-30m and 51.1cm/s2 of G.L.-77m respectively. By amplification propertiesof the surface layer, PGA of shallow depth is bigger than deep depth. On the other hand, PGD of eachseismometers are 10.6cm of G.L.-2m, 11.2cm of G.L.-30m and 9.7 of G.L.-77m respectively. PGD has
an almost same value at three depths.
Fig.6 shows pseudo velocity response spectra (PVRS) of horizontal direction with damping factorof 0.05. The predominance of the long period component from 6 to 8s is seen in a PVRS. It was
known that a surface wave with long period component occurred in the Kanto plain by basin structureof the Kanto district. When the Niigataken Chuetsu-oki Earthquake in 2007 occurred, a long period
ground motion with 7 seconds predominance was observed in Tokyo. Therefore it is thought that asurface wave was included in ground motion at Shinkiba site.
A broken line in Fig.6 is standard pseudo velocity response spectra (SPVRS) converted fromstandard acceleration response spectra of level 1 ground motion to use for collation of theearthquake-resistant performance of the land bridge (JRA 2002). The PVRS at G.L.-2m is at the samelevel as SPVRS.
Fig.7 shows transfer function of G.L.-2m for G.L.-77m. The transfer function means the Fourier
spectrum ratio smoothed in Parzen window with bandwidth of 0.2Hz. The red line indicates the Great
East Japan Earthquake and blue line indicate small earthquake (M3.8, 9th May 2010). PGA of
G.L.-77m of the small earthquake is less than 2cm/s2. The predominate frequency of the small
earthquake and the Great East Japan Earthquake is 0.83Hz and 0.74Hz respectively. It is estimated that
surface layer become nonlinearity during the Great East Japan Earthquake and shear stiffness
decreased. Equation (1) is derived by 1/4 wavelength rule when we supposed that behavior of thesurface layer controlled by a primary mode.
G* =Gs  f s =
Gw  f w 

2

(1)

Where, G* is decreasing rate of shear stiffness, Gs and Gw are shear stiffness at strong event andweak event, fs and fw are predominate frequency at strong event and weak event. The decreasing rateof shear stiffness is estimated 0.8 by equation (1).
Similarly, we estimate maximum shear strain of the surface layer using maximum relative
displacement and distance between the seismometer of the top and bottom. Table 3 shows maximum
relative displacement and maximum shear strain. The maximum shear strains are less than 10-3.
Therefore it is thought that the nonlinearity which occurred on the ground of the Shinkiba site during
the Great East Japan Earthquake was not so large.

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(a)Y direction(b)X direction
Fig.5 Waveform of observation record at Shinkiba site from main event

Table 2 Maximum amplitude of observation record at Shinkiba site
during the Great East Japan earthquake
Depth
PGA(cm/s2)
PGV(cm/s)
PGD(cm)G.L.
X
Y
XYmax ZX
Y
XYmax ZX
Y
XYmax Z-2m 102.7 115.7 126.8 60.4 24.9 26.2 26.5
6.6
9.5
10.3 10.6
3.5-30m 72.0 90.8 96.9
19.7 18.6 20.0
9.7
10.6 11.2
9.7-77m 44.8 46.2 51.1 27.4 14.0 13.2 14.2
4.8
8.3
9.7
3.2
XYmax: (X2+Y2)0.5 max
The UD record of G.L.-30m haven't recorded for the instrumental trouble

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Y Direction

X Direction

G.L.- 2m
G.L.-30m
G.L.-76m
Level 1

G.L.- 2m
G.L.-30m
G.L.-76m
Level 1

Fig.6 Pseudo velocity response spectra (PVRS) with damping ratio of 0.05 of ground motion recorded
at Shinkiba site during the Great East Japan earthquake. The standard pseudo velocity spectra of Level
1 ground motion of Specifications for Highway Bridges (Japan Road Association 2002) is indicated ina broken line.

GEJ (M9.0)
Small event(M3.8)

Fig.7 Comparison between the Great East Japan earthquake (M9.0) and small event(M3.8) transfer
function

Table 3 The relative displacement calculated from observation record andmaximum shear strain calculated from relative displacement
Maximum relative
Depth of seismometer
Maximum shear strain
displacement(cm)upper
BottomY-direction
X-direction
Y-direction
X-direction-4
G.L.-2m
G.L.-30m
2.30
2.49
8.2× 10
8.9× 10-4
G.L.-30m
G.L.-77m
2.25
3.05
4.9× 10-4
6.6× 10-4

