京 都 大 学 防 災 研 究 所 年 報 第 49 号 C 平 成 18 年 4 月
Annuals of Disas. Prev. Res. Inst., Kyoto Univ., No. 49 C, 2006

Landslide Disasters Triggered by the 2004 Mid-Niigata Prefecture Earthquake in Japan

Kyoji SASSA, Hiroshi FUKUOKA, Fawu WANG,
Gonghui WANG, Kenji OKADA*, Hideaki MARUI***

Japan Meteorological Agency
Research Center for Natural Hazard and Disaster Recovery, Niigata University, Japan**

Synopsis
During the 2004 Mid-Niigata Prefecture earthquake, thousands of landslides were triggered.
Among them five catastrophic landslides were introduced in this paper. In addition, the influence ofrainfalls in prior to the earthquake and the urgent response to prevent disastrous failure of landslide
dams were also discussed and presented. Higashi Takezawa and Terano landslides occurred withinpast landslide masses and dammed the river at the toe of the landslides, causing great damages to theproperties. Detailed field investigation was performed on these two landslides. To examine thetriggering and movement mechanisms, samples were taken from these landslides. By using a ringshear apparatus, real earthquake wave loading test and cyclic loading tests were performed on thesesamples. The test results revealed that those sand samples from both landslides can suffer from sliding
surface liquefaction phenomenon with very lower final apparent friction angles, while the silt sample
from Terano landslide showed no liquefaction failure, indicating that landsliding during thisearthquake time was triggered within the sand layer above the silt layer in the past landslide masses.
Finally, the movement of Higashi Takezawa landslide was simulated.
Keywords: landslide, earthquake, soil water index, ring-shear test, simulation, disaster recovery

1. Introduction
The M6.8 Mid-Niigata Prefecture earthquake,occurred at an epicentral depth of 13 km near Ojiya city
in Niigata Prefecture at 17:56 on 23 October 2004. 362
landslides with widths of more than 50 m and 12
large-scale landslides with individual volumes of morethan 1 million cubic meters were triggered by thisearthquake (Ministry of Land, Infrastructure andTransport 2005). It was the greatest earthquake disasterin Japan after the 1995 Hyogoken-Nambu earthquake(M7.2). Mostly, the landslides were reactivation of pastlarge-scale landslide masses. Forty five landslide dams
were formed by this earthquake. Within those, fiveFig. 1 Locations of the four major landslides triggered by the M6.8 Mid-Niigata Prefecture earthquake－119－Fig. 2 General view of the Higashi Takezawa landslide. (a) Oblique airphoto taken by Marui on 3 November 2004,(b) Oblique airphoto taken by Sassa on 6 November 2004, (c) Debris intruded into a school building from thelandslide mass, d) Side view of the Higashi Takezawa landslide taken on 16 November 2004.
major landslides occurred on Higashi Takezawa,
Terano, Dainichisan, Haguro, and Myoken areas (Fig.
1) were the most catastrophic. The Higashi Takezawa
landslide and the Terano landslide which formed
landslide dams more than 25 m high were investigatedin detail. This research presents the dynamic propertiesof these landslides and the mechanism of large-scale
rapid landslides induced by earthquake within pastlandslide masses, as well as the landsliding computer
simulation for the Higashi Takezawa landslide.
2. Major landslides triggered by the 2004 MidNiigata Prefecture earthquake
The landslide that posed the greatest threat to thearea was the Higashi Takezawa landslide, whichblocked the Imokawa River, forming a landslide dam.
Fig. 2a, b show the general view of this landslide takenfrom a chartered helicopter. Photo a) was taken on 3
November, and photo b) was taken on 6 November.
The road bridge that can be seen in photo a) was
submerged in the landslide-dammed lake in photo b).

Both photos clearly showed that the landslide mass was
not disturbed through its approximately 100 m distanceof rapid movement. As can be seen in photo b), trees on
the landslide generally remained straight. However, theforest showed backward rotation at the toe of thelandslide (photo a), because the landslide mass filled
the Imokawa River and moved upward along the slopeof the opposite bank. The building in Fig. 2a was anelementary school. Fig. 2c shows debris that flowed
into the entrance and a window of the school building,and mud that scattered onto the building wall. The
phenomenon suggested the rapid movement of thislandslide. The debris and scattered mud at the schoolbuilding probably came from a small landslide (“A” in
Fig. 2a) at the toe of large displaced landslide mass.
This part of the mass did not include the forest, but
probably was soils from/around the Imokawa Riverthat was scraped and carried by the displaced landslidemass. Fig. 2d shows a side view of this landslide fromwhich high mobility can be hypothesized based on theangle from the head scarp to the toe of the landslide.
The mobilized apparent friction angle was 7.5 degrees.

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The toe of landslide climbed up the opposite bank. The
height of the lowest part of the landslide-dam crest was
raised by piled sandbags that were transported byhelicopters of the Japanese Self-Defense Force toprevent overflow of the dam. Standing trees suggested
a smooth movement along a liquefied sliding surface.
The upper part of the landslide mass was notcompletely saturated although the sliding surface itself
probably was.

layer. The sand layer over this stiff silt layer was soft. It
was probably a part of previously moved landslidemass. A sample (H2) was taken from this sand layer
behind the current sliding surface as seen in Fig.5.

