Bulletin of the Seismological Society of America, 91, 5, pp. 1267–1282, October 2001

S-Wave Velocity Structure of the Taichung Basin, Taiwan, Estimatedfrom Array and Single-Station Records of Microtremors
by Toshimi Satoh, Hiroshi Kawase, Tomotaka Iwata, Sadanori Higashi,
Toshiaki Sato, Kojiro Irikura, and Huey-Chu Huang

Abstract The objective of this study is to estimate the S-wave velocity structureof the Taichung basin in a near-fault region, which is needed for strong-motion
evaluation for the 1999 Chi-Chi earthquake. We have conducted array measurementsof microtremors with a total of 12 arrays at four sites and single-station measurementsof microtremors at 48 sites in and around the Taichung basin. Based on the Rayleighwave inversion technique using phase velocities estimated from array records ofmicrotremors, we find that a thick layer (the thickness of about 1000 to 1400 m)
with an S-wave velocity of VS 1100 m/sec exists in the east-central part of the Taichung basin. We estimate the thicknesses of sedimentary layers above the preTertiary bedrock at 48 sites to fit calculated peak and trough frequencies ofhorizontal-to-vertical spectral ratios of Rayleigh waves to observed peak and troughfrequencies, assuming the same S-wave velocities estimated using array records. The
pre-Tertiary bedrock depth was estimated to be about 5 to 6 km in this region. The
estimated thickness of the layer with VS ⳱ 1100 m/sec is largest in the east-central
part of the basin and rapidly decreases to less than 400 m in the northeastern andwestern parts inside the basin. The estimated S-wave velocity structures reasonably
explain arrival time of initial P and S waves of aftershock records observed by
Higashi et al. (2001).

Introduction
The 1999 Chi-Chi earthquake caused severe damage in
the near-fault region. To evaluate near-fault strong motions,
it is important to estimate source processes and site effects
quantitatively. Theoretical evaluation of strong motions andsite effects requires information on velocity structures, especially for sedimentary layers above the bedrock. Theoretical simulation of strong motions for the 1994 Northridge
earthquake (e.g., Graves, 1996) and the 1995 Kobe earthquake (e.g., Pitarka et al., 1998; Kawase and Matsushima,
1998; Iwata et al., 1998) had shown that site effects causedby the 3D basin structure have an important role in strong
motions. However, the literature on velocity structures in thenear-fault region of the Chi-Chi earthquake does not provide
enough information for theoretical strong-motion evaluation. Therefore we conducted array and single-station measurements of microtremors in and around the Taichung basin, which is the most populated area in the near-fault region,and estimated S-wave velocity structures to construct a basinmodel needed for theoretical strong-motion evaluation.
The exploration method to estimate deep structures using array records of long-period (⬎1 sec) microtremors wasestablished by Horike (1985) and Matsushima and Okada(1990) after the pioneering work done by Aki (1957). Since

that time, it has been extended to estimate shallow structuresusing short-period (⬍1 sec) microtremors (e.g., Sato et al.,
1991; Tokimatsu and Miyadera, 1992). Based on the successes of these studies, the array method has been appliedin recent years to estimate not only shallow but also deep
structures to the bedrock using short- and long-period microtremors (e.g., Kawase et al., 1998; Kudo et al., 2000;
Satoh et al., 2001a).
Array measurements of microtremors are appropriate toestimate an S-wave velocity structure, which is the most influential structure on strong motions. In addition, array measurements of microtremors are less expensive than conventional methods such as reflection and refraction exploration
methods to explore deep sedimentary layers. So we conducted array measurements of microtremors at four sites in
Tali City inside the basin and estimated the S-wave velocity
structures below these sites.
Since four sites are not enough to construct a 3D basinmodel, we then conducted single-station measurements of
microtremors at 48 sites in and around the Taichung basin.
In previous studies, peak or trough frequencies of horizontalto-vertical spectral ratios (HVRs) of microtremors haveagreed with calculated ones of the fundamental Rayleigh

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T. Satoh, H. Kawase, T. Iwata, S. Higashi, T. Sato, K. Irikura, and H.-C. Huang

wave (e.g., Yamanaka et al., 1994; Ishida et al., 1998; Satohet al., 2001b). In a few basins the bedrock depth has been
estimated to fit the calculated peak frequencies (Yamanaka
et al., 1994; Satoh et al., 2001c) or trough frequencies (Ishida et al., 1998) of Rayleigh wave to the observed frequencies. Satoh et al., (2001c) constructed a 3D velocity modelof the Sendai basin in Japan by interpolating S-wave velocity
structures estimated from array records of microtremors at
six sites by using single-station records of microtremors
from 61 sites. They successfully simulated strong-motion
records of an earthquake of MJ 5 using the 3D basin model.
Thus single-station records of microtremors are useful for
estimating a spatial basin structure if S-wave velocity structures at several sites in the basin are explored.
In this study we estimated the S-wave velocity structuresat the 48 sites using both peak and trough frequencies of
HVRs. Note that our method of HVRs was different
from Nakamura’s method (Nakamura, 1988). Nakamura’s
method assumed observed HVRs to be S-wave amplification,
whereas we fit calculated Rayleigh wave HVRs to observed
HVRs. The idea of the method used in this study is the sameas in the previous studies referred to earlier (Yamanaka etal., 1994; Ishida et al., 1998; Satoh et al., 2001c), but they
estimated only one parameter, bedrock depth, because theyused only one peak or one trough frequency of HVRs. In
contrast, we estimated several parameters (thicknesses of six
layers) using both several peak and trough frequencies of
HVRs in a relatively wide frequency range.
Finally, we examined the initial P- and S-wave travel
times of strong-motion records of an aftershock (M 5.5) recorded by 10 temporary strong-motion stations (Higashi etal., 2001) to check the validity of the velocity structures
estimated from microtremors.

