Streamflow Changes in the Vicinity of Seismogenic Fault After the 1999 Chi–Chi Earthquake
Streamflow Changes in the Vicinity of Seismogenic Fault After the 1999 Chi-Chi Earthquake CHING-YI LIU,1 YEEPING CHIA,1 PO-YU CHUANG,1,2 CHI-YUEN WANG,3 SHEMIN GE,4 and MAO-HUA TENG1
Pure Appl. Geophys.
0 Department of Earth and Planetary Science, University of California , Berkeley, CA 94720 , USA
1 Geotechnical Engineering Research Center, Sinotech Engineering Consultants, Inc , Taipei 114 , Taiwan
2 Department of Geosciences, National Taiwan University , No. 1 Section 4 Roosevelt Road, Taipei 106 , Taiwan
3 Department of Geological Sciences, University of Colorado , Boulder, CO 80309 , USA
-Changes in streamflow have been observed at 23 stream gauges in central Taiwan after the 1999 MW 7.6 Chi-Chi earthquake. Post-earthquake increases, ranging from 58 to 833% in discharge, were recorded at 22 gauges on four rivers and their tributaries. The streamflow increase typically peaked in 2-3 days and followed by a slow decay for a month or more. An increased groundwater discharge to the river after the earthquake can be attributed to rock fracturing by seismic shaking as well as pore pressure rise due to compressive strain. A large decrease in discharge was recorded immediately after the earthquake at the gauge near the earthquake epicenter. Further analysis of long-term data indicates that the post-earthquake discharge at the gauge reduced to a level smaller than that at an upstream gauge for 8 months. Such a streamflow decrease might have been caused by a discharge to the streambed due to a co-seismic decrease in pore pressure induced by crustal extension during the rupture of the thrust fault.
Streamflow; gauge; Chi-Chi earthquake; deformation; groundwater
CHING-YI LIU,1 YEEPING CHIA,1
PO-YU CHUANG,1,2 CHI-YUEN WANG,3 SHEMIN GE,4 and MAO-HUA TENG1
Hydrological changes associated with earthquakes
have been observed at many places in the world.
Generally, changes in groundwater level occur
(Waller 1966; Roeloffs 1998; King and
Igarashi 1999; Chia et al. 2001; Cox et al. 2012;
Weingarten and Ge 2014, He et al. 2017)
changes in streamflow or spring flow appear after
major earthquakes (Rojstaczer and Wolf 1992;
Wood and King 1993; King et al. 1994; Manga 2001;
Manga et al. 2003; Montgomery et al. 2003; Wang
et al. 2004a, b; Manga and Rowland 2009; Mohr et al.
Earthquake-triggered hydrological changes may
provide insight into the mechanism underlying the
crustal processes during the fault rupture.
Groundwater-level changes in the vicinity of seismogenic
fault are generally attributed to the redistribution of
stress–strain due to fault movement
Montgomery and Manga 2003)
Rojstaczer and Wolf
proposed that streamflow increases are likely
caused by the increase in permeability of near-surface
rock due to fracturing by seismic shaking.
MuirWood and King (1993) related the hydrological
changes following major earthquakes to the style of
faulting. Significant streamflow increases were found
to accompany major normal fault earthquakes, while
small decreases were associated with reverse fault
earthquakes. Soil liquefaction has been proposed for
the streamflow increase
(Manga 2001; Wang et al.
2001; Montgomery et al. 2003)
, but possibly limited
to shallow sediments in the river valley of the
mountain area (Mohr et al. 2012).
During the 1999 MW 7.6 Chi–Chi earthquake, the
largest and most destructive inland earthquake in the
history of Taiwan, stream discharges recorded by a
dense network of gauges on major rivers and their
tributaries provide comprehensive data for exploring
earthquake-triggered hydrological changes. In this
paper, we presented the streamflow changes in the
vicinity of the seismogenic fault before and after the
Chi–Chi earthquake. Long-term data analysis and
further investigations provide a basis for a better
understanding of the natural processes of the
earthquake hydrological anomalies.
