13th World Conference on Earthquake Engineering
Vancouver, B.C., Canada
August 1-6, 2004
Paper No. 3453

TOPOGRAPHIC SITE RESPONSE AT A QUATERNARY TERRACEIN HACHIOJI, JAPAN, OBSERVED IN STRONG MOTIONS
AND MICROTREMORS
Takumi TOSHINAWA1, Yoshiaki Hisada2, Katsuaki KONNO3, Akihiro SHIBAYAMA4,Yuya HONKAWA5 and Hiromichi ONO6

SUMMARY
We investigated a topographic site effect of a Quaternary terrace in Hachioji, Japan, on ground-motion
amplification. The terrace discussed herein is about 12-m high consisting of volcanic ash, clay, silt andsand. 7 accelerographs were deployed in the site: 6 on the crest and 1 on the base of the terrace. Themaximum PGA and PGV ratios with respect to the base are 3 and 2.7, respectively, demonstrating that thesite amplification due to the topographic effect of the terrace is significant. Next we evaluated crest/basespectral ratios in order to investigate the relative site response. It is found that ground motions atfrequencies above 5 Hz are amplified and the fundamental frequency of the terrace is about 7 Hz. The
crest/base spectral ratios also show that ground motions at frequencies above 10 Hz are affected bydirectional topography of the terrace and the maximum difference of horizontal ground motions is about 5.
Additional microtremor observations and eigenvalue analysis demonstrated that the crest edges tend to
oscillate higher in the direction perpendicular to the edges.
INTRODUCTION
Earthquake damage is sometimes worsened due to ground-motion amplification. A topographic effect is
one of the main factors of the ground-motion amplification. During the 1994 Northridge earthquake, forinstance, high PGA of 1.78 g was observed at a 15-m high and 130-m wide hill at Tarzana (Shakal [1]).
The unusual seismic response was probably caused by the resonance of the hill (Spudich [2]). Anotherexample is that according to an observational case study conducted in Yokohama, Japan, a strong-motion
station atop 25-m high plateau observed stronger ground motions by a factor of 2 in comparison to anadjacent lowland (base) station (Toshinawa [3]). This was presumably because ground motions wereamplified in the plateau that mainly consists of soft volcanic ash flown from Mt. Fuji during the1

Associate Professor, Meisei University, Tokyo, JAPAN. Email: tosinawa@ce.meisei-u.ac.jp
Professor, Kogakuin University, Tokyo, JAPAN.
3
Associate Professor, Shibaura Institute of Technology, Tokyo, JAPAN.
4
Graduate student of Kogakuin University
5
Former undergraduate student of Meisei University
6
Undergraduate student of Meisei University
2

Quarternary period. As is evident from these examples, upland sites have possibilities to be amplified
during earthquakes. Thus we made an investigation on the topographic effect on ground-motion
amplification at a Quaternary terrace in Hachioji, Japan,by using strong-motion observation, microtremor
observation and 3-D FEM calculation.
SITE DESCRIPTION

The site considered herein is located about 30 km west of the center of Tokyo (Figure 1). This
site is mapped as a part of hills that cover the western part of Kanto area. Figure 2 shows a planand side views of the terrace. For a land use, an original terrace was artificially cut and filledforming a 12.5-high and southwestern-cornered terrace. On the top of the terrace, low story
buildings were built, the locations of which are depicted in the figure. Figure 3 shows soil, SPT
N-value and PS profiles at GR1, GR4, and GR3(GR0). GR1 sits on cut ground while GR4 iscovered by 4-m artificial fill with shear-wave velocity of 90-200 m/s. The depths to sandy gravel
are 8.5-9 m. 7 accelerographs (GR1-GR7) were deployed on the ground surface and one
accelerograph (GR0) was installed in the ground beneath GR3 (Kawakami [4]). GR7 wasinstalled on the base of the terrace.

Figure 1. Location of the site and epicenters of the events used.

Figure 2. Plan and side views of the terrace with instrument layouts.

Figure 3. Soil, SPT N-value and PS profiles at GR1, GR4 and GR0.

