10th Asian Regional Conference of IAEG (2015)

Fluidized Landsliding Phenomenon on Volcanic Ash Slope during Heavy Rainfall:
A Case Study of Izu Oshima Landslides
Gonghui WANG(1) and Yao JIANG(2)(1) Disaster Prevention Research Institute, Kyoto University, Japan
E-mail:wanggh@landslide.dpri.kyoto-u.ac.jp(2) Graduate School of Science, Kyoto University, Japan
Abstract
On Oct. 16, 2013, catastrophic shallow landslides were triggered on a wide area of
the west-side hill slopes in Izu-Oshima Island, Japan, by the heavy rainfall accompanying
Typhoon Wipha. The displaced landslide materials traveled long distance with rapid
movement, resulting in 36 dead, 3 missing, and 46 buildings being completely destroyed on
the downstream area of Motomachi area. To understand the initiation and movement
mechanisms of these shallow landslides, we took sample from the source areas andexamined the shear behavior of these samples under partially drained or undrained
condition. We also triggered landslides within soil layers made up of these samples by
rainfall in a flume, and examined the variation of soil moisture, pore-water pressure andlandslide movement with the introduction of rainwater. Test results showed that highpore-water pressure can be built up and maintained within the displaced landslide material,
and then elevate the landslide mobility.

Keywords: shallow landslides, heavy rainfall, mobility, liquefaction, tephra slope
1.

Introduction

fatalities and left 3 others missing, and destroyed
more than 40 buildings.

Typhoon Wipha started as a tropical storm on 9
October 2013 over the sea near the island of Guam,and strengthened on October 11, and later on October
12, 2013, it intensified into a typhoon. Typhoon
Wipha weakened as it skirted Japan’s eastern
coastline on October 15 as the ocean waters
surrounding the typhoon began to cool, and reachedextra-tropical status late on October 16, and finallydisappeared on the north-east of Pacific Ocean.
Typhoon Wipha dumped very heavy rain andwhipped up strong winds in many areas of East Japan.
The 24-hour amount of rainfall through 8:20 a.m. in
Oshima on Oct. 16 totaled 824 mm, setting up a newrecord. This daily precipitation is equivalent to morethan twice the amount that Oshima had in all of
October in an average year. The maximum hourly
rainfall reached 125 mm. Also a maximum windvelocity was recorded as 35 m/s. As a result of therainfall, several landslides were triggered in the thin
volcanic ash layer of hillside surface. Among them, amassive one from an area perched above theMotomachi district traveled approximately 2.2 km,
demolished a large area of the town, caused 36

Fig. 1 Landslides in Izu Oshima during Typhoon
Wipha, 2013 (after Geospatial Information Authorityof Japan, 2013). S: sampling point.

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10th Asian Regional Conference of IAEG (2015)

Fig. 3 Geological map of the landslide area (after Kawanabe (1998) and Geological Survey of Japan (2013)stronger gradually. During the next day as Wipha
moved towards the north-northwest it dumped very
heavy rain. In the town of Oshima, an hourly rainfall
total of 100 mm and a 24-hour total of 824 mm wererecorded (Fig. 2). As a result, several shallow
landslides over large areas were triggered in the thin
volcanic ash layer on the hillsides perched above thetown.
These landslides are located on the outer slope ofthe caldera wall that was formed during the eruptionof 1,300-1,500 year ago and featured by sharp slope.
In the landslide area a crater was formed due to thefissure eruption in the era of 1330 (Y5) (Fig. 3). Thelava of this eruption was covered by tephra (scoria
and ash) layers that came from 4 times of eruption(Y4-Y1) (Fig. 4). These tephra layers have athickness reaching several meters in maximum andshow stratifications nearly parallel to the slope
surfaces.
The landslides are mainly localized on two
catchments where the slopes are steeper than 20
degrees and the relief within a square of 1km×1km isabout 200-300m. The most upper part of the failed
slope originated on the elevation of 450 m. The
failures on the upper slope of the road were about
10-40 m wide, and widened to 50-100m on the slope
below the road, and finally combined as a whole witha width of about 250m on the elevation of 300-400m.
Our field investigation revealed that the thicknessof the displaced landslide material on the source area
was about 70-120 cm. The source area of the biggest
one involved a failure area of about 0.13 km2. The
outcropped sliding surfaces are loess, above which is
the tephra of Y2 formed during the 1684 volcanic
eruption. According to Terajima et al. (2015), thepermeability of this loess layer ranges approximatelyfrom 10-4 to 10-5 cm/s, while that of the overlaying

