Proceedings of the International Symposium on Engineering Lessons Learned fromthe 2011 Great East Japan Earthquake, March 1-4, 2012, Tokyo, JapanAPPLICATION OF THE H/V SPECTRAL RATIOS FOR
EARTHQUAKE GROUND MOTIONS AND
MICROTREMORS AT K-NET SITES IN TOHOKUREGION, JAPAN TO DELINEATE SOILNONLINEARITY DURING THE 2011 OFF THE
PACIFIC COAST OF TOHOKU EARTHQUAKEFumiaki NAGASHIMA1, Hiroshi KAWASE2, Shinichi MATSUSHIMA3, Francisco J.
SANCHEZ-SESMA4, Takashi HAYAKAWA5, Toshimi SATOH6 and Mitsutaka OSHIMA71Student, Graduate School of Engineering, Kyoto University, Kyoto, Kyoto, Japan,
nagashima@zeisei.dpri.kyoto-u.ac.jp
2
Professor, DPRI, Kyoto University, Uji, Kyoto, Japan,
kawase@zeisei.dpri.kyoto-u.ac.jp
3
Associate Professor, DPRI, Kyoto University, Uji, Kyoto, Japan,
matsushima@zeisei.dpri.kyoto-u.ac.jp
4
Professor, Instituto de Ingenieria, Universidad Nacional Autonoma de Mexico, Mexico D.F., Mexico,sesma@unam.mx
5
Senior Researcher, Institute of Technology, Shimizu Corporation, Koto, Tokyo Japan,takashi.hayakawa@shimz.co.jp
6
Senior Researcher, Institute of Technology, Shimizu Corporation, Koto, Tokyo Japan,
toshimi.satoh@shimz.co.jp
7
Researcher, Institute of Technology, Shimizu Corporation, Koto, Tokyo Japan,m.oshima@shimz.co.jp

ABSTRACT: To understand the reason why observed accelerations in Miyagi Pref.
during the main shock were so high, we deployed several aftershock observation stations
near two K-NET sites. We determined detailed velocity structures based on the H/Vspectral ratios of aftershock records. Then we compared H/V spectral ratios of strong
motions during the main shock with those averaged weak motions to see soil nonlinearity
effects. We found strong peak frequency shift during the main shock in comparison toweak motions, which suggests shear rigidity reduction of approximately 25 %.
Key Words: The 2011 Off the Pacific Coast of Tohoku Earthquake, H/V spectral ratio,strong motion, microtremor, grid-search, velocity structure, nonlinearlity

INTRODUCTION
During the 2011 Off the Pacific Coast of Tohoku earthquake which occurred on March 11, 2011, themaximum acceleration of the NS component reached 2,700 cm/s2 and JMA seismic intensity 7 was361

recorded at K-NET Tsukidate (MYG004) station of the K-NET strong motion network (for thenetwork description see Kinoshita, 1998). However, damages near the observation site were rather
small. The authors, therefore, observed aftershocks around MYG004 in order to identify factors forgenerating high acceleration and investigate the relationship between high acceleration and damages inthe vicinity. Also, prior to the aftershock observation, we measured microtremors to focus on the effectof cliff topography near MYG004. The H/V spectral ratios (HVRs) of these data were compared toidentify the ground motion characteristics of the area, and the HVRs of ground motion records wereused to identify the soil structure directly below MYG004. Also, we deployed temporary aftershockobservation sites at Furukawa in Ohsaki City, which is near the damaged area in Furukawa, Miyagi
Prefecture. Based on the HVRs of the ground motion recordings, the ground motion characteristics atFurukawa were investigated and compared with those in Tsukidate.

TEMPORARY AFTERSHOCK OBSERVATION AT TSUKIDATEOn April 29 and 30, 2011, four survey sites were established around MYG004. One site was set at thecenter and other three sites were set on the circumference of a circle with a diameter of approximately
400 m around MYG004. Figure 1 shows the locations of MYG004 and temporary aftershockobservation sites (TKDZ01-TKDZ04). At these sites, data have been continuously recorded using
SMAR-6A3P (Mitutoyo Corporation) which data logger was replaced by LS8800 (HakusanCorporation). These sites were set under eaves of residential properties with AC power. GPS was used
for time correction and data sampling had been performed at 100 Hz.