Fig. 5 Sampling from the sand layer behind the sliding
surface and over the stiff silt (stone) layer
Fig. 3 Higashi Takezawa landslide and the head scarpof past landslides (Taken by Sassa)

Fig. 4 Stiff silt (stone) layer outcropped in the head
scarp (it inclined around 20 degrees to the landslide
moving direction and groundwater flowed over thelayer)
Fig. 3 presents another view of the Higashi
Takezawa landslide. The head scarp of the current
landslide is shown on the figure by red arrows, and thehead scarp of the previous landslide is shown by acurved line with red arrows. We investigated the head
scarp of this landslide. A very straight, gently-dipping(around 20 degrees) stiff silt (stone) layer outcropped asshown in Fig.4. This layer seemed to be a part ofsliding surface of this landslide. It is relatively
impermeable and groundwater flowed over this stiff silt

Three days after (26 October 2004) the earthquake,an air borne laser scanning survey was conducted, andFig. 6a presents its result. The gentle slope found in thecontours clearly presented the previous landslide
topography above the head scarp of this landslide. The
central section A-A of the landslide is presented in
Fig.6b that includes an interpretation of landslide
process based on the topographical map made by the
air borne laser scanner after the landslide, the 1/25000
topographical map before the landslide, and fieldobservations. In Fig. 6b, the black line presents theground surface before the landslide, and the green line
shows the ground surface after the landslide. The mass
A (red dots) and the mass B (black dots) are theunstable mass before the current landslide movement.
The initial average slope angle between P1 and P2
is around 14.6 degrees, and the average slope angle
between P1 and P3 is around 13.5 degrees. The
mobilized energy line between P1 and P4 (highest
point) is around 7.5 degrees. Hence, this rapid landslide
occurred in a gentle slope less than 15 degrees, and themobilized average apparent friction angle duringmotion was 7.5 degrees. The large difference between
the energy line and the center of gravity of the moving
mass suggests a rapid motion.

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(b)

Fig. 6 (a) Plan of the landslide (the map was made by
the air borne laser scanning in the courtesy of Aero
Asahi Corporation. Contour lines are 2 m spacing); (b)Section of the landslide along A-A line in Fig. 6a
For this landslide, the first question arose on theposition of sliding surface from this observation. The
outcropped silt (stone) layer was very stiff andapparently too strong to slide. However, the silt near theborder to the sand layer could be soft due to weathering
because an adequate groundwater flow existed.
Accordingly there are two options for the location ofsliding surface of this outcropped head scarp, 1)
weathered top part of lower silt layer, 2) bottom ofupper sand layer. This will be examined later in thispaper based on the dynamic loading tests and detailed
section.
The second question arose on the sequence oflandslide movement. There are two possible scenarios.
1) Block B was firstly liquefied, then the block A
started to move due to loss of support in its toe, 2)
Block A initially started to move due to seismic loading
and the resulted excess pore pressure generation. Thus,
the movement of block A then gave an undrained
loading to block B. This undrained loading effect canbe greater than earthquake loading for the Block B.
Then, the block B started to move with the block A andcrossed the Imokawa River and hit the opposite bank.
Block B is near the river water, and the lower half of

this mass was apparently continually submerged under
the groundwater. Such part was deoxidized and became
blue or gray in color, while the oxidized parts arebrown. The deoxidized gray color sand was found on
the opposite bank and around the school (into whichsome part of soils entered).
Pore pressure generation on the shear surface incrushable sands is greatly affected by shear
displacement (Wafid et al 2004, Sassa et al 2004).
Block A is loaded by shear stress due to self weight andseismic loading. Block B is loaded almost exclusively
by seismic loading due to its flat position. Therefore,
when the bottom of Block A was saturated, the shear
zone should be subjected to greater shear displacement
and generate greater pore pressure compared to Block
B. There were many terraces along the river, but noliquefaction failure in such terraces near the river was
noticed. Another reason is the hardness of sand in thisarea. The sand is a marine deposit from the Tertiary
period. It is much stronger than tuff, pumice or
weathered granitic sands, because those sands
experienced a long downriver transportation andweaker parts was necessarily removed already throughthis flow with water. The depth of Block B (around 13
m above the river bed) is much smaller than Block A(around 40 m). Shaking or shearing under a greater
overburden pressure could facilitate grain crushing andresulting pore pressure generation. The rapid
movement landslides were the cases for more than 25
m in depth such as the Higashi Takezawa landslide, theTerano landslide and the Dainichisan landslide (thebiggest landslide in this earthquake). Accordingly, andbased on this consideration, it has been concluded
regarding the second question that Block A moved
firstly due to strong seismic shaking plus the static
shear stress due to self-weight of the around 40 m thicksoil layer. Therefore, dynamic loading ring shear tests
were conducted to simulate the initial movement ofBlock A.
The Terano landslide (Fig. 7) also formed alandslide dam 4.5 km north of the Higashi Takezawa
landslide (Fig. 1). The Terano landslide occurred as
landslide blocks in a repeatedly moved residual state.
Fig. 7a shows an overview of the Terano landslide,which occurred on a gentle slope; the displaced
landslide mass blocked the Imokawa River, forming alandslide dam. This area had been the site of rice paddy
fields. Because there was no house or other structure－122－

