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Engineering Geology xx (2006) xxx – xxx
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Post-seismic surface processes in the Jiufengershan landslide area,
1999 Chi-Chi earthquake epicentral zone, Taiwan
Kuo-Jen Chang a,⁎, Alfredo Taboada a , Yu-Chang Chan b , Stéphane Dominguez a
a
Laboratoire Dynamique de la Lithosphère, Université Montpellier II, Montpellier, France
b
Institute of Earth Sciences, Academia Sinica, Nankang, Taipei, Taiwan, ROC
Received 22 December 2004; accepted 20 February 2006
Abstract
The Jiufengershan rock avalanche was one of the largest and most damaging landslides triggered by the Chi-Chi earthquake
(ML = 7.3, MW = 7.6) of 21 September 1999. The rock avalanche transported a mass of sedimentary rock 50 m thick and 1.5 km
long, located on the western limb of the Taanshan syncline. The surface of rupture coincides with the bedding plane and dips
moderately toward the Jiutsaihu valley.
This paper is mainly devoted to the study of post-seismic surface processes that affected the sliding surface as well as the debris
deposit, from September 1999 to February 2003. Large fractured blocks and a debris layer observed on the surface of rupture were
subjected to mass wasting processes and denudation. The quantification of erosion was made using two different approaches.
First, the subpixel correlation method was used to determine the horizontal displacement field from aerial photographs taken,
respectively, 2 and 3.5 months after the earthquake. Displacements ranging from 1 to 6 m were observed around unstable blocks
located at the western flank of the surface of rupture. Second, the co-seismic and post-seismic volume distributions in the sliding
zone were determined from three digital elevation models, including a LiDAR image taken in 2002. Post-seismic erosion and
deposition from September 1999 to April 2002 were mainly associated with mass wasting and denudation at the surface of rupture,
deposition in small basins and lakes located in the debris deposit, and evacuation of materials from the debris deposit along natural
and artificial drainage channels. The vertical compaction is 1% of the initial height of the deposit.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Landslide; Post-seismic; Taiwan; Cut and fill volume; Chi-Chi earthquake; Subpixel correlation
1. Introduction
Large shallow earthquakes are one of the main
triggering mechanisms of rapid landslides (Keefer,
1984; Wieczorek, 1996), which are commonly located
near the activated fault segment. The 21st September
1999 Chi-Chi Taiwan earthquake (ML = 7.3, MW = 7.6),
⁎ Corresponding author.
E-mail address: [email protected] (K.-J. Chang).
triggered more than 10,000 landslides with surface areas
larger than 625 m2, causing severe destruction and loss
of life (Liao, 2000). The Jiufengershan rock-and-soil
avalanche (located at 120.84°E, 23.96°N) is one of the
major landslides triggered by this earthquake, affecting
a large-scale monocline structure (Chang et al., 2005b).
The slide is located 12 km north of the epicenter, and it
mobilized jointed rock and soil materials along bedding
planes creating an avalanche.
Studies on earthquake-triggered landslides are generally focused on the co-seismic sliding process. In this
0013-7952/$ - see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.enggeo.2006.02.014
ENGEO-02547; No of Pages 16
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way, the morphological and geological description of
the Jiufengershan landslide as well as the mechanical
analysis of the initiation and propagation of the
avalanche have been studied using different approaches
(Chang et al., 2005a,b). Nevertheless, few studies deal
with the post-seismic mobilization of unstable geological materials that are frequently observed in the
hillslopes after an earthquake. Thus, this paper is mainly
concerned with the study of post-seismic surface
processes observed in the Jiufengershan landslide area.
The choice of this example is justified given the size of
the avalanche and the quality of the available morphological and geological data.
This study is based on field observations and detailed
morphological analysis of mass wasting processes that
affected the sliding surface as well as the debris deposit,
from September 1999 to February 2003. Post-seismic
surface processes are correlated with data on seismicity
and precipitation during a period extending from several
months before to 1–2 years after the main shock. Two
complementary methods are used to quantify postseismic surface processes in the sliding area.
The first method consists of calculating the horizontal displacement field on the surface by comparing two
aerial photographs, using the subpixel correlation
technique (Van Puymbroeck et al., 2000; Michel and
Avouac, 2002; Dominguez et al., 2003). The photographs were taken in November and December 1999, 2
and 3.5 months after the main shock, respectively.
The second method concerns the volume estimation
of surface materials that have been eroded or
deposited. Sediment budget assessments of natural
systems such as landslides, glaciers or river channels
have been studied using remote sensing, sequential
aerial photography and digital elevation models (e.g.,
Etzelmüller, 2000; Reid and Page, 2003; Wieczorek,
1996; Gaeuman et al., 2003). We present new
estimations of the co-seismic and post-seismic volume
distributions in the sliding zone, based on the analysis
of three digital elevation models (DEM). The DEMs
were taken in 1986, 1999 (after the earthquake) and in
April 2002. The latest one is high-resolution Light
Detection and Ranging (LiDAR) image (Lillesand and
Kiefer, 1994; Priestnall et al., 2000). The co-seismic
sliding process is illustrated by means of an isopach
map showing the difference in height between the
1999 and the 1986 DEMs. Post-seismic surface
processes are illustrated by an isopach map showing
the difference in height between the 2002 and the 1999
DEMs.
