AND Wen-Gee Huang

YEONG TEIN YEH Kuo-Liang Wen

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Bulletin of the Seismological Society of America, Vol. 82, No. 2, pp. 1124-1133, April 1992 
S H O R  T N O T E S  
EFFECTS OF AN ALLUVIAL BASIN ON STRONG 
GROUND MOTIONS 
BY Kuo-LIANG WEN, YEONG TEIN YEH, AND WEN-GEE HUANG 
It has been reported that  structures on loose soil sustain more damage during 
ear thquakes  than those on solid rock. This phenomenon has long been ascribed 
to the influence of local site geology. Gutenberg (1956) observed stronger 
shaking and longer duration at sites on alluvium than at those on crystalline 
rock, and large variat ions in ground shaking even at sites only 1000 feet apart. 
During the recent Michoacafi ear thquake of 1985, most of the damages occurred 
in Mexico City more than  300 km from the epicenter (Anderson et al. ,  1986). 
The city is s i tuated atop very soft sediments tha t  caused dramatic amplification 
of strong ground motion, severe damage to the structures, and considerable loss 
of life within the area of the city located on top of the soft sediments. 
On 20 May 1986 a magnitude M L = 6.5 ear thquake occurred near Hualien in 
eastern Taiwan. The epicenter was located at 24°4.9'N and 121o35.49 ' E, and 
the focal depth was 15.8 kin. This ear thquake caused some damage in the 
Hualien and Taipei areas (Tsai et al. ,  1986). Accelerographs at 37 sites of the 
Strong Motion Accelerographs Array in Taiwan phase 1 (SMART1 array, Fig. 1; 
Bolt et al. ,  1982) were triggered. The Lotung Large Scale Seismic Test Array 
(LSST Array, Figs. 1 and 2; Wen et al. ,  1986) is si tuated within the southwest- 
ern quadrant  of the SMART1 array. Surface recordings and two groups of 
downhole recordings of the LSST array were also obtained. In this study, we use 
the frequency-wavenumber (f-k) analysis to study the variations in wave propa- 
gation direction and velocity. The change in peak acceleration, velocity, and 
displacement with respect to depth and the frequency-dependent ground motion 
amplification are studied by using the downhole and surface recordings of the 
LSST array. 
THE SMART1 ARRAY DATA 
From the SMART1 array center (CO0), the epicentral distance and azimuth of 
the Hual ien ear thquake are 68 km and 195 °, respectively. Figure 3 is an 
il lustration of some horizontal accelerograms recorded by the SMART1 array. 
In this figure, the t ime traces are plotted in a form commonly used with 
exploration geophone spreads. Because absolute t ime is available, the records 
can be correlated against  epicentral distances in real time. Note the coherent S 
pulses on the horizontal components can be seen clearly across the whole array. 
Figure 4 is the contour map of peak ground accelerations for the EW compo- 
nent in the SMART1 array area caused by this earthquake.  The contour 
interval is 10 gals. The symbols " + "  and " - "  in the map represent  the contour 
high and low, respectively. The largest peak acceleration is about 0.21 g and it 
occurred near stations 004,  005,  and M06; the lowest peak acceleration is about 
0.09 g and it occurred near station O12. The differences in maximum accelera- 
tion can be as much as 0.13 g within an area of 12 km 2. This large variation 
over a distance of less than 4 km accentuates the difficulty in describing an 
1124 
SHORT NOTES 1125 
• SMART1 site 
A LSST array  
ILAN 
M1 
MO9 e 
M06 
oM03 
'06 
003 
Q 
~UCHE 
eE01 
TUNGSHAN 
E02 
"a ~looosoo o ~- - Jm ?~-~ .... ',,,,I 
SUAO 
1 2km 
I I 
121°44"  121 ° 48" 
Fro. 1. Layout of the SMART1 array. 
24o43" 
24°38" 
accelerogram simply by its peak value. The reasons for these differences may be 
the differences of the local site geology or the properties of the subjacent strata. 
f-]~ ANALYSIS 
Using the algorithm of Capon (1969), the f k  spectrum was computed at a 
discrete frequency as a function of wavenumber  k and was decomposed into two 
orthogonal components. A 5-sec time window of the S wave (shown in Fig. 3) for 
the recordings of CO0, M-ring, and O-ring (Fig. 1) was used. The f-k analysis 
was conducted on the frequency band of 0.6 to 1.4 Hz, for both EW and NS 
components. The results from the NS component show that  within the 95% 
confidence interval the wave propagated across the array with an apparent 
1126 
N 
T 
3 2  
DHA 
0 IOM 
I I I I | I 
SHORT NOTES 
1 .5 
DHB 4-.i2. 3 
5 
© 
1 / 1 2 M O D E L  
FA1 
4 
]3 
12 
"1"5 
~ / 4  M O D E L  
2 3 4 FA2 
[ ]  
5 
FIG. 2a. The surface instrumentation of the LSST array. 
