こうえいフォーラム第 12 号 / 2004. 1

Landslides on an ancient Emperor's tomb mound induced by anearthquake in the 16th century
16世紀の地震によって発生した継体天皇陵前方後円墳（今城塚古墳）に認められる地すべりについて

守随治雄*・釜井俊孝**
Haruo SHUZUI and Toshitaka KAMAI

A landslide that occurred on an Emperor's tomb mound is described and discussed from an engineering
geology viewpoint. The remnant horizontal 'fish-scale mosaic' structure of soil blocks that was originallyformed may be a valid indicator of the deformation. The main part of the landslide moved a distance ofbetween 2 and 5 m along a slip surface that was inclined 1 to 3 degrees against the dip of the mound. The toe
fell into an adjacent moat as a slump and became a rapid earth flow that filled the moat. The 'KeichoFushimi' earthquake of 5 September 1596 is considered to have triggered the landslides which deformed the
Imashiro-zuka Emperor's tomb mound. The cumulative permanent displacement was estimated byNewmark's method. The estimated permanent displacement of approximately 200 cm is the lower limit of thegeological cross-section observed. This is a good approximation given that the landslide occurred four
hundred years ago and considering the limitations of information based on deformations, swelling of clay
minerals, smectite, groundwater, and drainage measures.
Key Words： landslide, earthquake, archaeological research, tomb mound, kofun

1. INTRODUCTION'Kofun', keyhole-shaped tomb mounds, are famous monumentsof ancient East Asia, found mainly in Korea and Japan. They are
impressive large-scale fill structures, and were constructed only
during the 300 years between the 3rd century and the 6th century
AD. These kinds of tomb mounds were typical of ancient powerfulclans or Emperors who were at the top of the hierarchy of ancientsociety. The Imashiro-zuka mound is one of the largest tomb

1996, the first detailed research on an Emperor's tomb mound,and remarkable achievements have been made. In the autumn of2000, it was discovered that a landslide had drastically deformed
the tomb mound (Shuzui et al., 2000).
In this paper, a geological description of landslides on ancient
tomb mounds (Kofun) is presented, and a simple model of thelandslide on the Imashiro-zuka mound, one of the most important
historical heritage sites in Japan, is discussed.

mounds of the 6th century in the northern Osaka region. Based onarchaeological research, it is considered to be the tumulus of theGreat Emperor Keitai who died in 531 AD (Miyazaki, 2000).

2. STRUCTURE OF THE EMPEROR'S TOMBMOUNDThe Imperial Household Agency of the Japanese Government

The Imashiro-zuka mound was built on an alluvial plain in

has selected and listed Emperors' tumuli from among many tomb

Takatsuki City, in the northern part of the Osaka region. The

mounds that were located in the ancient capitals of Japan. The

capital of the Yamato dynasty (ancient Japan) was located close

government intends to preserve Emperors' tumuli from any

to the Imashiro-zuka mound in the first half of the 6th century.

excavation, and does not give permission for research on

One of the most active faults in Japan, the 'Arima-Takatsuki

Emperors' tumuli. The Imashiro-zuka mound has fortunately

tectonic line', runs beneath the Imashiro-zuka mound, which

failed to be listed by the Imperial Household Agency as anmeans that the Imashiro-zuka mound is one of the largest fill

Emperor's tumulus because of its deformed shape. Instead of thestructures located in the region of the highest seismic activity in

Imashiro-zuka mound, a mound of better shape was officially

the world.

appointed as the Emperor Keitai's tumulus.
Archaeological research on the Imashiro-zuka mound started in

The plan view of the Imashiro-zuka mound is keyhole shaped.
This is the most popular and original shape of ancient tomb
mounds in Japan, a combination of a rectangular shape at the***

大阪支店 技術部
京都大学防災研究所 地盤災害研究部門

front and a round shape at the back of the mound. The tomb, 186m long and 10 to 15 m high, is surrounded by a double water19

Landslides on an ancient Emperor's tomb mound induced by an earthquake in the 16th century

filled moat 1 to 4 m deep. The mound sustained an artificial
landform change by the Load Nobunaga in the 16th century. He
constructed a fortress for his corps on the mound.

