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

ESTIMATION OF A DEEP SHEAR WAVE VELOCITYSTRUCTURE OF CHIBA CITY BASED ON ARRAYS
OF ACCELEROMETERS
Shoichi NAKAI1, Toru SEKIGUCHI2 and Hiroki ISHINO3
1Professor, Department of Urban Environment Systems, Chiba University,Chiba, Japan, nakai@faculty.chiba-u.jp
2Assistant Professor, Department of Urban Environment Systems, Chiba University,Chiba, Japan, tsekiguc@faculty.chiba-u.jp
3
Graduate Student, Department of Urban Environment Systems, Chiba University,Chiba, Japan, hiro_ishino@chiba-u.jp

ABSTRACT: During the 2011 Tohoku earthquake, strong ground motions have beenrecorded at a number of stations in Chiba City that were installed by the authors andother institutions. From the viepoint of its magnitude and the epicentral distance, it ispossible to make an assumption that the surface wave is dominant in the record. In thisstudy, seismic records have been analyzed to obtain a phase velocity dispersion curve
based on the high resolution F-k analysis and the CCA analysis. The results match wellwith the existing results found in a literature.
Key Words:

2011 Tohoku earthquake, array of accelerometers, deep soil profile, phasevelocity dispersion curve, F-k analysis, CCA analysis

INTRODUCTION
A long period ground motion during an earthquake sometimes has a serious impact on large structures
such as bridges, high-rise buildings and oil storage tanks. During the 2011 Tohoku earthquake(Great East Japan earthquake), Chiba City, which is located east of Tokyo, has been hit by a fairly
strong ground motion with a very long duration time and has suffered from serious damage. An
explosion in flames of a liquefied petroleum gas tank is one of the most hazardous incidents that
occurred during the event, as shown in Photo 1.
It is well known that the evaluation of ground motion is critical when estimating damages due toearthquakes. Since the ground motion during an earthquake is largely affected by the so-called site
effect which is determined by the soil structure, it is important to obtain its profile, especially the shear
wave velocity profile. There are a number of methods available that are used for this purpose.
Among others, microtremor array measurement is one of the most popular ways and has been
conducted extensively. This measurement gives phase velocity dispersion curves from which it ispossible to estimate the soil profile through an inversion process. This process, however, is very
sensitive to changes in dispersion curves (Kurose and Yamanaka 2006). In addition, it is believedthat the power of microtremors in the longer period range is in a low level compared to shorter period339

ranges. To overcome this limitation, the joint inversion of more than one function has been pursued
by a number of researchers (e.g., Arai and Tokimatsu 2005, Kurose and Yamanaka 2006).

Photo 1 An explosion in flames of a liquefied petroleum gas tank (Asahi.com 2011)
In this study, an attempt has been made to estimate an shear wave velocity profile by seismometer
array measurements (Tomishima et al. 2010) during the 2011 Tohoku earthquake and its after shocks
in addition to the earthquakes prior to it, in which Chiba area was chosen as a target area. The result
was compared with that of microtremor measurement and the existing profile for deep soils.

OBSERVATIONS
Target area
The area of study is Chiba City, which is located about 50 kilometers east of Tokyo, Japan, as shownin Fig. 1. Fig. 2 shows a map of landform classification in this area, in which the location of thecensors used in this study is also plotted. As can be seen in Fig. 2, the landform of this area consistsof three categories: terrace, river valley and reclaimed ground.

Fig. 1 Locations of Chiba City and epicenter of the earthquakes considered in the study	 340

Fig. 2 Map of landform classification and sensor location
Earthquake observations
A total of 12 sensors (accelerometers) are deployed in this area, their locations are shown in Fig. 2.
Of these, two sensors marked as CHB009 and CHB024 are installed by the National Research Institutefor Earth Science and Disaster Prevention, or NIED, and are called K-NET sensors, while other two ofthem denoted by CKJ and KSN are installed by Japan Meteorological Agency, or JMA, and are called
seismic intensity meters. The rest of the sensors have been installed by the authors. As can be seenfrom the figure, these sensors are deployed in such a way that the distances between the sensors varyfrom 0.2 to 2 kilometers, which makes it possible to form a different size of array of sensors that cancope with the measurement of phase velocities of the wave in the period range of 1 to 5 seconds.
Recording time of our sensors is set to 300 seconds so that the coda part of the ground motion can berecorded.
Microtremor measurements
Although we are interested in a deep soil profile, it is believed that the shallow soil profile at the site
plays an important role when looking at the dynamic characteristics of the ground such as the phasevelocity dispersion curve. In addition, it is worthy of consideration to compare the earthquake
observation-based results with those from microtremors. From this point of view, a series of
microtremor array measurements were also conducted in the Nishi-Chiba campus of Chiba University(Tomishima et al. 2010). The array consists of five sensors on the circumference of a circle and one
at its center with the radius ranging from 22 to 222 meters.
Sensors we have used to measure microtremors are velocimeters.