This multidisciplinary approach, based on geomorphological observations and precise topographic analysis, allows us to investigate the occurrence of postseismic surface processes in the surface of rupture and in
the debris deposit of a large avalanche.
2. Setting and landslide occurrence
2.1. Seismotectonic setting
The rock avalanche was triggered by the 21
September 1999 Chi-Chi earthquake, which activated
the Chelungpu fault located at ∼ 10 km from the
landslide (see inset, Fig. 1). This major thrust trends NS
and dips eastward. Faulting is associated with rapid
convergence of the Philippine Sea Plate toward the
Eurasian Plate (see inset in Fig. 2) (Seno, 1977; Suppe,
1981; Seno et al., 1993; Yu et al., 1997; Malavieille et
al., 2002).
The Jiufengershan landslide destabilized rocks from
the western limb of the Taanshan syncline in Central
Taiwan. This asymmetric fold is part of an imbricated
west-vergent fold-and-thrust belt, bounded by the
Chelungpu and Shuilikeng faults. Fig. 2 shows the
geological map of the sliding area, as well as a hillshading model obtained from a high-resolution DEM
(details on the DEM are given in the following section).
The fold axis plunges gently southward. The dip angle
of the western limb varies between 15° and 25°, whereas
the dip of the eastern limb is steeper (between 50° and
75°). The folded strata are composed of early to middle
Miocene sandstones with interbedded shale layers. The
stratigraphic formations from bottom to top in the study
area are defined as follows: Tanliaoti Shale (TL),
Shihmen Formation (SMB, SMm, SMt), Changhukeng
Shale (CHb, CHm, CHt) and Kueichulin Formation
(KC) (Huang et al., 2000, 2002; Wang et al., 2003;
Chang et al., 2005b). The Tanliaoti shale is an Early
Miocene formation composed mainly of thick shale
beds and subordinated interbeds or laminations, overlain
by alternations of shale and siltstone beds. The Shihmen
Formation is composed of light-gray, thick to massive
sandstones and thin shale layers. The Tanliaoti Shale is
bounded by sub-metamorphic rocks of the Hsuehshan
Fig. 1. Map of the Chi-Chi earthquake aftershocks from the 21/09/1999 to the 31/12/2000. The coordinates are specified in kilometers using the
2DTM system. The inset in the top-right corner shows the location of the landslide in relation with the Chelungpu fault trace and the main shock
epicenter (the rectangle indicates the location of the map of aftershocks). The inset in the top-left corner indicates the zone around Taiwan in which the
seismic energy release is calculated in Fig. 3.
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Range Belt located on the footwall of the Shuilikeng
fault.
The Changhukeng Shale is an Early to Middle
Miocene formation composed mainly of shales and
sandstones. The sliding materials of the rock avalanche
involve the top layers of the middle member (∼ 10 m)
and the overlying layers of the top member (CHt)
(∼ 40 m). The middle member (CHm) is mainly
composed of several thick to massive clayey sandstone
layers, while the top member (CHt) consists dominantly
of shales with alternations of sandy silts and shales (Wu,
1986; Huang, 1986). The surface of rupture was
localized at a clay seam, which was sheared during the
earthquake.
The Pliocene Kueichulin formation is exposed in the
axial zone of Taanshan syncline, and it is composed of
calcareous, massive, fossiliferous sandstones. The
contact between the Changhukeng shale and the
Kueichulin formation is probably a disconformity
(Tang, 1977; Huang, 1986).
The landslide began as a translational rock slide in
which the main body slid over a weak stratum. The slide
transformed into an avalanche as the shear strength at
the surface of rupture decreased and the velocity of the
displaced mass increased. The shear strength drop was
probably associated with vaporization and fluidization
processes at the surface of rupture (Chang et al., 2005a).
The rocks were deformed and disrupted as they
approached the toe of the monocline. The displaced
mass was transported through a valley incised by the
Jiutsaihu creek (Fig. 2). The avalanche propagation was
blocked by neighboring steep ridges, which trend NS,
composed of sandstones layers located in the opposite
limb of the Taanshan syncline. The Jiutsaihu valley was
partially filled by the debris deposit. The morphology of
the deposit area shows a sequence of smooth ridges and
depressions. The geometry of the landslide accumulation in the field has an irregular star shape. Three small
dammed lakes were formed upstream of the debris
deposit (Fig. 2).
2.2. Summary of the landslide event
2.3. Aftershocks
The landslide destabilized a ∼ 50-m-thick and
1.5-km-long mass of rock composed of shales and
sandstones, which dip ∼ 22° SE (Fig. 2). It involved a
flatiron remnant, which was almost entirely mobilized
during the Chi-Chi earthquake. Even though the
avalanche was triggered by earthquake shaking, other
factors contributed to the destabilization of the
monocline limb. Borehole data suggest that the
water table was located ∼ 30 m above the surface
of rupture, decreasing the effective normal stress on
the décollement level. Note that for frictional
materials, the shear strength is proportional to the
normal effective stress. No important mass movement
had occurred in the Jiufengershan area before 1999
and, in particular, foothill erosion was not observed
along the flatiron remnant before the earthquake.
