GEOPHYSICAL RESEARCH LETTERS, VOL. 30, NO. 17, 1890, doi:10.1029/2003GL017601, 2003
Field relations among coseismic ground motion, water level change and
liquefaction for the 1999 Chi-Chi (Mw = 7.5) earthquake, Taiwan
Chi-Yuen Wang,1 Douglas S. Dreger,1 Chung-Ho Wang,2 Daniel Mayeri,1
and James G. Berryman3
Received 23 April 2003; revised 25 July 2003; accepted 25 July 2003; published 6 September 2003.
[1] Field-based, basin-wide relations are discovered
among coseismic ground motion, water-level change and
liquefaction induced by the 1999 (Mw = 7.5) Chi-Chi
earthquake, Taiwan. The relations imply thresholds of
4.4 m/s2 and 0.6 m/s in the spectral acceleration and
velocity at 1 Hz, respectively, below which pore pressure
does not change appreciably and above which pore pressure
increases exponentially with increasing acceleration and
velocity. Liquefaction occurs when the 1-Hz spectral
acceleration and velocity and the peak ground acceleration
exceed 16 m/s2, 2.4 m/s, and 2.5 m/s2 (0.26 g),
respectively. Broad consistency between the present and
previous results suggests that the relations may be
transferable to other areas and may provide fundamental
constraints for modeling coseismic pore-pressure increase
INDEX TERMS: 5104 Physical Properties of
and liquefaction.
Rocks: Fracture and flow; 7212 Seismology: Earthquake ground
motions and engineering; 7223 Seismology: Seismic hazard
assessment and prediction; 8045 Structural Geology: Role of
fluids; 8105 Tectonophysics: Continental margins and sedimentary
basins (1212). Citation: Wang, C.-Y., D. S. Dreger, C.-H. Wang,
D. Mayeri, and J. G. Berryman, Field relations among coseismic
ground motion, water level change and liquefaction for the 1999
Chi-Chi (Mw = 7.5) earthquake, Taiwan, Geophys. Res. Lett.,
30(17), 1890, doi:10.1029/2003GL017601, 2003.
reported ground motion, coseismic pore pressure, and
liquefaction at a single site in Imperial Valley, California.
Here we report field-based, basin-wide relations among
coseismic ground motion, pore-pressure buildup, and liquefaction, derived from data for the 1999 Chi-Chi earthquake.
[3] Taiwan is a young mountain belt formed by the
oblique collision between the Luzon arc of the Philippine
Sea plate and the continental margin of the Eurasian plate
since 5 Ma. The 1999 Chi-Chi (Mw = 7.5) earthquake
ruptured the crust along a 80 km segment of the Chelungpu fault (Figure 1). Extensive liquefaction occurred in
two Holocene sedimentary basins near the ruptured fault
[Figure 1; National Center for Research on Earthquake
Engineering, 1999; Central Geological Survey, 2000;
Steward, 2001]. Seventy evenly distributed hydrologic
stations (Figure 1) with 188 monitoring wells documented
widespread coseismic changes in water-level across a large
sedimentary basin [Hsu et al., 1999; Chia et al., 2001;
Wang et al., 2001]. At the same time, a network of 60
broadband (nominal DC to 50 Hz) strong-motion stations
(Figure 1c), sampling at 200 Hz, recovered an unprecedented amount of near-field ground-motion data for the ChiChi earthquake [Lee et al., 2001; Shin and Teng, 2001].
These data provide a rare opportunity for studying the
relation among coseismic ground motion, water level
change and liquefaction.
1. Introduction
[2] Liquefaction, manifested by a loss of stiffness as a
result of rising pore pressure in sediments, commonly
occurs during or immediately after earthquakes. Fundamental understanding of the relation between dynamic loading
and liquefaction in soils has been largely derived from
laboratory studies begun in the 1960s [Seed and Lee,
1966]. Significant differences, however, occur between the
laboratory and field conditions; thus field-based studies of
the relations among seismic loading, pore-pressure buildup
and liquefaction are very much needed for better understanding these processes. A critical obstacle to advances in
this direction has been the scarcity of field data for ground
motion and water level change in the same sedimentary
basin during the same earthquake. Holzer et al. [1989]
1
Department of Earth and Planetary Science, University of California,
Berkeley, California, USA.