Fig.7 View of the Terano landslide and landslide distribution map of the vicinity. a) Oblique airphoto taken by Sassa
on 6 November 2004, b) Side view of the Terano landslide, c) Landslide distribution map and the Terano landslide(Inokuchi et al. 2004)
within the area of the Terano landslide, it was notdesignated as a landslide risk area. However, theadjacent areas were designated as landslide risk areas.
A house stands at the margin of the Terano landslide(Fig. 7b). Fig. 7c shows the location of the landslide
mass triggered by the earthquake overlaid on an earlier
landslide in a previously made landslide distributionmap (Shimizu et al. 2004). In this figure, brown dashedlines define the margins of the previously moved
landslide mass identified by airphoto interpretation of
the topography. The black dashed lines delineate thehead scarps. The blue dashed lines are the main scarpof the landslide before the motion by the Mid-Niigata
Prefecture earthquake and the red line presents thelandslide block after the movement (Inokuchi et al.
2004).
Fig.8 presents the view from the head scarp to thetoe of the Terano landslide. It created a large landslide
dam and resulted lake. The landslide is a part of big
repeatedly-moving type of landslide complex area,
which is typical in the Tertiary mudstonelandslide-prone area. Fig. 9a, b show the plan and thesection of this landslide. This landslide was also

investigated. The main body of the landslide block at
the toe was composed of sands (T1) that seem to be
similar to the samples of H1 and H2 of theHigashi-Takezawa landslide. In contrast to the Higashi
Takezawa landslide, the silt sample (T2) taken from thehead scarp was well weathered and soft. The head
scarp and Sample T2 was inside the previously moved
landslide mass as suggested by the section shown in
Fig. 9. The silt was well softened due to landslide
motion and well oxidized which was suggested by its
brown color. While the siltstone outcropped in the head
scarp in the Higashi Takezawa landslide was very stiff
and in the stable layer below the landslide mass andprobably always saturated suggested by its gray colorof deoxidization. The grain size distributions ofsamples T1, T2 and H2 are shown in Fig.10. Samples
H2 and T1 are rather similar though sample T1
includes a slightly greater portion of finer grains. Whilesilt sample T2 is quite different from samples T1 andH2, it is much finer than those sands.
The initial slope angle was 14.9 degrees between
P1 and P2 at the top of river bank, and 17.5 degrees
between P1 and P3 along the river. The mobilized－123－average apparent friction angle during motion between
P1 and P4 was 12.7 degrees. The difference between
the energy line from P1 to P4 and the center of gravity
suggests a slower landslide than the Higashi Takezawa
landslide.

Fig.10 Grain size distribution of three collected
samplesFig. 8 Overview of the Terano landslide (Taken by
Sassa on 6 November 2004)

Fig. 9 Plan and Section of the Terano landslide (a: Plan
obtained from air bone lazar scanner. Contour lines are
2m pitch. b: Central section A-A of the Fig.9a)
The largest landslide in area and volume triggeredby the Mid-Niigata Prefecture earthquake was theDainichisan landslide (Fig. 11), which is located 1 km
west of the Higashi Takezawa landslide (Fig. 1). As
indicated in Fig. 11a, the Dainichisan landslide is atypical slump. This landslide did not reach the
Imokawa River, and thus did not cause a landslide dam.
Fig. 11b shows the exposed probable sliding surface,which is an inclined siltstone bed.

Fig. 11 Dainichisan landslide (a) and head scarp (b)Sand covered the siltstone bed. Almost all of thelarge landslides triggered by the Mid-Niigata Prefectureearthquake occurred in this sand layer and/or a silty
sand or silt layer. Whether the sliding surface formed at
the bottom of the sand layer or at the top of the siltstone
is in question. The situation was the same for theHigashi Takezawa landslide.
The Higashi Takezawa, Terano and Dainichisan
landslides were 25-50 m deep-seated landslides. The
Haguro landslide (Figs. 1, 12a) was relatively shallow,
and the landslide mass was very disturbed because of－124－

the steep slope and its shallow depth. This landslide
destroyed a road and many parked cars, but fortunately
no one was killed. The relatively small rock slide in the
Myoken area (Fig. 12b) covered a main road andburied a car occupied by a mother and two children.
The rescue team noticed the car buried under the
displaced landslide mass, and tried to rescue theoccupants. However, the rescue operation took threedays because of the very unstable landslide rock debris.
The rescue activities were monitored and broadcasted
every day by TV and other media. Finally a 2-year-old
boy was safely extracted from the buried car. The
Myoken landslide occurred on a convex slope. In
addition, it was dry, which was one reason the boy
could survive for 92 hours under the landslide mass.