//%/,~./,~ 
3.2m 
>,1 °.A 
. I [ 3 "  
45.7m ='l 
__6m 
2 J m  
[ ]  J7  m- 
[ ]  47m 
DHB = N l[ L[] 
[] 
[] 
. [ 3  
FIG. 2b. The downhole instrumentation of the LSST array. 
SHORT N O T E S  1 1 2 7  
E 
V 
LU 
C) 
Z 
<~ 
U) 
4 ~ 001 
n 002 
- o J 2  
- 0 0 3  
3-- M 0 1  
M 1 2  
- M03 
- M l l  
- 1 0 1  
2-- CO0 
- M I O  
- 1 0 7  
- M05 
- , M 0 9  
M 0 6  
1 -  M 0 7  
L 006 
_ 0 0 8  
O-  007 
SMART1 PLAYBACK TRACES 
EQ.  NO. 4 0  ( 1 9 8 6 ,  5 ,  2 0 , 5 : 2 6 )  EW COMP. 
10 15 20 
I I I I I I , I I I I I I I I I I I 
2 0 0 g a l  
W 1 -~ 
[ / o  
f I I I I I I [ I i I I I I I i ] 
10 15 20 
TIME ( s e c )  
FIG. 3. Acceleration waveforms of EW component recorded at  the  SMART1 array. The records 
are aligned in absolute t ime and are plotted according to increasing epicentral  distance. The t ime 
window W1 is used for the f-k analysis.  
1 1 2 8  SHORT N O T E S  
FIG. 4. Contour map of peak acceleration of EW component in  the  SMART1 array area. The 
contour interval  is 10 gals. 
velocity of 3.1 + 0.5 km/sec and azimuth of S17 ° + 7°W. For the EW compo- 
nent, the results are 3.5 + 0.4 km/sec and $19 ° + 7°W. The epicentral azimuth 
(S15°W) is within the 95% confidence interval of the calculated S-wave propa- 
gation direction ($10 ° to $26°W). This indicates that the S wave does propa- 
gate along the path connecting the focus and the array. 
f-k analysis was also performed by using four subarrays, defined by the 
principal quadrants, in the same frequency range and time window. The wave 
propagation direction and apparent velocity in each quadrant, calculated from 
the EW component, for the frequency band of 0.6 to 1.4 Hz are given in Figure 
5, and the 95% confidence intervals of velocity and azimuth are also shown in 
Figure 5 with an error ellipse. It is clear that the mean wave propagation 
direction in the northern array area was pointed to the NE and the apparent 
velocity varied from 2.5 km/sec in the southern area to greater than 4.0 km/sec 
in the northern area. The analyses of the 29 January 1981 Suao earthquake by 
Bolt et al. (1984) also point out the significant change in wave propagation 
direction occurring over a short distance with significant rotational motion 
present. This rotational motion suggests the existence of geometrical focusing 
effects in a three-dimensional basin. These focusing effects and three-dimen- 
sional caustics are highly developed by the spherical geometry of the circular 
basin model considered as shown in the numerical study by Lee and Langston 
(1983). The basin center of the Lanyang plain is the northeast of the array, and 
this may explain why the wave path turned to the northeast. 
DOWNHOLE RECORDINGS OF THE LSST ARRAY 
The decrease of peak horizontal acceleration, velocity, and displacement with 
increasing depth was studied using the DHB group (see Fig. 2) of downhole 
recordings of the LSST array. The maximum of the two horizontal peak 
SHORT N O T E S  
N 
1129 
N 3 1  ° -+ 8 . 4 ° E  
...'"'.. 
N 3 8  ° + 8 . 4 ° E  
: ....... . .  