×TKDZ04

×TKDZ02×

TKDZ01× MYG004

× TKDZ03
Fig. 1 Locations of the observation sites in Tsukidate

INFLUENCE OF THE CLIFF TOPOGRAPHY
Microtremor observation
Before the establishment of the temporary aftershock survey sites, in order to determine the effects ofthe cliff topography near MYG004, microtremor array observation was performed at five sites: twosites were set on top of the cliff and three sites were set at the base of the cliff. Figure 2 shows thearray arrangement. M01 and M02 were set on top of the cliff with M02 located right in front ofMYG004, and M01 was set approximately 10 m south of M02. M03, M04 and M05 were set at thebase of the cliff with M03 located immediately beneath the cliff and M4 and M05 located 10 m and 30
m apart from M03, respectively. Microtremors were recorded for 30 minutes.

362

M01

10m

M02
MYG004

M03

10m

M04

20m

M05

Fig. 2 Schematic figure of microtremor array arrangement across the cliff

Microtremor HVRs
A time window of 40.96 seconds was extracted (50% overlapped), and HVRs were computed for theNS and EW components from average Fourier spectra, separately. Figure 3 shows the HVRs for
NS/UD and EW/UD respectively. While the HVRs were similar among the sites up to 7 Hz, atfrequencies higher than 7 Hz, the HVRs for the cliff-top sites (M01 & M02) were two to three times ashigh as the HVRs for the cliff-base sites. This is believed to be the effect of the cliff topography.

Aftershock NS

M01

M02

M03

M04

M05

M04

M05

H/V Spectral Ratio

1010.10.1

110Frequency [Hz](a) NS/UDAftershock EW

M01

M02

M03

H/V Spectral Ratio1010.10.11

10Frequency [Hz](b) EW/UDFig. 3 Microtremor HVRs with aftershock HVR for MYG004

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AFTERSHOCK DATA AT TSUKIDATE
Comparison of weak motion HVRs at temporary observation sites
From the aftershock data recorded between April 30 and September 20, 2011 at four temporary
observation sites shown in Figure 1, a time window of 81.92 seconds from the onset of the S wave wasextracted and the average Fourier spectrum of each component was computed to determine the HVRs,
that is, NS/UD and EW/UD, for the 77 earthquakes recorded at MYG004. Figure 4 shows theaftershock HVRs for NS/UD and EW/UD as well as those for MYG004 respectively. The aftershock
HVRs has peaks at around 0.3 and 1.0 Hz at all five survey sites including MYG004, which isbelieved to be the effect of the deep soil structure. On the other hand, at frequencies higher than 1.5 Hz,
HVRs are significantly different among the observation sites, except for TKDZ01 and TKDZ02. The
NS component at MYG004, in particular for 7-10 Hz, showed higher HVRs than those at otherobservation sites. This is consistent with the results discussed in the previous section, where the effectsof the cliff topography were apparent at frequencies higher than 7 Hz. Note that at TKDZ04 where
there were some damages occurred to buildings, a peak can be seen at around 2.5 Hz, indicating therelationship between dominant frequency and damages.

MYG004TKDZ01TKDZ02TKDZ03TKDZ04

H/V Spectral Ratio

1010.10.1

1Frequency [Hz]10(a) NS/UD

MYG004TKDZ01TKDZ02TKDZ03

TKDZ04

H/V Spectral Ratio

1010.10.1

1Frequency [Hz](b) EW/UDFig. 4 Aftershock HVRs in Tsukidate

364

10

Comparison between the main shock, the April 7 strongest aftershock and average of otheraftershocks at MYG004
Next, we compare the strong motion HVRs of the main shock, and the largest aftershock that occurredat 23:32 on April 7 (M7.1) with weak motion HVRs of average of other aftershocks. Figure 5compares NS/UD and EW/UD of these HVRs. With regard to the maximum acceleration (PGA), theNS and EW components for the main shock were 2,700 cm/s2 and 1,268 cm/s2, respectively, and thosefor the April 7 aftershock were 1,242 cm/s2 and 886 cm/s2, respectively, showing large differences
between the two events (1.5~2 times), but the HVRs were similar between them. Compared with theweak motion HVRs, the peaks of the strong motion HVRs were shifted to lower frequencies,
indicating the effect of nonlinearity. The peak frequency was shifted from 9 Hz to 4~5Hz for strong
motion HVRs in comparison to weak motions HVRs, and shear rigidity may have dropped toapproximately 25 % of the linear values during the strong shaking.
AverageNS1103111446NS1104072332NS

H/V Spectral Ratio

100

1010.10.1

1Frequency [Hz]10(a) NS/UD

AverageEW

H/V Spectral Ratio

1001103111446EW

1104072332EW

1010.10.1

1Frequency [Hz]10(b) EW/UDFig. 5 HVRs of main shock, largest aftershock and average of other aftershocks
at MYG004