Field Measurements and Data
Figure 1 shows the locations of the epicenter of the ChiChi earthquake of MW 7.6 and one of its aftershocks of M
5.5 that occurred on 19 October 1999, together with the surface fault traces, most of which coincide with the Chelungpu
fault. We plotted the epicenter of the mainshock estimatedby Shin et al. (2000) and that of the aftershock estimated by
the Taiwan rapid earthquake information release system (Wuet al., 1997). Figure 2 shows the locations of the array andsingle-station microtremor observation sites together with
the Central Weather Bureau (CWB) strong-motion stations(Shin et al., 2000) and the temporary stations referred toearlier (Higashi et al., 2001) in and around the Taichungbasin. The Taichung basin is bounded by the Chelungpu
fault on the east and the Changhua fault on the west. Bothof these are thrust faults dipping to the east accompanied by
anticlinal folds on the hanging wall (e.g., Lee et al., 1996).
We conducted array measurements of microtremors at
TCU062, TAL001, TAL002, and TCU067 in the middle of
October 1999. The four sites are located over Holocene sediments. Stations TAL001 and TAL002 are located near theheavily damaged area in Tali City, where several middle-

Figure 1.

Locations of the epicenter of the ChiChi earthquake of MW 7.6 and one of its aftershocksof M 5.5, which occurred on 19 October 1999, together with the surface fault traces, most of whichcoincide with the Chelungpu fault (blue line). Rectangle denotes the studied region.

rise buildings have collapsed during the Chi-Chi earthquake.
At these two sites we also conducted aftershock observation(Higashi et al., 2001). Stations TCU062 and TCU067 are
CWB strong-motion stations, although the mainshock was
not recorded by TCU062. The peak ground acceleration of
the east–west (EW) component at TCU067 during the ChiChi earthquake was about 490 cm/sec2, and the peak ground
velocity integrated from the acceleration record was about
100 cm/sec. In the middle of February 2000, we conducted
single-station measurements of microtremors at 48 sites including 23 CWB strong-motion stations in and around theTaichung basin. The single-station measurements were conducted basically along one north–south line (NS) and two
EW lines (EW-1 and EW-2) with a spacing of 1 to 3 km
across the Taichung basin.
Array and single-station measurements of microtremors
were done using portable instruments. One set of instrument
was composed of an overdamped moving coil type threecomponent accelerometer developed by Kudo (1998), an
amplifier, and a 24-bit digital recorder. The accelerometer
had flat amplitude from 0.07 to 140 Hz. The accuracy of theinternal clock was within 1 ppm and was corrected by the

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S-Wave Velocity Structure of the Taichung Basin, Taiwan, Estimated from Array and Single-Station Records of Microtremors

Table 1Array Size and Number of Stations

Name

Figure 2.

Locations of the array (red big circles)
and single-station (black open circles) microtremor
observation sites together with the CWB strongmotion stations (triangles) (Shin et al., 2000) andaftershock strong-motion stations (squares) (Higashi
et al., 2001) in and around the Taichung basin. The
HVRs at stations along the NS line, the EW1 line, andthe EW2 line are shown in Figure 12.

Global Positioning System (GPS) before each measurement.
The timing accuracy from GPS correction was within 1 msec.
Starting and ending schedules of measurement were set before the measurement using portable personal computers.
We measured microtremors at the 4 array sites with a totalof 12 arrays in six days. The array size and number of instruments for each array are listed in Table 1. The radii ofS, M, L, and X arrays were 20 to 25 m, 40 to 80 m, 130 to
260 m, and about 700 m, respectively. In Figure 3 we showarray configurations of S and M arrays at TAL002. We tried
to set up the instruments such as shown in Figure 3a, but theconfiguration of larger arrays tended to be distorted as in
Figure 3b due to obstacles such as buildings and roads. We
brought seven sets of instruments, but one set did not work
properly for the later half of the measurements. Two or threesets of 15-min observations with a sampling frequency of
100 Hz were performed for each array. A high-cut filter of
10 Hz was used for S- and M-array measurements, while thatof 5 Hz was used for L- and X-array measurements. The
relative coordinates of stations were measured by measuring

Smallest
Station
Separation(m)

LargestStation
Separation(m)

Numberof
Stations

TCU062

S array
M array
L array

12.5
25.0
106.7

43.3
86.6
449.4

6
6
6

TAL001

S array
M array
L array
X array

12.5
25.0
69.5
317.4

43.3
141.6
237.5
1192.17
7
6
6

TAL002

S array
M array
L array

10.0
19.8
84.0

34.9
61.2
227.6

7
7
7

TCU067

S array
M array

21.7
35.0

40.4
84.0

5
6

tapes for S and M arrays and calculated from latitude andlongitude observed by a differential GPS for L and X arrays.
The relative station location accuracy was a few meters forthe differential GPS. At 48 single-station microtremor sites,
we observed microtremors for 15 min with a sampling frequency of 200 Hz and a high-cut filter of 30 Hz.
Figure 4a and b show Fourier acceleration amplitude
spectra at the center station of the S array at each site. Here,observed records were divided into time segments of 40.96
sec in length by overlapping the half of the window length,
and the Fourier amplitudes were averaged over the time segments after removing a few time segments clearly contaminated by instrumental or locally generated noise. Horizontal
spectra are geometric means of NS and EW components.
Both horizontal and vertical spectra have clear troughs in thefrequency range from 1 to 2 Hz. For comparison we show
Fourier acceleration amplitude spectra of vertical components observed inside the Los Angeles and San Fernando
basins in the United States (Kawase et al., 1998) and thoseinside (OKIN) and outside (TAMA) the Sendai basin in Japan(Satoh et al., 2001a) in Figure 4c. Since Kawase et al. (1998)
and Satoh et al. (2001a) observed velocity records, we calculated acceleration spectra by multiplying velocity spectraby the circular frequency. In the frequency range from 0.2
to 0.5 Hz, the amplitudes inside the Taichung basin weremuch larger than those observed at the other sites. Such ahigh-amplitude level of long-period microtremors is helpful
for estimating a deep basin structure.