V = 3 .76  +_ O , 5 8 k m  / s 
N 6 ° _ + 6 . 3 ° E  
..,% l': 
V = 2 . 2 2  + 0 . 2 8 k m / s  
..°.,,*" 
V = 4 . 6 5  -+ 0 . 7 0 k m / s  
l 
N 6  ° + 4 . 0 ° E  
V =  2 . 4 8  + 0 . 2 7 k i n / s  
E 
FIG. 5. Average wave propagation direction and  apparent  velocity of S wave in the  frequency 
band of 0.6 to 1.4 Hz as calculated from EW component. The 95% confidence interval  is shown by an  
error ellipse. 
accelerations, velocities, and displacements were designated as the PGA, PGV, 
and PGD, respectively. To understand the relationships between the peak 
amplitudes and depth, the peak amplitudes recorded at the downhole sites were 
normalized with respect to the surface sites and shown in Figure 6. Using linear 
regression analysis, we obtain the empirical relationships between the normal- 
ized parameters,  NPGA, NPGV, and NPGD, and depth. The relationships are as 
follows: 
N P G A  = e -0"1827H0"4068, 
N P G V  = e-°°Z°SH12~6,  
N P G D  = e -°°°26H15°6~, 
where H is depth in m. These equations are plotted as dashed lines in Figure 6. 
The PGAs at the surface are about 2.1 times greater than those at 47-m depth. 
This ratio agrees with the results in the Richmond area in California, where the 
PGA at the free surface is about 1.6 to 2.8 times greater  than that  at  40-m 
depth (Johnson and Silva, 1981). The shear wave velocity at Richmond between 
0 and 36 m depth is about 100 to 300 m/sec  (Johnson and Silva, 1981), which is 
similar to that  under the LSST array ( V s  = 120-280 m/sec in the upper 50 m 
soil; Wen and Yeh, 1984; HCK, 1986). 
The effects of a soil layer can be examined by comparing the Fourier  spectrum 
of an alluvium site to that  of a rock site or of a surface site to that  of a downhole 
1130 
<~ 
0 
13. 
Z 
> 
rt 
Z 
Q 
Z 
1 . 2  m 
' I 
1 . 0  
O .  8 -~,, 
0 . 6  - 
0 . 4  
0 . 2  = I 
0 10  
1 . 2 ~ .  , I 
1 . 0  ~ . , . ,  
o . 8  - " ~ .  
o,6 
0.4 
0 . 2  i I 
0 10 
1.2 
1.0 
0 . 8  
0 .6  
0 .4  
0 . 2  
SHORT NOTES 
I • " ! 
L... 
! I 
DHB 
| .... 
I i I i I 
20  30  40  
D E P T H  ( m )  
I ' I ' I 
@ DHB 
50 
• I , I , I , I I 
0 10 20  30  40 
• D H B  
% %  
50 
DEPTH ( c m )  
FIQ. 6. Variations ef the normalized peak ground motions with respect to depth in the LSST 
array. (a) Acceleration, NPGA; (b) velocity, NPGV; and (c) displacement, NPGD. 
I i I , i , 
20  30  40  50 
D E P T H  ( m )  
I ' I ' "I  ' .... 
SHORT NOTES 
EW COMPONENT 
1 2 0  I . . . .  1 I ' I I I i I I I I ' I ' I 
I I 
,~ . . . . . . . . . . . .  F A 1  - 5 - '  
~ "  : :  . . . . . . .  D H B 4 7  
U : :  : :  u~ : : L H A S K E L L  
9 0  : :  ~ 
l : :  : :  
° I - 
Q , 
P_ 6o 
/ 
~ ;  i 
W ' 
~.j 
LL 
1 2 3 4 5 6 7 8 
FREQUENCY ( H z )  
1131  
1 2 0  
U 
~ ' °  
2 ~. 6o 
0 
NS COMPONENT 
' I i I ' I i I i I I I I I ' 
. . . . . . . . . . . .  F A 1 - 5  
. . . . . . .  D H B 4 7  
H A S K E L L  
i i! /i 
"" " . - 
- ~ "....  -'.. _ 
0 1 2 3 4 5 6 7 8 
FREQUENCY ( H z )  
FIG. 7. The Fourier spectra of surface and DHB47 downhole recordings. The solid line show the 
results estimated by using the record of DHB47 as input. 