ANALYSIS AND DISCUSSIONS
Theoretical dispersion curves
Fig. 3(a) shows the shear wave velocity profile of deep soils at Nishi-Chiba campus of Chiba
University found in literatures (J-SHIS 2010, Sato et al. 1999), while Fig. 3(b) shows that of shallow
soils constructed from a PS-logging carried out in the campus. Fig. 4 compares the two of thedispersion curves based on the theory of surface wave propagation in an elastic medium (Tokimatsu etal. 1992), in which each soil profile consists of the shallow part, i.e. Fig. 3(b), and the deep part, i.e.
one of the two plots in Fig. 3(a). Fig. 5 shows the phase velocity and the medium response	 

341

(Harkrider 1964, Saito et al. 1993) of each mode of Rayleigh wave for the soil model with deep soils
from J-SHIS.
From Fig. 4, it is found that the two dispersion curves are slightly different depending on thedifference between the corresponding deep soil profiles. It is also worthy of note that the first higher
mode of Rayleigh wave dominates over its fundamental mode in some of the period ranges such asnear 0.1 seconds, 2 to 4 seconds and 6 seconds or longer, in which the synthesized dispersion curve inFig. 4 shows fluctuation.
V [m/s]V [m/s]

SS

00

2000

4000

00500

Sato et al.

1000

PS-logDepth (m)J-SHIS

1000

10200020300030(a) Deep Soil(b) Shallow Soil

4000

40Fig. 3 Shear wave velocity profile of the target site (Nishi-Chiba campus of Chiba University)
2500
Sato et al.

Phase Velocity (m/s)

2000

J-SHIS

1500
1000
500
00.1

1Period (s)10Fig. 4 Theoretical dispersion curves
Recorded ground motions
From the last two and a half year of observation, we have recorded a number of seismic activities
including the 2011 Tohoku earthquake and its after shocks in addition to the earthquakes prior to themain shock. The locations of the epicenter of the earthquakes along with their magnitudes (Mj) arealso indicated in Fig. 1. It is seen that most of the earthquakes occurred in the region north to	 342

northeast of the target area except the one which occurred in the region southwest of the target area.
Table 1 summarizes the maximum accelerations and velocities (PGA and PGV) of the groundmotions recorded within this time period. If you take a closer look at the peak values of the 2011
Tohoku earthquake and then compare them with landform classification of the sensor locations shownin Fig. 2, it is understood that PGA’s observed on terrace and in lowland are of the same order, butthat PGV’s at lowland are larger than those at terrace. This implies that soil nonlinearity occurred in
the soft soil found in lowland.

Phase Velocity (m/s)

2500
Synthesized
Fundamental mode
1st higher mode
2nd higher mode
3rd higher mode

2000
1500
1000
500
0Medium Response

10-610-710-810-90.1

1Period (s)10Fig. 5 Phase velocity and medium response for each mode
Method of analysis for estimating phase velocities
There are a number of methods for estimating phase velocity of the soil. In this study, two methods
were used: high resolution F-k and CCA methods. The high resolution F-k spectrum analysis (Capon1969) together with the SPAC analysis (Aki 1957) is one of the most popular methods to calculate thedispersion curve of Raleigh waves. There is another method called the centerless circular array
analysis, or CCA, that is an extension to the SPAC analysis and is proved to be robust in thecomputation of this curve (Cho et al. 2006).
These analyses have their base on the correlation between the vertical recordings of the sensors
that constitute an array. The vertical recordings are used since they contain Rayleigh waves. This
notion is widely accepted in the case of microtremor. Seismic ground motion, however, contains not
only surface waves but also body waves. Thus, in order to apply these methods, a portion of data has
to be extracted from seismic recordings so that it contains surface waves.
We have looked at non-stationary spectra of the vertical component of the seismic record for thispurpose. A portion of the recorded accelerogram has been extracted where the spectrum shows adispersive nature which the surface wave is characterized by. Some of the example time histories
and their corresponding non-stationary spectra are shown in Fig. 6. Fig. 6(a) shows the groundmotion recorded at CHB024 and Fig. 6(b) shows the recordings at MAM, both during the main shock.
From the viewpoint of landform, CHB024 is located in the reclaimed ground and MAM is located on
the terrace as shown in Fig. 2.	 