2. Seismicity, Geology and Hydrology
As a part of the Circum-Pacific seismic belt,
Taiwan is located at the convergent boundary
between the Philippine Sea plate and the Eurasian
plate. As a result of movement and collisions of the
two plates, Taiwan is one of the most seismically
active regions in the world. Historical destructive
earthquakes could be documented back to 1644,
while earthquakes recorded by seismic instruments
started after 1897
. From 1990 to 2006,
there were 899 earthquakes cataloged with magnitude
greater than MW 5.5
(Chen and Tsai 2008)
The island of Taiwan is located at the margin of
young orogenic belt containing a series of
northnortheast trend geologic formations
western plain consists of a thick sequence of
undeformed unconsolidated deposits, while sedimentary
and metamorphic rocks appear along with complex
fold and fault structures in the mountainous area. As
mountain ranges in the island stretch from north to
south, most rivers flow in the east–west direction.
Groundwater storage is different in the consolidated
formation and the unconsolidated sediments (WRA
2016). In the mountains area, groundwater is stored in
the pores of sedimentary rocks as well as fractures,
such as joints and faults, of the metamorphic rocks.
Major aquifers, however, are located in gravel and
sand layers of unconsolidated deposits in the western
3. Chi–Chi Earthquake and Co-seismic Groundwater-Level Changes
The 1999 Chi–Chi earthquake occurred at 1:47
am on September 21, 1999, local time (17:47 on 20
September UTC). The epicenter of the MW 7.6
earthquake is located at 23.85 N; 120.82 E, near the
town of Chi–Chi in central Taiwan (Fig. 1). The
depth of the hypocenter of the earthquake is about
8 km. The focal mechanism of the mainshock was of
a thrust type with strike 5 , dip 34 . Surface rupture
appeared to extend approximately 100 km in the
north–south direction along the traces of the
Chelungpu thrust fault
(Angeliera et al. 2003)
and vertical offset caused by crustal deformation
ranged from 2.4 to 10.1 m and 1.2 to 4.4 m across the
Chelungpu fault, respectively. In the epicentral
region, the crustal deformation was essentially a
uniaxial compressional strain of 0.36 micro-strain/
year in the direction of 114 over several years before
(Yu et al. 2001)
. After the mainshock,
more than 10,000 aftershocks were recorded in the
first 3 weeks
(Ma et al. 1999)
In the previous studies, examination of available
data recorded at monitoring wells in the footwall of
the Chelungpu fault revealed widespread co-seismic
(Wang et al. 2001)
Widespread sustained co-seismic groundwater-level
changes were observed in monitoring wells in the
(Chia et al. 2001, 2008a; Wang et al. 2001)
As shown in Fig. 1, groundwater-level falls were
recorded in wells near the ruptured fault, while rises
were recorded in most wells farther away to the west
in the coastal plain consisting of unconsolidated
deposits (Chia et al. 2001). Generally, larger changes,
up to 11.09 m, were found in wells closer to the fault.
These co-seismic changes were likely caused by
coseismic strain induced by fault displacement
1975; Roeloffs 1988; Grecksch et al. 1999; Ge and
Stover 2000; Chia et al. 2001, 2008b)
distribution of the sustained changes in the footwall revealed
that crustal extension dominated near the Chelungpu
fault during the earthquake, while compression
prevailed away from the fault.
4. Rainfall and Stream Gauges
In central Taiwan, the average annual
precipitation is about 2154 mm, but it may reach to 4000 mm
in the mountainous area. The climate of the area is
characterized by a distinct wet and dry season, with
most of the rainfall occurring between May and
September. Heavy rainfall usually occurs in short
duration, and thus, most stream discharges peak
shortly after rainfall. The river usually takes a few
days to return to the base flow. Significant
rainfallinduced changes in streamflow are frequently
recorded during wet seasons. The Chi–Chi earthquake
occurred at the end of the wet season. During the dry
season, the gradual decay in discharge reflects
diminishing supply from the groundwater discharge.
During the Chi–Chi earthquake, streamflow
discharges were recorded at 32 gauges on four major
rivers, including Daan River, Dajia River, Wu River
and Choshui River, and their tributaries in the vicinity
of the Chelungpu fault (Fig. 1). The four rivers
originated from the Central Mountain Ranges,
flowing westward through the coastal plain to the
Taiwan Strait. The length of these rivers ranges from
96 to 187 km, and the watershed area ranges from
758 to 3157 km2 (Table 1). Of those, the Choshui
River is the longest river in Taiwan.