This is in the opposite direction to the landslide movement.
There are two horizons of slip surfaces in the main part of thelandslide. The upper slip surface has a horizontal shear zone of 1

Horizontally layered small soil blocks of 40 cm width and 10

to 3 cm thickness in which soil blocks are thinly elongated. The

cm thickness built up the fill of the mound. The remains of 'scaly

trace of the upper slip surface continues for about 15 m along themosaic' (fish skin) structures corresponding to each soil block arebase of the mound, and is limited in the main body of theobserved almost horizontally in the non-disturbed part of thelandslide. The lower slip surface is a smooth shear surface cutting

mound. These soil blocks, produced from mixtures of silt, clay andalluvial soft blue-coloured clay, and lies about 10 cm beneath thecharcoal, were thoroughly consolidated by being rammed hard,

upper slip surface. The continuity of the lower slip surface is better

and were even difficult to excavate by bulldozer. The wet density

than that of the upper slip surface. It can be considered that theof soil blocks ranges from 2.02 to 2.06 g/cm3, and the range ofshaking of the earthquake initially formed the upper slip surface,void ratio is from 0.514 to 0.561 (Shuzui et al., 2000). Fourand the landslide body was replaced along the lower slip surface.

trenches were dug on the mound for the archaeological survey.

Based on both the elongate structure of soil in the shear zone and

The trench investigation revealed that the base of the moundthe blue-coloured alluvial clay of the original ground beneath theinclined slightly to the central part of the mound. It appears thatlower slip surface, the ground water table at the landslide movement

the inclination of the base is caused by ground subsidence in

can be considered to be higher than the upper slip surface.

alluvial clayey deposits under the excess load of the mound.

3）Deformation of main partThe original scaly mosaic structure is maintained almost

3. LANDSLIDE

horizontally in the main part of the landslide mass. Coherent（1）DISTRIBUTION

landslide movement along the almost-horizontal slip surface canFive landslide blocks developed on the slope of the tomb
mound. A trench related to the archaeological survey of 2000
traversed one of these (block # 2) longitudinally. A geologicalcross-section of # 2 landslide mass with a length of 60 m and awidth of 50 m was clearly observed along this trench. The foot of
the landslide, the accumulation region, spread across the inner
water-filled moat from the edge of the mound slope. It has alimited vertical displacement of 1 to 2 m, because of the nonrotational movement of the landslide. The head scarp developedin obscurity. The depth of the slip surface is between 2 and 6 m.

be recognized based on the slight deformation in this part. In thehead (upper part) of the landslide, the scaly mosaic structure of
the filling material is inclined steeply towards the central part ofthe mound. The structure dips at up to 50-70 degrees. It isconsidered that the upper part of the head of the coherent
landslide body fell with a rotational movement (toppling) into theopen space that was formed by the landslide movement. The
opposite inclination of the slip surface was convenient for thatmovement (toppling). The toppling of the upper part of the headoccurred immediately after the initial landslide movement.
Several minor gravitational faults developed in the lower part（2）STRUCTUREof the main body of the landslide. These faults have a net slip
(vertical dislocation) of up to 4 cm, cut the upper slip surface, and

1）The 'scaly mosaic' structure of soil blocks
The tomb mound is composed of an accumulation of wellconsolidated small soil blocks that are stacked together to form
the 'scaly mosaic' (fish skin) structure' in almost horizontal layers.
Thus, the deformation of the horizontal units of the scaly mosaic
structure is a valid indicator of the deformation caused by thelandslide and the artificial disturbance of the mound. Figure 5 is a
sketch showing the geology along a lateral wall of the trench.
2）Slip surface
A slip surface developed mainly along the boundary between
the original ground surface and the fill of the tomb mound. Thebase of the tomb mound dips slightly to the centre (the core) fromthe rim of the mound, therefore the slip surface in the upper partof the landslide is inclined at 1 to 3 degrees towards the centre.