Inversion of S-Wave Velocity Structures Using
Array Records of Microtremors
f-k Analysis
We divided the observed records of vertical components
into time segments of 40.96 sec in length for S- and M-array

1270

T. Satoh, H. Kawase, T. Iwata, S. Higashi, T. Sato, K. Irikura, and H.-C. Huang

Figure 3. Array configurations of (a) S-array and (b) M-array at TAL002. The number near the station denoted by circle in is the station number.

Figure 4.

(a) Fourier acceleration amplitude spectra of horizontal components at thecenter station of S-array at four sites in the Taichung basin. (b) Fourier acceleration
amplitude spectra of vertical components at the center station of S-array at four sites
in the Taichung basin. (c) Fourier acceleration amplitude spectra of vertical components
observed inside the Los Angeles and the San Fernando basin (Kawase et al., 1998) andthose inside (OKIN) and outside (TAMA) the Sendai basin (Satoh et al., 2001a).

records and 81.92 sec in length for L- and X-array records
by overlapping the half of the window length. After removing a few time segments that had been clearly contaminatedby instrumental or locally generated noise, we estimatedphase velocities by f-k (frequency-wavenumber) spectral
analysis based on the maximum-likelihood method (Capon,
1969). Figure 5 shows examples of f-k spectra using micro-

tremor data from TAL001. The f-k spectra were calculated
at 101 ⳯ 101 grid points at each frequency. The phase velocity was estimated from the wavenumber of the maximum
peak in the f-k spectrum. Since the maximum peaks weresearched within the wavenumber windows set by us a priori,
minimum phase velocities were limited, but maximum ones
were not. The phase velocity of the Rayleigh wave is less

S-Wave Velocity Structure of the Taichung Basin, Taiwan, Estimated from Array and Single-Station Records of Microtremors

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Table 2Estimated Structure

Layer
No.

S-WaveVelocity(m/sec)P-WaveVelocity(m/sec)Density(g/cm3)

1
2
3
4
5
6
7

116
278
657
1100
1580
2560
3290

1419
1599
2019
2511
2808
4551
5849

1.7
1.8
1.9
2.1
2.3
2.5
2.6

Thickness(m)
TCU062

TAL001

TAL002

TCU0674(5)0(2)
65(76)
(900)

4(5)
3(4)
11(10) 22(24)67(76)
0(0)1202(1020)(1800)(2700)3(4)14(8)
72(70)(1370)

Bold numbers indicate values inverted from phase velocities; parentheses
indicate values estimated from HVRs.

Figure 5.

The f-k spectra using observed data at
TAL001. The number after “FREQ” means frequency
in Hz. The numbers after “PEAK⳱” mean the wavenumber in the east (E) direction, wavenumber in thenorth (N) direction, the phase velocity in m/sec, andthe propagation direction counterclockwise from theeast in degree. The phase velocity is estimated fromthe frequency and the wavenumber of the maximum
peak in the f-k spectrum. The f-k spectra at 8.0 Hz,
4.5 Hz, 2.5 Hz, and 0.3 Hz are calculated from records
observed by S, M, L, and X arrays, respectively.

than the S-wave velocity, so we exclude phase velocities
higher than 4.5 km/sec. We also excluded unreliable phasevelocities based on Thompson’s test (Thompson, 1935) withthe significance level of 5% and took a logarithmic averageover the reliable phase velocities.
The number of instruments ranged from 5 to 7 as shownin Table 2, so we examined the effects of the number of
instruments on average and standard deviation of phase velocity using M-array records at TAL002. Figure 6 shows theaverage and the logarithm of the standard deviation of phasevelocities at the frequencies of 4.0, 4.5, and 5.0 Hz, whichwere the frequencies used for the following inversion analysis. The horizontal axis shows the number of removed instruments (See Fig. 3b). When there were five instruments(two instruments were removed), the standard deviation became larger, and the average in some cases was much different from the one estimated from seven instruments. Whenthe number of instrument was six, the average was not so
different from one estimated from seven instruments, although the standard deviation tended to be slightly larger.
Inversion Analysis
We inverted S-wave velocity structures based on a Rayleigh-wave inversion technique to minimize the following
misfit J({r}) using a quasi-Newton method (Fletcher, 1972).

Figure 6.