2
Institute of Earth Sciences, Academia Sinica, Nankang, Taiwan,
People’s Republic of China.
3
University of California, Lawrence Livermore National Laboratory,
Livermore, California, USA.
Copyright 2003 by the American Geophysical Union.
0094-8276/03/2003GL017601$05.00
HLS
2. Coseismic Water-Level Change (Cw) and
Liquefaction
[4] The Choshui River fan, a Holocene alluvial fan, lies
on the west of the uplifted Pleistocene Pakuashan anticline
(Figure 1). In the upper aquifer, the focus of this study, Cw
was less than 0.5 m over most part of the Choshui River fan;
but in an area of 100 km2 west of the Pakuashan anticline
Cw exceeded 3 m.
[5] The distribution of liquefaction was highly uneven.
Most of the 75 reported liquefaction sites occurred within
a distance of 30 km from the ruptured Chelungpu fault;
3 sites along the coast occurred in landfills. No liquefaction occurred on the uplifted Pleistocene ridges where
consolidated sediments are exposed. Of the 13 reported
liquefaction sites on the Choshui River fan, nine occurred
in the 100 km2 area where Cw in the upper aquifer
exceeded 3 m (Figure 1a). This corresponds to an average of
101 site/km2, as compared with 103 site/km2 over the
rest of the Choishui River fan. Since no Cw data are
available for the Taichung Basin, a similar correlation
cannot be made. The patterns of Cw in the lower aquifers,
discussed earlier [Hsu et al., 1999; Chia et al., 2001; Wang
et al., 2001] showed no correlation with the occurrence of
1
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WANG ET AL.: FIELD RELATIONS FOR CHI CHI EARTHQUAKE
liquefaction, suggesting that liquefaction occurred in the
upper aquifer. This is consistent with the fact that the nearsurface sediments are the youngest and least compacted, and
thus most liquefiable.
[6] The water pressure 1 hr prior to the earthquake at a
depth of 10 m, normalized by sediment overburden, is
shown in Figure 1b. No apparent correlation appears between this and the coseismic pore-pressure change or
liquefaction occurrence.
3. Coseismic Ground Motion and Liquefaction
[7] After the effect of instrument drift was corrected
[Iwan and Chen, 1995], the peak ground acceleration
(Pga), the spectral acceleration (Sa) and the spectral velocity
(Sv) are calculated from the seismic records [Jennings,
1983]. Figure 1c shows that Pga decreases with distance
from the ruptured fault. It also shows that all liquefaction
sites (except 3 sites along the coast in landfills) lie within
the area where Pga exceeded 2.5 m/s2, or 0.26 g. However,
some basin areas, e.g., around hydrological stations ES, HH
and KC (Figure 1c), where Pga exceeded 0.26 g, show no
evidence of liquefaction.
[8] Figure 2a shows the distribution of Sa at 1 Hz with
zero damping. All but one inland liquefaction sites occurred
in areas where Sa at 1 Hz > 16 m/s2. The areas around the
stations ES, HH and KC, where no liquefaction occurred,
are below the Sa-threshold for liquefaction. Similarly, all but
one inland liquefaction sites occurred in areas where the
1-Hz Sv 2.4 m/s (Figure 2b) and the stations ES, HH and
KC are also below the Sv-threshold. At frequencies away
from 1 Hz, neither Sa nor Sv show a correlation with
liquefaction (Figure 2c, for Sa at 0.1 Hz), suggesting that
coseismic consolidation and liquefaction of sediments may
be a function of the frequency of the impinging seismic
waves.