Fig. 12 Oblique airphoto of the Haguro landslide (a)and the Myoken landslide (b). Both photos were takenfrom a helicopter by Sassa on 6 November 2004.
3. Influence of rainfalls in prior to the earthquake
The Mid-Niigata Prefecture earthquake in October
2004 occurred 3 days after 100 mm of rainfall was
triggered by typhoon No.23. Its effects differed fromthose of the Hyogoken-Nambu earthquake, which
occurred in the dry season (January) of 1995 when
there was no rain immediately prior to the earthquake.

The Hyogoken-Nambu earthquake was M7.2 and its
epicentral depth was 17 km. The Mid-Niigata
Prefecture earthquake was M6.8 with an epicentral
depth of 13 km. Thus, the earthquake energy was
smaller in the Mid-Niigata Prefecture earthquake thanin the Hyogoken-Nambu event, but the number andscale of landslides were much greater in the
Mid-Niigata Prefecture earthquake. 362 landslides witha width of more than 50 m and 12 large-scale landslides
with volumes of more than 1 million cubic meters were
triggered by the earthquake (Ministry of Land,
Infrastructure and Transport 2005), while the only
remarkable landslide triggered by the HyogokenNambu earthquake was the Nikawa rapid landslide(volume of 1.1-1.2 x 105 m3 and a width of 125 m),which killed 34 peoples (Sassa et al. 1996; Sassa 1996).
A possible reason for this difference was the influenceof rainfall in prior to the Mid-Niigata Prefectureearthquake.
The Japan Meteorological Agency uses theso-called Soil Water Index (SWI) as an index toevaluate the water stored beneath the surface of theground (Okada 2001). Fig. 13 illustrates the concept of
the SWI. SWI is an effective technique for estimating
the landslide disaster potential by comparing the valuesof Soil Water Index for past landslide disasters. SWI
uses the Radar-AMeDas (name of radar system for
precipitation monitoring) precipitation analysis as an
input rain value for the three steps of a serial storagetank model to express water runoff to the groundsurface, the surface soil layer, and the deeper soil layer.
This model expresses the shift of rain to the groundwater with time delay (Ishihara and Kobatake 1979).
The groundwater will be stored in each tankcorresponding to different soil layers. SWI is defined tobe the total storage thickness of the three tanks. The
index value is decided for each 5 km×5 km cell. For
Japan, number of cell in the mesh is 14,132.
SWI can establish a suitable standard for each 5 km
× 5 km cell based on the record of past landslidedisasters and amount of precipitation prior to the events.
When the record of landslide disasters is not available,
SWI can present the extent of danger based on the
ranking of SWI value in the past 10 years by quoting
the past ten years historical ranking value among its
surrounding meshes in which landslides occurred.

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Fig. 13 Soil Water Index (SWI) used by the Japan Meteorological Agency. Height of the upper outlet in the first tankis 60 mm, and the height for the other outlet in the first tank, second tank and third tank is 15 mm.

Fig.14 Time sequences of SWI at Yamakoshi Village
from July 1st to Nov. 20, 2004 (Rainfall in blue; SWI
in red.)As an example, the present SWI in a certain area
showed the highest value (1st rank) in the past 10 years
or 2nd highest value within the past 10 years. Landslidedisasters occurred in the 1st and 2nd rank of SWI in theneighboring areas. Therefore, caution and preparedness
is necessary for possible landslide disasters.
As a result of SWI analysis for landslides that
occurred from 1991 to 2000:
・About 60 percent of the landslide disasters occurred in
the areas with 1st rank historical rain, both multiple
rain areas and light rain areas.
・Most of the large-scale landslide disasters occurred in
1st rank historical rain.

・More than 80 percent of landslides with fatalities
occurred in 1st rank historical rain.
Thus, when 1st rank SWI value in the past 10 years
is recorded for a certain city, the heavy rain warning of“The danger of landslide disaster for this city is thehighest in past several years” is announced. The effectof the historical ranking of SWI is confirmed by thefact that 96 percent of large-scale landslides occurred in
the cities of its 1st rank of SWI appeared in Niigata
prefecture during 2004 Niigata heavy rain.
Fig. 14 shows the time sequences of SWI at
Yamakoshi Village from July 1 to November 18, 2004.
During 2004 many heavy-rain events struck Japan, andsome of these heavy rains occurred at Yamakoshi
Village, resulting in major landslide disasters. For the
Niigata-Fukushima Heavy Rain from July 10 to July 14,
the total amount of rain recorded by Rader-AMeDas
Precipitation was 428 mm, and the maximum index
value was 124 at July 13, 1900 hrs. For the Fukui
Heavy Rain from July 16 to July 18, total amountrecorded was 272 mm by Rader-AMeDas Precipitation,and the maximum index value was 195 at July 17, 2000
hrs. The past maximum index value during 1994 to2003 was 142. So the maximum value in this area wasexceeded in 2004.