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EVALUATION OF THE EARTHQUAKE BEHAVIOR OF THE SURFACE LAYERBASED ON THE EARTHQUAKE RESPONSE ANALYSIS
Soil investigation including laboratory test using the sample taken from boring hole was carried out at
the Shinkiba site, and data to set a parameter necessary for earthquake response analysis were
provided. We attempted to estimate earthquake behavior of the surface layer from earthquake response
analysis. The equivalent linear analysis (P. B. Schnabel et al. 1972) is selected for earthquake response
analysis method because the level of maximum shear strain of surface layer is about 10-3 (Nishimotoand Ikeda 2011). Table 4 shows analysis model for equivalent linear analysis. The analysis model iscomprised of 13 layers based on soil profile. Nonlinear property obtained laboratory test for the
sample taken from 9 depths on the site. Fig.10 shows nonlinear property used in this study.
Fig.11 shows depth distribution of maximum acceleration, maximum velocity, maximum
displacement, maximum shear strain and decreasing rate of shear stiffness (G/Go). G/Go is evaluatedfrom the convergence value of the equivalent linear analysis. The black filled circle overwritten on a
distribution is an observation result. The analysis results of maximum response can almost reproduce
an observation results. The average maximum shear strain calculated from observation record issuperimposed on the depth distribution.
The average shear strain of G.L.-2m to -30m and G.L.-30m to -77m calculated from earthquake
response analysis are 6.6 × 10-4 and 4.8 × 10-4 respectively, these values are consistent with average
shear strain calculated from observation record. The range of decreasing rate of shear stiffness is 0.71
to 0.91 and average is 0.83, this average value is in good agreement with estimation value fromequation (1)Fig.12 shows waveforms that superimposed an analysis on the observation. The PVRS with
damping factor of 0.05 at G.L.-2m and G.L.-30m are shown in Fig.13. The analysis results can almost
reproduce an observed one.
As a result of these considerations, it is thought that the earthquake behavior of the Shinkiba site
during the East Japan earthquake can almost reproduce by earthquake response analysis using
equivalent linear analysis. It is thought that the maximum shear strain is 1.3 × 10-3 and the surface
layer didn't happen rapid nonlinear during the Great East Japan Earthquake. The maximum shear
strain to occur in the sand layer where liquidizing is 5×10-4. This result is consistent with theliquefaction investigation result that is not finding liquefaction around Shinkiba site.

390

Table 4 Model parameters for earthquake response analysis (equivalent linear analysis) at Shinkiba site
Thickness
Dense
Vs
G
No.
Soil profile
Non-linearly
3)
(m)
(t/m
(m/s)
(kN/m2)
5.0
1
Fill
1.80
95
16245
Type1
7.5
2
Fill
1.85
125
28906
Type1
5.5
3
Fine sand
1.85
170
53465
Type2
10.0
4
Silt
1.70
155
40843
Type3
9.0
5
Silt
1.55
140
30380
Type4
4.2
6
Silt
1.60
160
40960
Type4
3.8
7
Fine sand
1.85
235102166Type5
1.5
8Fine sand
1.90
235104928
Type5
7.5
9
Silt
1.70
290
142970Type8
9.7
10Fine sand
1.90
290
159790Type6
7.1
11
Sand and gravel
1.95
400
312000
Type7
4.2
12
Sand
1.75
300
157500Type8
5.0
13Sand and gravel
2.00
400320000Linear
14
Base
2.00
400
320000
Vs: Shear wave velocity, G: Shear stiffness

Type1
Type2
Type3
Type4
Type5
Type6
Type7
Type8

Fig.10 Nonlinear properties which are G/Go – γ and h -γ of each layers

Fig.11 Depth distribution of maximum response calculated by earthquake response analysis.
A filled circle and red line had indicated observation results.

391

Fig. 12 Comparison of an observation waveform and an analysis waveform at G.L.-2m and G.L.-30m.
A black line means observation and a red line means analysis.

(a) G.L.-2m(b) G.L.-30m
Fig.13 Comparison of the pseudo velocity response spectra (PVRS) with damping factor of 0.05 of theanalyzed motion and observed motion at G.L.-2m and G.L.-30m. A black line means observation and a
red line means analysis.

392

CONCLUSIONSThis paper evaluated the earthquake behavior of surface layer at Tokyo bay area during the Great East
Japan Earthquake from downhole array observation records and earthquake response analysis using
equivalent linear analysis. The results are as follows.
(1) Horizontal maximum acceleration is 126.8cm/s2 of G.L.-2m, 96.9cm/s2 of G.L.-30m and 51.1cm/s2
of G.L.-77m.
(2) The pseudo velocity response spectra at G.L.-2m is at the same level as standard pseudo velocity
response spectra of earthquake ground motion of level 1 to use for collation of theearthquake-resistant performance of the land bridge.
(3) The predominance of the long period components were seen in an observation record of the 3
depths. It was thought the influence of the surface wave caused by the basin structure in the Kanto
district.
(4) Maximum shear strain during the Great East Japan Earthquake is estimated about 8 to 9×10-4 of
G.L.-2m to -30m and 5 to 7×10-4 of G.L.-30m to G.L.-77m by observation records. Similarly, thedecreasing ratio of shear stiffness is estimated to be around 0.8 times for initial value.
(5) Maximum shear strain of the surface layer estimated by earthquake response analysis is 1.3
×10-3.Therfore, It is thought that the surface layer didn’t show the rapid nonlinearity during theGreat East Japan Earthquake. This result is consistent with the liquefaction investigation result thatis not finding liquefaction around Shinkiba site.

ACKNOWLEDGMENTS
The ground motion records that we used in this study were obtained by the downhole arrayobservation which the Konagai Laboratory, IIS, the University of Tokyo has carried out bycommission from the Tokyo Metro Co., Ltd. We thank the Tokyo Metro Co., Ltd and everyone
concerned with Shinkiba downhole earthquake observation since 1976.

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