こうえいフォーラム第１０号／２００２. １

LANDSLIDE RISK AT INCA'S WORLD HERITAGE IN MACHU PICCHU, PERU
世界遺産マチュピチュにおける地すべり危険度評価

佐々 恭二*・福岡 浩*・釜井 俊孝*・守随 治雄**
Kyoji SASSA, Hiroshi FUKUOKA, Toshitaka KAMAI and Haruo SHUZUI

ユネスコ世界遺産の中で最も顕著な、ペルー国、マチュピチュのインカ遺跡において、ペルー国文化庁、自然資源庁、地球物理研究所地の協力を得て、予備調査として2000年3月に綿密な地質・地形踏査と空中写真判読を実施した。その結果に基づき簡易型伸縮計10台と自記式伸縮計 2 台を2000年11月に設置し予備観測を開始した。世界遺産マチュピチュは、大規模な岩盤崩壊地形の真上に位置し、クラックや小崩壊などの危険な兆候を示していることがわかってきた。
本論文は、地すべり危険度軽減と文化・自然遺産保全における、UNESCO/IGCPシンポジウム（2001年、 1 月15日〜19日）の国際会議にて発表したものである。
Key Words : Machu Picchu, World Heritage, Landslide, Monitoring, Risk-Mitigation

１．INTRODUCTION
The Inca Citadel in Machu Picchu, at northwest of Cusco,
Peru, (Fig.1) was declared a World Heritage of Humanity in

Cultural Heritage at Risk at UNESCO in September 1999, his
report on the subproject attracted considerable interest fromparticipants.

terms of both cultural and natural property by UNESCO in 1983.
It is also an ecological sanctuary because of its ecologicalrichness. The present style of citadel was probably built by theIncas in the 15th century. It remained untouched through thecolonial period because of its isolated location on the top of asteep mountain. Machu Picchu became known to the world after
its "scientific discovery" by Prof. Hiram Bingham in July 1911.
As time passed, it seemed that Machu Picchu citadel possibly
has been affected by landslides. Ten years ago, the director of
the Geophysical Institute of Peru (Instituto Geofisico del Peru =IGP) visited the Disaster Prevention Research Institute, Kyoto
University and requested an investigation of the area. And in

Fig. 1 Location and view of Machu Picchu Inca Citadel, Peru

1997, R. Carreno and C. Bonnard published a paper "Rock slide
at Machu Picchu, Peru." Carreno proposed a study of this slide

Sassa, Fukuoka, Shuzui, and others investigated this area andas a sub-project of IGCP-425 in 1998, and it was approved.

collected information in Lima and Cusco from 14 to 20 March

During the IGCP-425 meeting and UNESCO Conference on

2000 to evaluate the possibility of landslide risk and to make a
research plan. From 9 to 16 November 2000, Fukuoka, Kamai＊
京都大学防災研究所 地盤災害研究部門＊＊ 大阪支店 技術第二部

and others installed 12 sets of extensometers in Machu Picchu
citadel for preliminary monitoring of active / potential landslide37

Landslide Risk at Inca's World Heritage in Machu Picchu, Peru

blocks. The investigation was in cooperation with the Peruvian

Fig. 2. A line of cracks and deformation of structure (Principal

INC (Instituto National de Cultura), INRENA (Instituto National

Temple) was found in other side. Therefore, the block thatde Recursos Naturales), and IGP.

includes Intiwatana is expressed as a potential landslide block(No. 5) in Figs. 4, 6 and 11.

２．GENERAL VIEW

The center of the citadel seems to be slightly concave (i.e.,

Fig. 1 (photo) presents the Inca Citadel at Machu Picchu.

nearly flat). This flat area was allegedly used for festivals and

The citadel was constructed on top of a mountain. The east

games by the Inca people. Its topography is similar to that ofslope (at the right side of the photo) is relatively gentle, and the

double ridges (two mountain peaks) as marked in Fig.1. The

access road (H. Bingham Road) and sightseeing hotel are

double ridges are known as a feature of topography that isconstructed in this side of the slope. We call this side of slope as

formed as the result of creep movement of a mountain, including

Front Slope. As reported in Carreno and Bonnard (1997), thethe ridge. However, a detailed examination will be necessary to

Front Slope is the dip slope, in which the joints of the rockjudge whether this flat or concave area is the result of slope

mass are aligned almost parallel to the slope. In this slope, wemovement or not, and it can be considered to be a precursor

found a "zone of broken stone structure," as indicated in Fig.1.

slope deformation to a possible catastrophic landslide, or not.

The stone structures seemed not so strong type there, but still thezone of broken structures is likely to be a shear zone of apotential landslide in the precursor stage.

３．LANDSLIDE DISTRIBUTION AT MACHU PICCHU
Fig. 3 is a stereo pair airphoto of the entire Machu Picchu

The other side (west side) of the mountain is very steep, andarea. The access road (H. Bingham Road) climbing up to thehas no access road from the Urubamba river to the Inca Citadel.

Inca Citadel is easily visible. In addition, the area shows theWe call this slope the Back Slope (Fig. 2). The Back Slope isform of a large-scale landslide. The Urubamba River makes ana cross-dip slope, and has a high resistance to shear. As a result,almost full turn around this area. The influence of the fault

the slope angle is very steep. However, there are many large

system is very strong. Mount Putukusi on the opposite side ofand small pre-existing landslides. The landslide marked by an

H. Bingham Road may seem to be the displaced landslide massarrow in Fig. 2 is large and relatively young. The white trace inin this photo, but it is at almost the same elevation as the citadel.

the opposite bank is the deposit of a recent landslide. Most

The base of the mountain seems to be intact, although its surface

landslides on the Back Slope are rock falls or topples.

has been modified by rock slides. This photo has been publishedpreviously in Landslide News No.10 (Carreno and Bonnard1997).