4. Relations Among Coseismic Water-Level
Change and Ground Motion
Figure 1. (a) Contours show Cw in m. Hydrological
stations with coseismic water-level changes in upper aquifer
are shown in triangles, and liquefaction sites are in open
diamonds. Inset map shows location of study area; major
structures indicated by 1, 2, 3 and C are Coastal Plain,
Western Foothills, Central Ranges and Chelungpu fault,
respectively. (b) Contours show initial water pressure
(1 hour prior to earthquake) at a depth of 10 m, normalized
by sediment overburden, revealing no correlation between
initial condition and liquefaction. (c) Contours show Pga in
m/s2. Open triangles show strong-motion seismic stations.
All inland liquefaction occurred in basins where Pga
exceeded 2.5 m/s2 or 0.26 g. However, no liquefaction
occurred in some basin areas where Pga exceeded 0.26 g,
such as at hydrological stations ES, HH or KC.
[9] Since Cw , Pga, Sa and Sv all show spatial correlations
with liquefaction, we may expect correlations to exist
among Cw , Pga, Sa, and Sv . To test this hypothesis we
interpolate Pga, Sa and Sv at the locations of the
20 hydrologic stations that lie within the strong-motion
network on the Choshui River fan and examine the
relationship between the interpolated Pga, Sa and Sv and
the corresponding Cw in the upper aquifer. The wells with
negative Cw lie outside the strong-motion network and are
excluded from this analysis. Strong correlation appears
between Cw and Sa (Figure 3a) and between Cw and Sv
(Figure 3b), even though several relevant parameters, e.g.,
the spatial variations of permeability and the thickness of
the confining layer (Lorraine Wolf, personal communication, 2002), are not included in this analysis. Fitting the data
with an exponential function, we obtain the following
empirical relations:
Cw ¼ 0:048½expðSa =4:4Þ 1
and
Cw ¼ 0:036½expðSv =0:6Þ 1;
WANG ET AL.: FIELD RELATIONS FOR CHI CHI EARTHQUAKE
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1-3
Figure 3. (a) Cw plotted against Sa at 1 Hz. Data at
hydrological stations are shown in dots. Central curve is
best fit; bounding curves show confidence of fit. (b) Cw
plotted against Sv at 1 Hz. Central curve is best fit;
bounding curves show confidence of fit. (c) Cw plotted
against Pga. No correlation between these parameters is
apparent.
Figure 2. (a) Contours show Cw in m/s2 at 1 Hz, revealing
strong correlation between liquefaction and basin area with
Sa at 1 Hz exceeded 16 m/s2. Open triangles show strongmotion seismic stations. Hydrological stations ES, HH and
KC are in areas below this liquefaction threshold.
(b) Contours show Sv in m/s at 1 Hz, revealing strong
correlation between liquefaction sites and basin area with Sv
at 1 Hz exceeded 2.4 m/s; stations ES, HH and KC are in
areas below this threshold. (c) Contours show Sa in m/s2 at
0.1 Hz; no correlation occurs between liquefaction and Sa at
this frequency.
where Cw , Sa and Sv are in m, m/s2 and m/s, respectively.
The goodness of these fits is given by the correlation
coefficients of 0.98 and 0.97, respectively. However, no
correlation appears between Cw and Pga (Figure 3c). Thus
no Pga-threshold for the coseismic pore-pressure buildup
may be established.
5. Discussion
[10] Lee et al. [2002] calculated the coseismic elastic
strain in the Chi-Chi earthquake and showed that the
observed Cw in the upper aquifer has the expected polarity
from the coseismic strain model [Wakita, 1975; Muir-Wood
and King, 1993; Roeloffs, 1996]. However, the magnitude
of the calculated strain [Figure 4 in Lee et al., 2002] is much
too small to explain the Cw in the upper aquifer.