Thus, this mechanism did not contribute to the
generation of the landslide. Nevertheless, chemical
weathering affected the upper layers of the flatiron
slope, in particular near the foothill, reducing the
shear strength of rocks.
Seismic shaking is one of the main triggering
mechanisms of landslides. In this section, we present
data concerning seismicity during a period extending
from several months before to 1–2 years after the main
shock. The seismic parameters of the Chi-Chi earthquake are the following (Ma et al., 1999; Kao and Chen,
2000; Chang et al., 2005a) (Fig. 1): the epicenter
location is 120.89°E, 23.82°N; the GMT = 20th September 1999, 17:47; the local time = 21st September
1999, 01:47; the magnitudes are ML = 7.3, and MW = 7.6;
the depth of the hypocenter is 8–10 km; the peak ground
acceleration component in the three stations located
nearest to the landslide is PGA = 0.65 g.
The map illustrated in Fig. 1 shows the aftershock
distribution around the slide, classified according to
their magnitude and depth, during a period extending
from the main shock in September 1999 to December
2000. Seismic activity was recorded by a dense network
of seismological stations from the Central Weather
Bureau of Taiwan. Most aftershocks occurred at shallow
depths (< 20 km), and some of them had magnitudes as
Fig. 2. Geological and geomorphological map of the landslide area (indicated by the black square in the inset), and NW–SE cross-section across the
slide (profile A–A′). The sliding area (SA) and the deposit area (DA) are contoured by continuous lines. Geological formations and structures are
defined as follows: TL = Tanliaoti Shale; SMb, SMm, SMt = bottom, middle and top Shihmen Formation; CHb, CHm, CHt = bottom, middle and top
Changhukeng Shale; KC = Kueichulin Formation; SF = Shuilikeng fault; TS = Taanshan syncline; HR = Hsuehshan Range Belt. Main creeks are
indicated: SC = Sezikeng creek; JC = Jiutsaihu creek; L1, L2, L3 = lakes. In the cross-section, the landslide volume is decomposed in three parts:
EV = eroded volume; JV = volume of the junction area; DV = volume of deposits located above the initial topography. The coordinates are specified in
kilometers, using the ‘2 Degrees Transverse Mercator’ system (Taiwan Datum 1997, TWD97).
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large as 6.8. The Jiufenershan landslide is located in the
hanging wall of the Chelungpu fault that was ruptured
during the main shock (see inset in Fig. 1).
To analyze the incidence of seismic shaking on the
sliding processes in the Jiufengershan area, we have
calculated the energy released by earthquakes for a twoyear period (from January 1999 to December 2000). The
seismic energy released per month, within a circular zone
of 50-km radius around the landslide, is indicated in Fig.
3 by the graph with triangular points. This circular area
will be named the epicentral zone (EZ), since it covers
entirely the Chelungpu fault. The seismic energy
released by aftershocks is calculated from logarithmic
relations between magnitude and energy (Gutenberg and
Richter, 1956). The list of aftershocks in the EZ with
magnitudes greater than 6 is given in Table 1. We have
also calculated the seismic energy released at a much
larger scale, in a rectangular zone around Taiwan (inset,
in top-left corner, Fig. 1). This area will be referred to as
the Taiwan Zone (TZ). The energy plot in this zone is
indicated by the graph with diamond-shaped points.
Energy released before the earthquake in the EZ is
roughly three orders of magnitude smaller than in the TZ.
The level of seismicity before the main shock is quite
constant in both regions. The Chi-Chi earthquake
Table 1
Main shock and aftershocks of the Chi-Chi earthquake with
magnitudes greater than ML = 6.0
Date (day/month/year)
Depth (km)
ML
Distance to the
landslide (km)
20/9/1999⁎
22/9/1999
25/9/1999
20/9/1999
10/6/2000
20/9/1999
20/9/1999
20/9/1999
20/9/1999
22/9/1999
20/9/1999
22/9/1999
8
15.6
12.1
12.5
16.2
12.5
9.8
8.6
7.7
17.4
19.7
24.0
7.3
6.8
6.8
6.7
6.7
6.66
6.6
6.59
6.44
6.2
6.07
6
12.3
25.3
19.8
24.9
27.6
22.7
18.1
41.6
20.9
28.7
2.5
28.1
The main shock is indicated with an asterisk.
generates a large positive peak, which increases the
energy release by more than two orders of magnitude at
the scale of Taiwan Island.
Seismic energy released in the EZ shows a
fluctuating behavior after the main shock. The function
decreases rapidly during a 5-month interval, down to a
local minimum, which is followed by a progressive
Fig. 3. Seismic energy released per month in the epicentral zone (triangular points) and at the scale of Taiwan Island (diamond-shaped points),
between January 1999 and December 2000. Precipitation data at four recording gauges located near the slide (Fig. 1), between January 1999 and
December 2001. The bar diagrams show the monthly cumulated rainfall in the four stations. The vertical shaded stripes indicate the major typhoons
that had an impact on the precipitation values in the region.