Resource Agency for the water resource management
and flood prevention. Most of these gauges are
located on the mountainous east, or hanging wall,
side of the fault. They were used to measure the
elevation of the stream water surface. Water level
sensor system was designed to fit the characteristics
of the cross section of the river at these gauges. A
water-stage recorder was installed on or near the
bridge crossing the river for automatically recording
water level of the stream. The water level was then
converted to the discharge (volumetric flow rate)
based on the stage-discharge relation. Currently, the
river water level at these gauges is recorded at 1-h
intervals. In 1999, however, the water level was
recorded daily, with the exception of the CS44 gauge
where the data were recorded at 1-h intervals.
5. Streamflow Changes Induced by the Earthquake
Streamflow changes recorded at some of these
gauges after the Chi–Chi earthquake were reported
(Wang et al. 2004b)
. As additional data
became available and further investigations were
conducted, it was found that, among the streamflow
changes recorded at 32 changes, six anomalously
large increases were caused by upstream reservoir
release for the dam safety, one substantial decrease
was caused by a large landslide dam on the river due
to the earthquake, and two changes were
fluctuation. Thus, only 23 gauges recorded distinct
earthquake-related changes. Of those, 22 are
postearthquake streamflow increases and the remaining
one is post-earthquake decrease (Table 2). The
distance between these gauges and the epicenter ranges
from 4 to 69 km.
5.1. Post-earthquake Streamflow Increase
Six gauges on the Dajia River and its tributaries
recorded a streamflow increase, ranging from 58 to
833%, in 2 to 3 days after the earthquake (Table 2).
The rapid increase was followed by a slow decay
over the next month or more. The temporal variations
of daily discharge at these gauges are shown in
hydrographs, along with nearby rainfall data (Fig. 2).
The largest percentage increase was recorded at
DJ46, with an elevation of 550 m, located on a
tributary in the downstream of the Dajia River. The
discharge increased 833% to 2.8 m3/s 3 days after the
earthquake. The large percentage increase was
primarily caused by the relatively small discharge,
approximately 0.3 m3/s, on the day before the
earthquake. The other five gauges are located near
the source of the Dajia River with an elevation
between 1434 and 1629 m. For instance, DJ35 is
located on the mainstream for monitoring the sum of
upstream tributary discharges. An increase of 83% in
discharge, from 16.5 m3/s on the day before the
earthquake to a peak flow of 30.2 m3/s 3 days after
the earthquake, was recorded at the gauge (Fig. 2).
The peak discharge took more than 1 month to return
to the pre-earthquake value. The slow decay could be
disturbed by rainfall, such as an increase in discharge
following the heavy rainfall in mid-December.
The temporal variations of daily discharge at five
gauges on the Wu River and its tributaries show
streamflow increases, ranging from 63 to 413%, after
the earthquake (Fig. 3). At WU28, an increase of
413% in discharge was recorded on the mainstream.
The discharge increased from 9.6 m3/s on the day
before the earthquake to a peak value of 49.2 m3/s 2
days after the earthquake. The WU25 gauge, with an
elevation of 10 m, is located on the mainstream in the
coastal plain. It recorded a streamflow increase of
63%, from 103.0 m3/s to 168.0 m3/s, in 3 days after
Four gauges on the Daan River and its tributaries
recorded a streamflow increase, ranging from 139 to
311%, 1 to 4 days without rainfall after the
earthquake (Table 2). The temporal variations of
daily discharge at these gauges are shown in Fig. 4.
At gauge DA12, for instance, an increase of 214% in
discharge was recorded on a small tributary. The
discharge increased from 2.1 m3/s on the day before
the earthquake to a peak value of 6.6 m3/s 4 days
afterward. The peak flow took about 2 months to
return to the pre-earthquake level. At DA15, the
discharge increased to the first peak value of 34.1 m3/
s 2 days after the earthquake and then decayed during
the third day. Instead of a gradual decay, the
discharge increased again for a week to the second
peak. Further investigations indicated that several
small landslide dams and lakes were created in the
upstream of DA15 during the earthquake
(Lin et al.
2000; Chen and Shi 2000)
. The collapse of these
dams and the rainfall at end of September provided
the discharge for the second increase to the peak of
52.8 m3/s on October 1. A similar pattern of
postearthquake streamflow changes was observed at
DA11, which is located in the downstream of DA15.
Seven gauges on Choshui River and its tributaries
recorded streamflow increases after the earthquake
(Fig. 5). Of those, four gauges, CS11, CS63, CS57
and CS58, were installed on the mainstream, while
the other three, CS40, CS48 and CS49, were on the
tributaries where the discharge is relatively smaller.