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Fig. 15 SWI distributions in Yamakoshi Village and in
the Osaka-Kobe area. (a) SWI distribution (Fukui
heavy rain) from 16-18 July 2004 in the vicinity of
Yamakoshi Village, (b) SWI distribution on the day of
the Mid-Niigata Prefecture earthquake (23 October2004) in the vicinity of Yamakoshi Village, (c) SWI
distribution on the day of the Hyogoken-Nambu
earthquake on 17 January 1995 in the vicinity of the
Nikawa landslide.
However, large landslides did not occur in thisperiod of heavy rain. The parameters of the tank model
were decided based on monitoring of the granitic
slopes. Granitic slopes that have suffered from many

previous landslides during heavy rains have greater
permeability than slopes in the Tertiary weatheredmudstone area of Niigata. Thus, reconsideration of therainfall parameters is necessary. Landslides in the
mudstone area of Niigata often activate during thesnow-melt season. Long-term precipitation is effective
for causing landslides in this area rather than short-term
precipitation probably because of the low permeability
in the mudstone layer.
After these heavy rains, the SWI reached zero inAugust and thereafter the value was around 20-40.
However, typhoon No. 23 on 20 October renewed theheavy rainfall in this area. Three days after the typhoon,
the Mid-Niigata Prefecture earthquake occurred. At thattime, SWI value was 62, a high value in this area
except for the two extraordinarily heavy rains in July.
Fig. 15 presents the SWI distribution in the area
including Yamakoshi Village during the Fukui heavy
rain (highest SWI value in history) and during the
Mid-Niigata Prefecture earthquake, and the SWI for in
the area including the Nikawa landslide, which killed
34 persons during the Hyogoken-Nambu earthquake.
The SWI values were quite different in the
Hyogoken-Nambu earthquake and in the Mid-Niigata
Prefecture earthquake; however, the geology in the twoareas was somewhat different.
234 landslides more than 50 m wide occurred in the
Mid-Niigata Prefecture earthquake, while the Nikawa
landslide (more than 125 m wide) was only the large
landslide with long travel distance in the
Hyogoken-Nambu earthquake. This large difference
was likely caused by the heavy rainfall prior to the
Mid-Niigata Prefecture earthquake. The combined
effects of rainfall and earthquakes are necessary to bestudied in landslide risk evaluation for theearthquake-rainfall complex landslide disasters.
4. Urgent response to prevent disastrous failure oflandslide damsMore than 50 landslide dams were formed along
the main channel of the Imokawa River and its
tributaries by the landslides triggered by the
Mid-Niigata Prefecture earthquake. Because of theirsize, the two most critical landslide dams were theHigashi Takezawa landslide dam and the Teranolandslide dam. Both of these landslide dams were about
350 m long and had volumes of more than 1 million m3.

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Fig. 16 View (a) and cross section of the Higashi Takezawa landslide dam (b) along the Imokawa River, and elevationof the water level in the landslide-dammed lake (c)In both cases, the length of the buried river channel was
about 10 times the maximum water depth of the lake.
Therefore, the possibility of the destructive collapse ofeither dam by water pressure and/or piping wasestimated to be low.
However, there remained the possible danger ofovertopping and successive breaching of one or bothdams. Failure of these landslide dams could cause
outburst floods or debris flows which would endanger
downstream residential areas and roads. Therefore, theinhabitants of the downstream area would have toevacuate. It was urgently necessary to lower the level ofthe landslide-dammed lake. Emergency measures toprevent failure of the landslide dams were carried out
by the Ministry of Land, Infrastructure andTransportation with the cooperation of the Self-Defense
Force (Marui 2004). The Higashi Takezawa, the largest
landslide dam, which is located farthest downstream
among five main landslide dams along the ImokawaRiver, impounded a lake that was critical. Fig. 16a, b
presents the plan and the cross section of the Higashi
Takezawa landslide. The width of this landslide was
265 m, and its peak elevation was 174 m. The elevationof the lowest part of the dam crest was 158 m, and theriver-bed elevation was 134 m. In order to reduce thedanger of overtopping, various emergency measures
were undertaken. As a result, the lake level was kept
lower than the overflow elevation of 161 m, and lakelevel was lowered to a safer level, as shown in the
figure of monitored water-level elevation andprecipitation through the end of 2004 (Fig.16c).
Fig. 17 shows photos of some of emergency
measures. The Japanese Self-Defense Force transported
sand bags to shift up the overflow level by 3 m fromthe low point of the crest at 158 m to the top of the sandbags at 161 m (Fig. 17a). Then, the water level was
lowered by means of pumps and siphons. Twelve
pumps were installed. As the result, the monitored
water level was kept lower than the overflow level atelevation 161m. It is not suitable to use drainage pumps
for the long term; thus, they should be used for only
emergence purposes in the initial stages. Because ofmaintenance problems for the pumps, alternative
diversion pipelines (Fig. 17b) were installed to prevent
overtopping. These additional diversion pipelines werequite effective. Finally, an open channel (Fig. 17c) witha sufficient cross-sectional area for water discharge
including snowmelt during early spring wasconstructed. It was absolutely necessary to retain thestability of the earthquake-induced landslide against
secondary motion during construction of the channel.
Therefore, excavation of the upper part of the displacedsoil mass was immediately carried out as an
appropriate emergency countermeasure. Monitoring on
secondary displacement of the landslide was also
carried out for security during the construction works.