Owning to its high mobility and the characters of
its traveling path and deposits, we describe thelandslide at Motomachi as a debris avalanche,following the classification of landslide typepresented by Hungr et al. (2001) (hereinafter termed
as MDA). Understanding the initiation and movement
mechanisms of the MDA is of great importance to theprevention or mitigation of similar type of landslides
in volcanic deposits under extreme weather
conditions. To achieve this, we conducted detailed
field survey on those landslides in Izu Oshima Island
triggered by Typhoon Wipha, and took samples fromthe source area of MDA. We examined the undrainedor partially drained shear behavior of these samples insaturated state by ring shear tests. We also triggered aseries of landslides in an instrumented flume bysparkling water to examine the hydraulic condition
under which the slope instability was triggered, andalso to examine the post failure phenomena of thedisplaced materials.
2.

Landslides in Izu Oshima

Izu Oshima is an active volcano about 100 km
SSW of Tokyo and east of Izu Peninsula in the NW
Pacific Ocean. Many eruptive activities haveoccurred in the past. Most recently, a serious fissure
eruption occurred on 21 November 1986 and forced
all the residents of the island to evacuate for the firsttime. The outline of the island is oval shape elongatedin NNW-SSE direction with length about 15 km andwidth about 9 km. Mt. Miharayama is the highest
point of the island (764 m above the sea level).
Motomachi, as the main district on the island, islocated on the west side.
With the approach and passing of Typhoon Wipha,rainfall started on October 15 in Oshima and became210th Asian Regional Conference of IAEG (2015)

tephra is about 10-2~10-3 cm/s. As a possible
mechanism on the initiation of the shallow landslide,
we inferred that this permeability contrast enabled theformation of a perched saturated soil layer withincreasing water table, and finally resulted in the
instability of the tephra layers. We also inferred thatthe displaced materials started as slide and thentransformed into flow.

average with roots less than 70 cm below the groundsurface) might have suffered strong wind throw,which could enforce the instability of the shallow
slope. In addition, a huge number of trees moving
downhill together with the displaced materials could
greatly enforce the destructive power to the houses on
its traveling path. The driftwoods also blocked thedownstream bridges, causing floods and resulting infurther loss of life and property damage. It is notedthat during our field survey we monitored the groundvibration and wind velocity on the forest of hillside
slope aiming at understanding the possible effect ofwind throw on the instability. Although detailed
analyses of these monitored data are still in progress,
our field monitor by video showed that the wind
throw of trees may contribute greatly to the initiationof shallow landslide on the hillside.

Fig. 2 Rainfall in Izu-Oshima on 14-16 October 2014

3.

Methods

To examine the initiation and movement
mechanisms, we took samples from different soil
layers above the sliding surface on the source area.
Our sampling point has a soil layer of 90 cm thickabove the sliding surface (on the upper part of theloess layer). We took samples from the loess layer(Sample LL), from the tephra layer of coarse volcanic
sand immediately above the loess layer (Sample VS),and also from the tephra layers of silty-fine volcanic
sand locating 10 and 20 cm (Sample VL1 and VL2)
above sample VS, respectively. We also used thesamples (two tons) that were collected from thedeposited area in Motomachi town for flume tests.
A ring shear apparatus was used to examine theshear behavior of the samples taken from the sourcearea. This apparatus, which has a shear box with 120
mm inner diameter, 180 mm outer diameter, 115 mm
height, and a maximum shear velocity of 10 cm/s,
enables shearing at different types of loadings undereither drained or undrained conditions.
The specimens in ring shear tests were preparedby pouring oven-dried samples into the shear box inlayers and compacting the layers to different densities(Ishihara, 1993). All saturated test specimens weresaturated by CO2 and de-aired water. For undrained
shear tests, all specimens were consolidated under agiven normal stress and then sheared to residual stateusing a shear-speed-controlled method. In partiallydrained conditions, dissipation of generated
pore-water pressure from the shear zone was key tothe after-main-shock movement of the displaced
landslide mass. To examine the possible role ofdissipation of pore-water pressure, shear tests werealso performed under partially drained conditions on
a water-saturated sample and a M15-fluid-saturated
sample. Note that M15-fluid refers the viscosity of
the Metolose fluid is 15 times that of water. All thering shear tests are summarized in Tables 1-3.

Fig. 4 A standard columnar section of tephra andloess in Motomachi area (after Koyama andHayakawa, 1996)

Fig. 5 Wind velocities on 16 October 2014
Trees on the hillside might have also played an
important role in both triggering the slope instability
and elevating the destructive capacity of the displaced
materials. On October 16, wind velocity reachedabout 35 m/s (Fig. 5). Those trees (about 20 m high in3

10th Asian Regional Conference of IAEG (2015)

A flume (200 cm long, 25.5 cm wide and 50 cmhigh) with transparent sides was used to study the
infiltration process of rainwater through examining
the moisture variation within the volcanic ash layers,
and also to examine the post failure behavior of theshallow landslides due to the introduction of rainfall(Fig. 6). We glued the same soils as the sample to thesurface of the flume base to assure the same friction
between the particles of sample and the base of the
flume as of that of sample inside the flume.