The streamflow increases at these gauges, ranging
from 61 to 143%, are listed in Table 2. For instance,
at CS48, the discharge increased from 26.4 m3/s on
the day before the earthquake to a peak value of
42.4 m3/s, or an increase of 61%, over 2 days after
the earthquake. The increased streamflow took about
one and a half month to return to the pre-earthquake
level. At CS11 and CS49, however, two
postearthquake peak flows were recorded after the
earthquake. Further investigations indicated the
anomalous changes were also caused by the collapse
of landslide dams and local rainfall in the upstream.
5.2. Post-earthquake Streamflow Decrease
After the 1999 Chi–Chi earthquake, the only
streamflow decrease was observed at gauge CS64.
This gauge, located only 4 km to the southeast of the
epicenter, was placed to monitor streamflow in the
downstream of the Shueili Creek (inset of Fig. 1).
The Shueili Creek, a tributary of the Choshui River,
lies almost directly above the hypocentral area and
coincides approximately with the Shueilikun fault. Its
bedrock is composed of fractured quartzitic sandstone
and arkosic sandstone interbedded with argillite. The
discharge recorded at CS64, as shown in Fig. 6a,
displayed a large and abrupt decrease, from 162.0 to
58.8 m3/s, on the day of the Chi–Chi earthquake.
During the following few days, it further decreased to
There was another gauge, CS44, located near the
base of the Mingtan Dam approximately 3-km
upstream from CS64 along the Shueili Creek (inset
of Fig. 1). CS44 was installed by the Taiwan Power
Company to monitor the release of Mingtan reservoir
to the Shueili Creek. The hourly discharge at CS44,
as shown in Fig. 6b, was normally maintained below
50 m3/s, except for emergency need or during heavy
rainfall. It did not show either an abrupt
postearthquake decrease, as recorded by CS64. In fact,
the daily discharge at CS44 increased from 14.1 m3/s
immediately before the earthquake to 89.6 m3/s after
the earthquake. The hourly discharge also showed a
rapid increase, up to 202.5 m3/s, after the earthquake
(Fig. 6b). The increase in discharge or reservoir
release was imposed for dam safety. However, such a
large increase at CS44 did not cause an increase in
discharge at the downstream gauge CS64. Instead, the
post-earthquake discharge continued to decline.
The long-term data provide a more detailed
comparison in discharge between the gauges CS64
and CS44, as shown in Fig. 7a. Generally, the
discharge at the two gauges shows a similar seasonal
pattern that increases in the wet seasons and
decreases in the dry seasons. It is also noted that,
similar to most rivers that obtain their water from the
groundwater discharge during the dry seasons, the
discharge of the Shueili Creek at the downstream
gauge CS64 was usually larger than that at the
upstream gauge CS44. However, a reversed situation
appeared between September 1999 and June 2000
(Fig. 7b). The reversal event suggests that the Shueili
Creek was changed to a influent (losing) stream from
a effluent (gaining) stream immediately after the
occurrence of the earthquake on September 21, 1999.
Such a phenomenon lasts until the wet season arrived
in May 2000. Therefore, based on the long-term
monitoring data, the Shueili Creek had lost water to
the creek bed over the 3-km segment between the
gauges CS44 and CS64 in the 8 months after the
Near CS64 was the other gauge, CS63, which
recorded no streamflow decrease, but a
post-earthquake increase, like most other gauges (Fig. 5). This
is not surprising, because CS63 is located on the
mainstream of the Choshui River, the recorded data
represent the sum of discharges from all upstream
tributaries (inset of Fig. 1).
The post-earthquake streamflow increases
recorded by the 22 gauges are similar to those observed in
the previous studies. The only source of the increases
with no rainfall comes from the discharge of
groundwater. Based on Darcy’s law, the increase of
groundwater discharge to the river is caused by either
an increase of rock permeability in the hills or a rise
of pore pressure in the ground. The sudden increase
of permeability is often explained by rock fracturing
due to seismic shaking
(Rojstaczer and Wolf 1992;
Rojstaczer et al. 1995; Sato et al. 2000; Wang et al.
2004a, b; Charmoille et al. 2005; Elkhoury et al.