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increase to a secondary peak. The peak is associated
with the large aftershock of 10 June 2000 of magnitude
6.7 located ∼ 30 km to the SE of the slide (Table 1).
From here onwards, the released energy decreases by
three orders of magnitude down to a roughly constant
value. The released energy is greater by at least two
orders of magnitude, with respect to the values observed
before the main shock. This behavior is attributed to
aftershocks located near the Chelungpu fault.
There are no reports on major landslides triggered by
aftershocks, in particular in June 2000. Large aftershocks
that occurred on the days following the main shock may
have triggered large slides, yet the available information
does not allow us to discriminate these events.
2.4. Rainfall
Intense rainfall has a major effect on surface
processes such as denudation and landsliding. This
section summarizes the precipitation data at four
recording gauges located near the slide, which are
also plotted in Fig. 3. These stations, indicated in Fig. 1
by small umbrellas, are operated by the Water
Resources Agency from Taiwan. The bar diagrams
show the monthly total rainfall at the four stations,
between January 1999 and December 2001. The yearly
monsoon in Central Taiwan extends from May to
October, creating heavy rains. Precipitation peaks are
generally linked with destructive typhoons that may
create natural hazards such as landslides and floods.
Vertical shaded strips indicate the main typhoons
observed in the region.
The Chi-Chi earthquake occurred at the end of the
rainy season in 1999. The ground water table in the
sliding area was then probably at its highest level. Water
seepage and small springs were observed on the sliding
surface a few days after the main shock, indicating the
existence of saturated rock layers beneath the surface of
rupture. Note that shear strength is an increasing
function of the effective normal stress, which decreases
with excess pore-water pressure. Thus, the rocks
destabilized by the earthquake were weakened by the
increase in pore-water pressure. Seismic shaking was, in
any case, the triggering mechanism of the Jiufengershan
avalanche.
Shaking during the Chi-Chi earthquake created a
great deal of loose debris that accumulated along the
slopes in the epicentral zone (Dadson et al., 2004).
These unstable materials, which are particularly susceptible to erosion, were mainly mobilized by subsequent
typhoons. Among these, the Toraji typhoon, which
occurred on July 30, 2001, is considered one of the most
7
devastating meteorological events in the past several
years. It created large debris flows in the epicentral zone
and in Central-Eastern Taiwan.
3. Methods and data sets used
The quantification of post-seismic surface processes
has been made using two complementary methods,
which give information on the displacements of unstable
material and on volume changes in the landslide area.
The first method is an application of an optical image
correlation technique, in which the horizontal displacement field is measured by comparison of images
acquired at two different times. This method is based
on a subpixel correlation of orthorectified images, using
sliding windows. Each image is divided into a grid of
small sections known as interrogation areas (windows).
The corresponding windows within each of the two
images are then cross-correlated. The cross-correlation
function is a pattern-matching routine that determines
the relative displacement between images, which are
shifted according to the best overlap. The residual
offsets of image pairs are computed from the phase shift
of the Fast Fourier Transform of the sliding window
(Van Puymbroeck et al., 2000). This value corresponds
to the horizontal displacement vector of the window.
The correlation between two images can be efficiently
performed as long as their texture is similar. The method
allows calculation of displacements as small as 5–10%
of a pixel if the correlation between the images is of
good quality. In landslide areas located in tropical zones,
the analyzed images should be taken at time intervals of
a few months in order to preserve the similarity of the
texture.
Section 5 describes the results of this technique to the
study of the horizontal displacement field calculated for
a time interval extending from 2 to 3.5 months after the
Chi-Chi earthquake. Note that this was a time of very
little rainfall but relatively high energy release. The
photographs were taken in November and December
1999, 2 and 3.5 months after the main shock,
respectively. They were first orthorectified using a 9-m
DEM of the landslide, taken just after the earthquake.
The second step before applying the optical image
correlation technique consisted in resampling the
corrected images using a 0.4-m square grid (the size
of the pixel is 0.4 m). The orthorectification was
performed using precise photographic parameters (camera and lens), and more than 60 ground control points
defined outside the landslide area. The orthorectification
accuracy quality of the two aerial photos was controlled
within 5 pixels RMS (equal to 2 m RMS). The method
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was also tested on images taken at intervals of more than
1 year and the correlation was fuzzy, because of the
dissimilarity in the vegetation cover in the sliding area.
The second method consists of determining the coseismic and post-seismic volume of eroded and
deposited materials in the sliding area using three digital
elevation models (DEM). The volume estimations were
calculated by comparing the altitudes and integrating the
volumes in the landslide area. The DEMs were baseline
corrected in order to minimize the vertical discrepancy
between them.