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Array configuration
Both F-k and CCA methods require an array of sensors to be configured. As mentioned earlier, there
are a total of 12 sensors deployed in the target area. Unfortunately, however, not all the sensors havebeen functioning properly during the events. Fig. 7 shows the array configuration during the event of
the 2011 Tohoku earthquake. As illustrated in the figure, a total of four sensor arrays wereconfigured, each of which is composed of four sensors with different sensor distances ranging from1,500 to 4,500 meters. As for other events, one to four arrays were configured in each event.
Table 1 PGA and PGV of recorded ground motions
EQ

090809091218110311110319110411

Censor
No.2
No.3
No.4
No.6
MAM
NIS
No.1
No.2
No.3
No.4
No.6
CHB009CHB024
MAM
NIS
No.3
KSN
CKJ
MAM
NIS
No.3
No.4
No.5
No.6
CHB009CHB024
NIS
No.3
No.4
No.6

PGA (cm/s2)
EWNS44.7
37.8
25.1
31.4
42.8
34.2
10.1
8.4
14.6
22.0
9.7
9.9
11.9
14.4
14.2
12.0
8.3
8.4
12.0
12.6
139.7
178.7
202.5
234.4
206.4
188.2
343.6
480.5
172.1
157.6
282.3
177.4
163.8
306.0
24.7
35.7
63.2
71.0
30.2
22.0
26.4
23.7
27.5
39.1
29.0
26.4
32.0
43.1
34.9
79.1
90.4
39.6
34.4
33.7
34.3
54.3
39.5

UD
25.1
12.0
13.4
16.7
6.0
12.6
5.6
7.4
4.7
6.8
5.7
77.1
85.6
147.0
203.3
90.7
119.8
126.5
16.4
27.4
9.210.5
10.9
11.8
14.4
16.2
44.8
15.6
25.8
24.8

EW
1.4
1.2
1.5
0.3
0.4
0.3
0.3
0.3
0.3
0.4
22.1
31.5
21.5
23.5
22.6
21.7
22.8
1.2
2.4
1.1
1.0
1.2
1.5
3.5
3.4
3.7
3.0
2.5
3.4

PGV (cm/s)
NS
1.3
1.2
1.2
0.3
0.9
0.3
0.4
0.4
0.3
0.4
35.0
30.7
24.3
29.8
22.8
15.4
20.1
1.2
2.9
1.0
1.1
1.4
3.4
2.8
4.7
2.9
3.1
2.6

UD
0.5
0.5
0.5
0.5
0.2
0.2
0.1
0.2
0.1
0.1
0.1
10.7
14.0
12.6
14.6
11.1
10.3
12.1
0.5
0.7
0.5
0.5
0.5
0.5
1.2
1.5
1.4
1.1
1.2
1.1

F-k spectra
The extracted portion of an accelerogram has been further divided into a number of data segments,
each of which consists of 8,192 data points. These data segments are averaged so as to compute F-kspectra by following the high-resolution F-k analysis procedure. Fig. 8 shows some of the F-kspectra for several frequencies in the case of the Tohoku event. According to the figure, it is seenthat clear peaks are found in most frequencies within the scope of frequency intervals corresponding to	 344

the size of an array. In some cases, however, the shape of a spectral peak contour is an ellipse rather
than a circle due probably to an irregular or flat shape of the sensor array. It is also noted that thearrival direction of each F-k spectrum roughly coincides with the direction toward the epicenter of themain shock.

(a) CHB024(b) MAM

Fig. 6 Accelerograms and non-stationary spectra of the ground motion during the main shock

Fig. 7 Configuration of array of sensors for the main shock	 

345

(a) Main shock(b) Main shock(c) Main shock(d) Main shock(e) After shock / 110319(f) After shock / 110319