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inside diameter and 180mm in outside diameter, themaximum shear speed in the center of sample is 10
cm/sec, maximum frequency of cyclic loading is 5 Hz,
and the maximum rate of data recording is 200
readings/sec.
5.1 Real earthquake wave loading test for theHigashi Takezawa sand (H2)Seismic loading test was performed to simulate theinitiation of the Higashi Takezawa landslide using thereal monitoring record of the Mid-Niigata earthquake
in the closest site. The monitored record is from one
observation station of K-NET (Kyoshi Net), which isthe strong earthquake motion monitoring network by
the National Research Institute for Earth Science andDisaster Prevention (NIED). K-NET is a system which
sends strong-motion data on the Internet. The data are
obtained from 1,000 observatories (25 km mesh)deployed all over Japan. The nearest monitoring site to
the Higashi Takezawa landslide is the observation
station of NIG019 at Ojiya, around 10 km west of theHigashi Takezawa landslide, and WNW 7 km from theepicenter of the main shock. The Higashi Takezawa
landslide is ENE 3.6 km from the Epicenter. The
location of the epicenter is between the NIG019 and theHigashi Takezawa landslides, but closer to thelandslide.

Fig. 17 Emergency measures to prevent failure of theHigashi Takezawa landslide dam. (a) Helicopters of theJapanese Self-Defense Force carried sandbags andpiled them on the lowest part of the landslide dam crest
every several minutes during our observation, (b) and(c) Diversion pipe lines and open-channel spillway
constructed by the Ministry of Land, Infrastructure andTransportation
5. Landslide dynamics tests
To investigate the dynamic properties of twoearthquake-induced rapid landslides, dynamic (seismicand cyclic) loading ring shear tests were conducted by
DPRI-5 apparatus, one of five dynamic-loading
ring-shear apparatuses (DPRI-3,4,5,6,7) developed in
DPRI, Kyoto University (Sassa et al. 2003 and 2004).
The size of sample box in donuts shape is 120 mm inFig. 18 Normal stress and shear stress during the
Mid-Niigata earthquake working on the sliding surfaceof the Higashi-Takezawa landslide, which was
calculated from the monitored earthquake record at themonitoring site NIG019 in K-NET (NIED)－129－

is approximated as the one dimensional slope (20degrees and 40 m deep, and a half of the layer was
saturated, namely groundwater level was 20 m deep).
The total density of soil layer was assumed to be 1.8
Mg/m3 . Sample H2 was placed dry into the shear box
and saturated by firstly replacing pore air by CO2 gas
and then replacing CO2 by de-aired water. The porepressure parameter ( BD = Δu / Δσ ) was measured as

Fig.19 Undrained real earthquake wave loading test to
simulate the Higashi-Takezawa landslide (BD=0.98).
(a) Time series data, and (b) Stress path

Fig.20 Stress path obtained in the drained constant
speed test after the real wave loading test (Fig.19)
The real acceleration and its wave form that
affected the Higashi Takezawa landslide can not beknown in detail as it is complex due to inter alia
topography, geology, underground structure, anddistance from the fault and the epicenter. Therefore, themonitored earthquake record in NIG019 was applied as
the first approximation of input acceleration to thesliding surface. Using three components of acceleration
records, normal stress component and shear stresscomponent on the bottom of landslide were calculated
based on the section of the Higashi Takezawa landslide.
The test procedure is as follows. The sliding surface

the confirmation of full saturation (BD = 0.98). Then,the shear stress before the earthquake on the sliding
surface of 230kPa was loaded under drained conditions.
Thereafter, the shear box was shifted to the undrained
state by closing the drainage valve, and then thecalculated seismic stresses were applied. The data isshown in Fig.18. The result of seismic loading of thesample was presented in the time series monitoring
result of loaded normal stress, mobilized shear
resistance (not the same with the applied shear stressbecause failure occurred in the sliding surface at the
failure line), generated pore pressure and the resulting
shear displacement as seen in Fig.19a. Due to seismic
shaking, pore water pressure was generated, then shear
failure occurred and shear displacement started. As
shear displacement progressed, a typical sliding surfaceliquefaction phenomenon was produced (Sassa 1996,
Sassa 2000, Sassa et al 2004), namely grain crushingalong the shear zone proceeded and a higher porepressure was generated in progress with shear
displacement. The sand was originally a marine deposit
and much stronger than volcanic deposits such as
pyroclastic flow deposits and pumice, and also
weathered granitic sands. However, the shearing stressunder 40m overburden pressure is enough to cause
grain crushing and resulting volume reduction. It
caused sliding surface liquefaction. The mobilized
apparent friction angle is only 2.5 degrees at the steadystate. The stress path of the test is shown in Fig.19b (inwhich a red color stress path expresses the effective
stress path and a green color stress path presents thetotal stress path). The apparent friction angle isobtained from the ratio of mobilized steady state shear
resistance divided by initial normal stress. The effective
friction angle mobilized in the post failure process is
not so clear in Fig.19b. So after this test, pore pressure
was dissipated by opening the drainage valves. After
full dissipation, sample was sheared under the speed
control condition at 0.2 mm/sec. Normal stress was
reduced at the unloading rate of 0.5 kPa/sec after－130－reaching the peak shear resistance. The failure line
mobilized in the post failure process (residual state)gave 36.9 degrees for the friction angle (Fig.20).
In addition, a series of cyclic loading tests tocompare the dynamic properties of the sand H2 of theHigashi Takezawa landslide and the sand T1 of the
Terano landside, and the silt T2 of the Terano landslidewere conducted.