The first DEM was taken before the earthquake in
1986, and it has a resolution of 40 m in the NS and EW
directions, and a vertical precision of 1–2 m in plan and
∼ 5 m in ranges for absolute error, less than 1 m for
relative system error. The data set consists of three scalar
numbers indicating the altitude, and the geographical
coordinates of the nodes in a square grid. The second
DEM was taken 2 months after the Chi-Chi earthquake;
it has a horizontal resolution of 9 m × 9 m, and a vertical
precision of ∼1 m. The latest one is a LiDAR image that
was taken 2.5 years after the Chi-Chi earthquake (Chang
et al., 2005a). The precision for this data set is up to
0.12 m vertical (Shih and Peng, 2002). From the LiDAR
data we generated a 1-m × 1-m grid, which constitutes
our third DEM. Fig. 4 illustrates the differences in
resolution between the three DEMs by means of a field
example located near the sliding area. The LiDAR
image (on the right) was calculated from the last
reflected LiDAR signals, in order to determine the
ground topography. Very fine features such as country
paths and different agricultural fields are readily
identified in this image.
The comparison between the three DEMs required a
homogenization of the data sets. First, we resampled the
altitudes on a unique square grid with 5 m resolution.
This value is an average between the 9-m resolution data
for the 1999 DEM and the 1-m resolution for the LiDAR
image. Second, we shifted the DEMs in the vertical
direction in order to minimize differences in altitude
outside the sliding area. For this, the mean altitudes of
several stable areas were calculated. The vertical offsets
were nearly the same for all areas, with differences of
less than 1–2 m. The 1986 DEM was chosen as the
reference altitude. The average height correction was
roughly constant for each DEM.
4. Post-seismic morphology and processes
The morphology of the western limb of the Taanshan
syncline, which was largely modified during the
earthquake, may be decomposed in three zones (Fig. 2):
Fig. 4. Visualisation of a stable zone located near the slide, using three digital elevation models (see text for explanations). The LiDAR image (on the
right) has the best resolution; it allows identifying meter-scale features.
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(a) The surface of rupture, defined by the main
sliding plane of the rock avalanche. This surface is
composed of shales and sandstones from the
Changhukeng Shale middle member, which dip
∼ 22°SE toward the Jiutsaihu valley (sliding area,
Fig. 2).
(b) The western flank of the surface of rupture, which
is defined by an elongated area of disturbed rocks.
These rocks are located above the sliding surface
and belong to the Changhukeng Shale top
member. They are affected by brittle extensional
structures such as normal faults and open cracks.
(c) The eastern flank of the surface of rupture is
defined by fractured blocks with sizes ranging
from 10 to 30 m, mainly from the Changhukeng
Shale top member (this zone is included within the
deposit area in Fig. 2). This zone shows debris
accumulations deposited by the rock avalanche
and incipient block slides. These rocks are
bounded downslope by an adjacent ridge that
limits the reactivation of slides.
The surface of rupture (SR) and the western flank
have been encircled by a continuous boundary (sliding
area, Fig. 2). This area indicates schematically the zone
of the limb where rock sliding was dominant during the
avalanche (sliding in the eastern flank was restrained by
adjacent reliefs located downslope). Rock and soil
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layers located above the SR were almost entirely
mobilized during the earthquake and transported by
the avalanche. After the earthquake, a 1–5-m layer of
debris covered the SR. These deposits have been
progressively washed out by overland flow, largely
clearing the SR. Linear accumulations of debris with
long axes oriented downslope are still present on the
sliding surface, however, in particular above the Longnan path (Fig. 5). This path (crossing the center of Fig. 5
from left to right) traverses the SR at mid-altitude and is
roughly parallel to the strike of the slope. These deposits
have been colonized by weeds and shrubs that reduce
the effectiveness of erosion during the rainy season.
Clayey sandstone blocks show oxidation aureoles and
spheroidal weathering linked to chemical alteration
along fractures. Overland flow is concentrated along 1–
10-m strips, located in between vegetated strips. Rock
alteration and incipient incision are observed in the
uppermost layers, along the paths of runoff. Wash
processes below the Longnan path are localized
downslope of several culverts, which drain water from
the upslope area.
The debris deposits located upslope in the SR are
affected by slumps that join the sliding surface (Fig. 6).
These slides show curved scarps with several meters of
displacement. The principal scarps expose clay materials that are not covered by vegetation. In contrast, the
slumped blocks show tilted bamboos and shrubs
Fig. 5. View of the sliding surface showing linear accumulations of debris oriented downslope, separated by overland flow paths. The top-half of the
image shows the morphology of the basin located at the toe of the hillslope and the debris deposit. The lakes and artificial channels are also observed.
LP = Longnan path; L1, L2 = lakes (see Fig. 2); SF = small fan.
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Fig. 6. Detailed view of the crown of a 50-m slump affecting debris deposits that cover the sliding surface. The large dislocated blocks observed on the
top-right corner are located on the eastern flank of the surface of rupture.
consistent with block rotation. These observations made
in January 2003 suggest that the slides were activated
after the main shock, since the main scarps show fresh
unvegetated soils.
The avalanche deposit located downslope shows a
sequence of smooth transverse ridges and depressions
that contrast with the neighboring steep ridges (Fig. 2).