STRONG MOTIONS
Strong-motion records

Table 1 shows listing of earthquakes observed by the strong-motion system. JMA (JapaneseMeteorological Agency) magnitude (MJ) ranges from 3.5 to 5.8 and focal depth ranges from 20
to 80km. Peak ground accelerations (PGA) and peak ground velocities (PGV) at GR7 are 1.6-3.1
cm/s2 and 0.06-0.20 cm/s, respectively. Epicenters except for the event 4 are plotted with solidstars in Figure1. Figure 4 shows ground accelerations recorded during the #2 event (2002/5/19).
It can be seen that even in such a small area ground motions are significantly different
suggesting that site response effect should not be negligible in this site. From the records, 10.24sec shear-wave parts are Fourier transformed and smoothed with a 1.0-Hz Parzen window(Figure 5) In comparison to GR0 and GR7, ground acceleration on the top of the terrace arehighly amplified. Figures 6, 7, 8 show amplification factors of PGA and PGV, and difference ininstrumental seismic-intensity with respect to GR7. Except for the event 4, PGA and PGV are
highly amplified with the highest factors of 3 and 2.7, respectively. The instrumental seismicintensity difference ranges from 0 to 1.1, which is significant variation in ground-motion
intensity observed within small area.
Table 1. Listing of the parameters the events used.
#1
2
3
4

Origin Time (local)
2002/4/30 21:42
2002/5/19 5:00
2002/7/13 21:45
2002/7/24 5:05

MJ
3.5
4.7
4.8
5.8

D30 km
80 km70 km
20 km

∆
20km
80km
80km
320km

Azimuth
100°
93°
65°
58°

PGA at GR7

PGV at GR7

2.9cm/s

20.06cm/s

2.4cm/s

20.11cm/s

3.1cm/s

20.14cm/s

1.6cm/s

20.20cm/s

Figure 4. Horizontal ground accelerations recorded during the #2 event (2002/5/19).

Figure 5. Fourier amplitude-spectra of the accelerations for the #2 event (2002/5/19).

32110

Seismic Intensity Difference

3PGV Ratio

4PGA Ratio

4

2 0 0 2 /4 /3 0
2 0 0 2 /5 /1 9
2 0 0 2 /7 /1 3
2 0 0 2 /7 /2 4201234STATION #56Figure 6. PGA ratios withrespect to GR7.

7001234STATION #56Figure 7. PGV ratios withrespect to GR7.

71 .5
10 .50-0 .5012345STATION #67Figure 8. Instrumental seismic
intensity difference to GR7.

Terrace/Base spectral ratio
Fundamental resonance

Figure 9 shows spectral ratios of accelerograms on the terrace obtained for all the events withrespect to GR7. Consistent trend can be found at each station suggesting that the amplification
was caused due to each site-amplification effect. Figure 10 shows the arithmetic meanings thespectra obtained for the north-south (NS) and the east-west (EW) directions. It is clear fromthese figures that the ground-motion amplification occurs at frequencies above 5 Hz and eachstation has fundamental peaks at 6-8Hz. The spectral amplitudes at the fundamental peaks are 3to 5 and no significant difference in direction is found. Therefore, the 6-8 Hz site response canbe regarded as the overall resonant oscillation of the terrace. Assuming the averaged thickness tothe sand stones and fundamental frequency to be 9 m and 7 Hz, respectively, an averaged shearwave velocity of the terrace can be estimated to be 250 m/s by the quarter wavelength law. The
estimate is consistent with the P-S profiles shown in Figure 3.

Figure 9. Spectral ratios of strong motions with respect to GR7.

Figure 10. Averaged spectral ratios of strong motions with respect to GR7.

Resonance in higher mode

Spectral amplitudes at frequencies above 10Hz are quite different from station to station:
GR1: Peaks at 36 Hz in the NS and EW directions. No significant difference in direction.
GR2: Peaks at 13 Hz and 29 Hz in the EW direction.
GR3: Peak at 22Hz in the NS direction.
GR4: Peak at 15Hz in the EW direction. High spectral amplitude at frequencies above 20Hz.
GR5: Peaks at 13Hz and 33 Hz. No significant difference in direction.
GR6: Peaks at 13Hz in the NS and EW directions and at 27Hz in the NS direction.
Because marked differences in direction are found at some stations, NS/EW spectral ratios areobtained (Figure 11). From Figure 11, it is found that GR3 and GR6 tend to have higher NS
amplitudes while GR2 and GR4 to have higher EW amplitudes. The difference is as large as 5 atsome frequencies.