liquefaction as that shown in the real wave seismic
loading test. The mobilized apparent friction angle atthe steady state is 3.3 degrees, almost the same levelwith that shown in Fig.19b. The failure line estimatedfrom the effective stress path in the post failure stress
path is 36.9 degrees, see Fig.21b.
5.3 Cyclic loading tests for the sand (T1) and silt(T2) of the Terano landslide

5.2 Cyclic loading test for the Higashi Takezawa
sand (H2)

Fig.21 Undrained cyclic loading test of the Higashi
Takezawa sand (H2) (BD=0.98). (a) Time series data,and (b) Stress path
For sample H2, the sample preparation and test
condition were the same with the above mentioned test.
The main differences were the loading stress, frequency
and wave form. During the test, normal stress was kept
constant, because the effective normal stress practicallywould keep constant in the fully saturated undrained
condition when subjected to any change in the normal
stress. Shear stress with the wave form of sine curve at
1 Hz frequency was applied. To ensure the failure, theshear stress was increased step by step until 15 cycles.
The test result for the Higashi Takezawa sand ispresented in Fig.21.
The sample was subjected to the sliding surface

Fig.22 Undrained cyclic loading test on the Terano sand(T1) (BD=0.98). (a) Time series data, and (b) Stresspath
The slope angle of the ground surface of the Teranolandslide is 17.5 degrees at average from the toe of theslope at the Imokawa River (P3) to the top of head
scarp (P1) in Fig.9b. The slope condition for the cyclicloading test was approximated as a 17 degree slopewith the depth of landslide mass above the sliding
surface being 20 m. Around half of the landslide mass
was assumed to be saturated, namely the groundwatertable was assumed to be 10 m deep from the groundsurface.
During the test, normal stress was kept constant,and shear stress of sine curve of 1 Hz loading
frequency was applied. The shear stress was increased
step by step until 15 cycles. All test conditions were thesame for T1 sand and T2 silt.

－131－

The cyclic loading test result for the Terano sand(T1) is shown in Fig.22. The Terano sand was clearlysubject to sliding surface liquefaction after failure. The
mobilized apparent friction angle at the steady state is
3.8 degrees, slightly greater than H2 (2.5 degrees in Fig.
19b, and 3.3 degrees in Fig. 21b). The mobilized
effective friction angle that is estimated from the stress
path after the peak is 35.7 degrees, about one degree
less than the 36.9 degrees for the Higashi Takezawa
sand. This may be due to the fact that the Terano sand isslightly finer than the Higashi Takezawa sand as foundin Fig.10. However, the both sands are generally
similar.

(T2) is presented in Fig. 23. The dynamic properties of
the Terano silt are quite different to the sands. The
sliding surface liquefaction and its resulting rapid
landslide motion were not produced in this test. Sheardisplacement was increased only during the cyclicloading, precisely say, only in the time span when theloaded stress reached or exceeded the peak shear
strength. It was the repeated movements of stop andmove, and the shear movement stopped at the sametime with the final cycle of loading. High pore pressure
was probably not generated or at least not monitored as
seen in Fig.23a.
To obtain the effective friction angle in the residualstate or the post failure condition, the sample wassheared at a constant shear speed of 0.05 mm/sec byusing the shear-speed-controlled method, while thenormal stress from was increased from 50 kPa to morethan 250 kPa at a loading speed of 0.1 kPa/sec in thedrained condition. The residual friction angle was 23.9
degrees for the friction angle (Fig.24). The friction
angle was 13 degrees less than the Higashi Takezawa
sand, and around 12 degrees less than the Terano sand.
The silts and sands display quite different shear
characteristics.
6. Solution for the first question

Fig. 23 Undrained cyclic loading test on the Terano silt(T2) (BD=0.98) (a) Time series data, and (b) Stress path

Fig.24 The residual state of friction angle for Terano silt
(T2)
The results of cyclic loading test on the Terano silt