The deposit consists of a chaotic mixture of small rock
fragments and jointed blocks, ranging in size from a few
centimeters to more than 20 m on a side, as well as
weathered shales. After the earthquake, an elongated
depression was observed at the toe of the sliding surface.
This small basin was filled progressively with debris
transported through small fans from the sliding surface,
as shown in Fig. 5 (at the toe of the sliding surface).
Small lakes created during the earthquake in the
periphery of the debris deposit are indicated in Figs. 2
and 5. Lakes L1 and L2, which result from the damming
of the Jiutsaihu and the Sezikeng creeks, were drained
by a natural channel that incised into the debris deposit,
following the stream channel that existed before the
avalanche (Fig. 2). Subsequently, the natural channel
was transformed into a bulldozed ditch with trapezoidal
section in order to drain the ponds more efficiently and
prevent subsequent geological hazards. Sediment traps
were constructed along the main channel to restrain the
transport of large blocks. Two artificial channels were
dug in order to drain water from the basin located at the
toe of the surface of rupture.
5. Determination of post-seismic landslide
movement
To evaluate the rate of denudation of debris and
dislocated rock blocks along the monocline slope
(northwestern limb of the Taanshan syncline), we
compared two aerial photographs taken 2 and 3.5 months
after the main shock, respectively (Fig 7). This period of
the year corresponds to the dry season, which has low
temperatures and precipitation. Thus, the vegetation is
sparse and geomorphic features such as slide scarps are
easily recognizable. A priori, a time interval 1.5 months
may seem to be too short and non-representative of postseismic displacements. However, the results of this
analysis show that this time interval is long enough to
identify the location of the major unstable zones.
In order to calculate the displacement field the
images were analyzed by means of the subpixel
correlation method (see Section 3) (Van Puymbroeck
et al., 2000; Michel and Avouac, 2002; Dominguez et
al., 2003).
Fig. 7B shows the displacement field of the surface of
rupture and adjacent areas. Two opposite behaviors are
observed:
(a) Large displacements are concentrated around the
western flank of the landslide. Patches of sliding
debris, which measure more than 500 m in the
downslope direction, are observed above the
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11
Fig. 7. Orthorectified aerial photographs of the sliding area showing the horizontal displacement field calculated by the subpixel correlation method.
The photographs were taken 2 and 3.5 months after the earthquake. (A) Image of the sliding area taken the 18 November 1999. (B) Image of the
sliding area 1.5 months later and horizontal displacement field indicated by the arrows according to the scale defined in meters. The figures in (C and
D) are close-up images of the windows indicated in (A and B). The open arrows indicate the location of extensional fractures. LP = Longnan path. The
displacement field was indicated by arrows, according to the color scale in the bottom-left corner.
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Longnan path. The displacements are oriented
downslope and vary between 4 and 6.4 m. The
average horizontal transport rate in these areas is
close to 0.2 m/day. This zone was intensely
fractured during the earthquake, and it was
particularly susceptible to instabilities.
(b) The rest of the sliding area and adjacent zones
show negligible displacements with very few
exceptions.
Fig. 7C and D illustrate the initial morphology and
the displacement field of the inset indicated in Fig. 7A
and B. This sector is characterized by extensional scarps
oriented NS and NE–SW. The striped pattern of the
image is due to a tea plantation, which is also visible in
the uppermost part of the slope. The displacements are
roughly oriented EW, perpendicular to the NS-trending
fractures. Surprisingly, this direction of extension is not
oriented downslope as in the other patches. The
magnitude of displacement vectors increases from
west to east, as the extensional fractures are crossed.
Disturbed blocks are subjected to E–W extensional
stresses that are perpendicular to the NS-trending and
east-dipping scarp that bounds the western flank (Figs.
2, 7). This scarp can be considered as a free boundary
since block movements are not restrained by the surface
of rupture. The embankment located above the Longnan
path blocks the sliding processes. Both aftershock
magnitudes and rainfall intensities were quite low
during this time interval (Fig. 3). Sliding was probably
enhanced by underground water that drained laterally
from the disrupted blocks located in the flanks of the
surface of rupture.
6. Determination of post-seismic erosion and
deposition
In this section, we present new estimates of the coseismic and post-seismic volume distributions in the
landslide area, based on three digital elevation models
(DEMs) presented in Section 3. The volume of rocks
destabilized during the rock-and-soil avalanche (RV)
results from the calculation of rock materials located
above the basal shear plane before the avalanche. For
the sake of clarity, we have decomposed this volume in
two parts (cross section A–A′, Fig. 2): the depleted
volume (DV) and the depletion (D). The depleted
volume is the part of the displaced mass that overlies the
surface of rupture but underlies the original ground
surface. The depletion is the volume bounded by the
surface of rupture, the displaced mass and the original
ground surface. The total volume of debris (TV) is also
decomposed in two parts (Fig. 2): the depleted volume
and the accumulation (A). The latter is defined as the
volume of deposits that lies above the original ground
surface. The calculation of A and D is performed by
subtracting the 1986 DEM from the 1999 DEM: positive
and negative altitude differences are linked with deposit
and erosion processes. The estimation of DV requires
additional information concerning the geometry of the
shear surface at depth. The surface of rupture was
extrapolated by supposing a circular fault geometry
between the toe of the monocline sliding plane and the
toe of the surface of rupture, which is defined by the
Jiutsaihu river bed (cross-section A–A′, Fig. 2).