2006; Wang and Manga 2010; Manga et al. 2012;
Mohr et al. 2015)
. Such an explanation can be
supported by the occurrence of numerous landslides
in the mountain area during the 1999 Chi–Chi
earthquake (Hung 2000). For instance, the removal of
vegetation and subsoil took place at 99 peaks during
the earthquake, exposed the bedrock to the surface
and enabled the groundwater to move through
fractures more easily. Part of the streamflow increase
may also be attributed to a rise of pore pressure
induced by compressive strain during the fault
rupture. Such an increase can be supported by the
coseismic groundwater-level rise prevailed in the
footwall or the plain area. The mountain ranges in
Taiwan were formed primarily by the compression
between the Eurasian plate and the Philippine Sea
plate. However, groundwater-level data were not
recorded in the mountainous hanging wall during the
Chi–Chi earthquake to support the compressive
The earthquake-triggered streamflow increase is
similar to the rainfall-induced increase, typically
peaked in a few days. However, unlike a rapid
decrease in discharge following the peak flow after
rainfall, the post-earthquake peak flow is followed by
a slow decay which may last for several months
(Figs. 2, 3, 4). The slow decay is attributed to a
prolonged groundwater discharge increase, instead of
runoff, to the river. Such a groundwater discharge
increase is consistent with an increase in either
permeability or pore pressure proposed for the
postearthquake streamflow increase.
A large streamflow decrease triggered by the
earthquake is rarely observed.
Muir-Wood and King
suggested that hydrological changes
accompanying thrust fault earthquakes are most notable by
their absence, or a decrease of well water level and
spring flow. The field observation in the vicinity of
the Chelungpu thrust fault, however, indicated
postearthquake streamflow increases at most gauges in
the hanging wall. The only post-earthquake decrease
in streamflow was recorded at the gauge nearest to
the epicenter. Moreover, co-seismic falls in
groundwater level were found primarily at wells adjacent to
the thrust fault. Therefore, the crustal extension was
likely to dominate from the epicentral region to the
area adjacent to the thrust fault during the earthquake.
The long-term data indicate that the discharge at
CS64 in the downstream of the Shueili Creek is
usually larger than that at CS44 in the upstream. After
the earthquake, however, the discharge at CS64
reduced to a level smaller than that at CS44 for
8 months or more. Such a post-seismic streamflow
decrease might have been caused by a co-seismic
decrease in pore pressure induced by crustal
extension near the earthquake epicenter during the rupture
of the thrust fault. The sudden decrease created a
rapid downward discharge to the crust through the
opening of pre-existing fractures in the bedrock of the
Shueili Creek valley and new fractures generated by
the earthquake. Such a downward discharge due to
crustal extension must be far greater than the
recharge from nearby hills due to seismic shaking.
The sustained downward flow from surface water to
the crust along the creek is likely to result in a
streamflow in the downstream smaller than that in the
upstream. The crustal extension is likely to extend to
the area adjacent to the thrust fault, where co-seismic
groundwater-level falls were recorded.
The significant streamflow decrease at CS64 on
the Shueili Creek after the Chi–Chi earthquake is a
unique observation. The gauge is only 4 km from the
earthquake epicenter, and the Shueili Creek lies just
above the hypocentral area. The proximity of gauge
CS64 to the epicenter suggests that the streamflow
decrease is likely induced by the crustal extension in
the hanging wall during the rupture of the thrust fault.
In the previous study
(Chia et al. 2001)
co-seismic falls in groundwater level in the footwall near the
Chelungpu fault trace were also attributed to the
extensional deformation. Apparently, these
hydrological anomalies reveal the dominance of the crustal
extension in the area adjacent to the thrust fault in
both the hanging wall and the footwall during the
Beyond the zone of extension in central Taiwan,
post-earthquake streamflow increase was recorded at
all stream gauges and co-seismic groundwater-level
rise was observed in nearly all monitoring wells.
While the groundwater-level rise in the footwall was
attributed to the compressive deformation, the
streamflow increase in the hanging wall is possibly
caused by both the permeability increase due to rock
fracturing by seismic shaking and the pore pressure
rise due to compressive deformation induced by fault
The authors would like to thank the reviewers for the
helpful comments and suggestions. We gratefully
acknowledge access to stream discharge data of the
Water Resources Agency of Taiwan and the Taiwan
Power Company. This work is supported by the
Ministry of Science and Technology of Taiwan
(MOST 105-2116-M-002-023). Special thanks are
extended to the members of the hydrogeology
laboratory of the National Taiwan University for
their research support.
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