Average and the logarithm of the standard deviation of phase velocity, which are estimatedby removing one or two instruments of the M arrayat TAL002. Horizontal axis is the removed instrument
number (See Fig. 3b).

feJ({r}) ⳱冮 SD |c ( f ) ⳮ c ( f, {r})| df
fs

1

2

o

t

fe

冮 |c ( f )| df

(1)

2

fs

o

Here co(f ) and ct(f , {r}) are the average of the observed
phase velocities and calculated phase velocity for the fundamental Rayleigh wave, respectively. SD is the logarithmof standard deviation of the observed phase velocities and isused as a weighting function. The variables fs and fe are theminimum and maximum frequencies, and {r} refers to in-

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T. Satoh, H. Kawase, T. Iwata, S. Higashi, T. Sato, K. Irikura, and H.-C. Huang

verted parameters. The inverted values are shown in Table
2. Except for the S-wave velocity of the fourth layers in
Table 2, bold numbers were inverted using equation (1).
Figure 7 shows the observed and calculated phase velocities having initial and inverted S-wave velocity structures. From S-array records at TCU067, phase velocities
were estimated successfully only in the frequency range
from 5 to 7 Hz due to a smaller number (five) of instruments(Table 1). Since these phase velocities (f ⳱ 5–7 Hz) agreed
with those estimated from M-array records, we used phasevelocities estimated from M-array at TCU067 for the inversion. The frequency range of observed phase velocities for

Figure 7.

each array was determined by minimum and maximum station separations shown in Table 1. The minimum wavelength we could detect was twice the minimum station separation. This limitation was derived theoretically from the
Nyquist wavenumber. The maximum wavelength was empirically considered to be two to seven times the maximum
station separation (e.g., Horike, 1985; Matsushima andOkada, 1990; Satoh et al., 2001a). In Figure 7 two obliquelines show the detection limits derived from three and six
times the maximum station separation of the largest array ateach site. At three sites, observed phase velocities becamealmost flat with the phase velocity of about 1 km/sec in the

Observed phase velocities of S array (circles), M array (triangles), L array(squares), and X array (multiplication signs) and calculated ones having initial (thickdashed lines) and inverted (thick lines) S-wave velocity structures. Average Ⳳ one
standard deviations are shown by bars. Thin dashed and continuous oblique lines show
three and six times of the maximum station separation of the largest array, respectively.

S-Wave Velocity Structure of the Taichung Basin, Taiwan, Estimated from Array and Single-Station Records of Microtremors

wide frequency range from 0.5 to 2 Hz. This suggests thatat all three sites, a thick layer with an S-wave velocity of
about 1 km/sec was deposited. The phase velocities in thefrequency range from 1 to 2 Hz could not be estimated at
TAL001 and TAL002, and the standard deviation in thatfrequency was relatively large at TCU062. The main reason
is that the Fourier amplitude in this frequency was too small
to detect coherently propagating waves (see Fig. 4b). On theother hand, phase velocities of the frequencies lower than 1
Hz or higher than 2 Hz could be estimated up to about six
times the maximum station separation due to the large spectral amplitudes.
In Table 2, the S-wave velocities of the fifth to seventh
layers were assumed based on the crustal structure of southwestern Taiwan (Chung and Yeh, 1997). P-wave velocities(VP) of the fifth to seventh layers were calculated using a
relation of VP /VS ⳱ 1.78. The thicknesses of the fifth andsixth layers were estimated using peak frequencies of HVRs,
as will be explained later. Using the estimated structures ofthe fifth to seventh layers, we estimated the S-wave velocityof the fourth layer to be 1100 m/sec by fitting calculated
phase velocities to observed ones at TCU062, TCU001, andTAL002 by a forward-modeling procedure. To show thesensitivity of the S-wave velocity of the fourth layer, weplotted calculated phase velocities with S-wave velocities of
1.2 times and 1/1.2 times the 1100 m/sec at the three sites
in Figure 8. Here, the other parameters were given constantvalues shown in Table 2. The sensitivity of the S-wave velocity of the fourth layer was high, and it is confirmed thatthe phase velocities calculated by using an S-wave velocityof 1100 m/sec reasonably explained phase velocity data in
the frequency range from 1 to 2 Hz at the three sites.

Figure 8.

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Then we estimated S-wave velocities and thicknesses ofthe first to third layers and a thickness of the fourth layer at
TAL001. Because the highest observed phase velocity wasless than about 1000 m/sec, at TCU062, TAL002, andTCU067, the thicknesses of the fourth layer with VS ⳱ 1100
m/sec could not be estimated at three sites. The P-wave velocities (VP) of the first to fourth layers were calculated from
S-wave velocities (VS) using an empirical relation of VP ⳱
1.29 Ⳮ 1.11 VS in km/sec (Kitsunezaki et al., 1990). Since
we did not have any information on S-wave velocity structures for the first to third layers, the same S-wave velocity
of VS ⳱ 350 m/sec was assumed in the initial model. Then
we inverted S-wave velocities and thicknesses of three layers
to improve the agreement between observed and calculated
phase velocities. The initial thicknesses of the first to fourthlayers were 1, 4, 45, and 1000 m, respectively. To show thesensitivity of S-wave velocities of the first to third layers at
TAL001, we plotted calculated phase velocities with S-wave
velocities of 1.2 times and 1/1.2 times the inverted values in
Figure 9. Here, P-wave velocities were changed by the empirical relation proposed by Kitsunezaki et al. (1990).
Once the S-wave velocities were obtained, the thicknesses of the first to third layers at the other three sites wereinverted by fixing the S-wave velocities as obtained at
TAL001. Here we used thicknesses inverted at TAL001 as
initial values. It was possible to estimate different S-wave
velocities at each site to improve the agreement with observed and calculated phase velocities. However, we assumed the same S-wave velocities at all sites. The main
reason was that simple modeling with the same S-wave
velocities would be more convenient since our final objective of this study was to construct a 3D basin model of the

Calculated phase velocities with S-wave velocities of 1.2 times and 1/1.2
times of 1100 m/sec (dashed lines) at TCU062, TAL001; and TAL002. Observed (circles and vertical bars) and calculated (continuous lines) phase velocities are the sameas those shown in Figure 7.