The following values were estimated: D = 37.4 ×
106 m3, DV = ∼ 4.7 × 106 m3, and A = 45.8 × 106 m3. The
volume of destabilized rocks is RV = ∼ 42.1 × 106 m3,
while the total volume of debris is TV = 50.5 × 106 m3.
Given the differing resolution of the 3 DEMs, the error
for these volume estimates is less than ∼ 5%. The
difference between these values suggests that the
mobilized rocks were subjected to fracturing and
expansion during the avalanche. Thus, the average
density of the debris deposit is ∼ 17% less than the
density of the original rocks.
Fig. 8 shows an isopach map illustrating the
difference in height between the 1999 and the 1986
DEMs, which is representative of the co-seismic sliding
process. This map is useful for identifying the
provenance of post-seismic eroded materials. The
isopach lines are shown at 10-m intervals: positive and
negative values indicate net accumulation or depletion
of rock material, respectively. The average thickness of
the sliding strata is between 50 and 60 m. The debris
deposit is located within a 1-km-wide depression that
was filled by the rock avalanche. The average thickness
of the deposit is between 60 and 80 m, reaching 110 m
above the Jiutsaihu creek channel.
Fig. 9 shows an isopach map illustrating the
difference in height between the 2002 and the 1999
DEMs, which results from several processes: postseismic sliding, overland flow and denudation, river
incision and man-made constructions and excavations.
The landslide area has been subdivided into nine zones
with varying morphologies, in which erosional processes are different. The net volume increase or decrease in
each zone is given in Table 2. Erosion of debris in the
neighborhood of the surface of rupture is mainly
observed in the western flank of the sliding surface,
above the Longnan path (zone 2, Table 2). The
difference in height in this zone ranges between 5 and
20 m, which gives a mean erosion rate of ∼ 0.5 m/month
for the unstable area. This area roughly coincides with
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13
Fig. 8. Isopach map of the landslide illustrating co-seismic surface processes, determined by subtracting the 1986 DEM from the 1999 DEM. The
isopach lines are shown at 10-m intervals: negative and positive values indicate net accumulation or depletion of rock material, respectively. The
boundaries of the landslide (Fig. 2) and the Longnan path (LP) have been traced. The coordinates are specified in kilometers, using the 2DTM system.
the patches showing large horizontal displacements after
the main shock (Fig. 7). The net volume loss in zone 2 is
∼ 0.47 × 106 m3. Erosion and denudation in the surface
of rupture tends to smooth down morphological features
such as folds and faults ranging in size from 1 to 10 m
(Chang et al., 2005a). Material removal is also observed
downslope of the culverts located underneath the
Longnan path (zone 4). The eroded materials from the
sliding surface are deposited in an elongated depression
located at the toe of the hillslope (zone 5). This small
basin was filled progressively with debris from small
fans that collect materials resulting from overland flow
and denudation in zones 2, 3 and 4. The average
thickness of post-seismic deposits in this basin ranges
between 12 and 16 m. The estimated volume of the
deposit in zone 5 is 0.43 × 106 m3, which is roughly half
the volume loss in the sliding surface (zones 2, 3 and 4).
The rest of the material eroded from these zones was
transported through the two subsidiary channels that
drain the small basin (Fig. 2). Small debris accumulations are also observed above the Longnan path,
downslope of the unstable rock materials located near
the western flank.
Post-seismic erosion is also observed in the EW scarp
face that bounds the surface of rupture toward the north
(Figs. 2, 9, zone 1). A small creek flows along the Vshaped alluvial valley that bounds the flatiron remnant.
The eroded materials mainly consisted of co-seismic
debris accumulations that were deposited along the
hillslope and near the river bed. The thickness of eroded
debris in this zone varies between several meters on the
hillslope to more than 20 m at the toe of the scarp face.
The volume of eroded debris in zone 1 is ∼2.5 × 106 m3.
Removal and transport of material mainly occurred
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Fig. 9. Isopach map of the landslide illustrating post-seismic surface processes, determined by subtracting the 1999 DEM from the 2002 DEM. The
isopach lines are shown at 10-m intervals. The landslide area is decomposed in nine zones plotted in dashed lines, in which the volume losses are
estimated (Table 2). The boundaries of the landslide (Fig. 2) and the Longnan path (LP) have been traced. The coordinates are specified in kilometers,
using the 2DTM system.
during the first large typhoons that affected this region
after the earthquake.