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IDENTIFICATION OF THE SOIL STRUCTURE AT MYG004
Lastly, identification of the soil structure immediately below MYG004 was attempted based on theHVRs of weak motion data recorded at the site. First, for the 55 waves observed between March 9 andMay 17, 2011, average HVRs of the EW components which are believed to be less influenced by thecliff topography was used as the target to identify the soil structure at MYG004, by making theresidual sum of squares smallest with theoretical HVRs. For the initial model, the data published bythe National Research Institute for Earth Science and Disaster Prevention (NIED, 2011) were used forshallow soil layers and the results of previous study (Kawase and Matsuo, 2004) were used for deep
soil layers. Layers 2 and 3 of the eight-layered model of Kawase and Matsuo (2004) had similar PS
logging results and densities, so they are treated as one layer to make a seven-layered model (theInitial Model in Table 1). The theoretical HVRs were calculated based on the concept of Kawase et al.
(2011), and were identified by changing the thickness of layers 1 to 6. As a result, the velocity
structure that explains the observed HVRs well was identified as shown in Figure 6. The velocity
structure model is shown in Table 1 and Figure 7. Comparing the identified model with the initialmodel, layers near the surface got thinner and layers 5 and 6 got thicker, creating peaks at 0.3 and 1.0Hz, and hence quite nicely reproducing the observed HVR as shown in Figure 6.

H/V Spectral Ratio

Observed EW

Initial model

Identified model

876543
2
100.1

1Frequency [Hz]10Fig. 6 Comparison of HVRs for observed EW/UD, initial model and identified model

Table 1 Velocity structure of the initial and identified model

No
1
2
3
4
5
67

Initial Model
Vs
Thickness Depth(m/s)
(m)
(m)
100
1
1
240
34
550
6.25
10.25
1364.2920
30.25
2075.17110140.252874.2750
190.253400-

366

Identified Model
Thickness Depth
(m)
(m)
0.4
0.4
1.12
1.52
14.25
15.77
71.4
87.17
1624.71711.67
1200
2911.67-

0

Vs (m/s)
2000

0

4000

00

20

500

40

Vs (m/s)
2000

4000

1000
Depth (m)Depth (m)

60
80
100
120
140

1500
2000
2500
3000

160
18035002004000

Initial model
Identified model(a) Shallow part(b) Deep partFig. 7 Shallow and deep part of the velocity structure

TEMPORARY AFTERSHOCK SURVEY AT FURUKAWA
On June 6 and 7, 2011, five observations sites were deployed around K-NET Furukawa (MYG006)
station; two sites were set at the center and three sites were set on the circumference of the circle witha diameter of approximately 800 m close to MYG006. Figure 8 shows the locations of the temporary
aftershock observation sites (FRKZ05, FRKI02, FRKI04, FRKI05 and FRKI06) and MYG006. At
these temporary observation sites, data had been continuously recorded using SMAR-6A3P which datalogger was replaced by LS8800 at FKRZ05 and ITK seismometer with K-NET95 accelerometer(a-Lab) at the other four stations. GPS was used for time correction and data sampling had been
performed at 100 Hz.

FRKZ05 ××MYG006

FRKI06 ×

FRKI05×

×FRKI02

× FRKI04

Fig. 8 Locations of the observation sites in Furukawa

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AFTERSHOCK RECORDS AT FURUKAWA
Comparison of weak motion HVRs
Among the 63 aftershocks triggered at MYG006 between June 6 and September 21, 2011, theobserved data at the temporary aftershock observation sites shown in Fig. 8 were used. A time windowof 81.92 seconds from the onset of the S wave was extracted and average Fourier spectra werecomputed for the NS and EW components to determine the HVRs of NS/UD and EW/UD. Figure 9shows the aftershock HVRs at our temporary observation sites for NS/UD and EW/UD,
respectively, as well as those for MYG006. As for the aftershock HVRs, a peak was observed ataround 0.2 Hz at all six observation sites including MYG006, and all of the sites recorded similar
HVRs in the frequencies below 1.5 Hz. Only FRKI06 showed higher HVRs in the ranges 2-3 Hz and5-8 Hz. This may be because, 1) the observation site is located at the warehouse of an occupational
training center with a workshop containing processing machines in the north, 2) there is a gutter at thesite with water constantly flowing, 3) the highway located near the site (to the west) ascends toward
the south to cross over the railroad tracks, creating topographic effects.