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WANG ET AL.: FIELD RELATIONS FOR CHI CHI EARTHQUAKE
[11] The empirical relations between Cw and Sa and
between Cw and Sv imply thresholds of Sa 4.4 m/s2 and
Sv 0.6 m/s, below which there is little buildup
of coseismic pore-pressure; above which pore-pressure
increases exponentially with Sa and Sv . Manga et al.
[2003] calculated the pore pressure and the particle velocity
in a watershed at Sespe Creek, CA, induced by several large
earthquakes and estimated a threshold of particle-velocity of
0.05 to 0.2 m/s for the buildup of excess pore pressure. The
difference in magnitude between this and the Sv-based
threshold (0.6 m/s) reflects the fact that Sv at 1 Hz is
significantly greater in magnitude than the peak particle
velocity. Holzer et al. [1989] measured coseismic ground
acceleration and pore-pressure change at the Wildlife site in
the Imperial Valley, California, which underwent liquefaction during the 1987 Superstition Hills earthquake. They
showed that the onset of pore-pressure buildup occurred
when Pga exceeded 0.21 g, followed by liquefaction, in
agreement with the basin-wide result of the present study
(Figure 1c). Such agreement suggests that the results of this
study may be transferable to other basin settings.
[12] An empirical relation g 1.2 z (Pga)/Vs2, where z is
depth in m, Vs is shear-wave velocity in m/s, and Pga is in
m/s2, was established to estimate the shear-strain amplitude
required for the onset of pore-pressure buildup in sediments
under dynamic loading [Dobry et al., 1982; Vucetic, 1994].
This provides a lower bound for the shear-strain amplitude
required for liquefaction. The depth of the coseismic
liquefaction at the YL site is unknown but may be estimated
from Cw and the effective-stress principle [Terzaghi et al.,
1996], i.e., the effective overburden at the liquefaction
depth prior to the earthquake is equal to the coseismic
increase in pore pressure. The coseismic water-level rise at
the YL station was 6.55 m, implying a pre-seismic effective
overburden of 6550 Pa and a liquefaction depth of 4 – 7 m,
depending on the bulk density of the sediments and the
thickness of the saturated column above the liquefied layer.
Assuming an average depth of 5 m, Vs = 116 m/s at this
depth [Satoh et al., 2001] and a Pga of 2.5 m/s2 (0.26 g),
we obtain a lower bound of 1.1 103 for g required for
liquefaction at the YL site. Laboratory studies of a variety
of sands under cyclic loading show that liquefaction
occurs when the shear-strain amplitude exceeded 103
[Dobry et al., 1982; Vucetic, 1994]. Thus there is a very
good agreement between field and laboratory results on the
strain amplitude required for liquefaction, providing a
justification for using laboratory data in field assessment
of earthquake hazards.
[13] In summary, field-based relations among ground
motion, pore-pressure increase and liquefaction are presented. Good agreement between the field and laboratory
results provides justification for applying the laboratory
results to field conditions. While acknowledging the need
for further field testing, we believe the relations presented in
this study may be valuable in predicting the coseismic
buildup of pore-pressure and the liquefaction potential in
sedimentary basins and thus may be useful in earthquake
hazard assessment.
[14] Acknowledgments. We thank Michael Manga, James Kirchner,
Emily Brodsky, Lorraine Wolf and Yi-Ping Chia for helpful discussions,
and two anonymous reviewers for constructive comments. Nancy Lin
assisted in data procession. Work funded by NSF grant EAR-0125548,
IGPP grant 03-GS-008 and NSC grant NSC91-2116-M-001-017; work of
JGB performed under DOE contract W-7405-ENG-48.
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C.-Y. Wang, D. S. Dreger, and D. Mayeri, Department of Earth and
Planetary Science, University of California, Berkeley, CA 94720, USA.
([email protected])
C.-H. Wang, Institute of Earth Sciences, Academia Sinica, Nankang,
Taiwan, People’s Republic of China.
J. G. Berryman, University of California, Lawrence Livermore National
Laboratory, Livermore, CA 94551, USA.