Erosion is particularly visible in the main channel
that drains the dammed lakes as well as the two
subsidiary channels that drain the small depression
located at the toe of the monocline. The depth of the
main channel varies between 20 and 40 m, and its
volume is ∼ 0.96 × 106 m3 (zone 8). Note that most of
this material was transported downstream as the river
incised into the debris deposits of the rock-and-soil
avalanche. The volume loss in zones 6 is ∼ 0.96 ×
106 m3; it includes compaction and denudation of the
debris deposit as well as the volume of excavation of the
two subsidiary channels. The denudation of the debris
deposit was mainly observed in the river channel (zone
8 in Fig. 9), whereas compaction was observed all
through the zone of accumulation. The average
compaction of the debris deposit is a function of its
thickness and its composition. The altitude difference
linked to compaction varies between 0.3 and 0.5 m, and
the average vertical compaction is roughly 1% of the
initial height of the deposit. Compaction is probably
associated with an average decrease in the size of open
fractures and pores in between the disrupted blocks.
Most of the pores were certainly non-saturated with
water since the volume of the debris deposit was much
greater than the volume of the original rock layers.
The volume of the lakes located in zone 7 is
∼ 0.44 × 106 m3. The largest lake was 20 m deep. The
depth of the channels controls the base level of the lakes.
Lakes act as sediment traps that are progressively filled
with debris carried by creeks located upstream and from
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Table 2
Volume changes associated with post-seismic surface processes from
September 1999 to April 2002 in the nine zones shown in Fig. 9
Zone
Location
Volume (× 106 m3)
1
2
3
4
5
6
7
8
9
EW scarp face
Surface of rupture and western flank
Eastern flank
Lower part of the sliding area
Small basin
Avalanche deposit (upstream)
Dammed lakes (L1 and L2)
Main drain channel
Avalanche deposit (downstream)
− 2.5
− 0.47
− 0.25
− 0.11
0.43
− 0.14
0.44
− 0.96
− 0.05
The error for these volume estimates is less than ∼11%.
nearby deposits. The Jiutsaihu creek is dammed to
create lake 1, which is drained through a small channel
connected to lake 2 (Fig. 2). Lake 2 is drained by the
channel that crosses the debris deposit (zone 8).
7. Conclusions
Post-seismic surface processes (PSP) in the Jiufengershan landslide area were studied by means of a
multidisciplinary approach, which combines geomorphological observations and precise topographic
analyses.
After the Chi-Chi earthquake, large unstable blocks
and debris deposits located in the flanks and the crown
of the sliding surface were mobilized by slumps and
translational slides. Erosion was also observed at the
surface of rupture, which was covered after the
avalanche by a 1–5-m-thick layer of debris deposit.
These unstable materials, which are particularly susceptible to erosion, were subjected to denudational
processes, mainly during the annual period of heavy
rains and typhoons (from May to September). Overland
flow has denudated the surface of rupture, reducing
considerably the volume of unstable materials; presently, only linear accumulations of debris with long axes
oriented downslope are still observed on the sliding
surface.
The post-seismic displacement field was calculated
by means of the subpixel correlation method of
orthorectified images taken, respectively, 2 and
3.5 months after the earthquake. As far as we know,
this is the first application of this optical image
correlation technique to study landslide processes. We
determined horizontal displacements ranging from 1 to
6.4 m around the fractured zones located near the
western flank of the surface of rupture (SR). This
approach provides a promising tool to study slow sliding
processes such as creep, from satellite images with
15
submetric resolution. The technique should be applied
to images taken at time intervals of a few months, in
order to preserve the similarity of the texture. In tropical
environments such as Taiwan, the vegetation cover can
be modified rapidly (especially in a landslide area),
reducing the quality of the correlation between the
images.
PSP were also studied by means of three DEMs,
including a LiDAR image taken 2.5 years after the
earthquake. The co-seismic and post-seismic volumes of
eroded and deposited materials were estimated by
calculating the difference in altitudes between the
DEMs. PSP in the avalanche debris deposit are
characterized both by erosion and deposition. Large
volumes of materials were deposited in small basins and
lakes located upstream from the deposit. In particular,
the eroded materials from the sliding surface were
deposited in an elongated depression located at the toe
of the hillslope. These depressions were partly drained
by natural and artificial channels that cut across the zone
of accumulation. The debris deposit is also subjected to
vertical compaction, which reduces its thickness in
∼ 1%.
Future work should evaluate the impact of individual
events such as a typhoon or a large aftershock, on the
reactivation of landslides and the mobilization of debris
material. This type of analysis would require precise
topographic information as well as field observations
gathered in the sliding area.
Acknowledgements
This work was supported by Taiwan–France Cooperation in Earth Sciences (French Institute in Taipei and
National Science Council of Taiwan) and partially
funded by: INSU Reliefs de la Terre 2004–05 and INSU
ACI Catastrophes Naturelles 2002. We thank Dr. G.
Wieczorek, Dr. D. Keefer and an anonymous reviewer
for thorough reviews of the manuscript, as well as J.
Wasowski and V. Del Gaudio for comments on the
manuscript. The authors appreciate the valuable discussions with Prof. C.Y. Lu of Department of Geoscience,
Prof. F.S. Jeng and Prof. M.L. Lin of the Department of
Civil Engineering in National Taiwan University,
Taiwan, and Prof. J. Malavieille of Université Montpellier II, France.
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