As shown in Fig. 6, pore-water pressure (PWP)
and soil moisture were measured at 7 locations. In theplace immediately close to the location of PWP in the
middle of the three PWP transducers on the bottom of
the flume, a ball was laid so that it connected with alinear displacement transducer through a wire. During
test, the ball buried in the sample moves together withthe sample; in this manner, the time series of sliding
distance can be monitored. To ensure that the ballwould move together with the sample without relative
motion, a styrene foam ball 2 cm in diameter and 0.1g
in weight was adopted, and a 160-g counterweight
was attached to the other end of the wire to balance
the pulling resistance of the linear-displacementtransducer. Because of the resolution of this large
capacity (150 cm) linear displacement transducer,
displacements smaller than 1.5 mm cannot bemeasured correctly. Therefore, to get precise
measurement, a laser displacement sensor withresolution of 0.015 mm and capacity of 15 mm wasalso used by fixing a target on the wire and shining alaser beam upon the target. Above the flume two
spray-nozzles were placed, and a uniform artificial
rainfall was assured. Three video cameras were used
to monitor the entire test process from both sides andthe downstream front view of the flume.
In the flume tests, water was first added to theoven-dried samples to make the initial water content
rise up to 5 percent, and then the sand was stirred
evenly. After that, the sample was packed into theflume with an inclination of 30 degrees (see Fig. 6).
To make the sample uniform, while packing, thesample was placed in a series of layers of 2 cm
thickness parallel to the flume base, and then eachlayer was tamped. Finally, the superfluous parts ofplaced sample were removed and the shape was madeto be as shown in Fig. 6. Initial dry density wasdetermined from the oven-dried weight of the usedmass and the volume of the sample.

Fig. 6 Layout of the flume test
Table 1 Densities of tested samples for measuring thefriction angle
sampleVL1(Depth: 70-80 cm)
VS(Depth: 80-90 cm)
LL(Depth: 90-100 cm)

Density (g/cm3)

Normal stress
(kPa)
40
60
80
100
40
60
80
100
40
60
80
100

1.24
1.32
1.35
1.37
1.33
1.39
1.42
1.43
1.14
1.23
1.27
1.30

4.

Results and discussion

Table 2: Conditions of undrained tests
sampleLL(Depth: 90-100 cm)
VS(Depth: 80-90 cm)

Normal stress
(kPa)
60
60

Density (g/cm3)
1.19
1.33

Table 3: Conditions of partially drained tests
sampleLL(Depth: 90-100 cm)
VL1(Depth: 80-90 cm)
VS(saturated by M15)
(Depth: 80-90 cm)

Normal stress
(kPa)
60

Density (g/cm3)

60

1.36

60

1.44

1.21

Fig. 7. Grain size distribution
The grain size distributions of these samples arepresented in Fig. 7, where it is noticed that LL has
finest grains, VS is the coarsest, while VL1 and VL2locate between them VS and LL. The SEM photos

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10th Asian Regional Conference of IAEG (2015)

revealed that LL shows a loose structure with fines
attaching on the surface of coarse grains (Fig. 8a),while VS has fewer fines on the surface of coarse
grains, but has big air holes in the coarse grains (Fig.
8b), which probably had been formed due to thebust-out of gas and steam during the eruption.
Undrained ring shear tests were performed on all
the four samples, and results showed liquefaction
phenomenon can be triggered in all the tests. Fig. 9
gives an example of the test on saturated VL1. The
sample was consolidated under a normal stress of 60
kPa and shear stress of 38 (corresponding to a slope
angle of 32 degrees), and then was sheared under
undrained condition by increasing the shear stress totrigger failure. It is noticed that after failure, themeasured shear stress presents the shear resistance ofthe soil. As seen in Fig. 9, with increasing of shear
stress, pore water pressure (PWP) increased. After
failure, the PWP showed a sharp increase, and shear
resistance lowered to a very small value (almost zero),and the PWP reached almost the same value of
normal stress, i.e., full liquefaction was initiated.