20

continue to the lower slip surface. This means that thesegravitational minor faults are younger than the initial landslide
movement, which formed the opposite inclined upper slip surface.
The gravitational faults are concentrated in the part that was theshoulder of the bank of the inner moat existing before thelandslide event.
4）The secondary slide and its associated earth flow
The inner moat was filled by the secondary slide and its
associated earth flow deposits. The secondary slide was separated
from the main landslide on the bank of the inner moat that existed
before the landslide, and fell into the moat along its inner flank.
The structure of the soil blocks indicates that the head of thesecondary slide toppled forward, and the lower half of the slide
moved as a rotational slump that fell into the water-filled moat.

こうえいフォーラム第 12 号 / 2004. 1

The foot of the secondary slide flowed into the ditch, which has

can use øcu = 0°for soft saturated clay that should be sheared ina width of 5 m and a depth of 1.2 m. This ditch was artificiallyan undrained condition during shaking due to the earthquake. In

made at the moat floor in the Middle Ages (13th to 16th century),

this case, the factor of safety (Fs) can be expressed as

and had been filled with thick muddy deposits. The deposits were（2）

eroded by the foot of the slide, and rolled up vigorously in water.
The underwater turbidity flow consisted of the sliding blocks andmuddy deposits pouring out rapidly from the ditch, and rollingwhere ccu = 42 kN/m2 (1.2 times the static cohesion) can be

horizontally across the inner moat. The flow reached to theused as dynamic shear strength for the stability analysis. The

opposite side of the moat, and its tip bent upward slightly along

static shear strength (cohesion) was estimated from thethe flank to terminate.

relationship between N values and unconfined compression
strength (qu) of the alluvial clay (Japan Geotechnical Society,（3）MECHANISM

1995). A critical acceleration αc = 0.46 g corresponding to the1）Seismic deformation of main part of the landslide

critical failure condition of the slope (Fs = 1.0) was calculated forNewmark's method

the main part of the Imashiro-zuka landslide, which has an almost

Two unique features characterized the landslide of the Imashiro-

horizontal slip surface (θ= 0).

zuka mound: the opposite-inclined slip surface and the slide which

Earthquake records

changed to a rapid earth flow. Based upon Newmark's method, we

The most difficult aspect of conducting a Newmark analysis iswould like to explain the ground displacement caused by landslide

selecting an input ground motion. Selecting a time history

motion. Newmark (1965) proposed modelling a slope subjected torequires some knowledge of the shaking characteristics.

earthquake-induced accelerations as a friction block resting on a

Archaeological evidence shows that the inner moat was filled

slip surface subjected to the same accelerations as the modelled

between the 13th and 16th centuries. In the northern Osakaslope. Therefore, in each instance when the sum of the static andregion, the only earthquake during this period that had an

the dynamic forces exceeds the shear resistance of the slip surface,intensity of over 0.46 g was the 'Keicho-Fushimi' earthquake of 5

the slope will displace. The sliding-block analogy assumes that the

September 1596 (Usami, 1996). The Keicho-Fushimi earthquake

slope consists of rigid and perfectly plastic materials. The material

resulted from the tectonic movements of the Rokko and theof the main body of the Imashiro-zuka landslide is composed of

Arima-Takatuki tectonic lines, and was similar to the Kobe

well-consolidated and rigid soils that are not very common as

earthquake of 1995 (Sangawa, 2000).

filling material. Therefore, each assumption is capable of being

The Japan Meteorological Agency's (JMA's) Kobe observatory

accepted in the case of permanent displacement analysis on thehas almost the same geological conditions as the site of Imashiro-

main part of the Imashiro-zuka landslide.

zuka that was very close to the hypocentre of the Keicho-Fushimi

The critical acceleration

earthquake. The strong movement characteristics at the Imashiro-

Newmark originally employed an energy-based method forzuka site, maximum velocity and spectrum etc., might be similarcalculating cumulative slope displacement. A less numericallyto the strong movement record collected at the JMA observatory

cumbersome method was developed by Wilson and Keefer(Kobe NS) located in the high-shaking-intensity zone of the 1995