In the Higashi Takezawa landslide, a stiff silt(stone) layer outcropped in the head scarp. And a very
similar stiff silt (stone) layer outcropped in theDainichisan landslide (Sassa et al. 2005). Therefore,
whether the location of sliding surface is formed on theweathered part of silt or on the bottom of sand layer isof great importance for understanding the triggering
mechanisms of these landslides. The weathered part ofsilt can not be found, probably it is very thin. However,weathered silt (T2) was taken in the Terano landslide.
The effective friction angle is 12-13 degrees lessthan the sand. It means a landslide should occur in thesilt layer when rain or melting snow is the triggeringfactor of landslides. This area is known as a Tertiary
mud stone landslide area. Many landslides cover thisarea (Shimizu et al. 2004 or NIED Web page forlandslide map).
The area of Niigata Prefecture is known as a heavy
snow area and snow melt is the usual main source ofwater supply for the ground. Slow and limited shear
displacement is the common form of movement of－132－

landslides here. In the case of water supply due to snow
melting or long rain, it is certain that silt is much
weaker than the sand. Therefore, previous landslides
probably occurred at the sliding surface in the silt layer,
most likely in the weathered part of silt just below thesand layer. In contrast, the silt is strong against theearthquake, while sand is weak against earthquake
loading. Sand grains are much easy to crush andsusceptible for volume reduction. Therefore,
earthquake induced landslides should form its sliding
surface within sand. The dynamic loading ring shear
tests indicate that the most likely scenario to cause theHigashi Takezawa and the Terano landslides was one inwhich the sliding surface liquefaction phenomenon
occurred within the sand layer under a high overburdenpressure.
7. Landsliding simulation of Higashi Takezawa
landslide
A geotechnical landslide simulation model was
proposed by Sassa 1988. It was improved to acomputer code for general use during APERITIF
project especially in the data input andthree-dimensional presentation of output. This newcomputer code for rapid landslides (Rapid/LS) wasapplied to simulate the motion of theHigashi-Takezawa landslide. In addition to thesteady-state shear resistance which can be measured
with ring-shear apparatus, another parameter for
geotechnical simulation, the lateral pressure ratio K was
introduced to express softness or potential for lateral
spreading of the moving mass. According to theparameter, the lateral pressure acting on a soil column
is close to 1.0 if the material appears to be liquid.
Liquid spreads laterally without limitation on the flat
plane. In contrast, if the material is completely solid(like metal or a rock mass), the lateral pressure ratio K= 0. A hard rock can stand on the plane and neverspread laterally. Soils are in between. It will bereasonable to assume that the value inside a moving
soil mass will be between 0 and 1, though the lateralpressure ratio in the static soil mass can be more than
1.0 under the over consolidated state.
In the case of Higashi-Takezawa landslide, it was
assumed relatively lower (0.50–0.65) because thelandslide mass was normally less saturated. Shear
resistance of the soil sample taken from the sliding

zone of the landslide was employed in the simulation.
Figure 25 presents six steps of thethree-dimensional visual output of the simulation
results from the initial state to the final deposition state.
It shows that the sliding mass rushed into the river
rapidly, and horizontal flow occurred obviously
thereafter.
8. Conclusions
1.

2.

3.

4.

5.

㧙㧙

Two rapid major landslides triggered by the 2004
Mid-Niigata earthquake were investigated. Bothcreated landslide dam on the Imokawa River.
Both landslides were initiated within the landslide
mass of the previous landslides. The mobilized
apparent friction angles of both landslides were
7.1 degrees in the Higashi Takezawa landslideand 12.7 degrees in the Terano landslide.
Cyclic loading ring shear test on sands from theHigashi Takezawa landslide and the Teranolandslide proved that both sands can be liquefied
by the sliding surface liquefaction phenomenon
under 20-40 m over burden pressure. While silt
taken from the Terano landslide was proved thatthe silt was not subjected to high pore waterpressure generation by cyclic loading, and theshear displacement was only limited when theshear stress reached or exceeded the failure line,
the movement stopped at the end of cyclicloading.
On the contrary, the effective friction angle of silt
is 23.9 degrees, smaller than those of sands (36.9
degrees and 35.7 degrees). Therefore, landslide
movement due to snow melting or rainfalls
should have its sliding surface in the silt layer. It isestimated from these test results that previous
landslides in both areas probably slid in the silt
layer; however the rapid landslides triggered by
the 2004 Mid-Niigata earthquake formed theirsliding surface within the sand layer.
A dynamic loading ring shear test to simulate theHigashi Takezawa landslide using the monitored
seismic record in Ojiya (NIG019) on the sandcollected from the head scarp of the landslide
reproduced the initiation of rapid landslide
resulting from the sliding surface liquefaction.
The mobilized friction angle at the steady state
was only 2.5 degrees.

Fig. 25 Simulation results on the Higashi-Takezawa landslide

Acknowledgment
This research was conducted as a part of Theme 2:
Landslide Investigation of the 2004 Mid-Niigata
earthquake disaster investigation by MEXT (Ministryof Education, Culture, Sports, Science and Technology,
Japan) Special Coordinating Fund for Science andTechnology. The authors acknowledge its special

coordinating fund to conduct this investigation andthank all members of this investigation team fordiscussion. We are grateful for Dr. Gen Furuya andother post-doctoral students and graduate students of
the Research Centre on Landslides, Disaster Prevention
Research Institute of Kyoto University for cooperationin the field investigation and sampling.

㧙㧙References
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