Fig. 10 Shear test on M15-saturated volcanic deposit
under partially drained condition (initial density: 1.44
g/cm3)

Fig. 11 Pore-water pressure (PWP-3) anddisplacement on the bottom and PWP within sliding
mass after failure in flume test
The results of partially drained shear tests on the
M15-fluid-saturated sample are presented in Fig. 10
in the form of time series data. As shown, less PWP
was built up before the shear failure occurred.
However, after the shear failure PWP increased
continuously with progress of shear displacement,
and increased to a high value, and the shear resistancelowered to a small value (about 10 kPa) at the time of
20 second, thereafter remained approximately as aconstant. The continued increasing in monitored PWP
might result from the fact that shearing is localized onwithin the shear zone where high PWP might be built
up, however, the transducers to measure the PWP
were located outside of the shear zone. Thisdifference in location would result in the delay in themeasured data. From Fig. 10, we infer that thelandsliding with long travel distance could betriggered if the saturated soil layer above the sliding
surface is thick enough such that the dissipation ofgenerated excess pore-water pressure from the shear
zone can be retarded.
Retrogressive failure phenomena occurred in therainfall-triggered landslides in flume. Although there
are some small differences in each test due to thedifference in initial density of the soil layer, these
retrogressive failures were characterized by slow
deformation with small displacement, and thenFig. 8 SEM photos of sample LL and VS

Fig. 9 Undrained shear test on saturated volcanic
deposit5

10th Asian Regional Conference of IAEG (2015)

followed by quick downslope movement. Fig. 11
gives an example of monitored water pressures within
the soil layer and displacement along the base of thesoil layer. It is seen that small displacement occurredat the time of around 2000 second, there was a small
increase in the water pressure. The main failure
occurred at the rainfall duration of around 2700s.
During the main failure, water pressure showed quick
increase with fast movement. However, a detailed
examine on the PWP transducer P5 revealed that P5
showed a temporal reduce in PWP and then followed
by increase of PWP. This might result from the factthat during the failure moment, the upper soil layer
suffered tension deformation due to the retrogressive
failure, while the upper soil layer was still in
unsaturated state. This was verified by the monitored
moistures at different soil depth. Fig. 12 presents thevariation of soil moistures of soil layers at differing
locations. After 60 minutes of rainfall, the moisturesat differing locations increased with further
continuing of rainfall, but had different response. M3
at the bottom of the soil layer on the toe part had thegreatest value. This indicates that the soil layers onother areas were not in the fully saturated state. After
failure, compression might have occurred within thesoil layer, resulting further increase in the moisture
content and generation of higher PWP. These test
results as shown in Figs. 11-12 show that due to theinfiltration of rainwater, ground water level above thesliding surface will be elevated such that failure canbe initiated within the soil layer, and at the movementof failure, the upper soil layers are not fully saturated.
Nevertheless, the displaced materials can suffer frommovement with high mobility.

(2) The meteorological data and the forest state on
the slope suggest that strong wind might also
have played an important role in triggering the
instability of hillside in large area.
(3) High pore-water pressure can be built up after
the occurrence of slope failure, and thenreduce the shear resistance, even if the shallow
soil layer were under partially drained
condition.
(4) The displaced landslide materials can sufferfrom rapid movement even if surficial soil
layers were at unsaturated state. Once shear
failure occurred, high pore-water pressure canbe built up and maintained with delayed
dissipation.
Acknowledgements
This work was supported by MEXT KAKENHI(Grant Number: 25900002). Discussions and help in
the field trips given by many group members
participating in this KAKENHI are greatly
appreciated.
References
Geospatial Information Authority of Japan (2013):
Public release of aerial photographs showing
disaster areas damaged from a landslide caused byheavy rain due to typhoon No.26, 2013. In:
http://www.gsi.go.jp/kokusaikoryu/kokusaikoryu-e30023.html
Geological Survey of Japan (2013): The geologicalbackground of the shallow landslides triggered by
the November 16, 2013 typhoon Wipha on the west
part of Izu Oshima. In: https://www.gsj.jp/hazards/
landslide/131016oshima.html (accessed on May 15,
2015)
Hungr, O., Evans, S.G, Bovis, M. and Hutchinson,
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Ishihara, K. (1993): Liquefaction and flow failure
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volcano. Geological map of volcanoes, Geological
Survey of Japan, 1998 (in Japanese with English
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Koyama, M. and Hayakawa, Y. (1996): Syn- andpost-caldera eruptive history of Izu Oshima volcano
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Terajima, T., Matsushi, Y., and Hattanji, T. (2014):
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Fig. 12 Water content of soil layers at different
locations5.

Conclusions

The principal findings could be summarized asfollows:(1) The Motomachi DA occurred along theboundary between the loess layer andoverlaying tephra (Y2.0) that were formed
during the volcanic eruption in 1684. The
lower permeability of loess layer could lead tothe formation of a peached saturated zone
above it, and then result in the instability ofthis hillside.

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