(1983), where those parts of an earthquake accelerogram that

Kobe earthquake. Therefore, the earthquake accelerogram record

exceed the critical acceleration (αc) of a slope are doubleof the Kobe earthquake might be best for the estimation of

integrated. The critical acceleration (αc) should be estimated bylandslide displacement using Newmark's method.

back analysis of static slope stability. The factor of safety during

Permanent displacement
Once the critical acceleration of the Imashiro-zuka landslide

an earthquake is calculated by the following equations.

has been determined and acceleration-time histories have been（1）

selected, permanent displacement can be calculated by doubly
integrating those parts of the strong-motion record that lie above

These are based on a simple relationship between the force

the critical acceleration (Wilson and Keefer, 1983). Figure 6

balances just at the time that the landslide collapsed as a result ofshows the critical acceleration superimposed on the earthquake

the earthquake. As shown in Figures 3 and 4, the main slip surface

accelerogram (JMA Kobe NS) of the 1995 Kobe earthquake, thedeveloped in alluvial blue-coloured (iron-hydroxide-coloured) clay

integrated velocity-time history, and cumulative displacement ofthat was usually beneath the ground water table. In this case, we

the sliding block.
21

Landslides on an ancient Emperor's tomb mound induced by an earthquake in the 16th century

This double integration process was conducted on thefollowing assumptions:fell into the mud-filled ditch as a rotational slump. This slump
changed to a rapid flow slide, and filled the inner moat. Thei ）The mobilized ccu after the displacement reached 100 cm wasshoulder of the bank of the moat became a region of tension, andreduced to 90% of static cohesion, taking into considerationa minor gravitational fault system formed in this part. The space

the disturbance resulting from the large displacement.

formed by the landslide, between the head and the main scarp,

ii ）The critical acceleration of 0.35 g was used after 7 sec in thestrong-motion time series, corresponding to changes

was filled by the 'backward toppling' of the upper part of the headof the landslide.

mobilized ccu by the large displacement of 100 cm.
iii）The block was able to move to the both directions, to thedownward of slope and to the upward.
iv）The landslide velocity will not force to be zero during the
movement, if the acceleration will be below the critical

4. CONCLUSIONSThis study has described and discussed the landslide in an
Emperor's tomb mound from an engineering geology viewpoint.
The following remarks are concluded.

acceleration (αc). The integration of acceleration will bei ）It was possible to follow the landslide activity by using thecontinuing until the calculated velocity equal to be zero,deformation of the scaly mosaic structure of soil blocks that

corresponding to the rapid increase of pore water pressure

were horizontally layered before landsliding occurred.

and the effect of momentum of inertia during the landslideii ）The main part of the landslide moved a distance of between

movement.

2 and 5 m along the opposite inclined slip surface. A rapid

The calculation shows that the cumulative permanent

flow slide filling the inner moat derived from the toe that fell

displacement would have reached about 200 cm after the main

into the inner moat as a slump. The rear part of the head ofshaking. Geological evidence shows that the main part of thethe main landslide toppled backward, and filled the space

Imashiro-zuka landslide moved a distance of between 2 and 5 m, asthat was formed by the landslide between the head and theshown in Fig. 5. The estimation by Newmark analysis agrees withmain scarp.

the lower limit of the cross-sectional geological observation, whichiii）The cumulative permanent displacement was estimated bywill be a sufficient approximation given by the insufficient

Newmark's method. This would have reached about 200 cm

information on a landslide which originated four hundreds years ago

after the main tremors. Geological evidence shows that the2）Process of sliding

main part of the Imashiro-zuka landslide moved a distance

The external force necessary to produce a landslide that had anof between 2 and 5 m. The estimation by Newmark analysis

opposite-inclined slip surface would be so great that a strong

agrees with the lower limit of the geological observation,

earthquake tremor is considered to have triggered the landslide. It

and will be a sufficient approximation given by theis possible to follow the process of landslide activity by using theinsufficient information on a landslide which originated four

deformation of the original scaly mosaic structure of soil blocks

hundreds years ago.

as the tracer. Over a short period of time, the three stages ofiv）The 'Keicho-Fushimi' earthquake of 5 September 1596 waslandslide activity advanced as follows.