1274

T. Satoh, H. Kawase, T. Iwata, S. Higashi, T. Sato, K. Irikura, and H.-C. Huang

Figure 9. Calculated phase velocities with S-wave velocities of 1.2 times and 1/1.2
times of values inverted (dashed lines) for the first, second, and third layers at TAL001.
Observed (circles and vertical bars) and calculated (continuous lines) phase velocities
are the same as those shown in Figure 7.

Taichung basin needed for theoretical strong-motion evaluation. The other reason was that observed phase velocities
had some variation shown by standard deviations and so that
it was meaningful to invert structures within such data resolution. The results in Figure 7 indicate that phase velocities
corresponding to the inverted S-wave velocity structures forthe sedimentary layers fitted the observations reasonably.

Estimation of S-Wave Velocity Structures Using
HVRs of Microtremors
HVRs at Array and Single-Station Microtremor Sites

Using the subsurface structures inverted from phase velocities, we calculated HVR, that is, ellipticity for the fundamental Rayleigh wave. We used thicknesses estimatedfrom HVRs only if they were not estimated from phase velocities (Table 2). Figure 10 compares the calculated andobserved HVRs. The observed HVR at each site is the value
averaged over HVRs of S-array records in the same way as
was done for the f-k analysis. The peak and trough frequencies of calculated HVRs shown by bold dashed lines mostly
agree with observed ones. On the other hand, calculated peak
and trough amplitudes do not agree with observed ones in
the high-frequency range, possibly due to contamination ofLove waves in horizontal components. Therefore we usedpeak and trough frequencies but not the amplitudes for theestimation of thicknesses from HVRs by a method mentioned
later. The thin dotted lines denote the calculated HVRs having thicknesses estimated from HVRs. The thicknesses estimated from the phase velocities and HVRs are compared inTable 2. The maximum differences between them are small,that is, within 1, 6, and 11 m for the first, second, and third

Figure 10. Observed HVRs (solid lines) and calculated ones having structures inverted from phasevelocities (bold dashed lines) and estimated HVRs
(thick dotted lines).

S-Wave Velocity Structure of the Taichung Basin, Taiwan, Estimated from Array and Single-Station Records of Microtremors

layers, respectively. The variation of data themselves andcontamination of Love waves in horizontal components may
have caused these differences.
Figure 11 shows observed HVRs (solid lines) at singlestation microtremor sites along the three lines, EW-1, EW-2,
and NS, shown in Figure 1. Here we calculated HVRs for all
the available time segments and averaged them at each site.
The HVRs at TCU062, TAL001, TAL002, and TCU067
along the EW-2 line are the same ones shown in Figure 10.
The HVRs have common peaks in the frequency range from
0.1 and 0.15 Hz. At most of the sites inside the basin (i.e.,
below the arrow of the fault line in Fig. 11), the HVRs havetroughs in the frequency range from 0.2 to 1 Hz, and peaks
at about 1.5 Hz. The dotted lines indicate the calculated
HVRs with structures estimated using peak and trough frequencies by a method explained later.
Numerical Experiment and Estimation Method
We estimated thicknesses of sedimentary layers at the48 sites to fit the calculated peak and trough frequencies of
HVRs for Rayleigh wave to those of observed ones assuming
the same S-wave velocities estimated from the phase velocities of the array records. To know which layer contributed
to observed peak and trough frequencies of HVRs, we conducted a numerical experiment on HVRs for the fundamental
Rayleigh wave. Figure 12a–e shows HVRs calculated bychanging thicknesses of the layers with VS ⳱ 1580 m/sec,
and VS ⳱ 1100 m/sec, VS ⳱ 657 m/sec, VS ⳱ 278 m/sec,
and VS ⳱ 116 m/sec, respectively. Here the other parameters(P-wave velocities, S-wave velocities, densities, and thicknesses except for the changing thickness) were fixed by thevalues estimated from the phase velocities at TAL001. We
also assumed the thickness ratio between the layers with VS
⳱ 1580 m/sec and VS ⳱ 2560 m/sec to be 2:3, which is thesame ratio as the inverted thicknesses.
The larger the thickness of the layer with VS ⳱ 1580
m/sec was, the lower the peak frequency became from 0.3
to 0.08 Hz. The larger the thickness of the layer with VS ⳱
1100 m/sec was, the lower the trough frequency became
from 1 to 0.2 Hz. The other peak and trough frequencies did
not depend so much on the thickness of the layer with VS ⳱
1580 m/sec or VS ⳱ 1100 m/sec, as shown in Figure 12a
and b. In addition, when the thickness of one of the shallower
layers with VS ⳱ 657 m/sec, VS ⳱ 278 m/sec, or VS ⳱ 116
m/sec changed, the peak and trough frequencies controlled
by the deeper layers with VS ⳱ 1580 m/sec and VS ⳱ 1100
m/sec did not change, as shown in Figure 12c–e. This numerical experiment shows that the thicknesses of the layerswith VS ⳱ 1580 m/sec and VS ⳱ 1100 m/sec can be independently estimated using the peak and trough frequencies
below 1 Hz.
For the layer with VS ⳱ 1580 m/sec, however, the observed peak frequencies at about 0.15 Hz did not change somuch from site to site, as shown in Figure 11. In addition,
the observed peaks were not sharp, mainly because the amplitudes in such a low-frequency range were as small as