Fig. 8 F-k spectra

Phase velocity dispersion curve
Fig. 9 shows the phase velocity dispersion relationship obtained from both microtremor measurement
and earthquake observation results. Theoretical dispersion curves are also plotted in the figure. Of
the two theoretical plots, the one denoted as J-SHIS is computed based on the deep soil model which
was taken from the J-SHIS web site (J-SHIS 2010) and the shallow soil model given by a PS-logging.
The other one marked as Sato et al. is computed based on the deep soil model found in a literature(Sato et al. 1999) along with the PS-logging shallow soil model. In both cases, the dispersion curve
was computed by considering higher modes of Rayleigh wave in addition to its fundamental mode.
It can be pointed out from the figure that:
I In the period rage from 2 seconds to 4 seconds, the plot shows some variations, which may beexplained by the influence of the higher modes of Rayleigh wave.
I The phase velocity relationship based on microtremor measurements and earthquake
observations agrees very well with the theoretical dispersion curves.
I The observation-based result better matches with the theoretical one with the deep soil modeltaken from J-SHIS when compared with the one with the Sato et al. model.
I The relationship obtained from microtremor measurements and that from earthquake
observations overlap for a period range between 1.5 seconds and 3 seconds and are found to becontinuous.
From this result, it is possible to say that an array of seismometers can effectively be used toestimate the deep soil structure.	 

346

Fig. 9 Phase velocity dispersion curves for the target area

CONCLUSIONSIn this study, an attempt has been made to estimate a shear wave velocity profile by seismometer arraymeasurements during the 2011 Tohoku earthquake and its after shocks in addition to the earthquakes
prior to the main shock. The result was compared with that of microtremor measurement and theexisting soil profile. It was found from the study that:I The phase velocity dispersion curve obtained from earthquake observations overlaps smoothlywith that from microtremor measurements.
I The observation-based phase velocity agrees fairly well with the theoretical one computed basedon the previously reported soil profile.
I An array of seismometers can effectively be used to estimate the deep soil structure.

ACKNOWLEDGMENTS
The strong motion records at K-NET and JMA used in this study were provided by National ResearchInstitute for Earth Science and Disaster Prevention and Japan Meteorological Agency.

REFERENCES
Aki, K. (1957). "Space and Time Spectra of Stationary Stochastic Waves, with Special Reference toMicrotremors." Bull. Earthq. Res. Inst. Univ. Tokyo, 415-457.
Arai, H. and Tokimatsu, K. (2005). "S-wave velocity profiling by joint inversion of microtremor
dispersion curve and Horizontal-to-vertical (H/V) spectrum." Bull. Seism. Soc. Am., Vol.95, No.5,
1766-1778.
Asahi.com (2011). http://www.asahi.com/photonews/gallery/jishin/jishin1018.html
Capon, J. (1969). "High-Resolution Frequency-Wavenumber Spectrum Analysis." Proc. IEEE, Vol.57,
No.8, 1408-1418.
Cho, I., Tada. T. and Shinozaki, Y. (2006). "Centerless Circular Array Method: Inferring Phase
Velocities of Rayleigh Waves in Broad Wavelength Ranges Using Microtremor Records." J.	 

347

Geophys. Res., Vol. 111, B09315, doi:10.1029/2005JB004235.
Harkrider, D. G. (1964). "Surface Waves in Multilayered Elastic Media, I. Rayleigh and Love Waves
from Buried Sources in a Multilayered Elastic Half-Space." Bull. Seism. Soc. Am., Vol.54, No.2,627-679.
J-SHIS (2010). http://www.j-shis.bosai.go.jp/
Kurose, T., Yamanaka, H. (2006). "Joint inversion of receiver function and surface-wave phasevelocity for estimation of shear-wave velocity of sedimentary layers." Exploration Geophysics, Vol.
37, 93-101.
Saito, M. and Kabasawa, H. (1993). "Computations of Reflectivity and Surface Wave Dispersion
Curves for Layered Media." Butsuri-Tansa, Vol. 46, No. 4, 283-298 (in Japanese).
Sato, T., Graves, R. & Somerville, P. (1999). "Three-Dimensional Finite-Difference Simulations ofLong-Period Strong Motions in the Tokyo Metropolitan Area during the 1990 Odawara Earthquake(Mj 5.1) and the Great 1923 Kanto Earthquake (Ms 8.2) in Japan." Bull. Seism. Soc. Am., Vol. 89,No.3, 579-607.
Tokimatsu, K., Tamura, S. and Kojima, H. (1992). "Effects of Multiple Modes on Rayleigh Wave
Dispersion Characteristics." J. Geotech. Eng., ASCE, Vol.118, No.10, 1529-1543.
Tomishima, K., Sekiguchi, T. and Nakai, S. (2010). "Estimation of the Deep Soil Structure in the
North-Western Chiba Area based on Seismometer Records." Proc. Annual Meeting of AIJ, PaperNo. 21406, 811-812.	 

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