Figure 11. Spectral ratios of NS strong motions with respect to EW motions.

MICROTREMORS
Observation at strong-motion points

At the strong-motion stations, microtremor measurements were carried out. The recording
system consists of servo-type velocity sensors with a 16-bit analog-to-digital converter and anote-book computer. At each station, the NS and EW components of microtremors in velocity
were recorded with a sampling rate of 100 Hz and a recording length of 180 sec. At each station,
three sets of 10.24-sec-long samples from the 180-sec recordings were selected. Fourier spectraof the samples were calculated and then smoothed with a 1.0-Hz Parzen window. NS to EW
spectral ratios were obtained after which the spectra were averaged. Figure 12 shows theaveraged NS/EW spectra at each station. From Figure 12, it is found that GR3 and GR6 tend tohave higher NS amplitudes while GR2 and GR4 to have higher EW amplitudes. The results arequite similar to those of Figure 11.

Figure 12. NS/EW spectral ratios of microtremors at strong-motion stations.

Complementary observation

The similarity between NS/EW spectral ratios of strong motions and microtremors enables us toinvestigate vibration characteristics of the terrace by making use of microtremors. Thus
complementary microtremor observations were carried out along the edge of the terrace, thelayout of the sensors are depicted in Figure 2. NS/EW spectral ratios were obtained in the samemanner as the previous calculation. Figure 13 shows the results, and from this figure it is foundthat points 04, 07 and 10 (the southern edge) tend to have higher NS amplitudes while points 13,
16 and 19 (the western edge) to have higher EW amplitudes.

Figure 13. NS/EW spectral ratios of microtremors along the crest edges.

EIGENVALUE ANALYSISThe strong-motion observation demonstrated that site amplification occurred atop the terrace at
frequencies above 5 Hz. Besides, with additional observations of microtremors, directional
topographic response were observed. In order to investigate the directional characteristics ofdynamic behavior of the terrace, an eigenvalue analysis using a 3-D finite element method isconducted. Figure 14 shows the analytical model. The model is 50-m long, 40-m wide and 9-m
high. The finite element modeling was made by dividing E-, N- and U- lengths by 25, 20 and 5,respectively. The material properties of the model are tabulated in Table 2. Northern, eastern andbottom nodes are fixed. From this model, eigenvalues and eigenvectors are calculated. Figure 15
shows eigenvalues and eigenvectors for the 1st to 5th modes. The first- and second- mode
frequencies are relatively close, and the two modes can be assumed as a pair of the fundamental
oscillations of the terrace. The mode shapes show that:
Modes 1 & 2: Large deformation in the E-W and N-S directions, respectively.
Mode 3: Large deformation in the N-S direction along the southern edge.
Mode 4: Large deformations in the E-W and N-S direction along the western and southernedges.
Mode 5: Large deformations in the E-W and N-S direction along the western and southernedges.
From the eigenvalue analysis it is clearly found that the western and southern edges are highly
deformed in the direction perpendicular to the axis of each edge. The analytical results are
consistent with the results of the strong-motion and microtremor observations.
Table 2 Material properties of the analytical model.
mass density
1.5 t/m

3S-wave velocity

Poisson’s ratio

250m/s

0.3N

N

E40m

E50m
Figure 14. 3-D FEM model for eigenvalue analysis.

Figure 15. Eigenvalues and eigenvectors for the 1st to 5th modes.

CONCLUSIONSIn this study we investigated a topographic site response of a Quaternary terrace on ground-motion
amplification by using strong-motion observation, microtremor observation and eigenvalue analysis. Fromthese investigations, the following findings are obtained:
1. Due to the topographic site effect of the terrace, ground motions are amplified with the highest factorsof 3 (PGA) and 2.7 (PGV), respectively, with respect to the base of the terrace.
2. The site amplification occurs at frequencies above 5 Hz and the fundamental resonance oscillation of
the terrace is about 7 Hz .
3. At frequencies above 10 Hz, some points show directional amplification with the highest difference ofabout 5. The highly-amplified directions at the terrace edges are perpendicular to the terrace edges.

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