1275

those in the frequency range from 1 to 2 Hz, where coherently propagating waves were difficult to detect by the f-k
analysis. Therefore we estimated the common thickness of
the layer with VS ⳱ 1580 m/sec at all the sites by a forwardmodeling procedure.
To estimate the thickness of the layer with VS ⳱ 1100
m/sec, we calculated HVRs by changing the thickness of thislayer from 200 to 1400 m by a grid spacing of 20 m. Then
we selected all trough frequencies in the frequency range
from 0.1 to 1.5 Hz and selected the higher frequency if there
were two troughs by considering the numerical experimentshown in Figure 12b. For observed trough frequencies webasically selected the deepest trough in the same frequency
range at each site. If the difference of the amplitudes ofnearby troughs was less than 0.02, we took a geometrical
average of the two trough frequencies as in the case of WST02 shown in Figure 11a. When the observed trough was not
so clear or multiple troughs existed in this frequency range,
we selected a trough by considering the numerical experiment and spatial smoothness in the thickness distribution.
As shown in Figure 12b, the trough is theoretically unclear
when the thickness was less than about 400 m, and so thereliability of the estimated thickness would be lower when
the thickness was less than 400 m.
Peak and trough frequencies in the frequency range
from 1 to 20 Hz depend on the layers with VS ⳱ 657 m/sec,
VS ⳱ 278 m/sec, and VS ⳱ 116 m/sec as shown in Figure
12c–e. The trough frequencies at around 12 Hz shown in
Figure 12c and d do not change so much, so they are mainly
controlled by the layer with VS ⳱ 116 m/sec. However, otherpeak and trough frequencies below 12 Hz are influenced bymultiple layers. Therefore we estimated the thicknesses of
the three layers by fitting the calculated peak and troughfrequencies to the observed ones in the frequency range from
1 to 20 Hz by a grid search method. The searching range ofthe thickness was determined based on the calculated andobserved HVRs. The thickness of the layer with VS ⳱ 657
m/sec was changed from 0 to 150 m by a grid spacing of
2 m. The thickness of the layer with VS ⳱ 278 m/sec waschanged from 0 to 30 m by a grid spacing of 2 m. The
thickness of the layer with VS ⳱ 116 m/sec was changed
from 0 to 5 m by a grid spacing of 1 m.
The number of total grids was 7296 (76 ⳯ 16 ⳯ 6).
Theoretical peak and trough frequencies were automaticallyselected in the frequency range from 1 to 20 Hz. Observed
peak and trough frequencies at each site were selected bysight by considering the numerical experiment and spatialsmoothness in the thickness distribution. The circles and triangles in Figure 12 show the observed peak and trough frequencies, respectively. The thicknesses of the three layers
were estimated to select mth grid having the smallest values
of ERRm,
J1i
ERRm ⳱
Min |wj log(TPmi /OPj)|j⳱1 i⳱1→l1 j
J2iⳭMin |log(TDm(2)
i /ODj)|j⳱1 i⳱1→l2 j

兺

兺

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T. Satoh, H. Kawase, T. Iwata, S. Higashi, T. Sato, K. Irikura, and H.-C. Huang

Figure 11. Observed (solid lines) and calculated (thick dashed lines) HVRs at
single-station microtremor sites along the three lines: (a) EW2 line, (b) EW1 line, and(c) NS line. Thin dashed lines denote the baseline of a factor of one at each site. Big
triangles denote troughs of HVRs used for the estimation of the thickness of the layer
with VS ⳱ 1100 m/sec. Small triangles and circles denote troughs and peaks of HVRs
used for the estimation of the thicknesses of the layers with VS ⳱ 657 m/sec, VS ⳱
278 m/sec, and VS ⳱ 116 m/sec.

S-Wave Velocity Structure of the Taichung Basin, Taiwan, Estimated from Array and Single-Station Records of Microtremors

Figure 12. Calculated HVRs by changing thicknesses of the layers with (a) VS ⳱
1580 m/sec, (b) VS ⳱ 1100 m/sec, (c) VS ⳱ 657 m/sec, (d) VS ⳱ 278 m/sec, and(e) VS ⳱ 116 m/sec. Here the other parameters (P-wave velocities, S-wave velocities,
densities, thicknesses except for the changing thickness) were fixed by the values estimated from the phase velocities at TAL001. Circles denote peak frequencies mainly
controlled by the layer with VS ⳱ 1580 m/sec. Triangles denote trough frequencies
mainly controlled by the layer with VS ⳱ 1100 m/sec.

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T. Satoh, H. Kawase, T. Iwata, S. Higashi, T. Sato, K. Irikura, and H.-C. Huangm
Here TPmi and TDi are the calculated ith peak and troughfrequencies for the mth grid, respectively. I1 and I2 are thenumber of calculated peaks and troughs in the same frequency range, respectively; OPj and ODj are the observed
jth-peak and trough frequencies, respectively; and J1 and J2
are the number of observed peaks and troughs in this frequency range, respectively. Wj is the weighting constant, 0.3
for j ⳱ 1 and 1.0 for everything else. Wj was empiricallyintroduced because the first peak frequencies at most of thesites were in the frequency range from 1 to 2 Hz, where
Fourier amplitudes are small (Fig. 4). The validity of themethod was proved by applying it to the four array measurement sites (Fig. 10).