H/V Spectral Ratio

MYG006100

FRKZ05

FRKI02

FRKI04

FRKI05

FRKI061010.10.1

1Frequency [Hz]10(a) NS/UD

H/V Spectral Ratio

MYG006100

FRKZ05

FRKI02

FRKI04

FRKI051010.10.1

1Frequency [Hz](b) EW/UD
Fig. 9 Aftershock HVRs in Furukawa

368

10FRKI06

Comparison between the main shock, the April 7 strongest aftershock and other aftershocks atMYG006
Finally, the strong motion HVRs for the main shock and the aftershock that occurred at 23:32 on April
7 (M7.1) and weak motion HVRs of average of other aftershocks were compared. Figure 10 shows thecomparison of NS/UD and EW/UD, respectively. With regard to the maximum acceleration, the NSand EW components for the main shock were 444 cm/s2 and 571 cm/s2, respectively, and those for theApril 7 aftershock were 415 cm/s2 and 478 cm/s2, respectively. While these components were nearly
10% higher in the main shock, the HVRs were almost the same between the two events and the effectof nonlinearity of the strong motion HVRs was also observed. During the strong motion, dueto the effect of nonlinearity, the frequencies that are said to cause damages to buildings, 0.5-2 Hz,
were greatly amplified and this is believed to be one of the causes for the building damages inFurukawa. If we focus our attention to the troughs around 8 to 10 Hz, we can see weak frequency shift
from the average HVRs of small aftershocks to the two strong motion HVRs.

AverageNS

H/V Spectral Ratio

1001103111446NS

1104072332NS

1010.10.1

1Frequency [Hz]10(a) NS/UDAverageNS

H/V Spectral Ratio

1001103111446EW

1104072332EW

1010.10.1

1Frequency [Hz]10(b) EW/UD
Fig. 10 HVRs of main shock, largest aftershock and average of other aftershocks at MYG006

369

SUMMARY AND CONCLUSIONSWith the aim of identifying the factors for generating high acceleration recorded at two K-NET sites inMiyagi Prefecture (MYG004 and MYG006) and investigating the relationship between highacceleration and building damages, aftershocks were observed at sites near K-NET Tsukidate(MYG004) and K-NET Furukawa (MYG006). Based on the observed H/V spectral ratios, groundmotion characteristics due to the soil around the sites were determined. Small peaks were observed at
0.3 and 1.0 Hz for all sites near K-NET Tsukidate, which are believed to be the effect of deep soillayers. The HVRs varied among the survey sites at frequencies higher than 1.5 Hz and were, inparticular, significantly different at MYG004 over the range of 7-10 Hz. This is consistent with theinline microtremor array measurement results which confirmed clear amplification at frequencies
higher than 7 Hz only for the cliff-top points.
The sites near K-NET Furukawa all showed a peak at 0.2 Hz and recorded similar HVRs in thefrequencies up to 1.5 Hz. The HVRs for the March 11 main shock and April 7 aftershock werecompared with the average HVRs for small or moderate-sized earthquakes; both at K-NET Tsukidate
and K-NET Furukawa, showed peak with lower frequency (longer period), and this is believed to be
the effect of soil nonlinearity. Furthermore, the one-dimensional soil structure at K-NET Tsukidate wasidentified based on the concept of Kawase et al. (2011). By widening the layer with Vs=3,400 m/s to2,900 m to reproduce peaks at 0.3 and 1.0 Hz, the structure that reproduces the observed records well
was identified.
In the future, we will estimate incident seismic waves by removing the effects of the cliff
topography and nonlinearity. Using them, ground motion characteristics of the main shock near
K-NET Tsukidate will be estimated and the relationship between them and actual damages will befully investigated.

ACKNOWLEDGMENTS
We would like to thank all the people who cooperated in the measurement of aftershocks. We also
thank the National Research Institute for Earth Science and Disaster Prevention (NIED) for allowing
us to access to their K-NET data. Some of the figures were made using “Google Map”.
REFERENCES
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Kawase, H., Sánchez-Sesma, F. J. and Matushima, S. (2011). “The Optimal Use of
Horizontal-to-Vertical Spectral Ratios of Earthquake Motions for Velocity Inversions Based on
Diffuse-field Theory for Plane Waves.” Bulletin of the Seismological Society of America, Vol. 101,
No. 5, 2011-2014.
Kinoshita, S. (1998). “Kyoshin Net (K-Net).” Seismolgical Research. Letters, 69, 309–334.
National Research Institute for Earth Science and Disaster Prevention (2011). “A strong-motion
seismograph networks Portal site.” http://www.kyoshin.bosai.go.jp/index_en.html .

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