1132 SHORT NOTES 
site. In our study, the first 5.12 sec signals after the S arrival is extracted from 
the original accelerogram by a tapered box-car window to calculate the Fourier 
spectrum. The Fourier spectra of the FA1-5 surface and the DHB47 downhole 
recordings are compared. The frequency band in which the ground motion has 
been amplified by the sedimentary layer is shown in Figure 7. The soil 
amplification effect is significant in the frequency band of 0.5 to 2.0 Hz. The 
dominant frequencies are about 0.9 and 1.3 Hz, respectively, for the EW and NS 
components. According to the velocity-depth profile at the site (Wen and Yeh, 
1984; HCK, 1986), the downhole accelerographs are located in the top two 
sedimentary layers with thicknesses of 10 and 40 m and S-wave velocities of 
120 to 160 and 190 to 280 m/sec, respectively. We use the average velocity 
model of 140 and 250 m/sec for these two layers (assume constant Q equal to 
10) and the record of DHB47 as input. Based on Haskell's method (1960), we 
can see that the estimation (shown on Fig. 7) of the NS component fits the 
observations well. However, the estimation of the EW component shows a shift 
of the peak frequency from 0.9 to 1.25 Hz. The downhole recordings of this 
earthquake were analyzed for evidence of nonlinear soil response during strong 
shaking (Chang et al., 1989). Effective shear moduli or shear-wave velocities 
decreased as the level of shaking increased. For the soil layer between 11 and 
17 m, the effective shear wave velocities were 81 and 145 m/sec, respectively, 
for EW and NS components (Chang et al., 1989). The shear-wave velocity from 
in situ geophysical measurements is 187 m/sec. This may explain why the soil 
responses of the EW and NS components are different. 
ACKNOWLEDGMENTS 
This work was supported by the National Science Council and the Institute of Earth Sciences, 
Academia Sinica. The LSST array is funded by the Taiwan Power Company (R.O.C.) and the 
Electrical Power Research Institute. Discussions with B. S. Huang, G. B. Ou, and B. Y. Kuo are 
greatly appreciated. The authors thank Dr. C. B. Crouse and an anonymous reviewer for their 
constructive comments and suggestions. 
REFERENCES 
Anderson, J. G., P. Bodin, J. N. Brune, J. Prince, S. K. Singh, R. Quaas, and M. Ofiate (1986). 
Strong ground motion from the Michoacan, Mexico, earthquake, Science 233, 1043-1049. 
Bolt, B. A., N. A. Abrahamson, and Y. T. Yeh (1984). The variation of strong ground motion over 
short distance, in Proc. 8th World Conf. Earthq. Eng., San Francisco. 
Bolt, B. A., C. H. Loh, J. Penzien, Y. B. Tsai, and Y. T. Yeh (1982). Preliminary report on the 
SMART1 strong motion array in Taiwan, EERC Report No. UCB/EERC-82/13. 
Capon, J. (1969). High resolution frequency-wavenumber spectrum analysis, Proc. IEEE 57, 
1408-1418. 
Chang, C. Y., M. S. Power, Y. K. Tang, and C. M. Mok (1989). Evidence of nonlinear soil response 
during a moderate earthquake, in Proc. 12th Int. Conf. Soil Mech. and Foundation Eng., 
Brazil. 
Gutenberg, B. (1956). Effects of ground on earthquake motion, EOS 37, 757-760. 
Haskell, N. A. (1960). Crustal reflection of plane SH waves, J. Geophys. Res. 65, 4147-4150. 
HCK (1986). Geophysical survey report of Lo-Tung project for Taiwan Power Company, HCK 
Geophysical Company. 
Johnson, L. R. and W. Silva (1981). The effects of unconsolidated sediments upon the ground motion 
during local earthquakes, Bull. Seism. Soc. Am.  71, 127-142. 
Lee, J. J. and C. A. Langston (1983). Wave propagstion in a three-dimensional circular basin, Bull. 
Seism. Soc. Am.  73, 1637-1653. 
Tsai, C. C., Y. T. Yeh, K. L. Wen, S. N. Cheng, and M. H. Kao (1986). The Hualien earthquake of 
May 20, 1986: Strong ground motion data and response spectra, Bull. Inst. Earth Sci., 
Academia Sinica 6, 29-64. 
SHORT NOTES 1133 
Wen, K. L. and Y. T. Yeh (1984). Seismic velocity structure beneath the SMART1 array, Bull. Inst. 
Earth Sci., Academia Sinica 4, 51-72. 
Wen, K. L., Y. T. Yeh, and C. C. Liu (1986). The observation system and data processing of the 
LSST array, Institute of Earth Sciences, Academica Sinica, ASIES-CR8602, (in Chinese). 
INSTITUTE OF EARTH SCIENCES 
ACADEMIA SINICA 
P. O. BOX 23-59 
TAIPEI, TAIWAN 10764 
REPUBLIC OF CHINA 
Manuscript received 6 March 1990 


EFFECTS OF AN ALLUVIAL BASIN ON STRONG GROUND MOTIONS

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EFFECTS OF AN ALLUVIAL BASIN ON STRONG GROUND MOTIONS

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