Estimated Velocity Structure
The thickness of the layer with VS ⳱ 1580 m/sec wasestimated to be 1800 m by considering peak frequencies(around 0.15 Hz) of HVRs and previous information (Shaw,
1996). When we use this thickness (1800 m), the lower
boundary of this layer at TCU109 (about 2.2 km) is consistent with the lower boundary of the Toukoshan Formationin Pleistocene era obtained from a deep well near this site
(Shaw, 1996). Since the pre-Tertiary bedrock depth obtained
from the deep well there (Shaw, 1996) is about 5 km, thethickness of the layer VS ⳱ 2560 m/sec should be about 2.6
to 2.7 km. The observed peak frequency of about 0.15 Hz
is generally explained by using the thicknesses of 1800 and
2700 m for the layers with VS 1580 and VS 2560 m/sec,
respectively, as shown in Figure 11. Using 1800 m as the
thickness of the layer with VS ⳱ 1580 m/sec, the depth of
the upper boundary of the layer with VS ⳱ 2560 m/sec becomes about 3 km at four array microtremor sites, as shownin Table 2. This is consistent with the upper boundary of the
Pliocene layer (lower boundary of the Toukoshan Formation) estimated from onshore and offshore deep wells inwestern Taiwan (Shaw, 1996). The pre-Tertiary bedrock
depth estimated from microtremors becomes nearly 6 km
near array microtremor sites in the east-central part of theTaichung basin (Table 2).
Troughs controlled by the layer with VS ⳱ 1100 m/sec,
shown in Figure 11, are not so clear at ETC006, ETC005,
and TCU061 along the EW-1 line, at TCU109 and TCU107
along the EW-2 line, and at CTC-07 along the NS line, so
the accuracy of the estimated thickness of this layer would
not be so good at these sites. Stations ETC005 and ETC006
are located on the hanging wall of the Chelungpu fault. Stations TCU061, TCU107, and TCU109 are located near the
Changhua fault in the western part of the Taichung basin.
Station CTC-07 is located in the northern part of the Taichung basin. At most of the other sites inside the basin,calculated peak and trough frequencies of HVRs agree wellwith observed ones. Comparing the observed HVRs on bothsides of the Chelungpu fault along the EW-2 line, the troughfrequency at a hanging-wall site, TAL004 is clearly higher
than that at a nearby footwall site, TCU067. On the other

hand, the differences of the trough frequencies at both sidesof the fault along the EW-1 line, ETC002 and ETC003, arenot clear. This suggests that a sharp lateral discontinuity
exists between TCU062 and TAL004, but not between
ETC002 and ETC003.
The spatial distribution of the thicknesses of the layerswith VS ⳱ 1100 m/sec, VS ⳱ 657 m/sec, VS ⳱ 257 m/sec,
and VS ⳱ 116 m/sec estimated from HVRs at all the 52 sites
is shown in Figure 13. The layer with VS ⳱ 1100 m/sec isthickest, 1000–1400 m, near the array microtremor sites inthe east-central part of the Taichung basin. This thickness
rapidly decreases to less than 400 m in the northeastern andwestern parts inside the basin. The difference in thickness
on the two sides of the fault is noticeable along the EW-2
line but not along the EW-1 line as suggested from troughfrequencies. The difference in the lower boundary of thelayer with VS ⳱ 1100 m/sec is estimated to be about 800 m
between TCU067 and TAL004 (1370 m versus 560 m).
These results suggest that lateral discontinuity exists at botheastern and northern sides of array microtremor sites in theeast-central part of the basin. However, we should note thatthese thicknesses were estimated under the assumption that
S-wave velocities are common at all the sites. Since the Chelungpu fault is a thrust fault dipping to the east, this assumption may not be true at sites on the hanging-wall sides.
The thickness of the layer with VS ⳱ 657 m/sec is about
60 to 100 m inside the basin. The thicknesses of the layer
with VS ⳱ 278 and 116 m/sec are small in the northeasternpart and large in the southern and east-central parts. Thus in
the northeastern part inside the basin, the thicknesses of thelayers with VS ⳱ 1100 m/sec and the soft layers with VS ⳱
278 m/sec and VS ⳱ 116 m/sec are smaller than those at theother sites inside the basin. It is quite interesting to see thatnear the array microtremor sites in the east-central part ofthe basin where we observed high damage concentration
during the mainshock, those three layers are relatively thick.
The estimated basin structure shown in Figure 13 suggeststhat it is spatially very complex, and so its 3D site effects
should be taken into account for strong-motion evaluationin the Taichung basin.
Travel-Time Analysis
We examined the arrival times of the initial P and S
waves using records of an aftershock with a magnitude of
M 5.5 and a focal depth of 13.5 km (Fig. 1), which was the
only aftershock recorded at all 10 stations deployed by Higashi et al. (2001). In Figure 14, the relationship between
the observed travel time and the epicentral distance is shownby circles. The epicentral distance and the travel time were
calculated based on the epicenter and the origin time determined by CWB. Using the estimated velocity structures, we
calculated the propagating time from the lower boundary of
the layer with VS ⳱ 1100 m/sec to the surface assuming
vertical incident P and S waves. We estimated incident angles from particle orbits of S waves with durations of 2 sec
using radial and vertical components. The incident angle av-

S-Wave Velocity Structure of the Taichung Basin, Taiwan, Estimated from Array and Single-Station Records of Microtremors

Figure 13. Spatial distribution of the thicknesses of the layers with (a) VS ⳱ 1100
m/sec, (b) VS ⳱ 657 m/sec, (c) VS ⳱ 257 m/sec, and (d) VS ⳱ 116 m/sec estimatedfrom HVRs.

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T. Satoh, H. Kawase, T. Iwata, S. Higashi, T. Sato, K. Irikura, and H.-C. Huangeraged over 10 stations was 5.5⬚, and the standard deviation
was 2.8⬚. So no ray tracing was performed for simplicity. In
Figure 14, the triangles denote the travel time corrected bysubtracting the propagating time from the observed travel
time from the source to the station. We drew calculated
travel-time lines with VP ⳱ 5849 m/sec and VS ⳱ 3290 m/
sec of the bedrock (Table 2) to intersect the corrected travel
time at TAL004 (T4). The corrected travel time was consistent with the theoretical travel time. If the thickness from the
bedrock to the lower boundary of the layer with VS ⳱ 1100
m/sec can be assumed to be the same at all 10 stations, thisagreement would show the validity of the estimated velocity
structures above the layer with VS ⳱ 1100 m/sec.
Using aftershock records, Higashi et al. (2001) showedthat site amplifications with respect to TAL004 were noticeable at TAL001, TAL002, and TAL003 along the EW-2 line,
but not at ETC001, ETC002, ETC003, ETC004, ETC005,
and ETC006 along the EW-1 line. The differences between
these observed site amplifications may be due to the differences in the underground structures along these two lines
described in this article.