the major earthquake occurring in the northern Osaka regioni）Separation from the tomb moundafter the 13th century. This earthquake is considered to have

Vertical open cracks were formed at the foot of the mound by

triggered the landslides which deformed the Imashiro-zuka

strong earthquake tremors. Some blocks were separated from theEmperor's tomb mound. The Imashiro-zuka mound is nottomb mound by deep vertical cracks. Five of the separated blocks

only a site of ancient historical heritage as an Emperor's

developed to landslides after the following process.

tomb mound, but also a monument to a major earthquakeii）Landslide movement along the reverse-sloping slip surface

that seriously affected social and economic activities in

A shear surface was formed by out-of-phase tremor motion

16th-century Japan.

along the boundary between the original ground and the tomb
mound. This shear surface became the slip surface of the

Acknowledgements

landslide. The landslide moved a distance of between 2 and 5 m

Dr. Akira Sangawa of the Geological Survey of Japan, Mr.

along the opposite-inclined slip surface. A smooth slip surface

Yasuo Miyazaki of the Cultural Heritage Division of Takatsuki

was formed by this large displacement.

City, and Prof. Hiroshi Fukuoka brought the landslide case to theiii）Movements after the initial sliding

authors' attention and provided valuable information.

The lower part of the landslide that stuck out on the inner moat22

こうえいフォーラム第 12 号 / 2004. 1

References
1） Japan Geotechnical Society. 1995. Geotechnical exploration manual.
Tokyo, Japan Geotechnical Society.
2） Miyazaki, Y. 2000. The 4th archaeological study on the 'Imashirozuka' tomb mound, Explanatory text for site tour. Takatsuki, Cultural
Heritage Division of Takatsuki City. [in Japanese]3） Newmark, N. M. 1965. Effects of earthquake on dams andembankments, Geotechnique, 15, pp. 139-160.
4） Okada, A., Uemura, Y., Togou, M., Nakata, T. and Watanabe, M.
1996. 1:25000 active fault map in urban area 'KYOTOSEINANBU'.
Tsukuba, Geographical Survey of Japan.
5） Sangawa, A. 2000. Records of ground disaster on archaeological sites.
Disaster Prevention Research Institute Symposium. [in Japanese]6） Shuzui, H., Kamai, T. and Fukuoka, H. 2000. Causes and distributionof landslides on the tomb mound of the Emperor "Keitai". Disaster
Prevention Research Institute Symposium. [in Japanese]
7） Usami, T. 1996. Materials for comprehensive list of destructive
earthquakes in Japan. Tokyo, University of Tokyo Press.
8） Wilson, R. C. and Keefer, D. K. 1983. Dynamic analysis of a slope
failure from the August 1979 Coyote Lake, California, earthquake,
Bulletin of the Seismological Society of America, 73, pp. 863-877.

Fig.1．Location and geological setting of the Imashiro-zukamound（geological map was after Okada et al.,1996）.
Fig.3．Close-up of the slip surfaces and the remaining
scaly mosaic'（fish skin）structure of soil blocks.
F: Material filling the mound; B: Black（organic）soil,
pre-6th century;
A: Alluvial clay（blue）

Fig.2．Plan view and landslide distribution in the Imashirozuka mound.

Fig.4．The landslide observed along the trench in 2000.
The landslide mass（secondary slide）fell into theinner moat. The base of the mound（the slip surfaceof the main block）is inclined at 1-3 degrees towards
the centre of the mound as a result of the groundsubsidence in alluvial clayey deposits under theexcess load of the mound.（Photo by Dr. Sangawa）
23

Landslides on an ancient Emperor's tomb mound induced by an earthquake in the 16th century

Fig.5.

Cross-section of the landslide along the trench in 2000.

（A）

（B）

Fig.7．Process of collapse within the landslide.

（C）

Fig.6．Results of the Newmark analysis.
（A）Earthquake acceleration-time history withcritical acceleration（Kobe earthquake NS）. The
critical acceleration was reduced after 7 sec inthe strong-motion time series corresponding to
the large displacement of 100 cm.
（B）Velocity of landslide block vs. time（C）Displacement of landslide block vs. time24