Conclusions

Figure 14. Relation (circles) between the observed travel time of P and S waves and epicentral
distance for an aftershock (M ⳱ 5.5) observed at 10
aftershock strong motion stations (Higashi et al.,2001). T1, T2, T3, T4, E1, E2, E3, E4, E5, and E6
denote TAL001, TAL002, TAL003, TAL004,
ETC001, ETC002, ETC003, ETC004, ETC005, and
ETC006, respectively. Using the estimated velocity
structures, we calculate the propagation time from thelower boundary of the layer with VS ⳱ 1100 m/sec
to the surface assuming vertical incident P and S
waves. Triangles denote the travel time corrected bysubtracting the propagating time from the observed
travel time from the source to the station. Lines denote calculated travel time with VP ⳱ 5849 m/sec andVS ⳱ 3290 m/sec of the bedrock to intersect the corrected travel time at TAL004 (T4).

The objective of this study was to estimate the S-wave
velocity structure of the Taichung basin in the near-fault
region, which is needed for strong-motion evaluation for the
Chi-Chi earthquake. To achieve this goal, we have conducted array measurements of microtremors with a total of
12 arrays at four sites and single-station measurements of
microtremors at 48 sites in and around the Taichung basin.
Based on the Rayleigh-wave inversion technique using
phase velocities estimated from array records of microtremors, we find that a thick layer with VS ⳱ 1100 m/sec isabout 1000 to 1400 m at array microtremor sites. Above thislayer, soft layers with VS ⳱ 657 m/sec, VS ⳱ 278 m/sec,
and VS ⳱ 116 m/sec have been deposited with thicknessesof less than 70, 20, and 5 m, respectively.
We estimated the thicknesses of sedimentary layers
above the pre-Tertiary bedrock at 48 sites to fit calculated
peak and trough frequencies of horizontal-to-vertical spectral ratios of Rayleigh wave to observed ones, assuming thesame S-wave velocities estimated using array records. The
pre-Tertiary bedrock depth was estimated to be about 5 to 6
km in this region. It is found that the layer with VS ⳱ 1100
m/sec is thickest around the array microtremor sites in theeast-central part of the basin and that the thickness rapidly
decreases to less than 400 m in the northeastern and westernparts inside the basin. The difference in the thicknesses at
the two sides of the Chelungpu fault is noticeable along thesouthern line across the fault (EW2) but not along the northern line (EW-1). The difference in the lower boundary of thelayer with VS ⳱ 1100 m/sec is estimated to be about 800 m
at EW-2. The estimated S-wave velocity structures reasonably explain arrival time of initial P and S waves of aftershock records observed by Higashi et al. (2001).

S-Wave Velocity Structure of the Taichung Basin, Taiwan, Estimated from Array and Single-Station Records of Microtremors

We should note that the thicknesses were estimated under the assumption that S-wave velocities of sedimentarylayers were common at all the sites, and only their thicknesses were varying. This assumption might not be true atsites on the hanging wall. However, since most of the singlestation microtremor sites are located at the footwall side inside the basin, the estimated velocity structures would be
useful for constructing an initial 3D basin model for strongmotion evaluation.
As pointed out by Sato et al. (1999), it is very importantfor quantitative strong-motion evaluation to modify the initial basin model estimated by geophysical exploration methods through waveform modeling of strong-motion records
for a moderate earthquake. It would be worthwhile to consider an initial 3D basin based on the velocity structures
estimated in this study and modify it using waveform modeling of aftershock records in order to construct a realistic
3D model for strong-motion evaluation for the Chi-Chi
earthquake.

Acknowledgments
Microtremor measurements were conducted with Ken’ichiro Nagato,
Jun’ichi Miyakoshi, Hiroshi Yajima, and Kazuhiko Ishihara. We especiallywish to thank Kazuyoshi Kudo for kindly loaning us seven sets of instruments, Hung-Chie Chiu, Jin Anshu, Haruko Sekiguchi, Yasuhiro Hayashi,
Saburoh Midorikawa, and Ingemar Johanson gave us useful suggestions
for selecting observation sites. We also appreciate the valuable comments
and suggestions of two reviewers. Finally, we used Generic Mapping Tools(GMT) by Wessel and Smith (1998) to plot some figures.

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Izumi Research InstituteShimizu Corporation
2-2-2, Uchisaiwai-cho
Chiyoda-ku, Tokyo 100-0011, Japan(T. S., T. S.)

Department of Architecture and Urban Design
Graduate School of Human-Environment Studies, Kyushu University
6-10-1, Hakozaki
Higashi-Ku, Fukuoka 812-8581, Japan(H. K.)
Disaster Prevention Research Institute, Kyoto University
Gokasho, Uji
Kyoto 611-0011, Japan(T. I., K. I.)
Geotechnical and Earthquake Engineering Department
Central Research Institute of Electric Power Industry
Abiko 1646, Abiko, Chiba 270-1194, Japan(S. H.)
Institute of Seismology, National Chung-Cheng University
No. 160 San-Hsing, Ming-Hsiung.
Chia-Yi 621, Taiwan(H. H.)
Manuscript received 31 July 2000.