Fig. 2 Inca Citadel and the Back Slope.
(Taken from a chartered helicopter by K. Sassa on 19 March 2000)

Fig. 3 Airphoto of Machu Picchu.
(By the Geographical Institute, Peru on 28 July 1963)

We found the trace of a very recent small debris flow on the

The airphoto of Fig. 3 was used for interpretation, in addition

back slope of Intiwatana ridge (symbolic, most popular and theto other airphotos and visual observations from a charteredhighest part of the Citadel; see Fig. 1). However, the debris

helicopter. The landslide distribution, including active, inactive,

flow trace is not visible in the photo of Fig. 2 because it isand potential landslides in the precursor stage were delineated.

behind a small ridge. The location is marked "debris flow" inThis landslide distribution map provides kind of working

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こうえいフォーラム第１０号／２００２. １

Fig. 4 Landslides and potential landslides around the Machu Picchu Citadel.

hypothesis for the planning of future geological borings andthe Urubamba River was one step higher (at the elevation of thegeophysical monitoring. Locations of existing landslides andpresent surfaces of Blocks No. 2 and 3), as shown in Fig. 5.

potential landslides around in the vicinity of the Machu Picchu

The bedding of the rock is almost parallel to the Front Slope.

Citadel are presented in Fig. 4. The Macupicchu citadel area isexpressed as dark shaded part. Recently active landslides(Blocks No. 1, 4, 6, 7, the smaller sub-blocks of No. 2 and theupper half of Block No. 3), landslide debris (along the river),potential landslides (Blocks No. 2 and 5) and inactive landslides(the lower half of Block No. 3) are shown in Fig. 4.
Fig. 5 shows the Front Slope and the side view of major
landslide blocks No. 1, 2, and 3. All blocks moved toward the
Urubamba River, and Block No.1 is about 100 m lower than
Blocks No. 2 and 3 in its center. Thick landslide debris seems tohave been deposited in the lower part of Block No. 1. The
sliding surface of this landslide block (No. 1) seems to connect
to the present level of the Urubamba River. Therefore, the lastactivity of Block No.1 is relatively young. Activity in Blocks
No.2 and 3 probably took place during the time when the bed of

Fig. 5 Side view of Blocks No.1, 2 and 3 in the Front Slope

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Landslide Risk at Inca's World Heritage in Machu Picchu, Peru

４．HYPOTHESIS FOR THE LANDSLIDEPROCESS AT MACHU PICCHU

to supply this area of a rocky high mountain top. Springs
issuing forth as the result of landsliding (especially below theA hypothesis for the slope evolution at Machu Picchu ashead scarp of Block No.1) were used for irrigation. Thus, theobtained from the field investigation and airphoto interpretation

Inca Citadel with its adjoining farm land was able to be

is explained using Figs. 4, 5, and 6. Fig. 6 illustrates the slope

developed. After the Inca Citadel construction, many smaller

evolution (in the sense of working hypothesis) along schematic

landslides have gradually occurred in the vicinity. Now thesections A-A' of Block No. 1 and B-B' of Block No. 2. The

citadel itself seems to be at risk from landslide activity as

locations of these sections are shown in Fig. 5. The slope ofillustrated in section B-B' of Block No. 2 in the second part ofMachu Picchu slid along joint in granite that are inclined toward

Fig.6. The second large landslide ② could include the entire

the Urubamba River at depth. As shown in Fig. 4, the sliding

area of the Inca Citadel or could split the citadel. In addition,

surface of the 1st slide ① in Block No.1 is likely to be at thesmaller landslides are gradually approaching the citadel frompresent ground level of Block No. 2 including the Inca Citadel

both slopes. The schematic cross section through Blocks No.1, 2

and its farm area, and also at the present ground level of Blockand 3 along section C-C' is shown at the bottom right of Fig. 6.

No. 3 (see section C-C' at the bottom right of Fig. 6). The

The level of Block No.1 is lower than that of Blocks No.2 and 3

second landslide ② occurred in Block No. 1 due to river erosion

because of the second slide ② in Block No. 1.

and the lowering of the level of the river bed. The present

Fig.7 is an oblique airphoto of Block No. 2 and the Machu

situation at Block No.1 seems to be a "reactivation" of thisPicchu Citadel, it shows some part of landslides and potential

earlier landslide mass. The previous landslide ① in Block 2

landslides of Fig. 4. Block No.1 is a reactivated landslide; theremoved the mountain ridge (Fig. 3) and formed topography

landslide topography is visible, the border being indicated by asuitable for construction of the citadel. Similar to the case atsolid line. Another long solid line at the top of the figure is theother landslide areas, landslide activity formed a relatively

Hynapicchu fault, between the rock cliff and the forest. Thisgentle slope and soils suitable for farming in this rocky area.

was the border of the first landsliding ① which removed the

The effect of landsliding aided successful excavation for

mountain ridge at this location and provided a site suitable forground water in Blocks No. 1 and 2. Enough water was available

the citadel. The possible head scarp, forming a double ridge

Fig. 6 Hypothesis of the landslide process in Machu Picchu.

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こうえいフォーラム第１０号／２００２. １

６．BASIC INVESTIGATIONA detailed topographical map at a scale of 1/1000 or 1/2000,
based on aerial photographs, is absolutely necessary. The
present available map at a scale of 1/25,000 is not enough forlandslide investigations. Mapping should carefully express thefeatures of landslides and related phenomena. A detailed
geological map based on reconnaissance also is necessary,
especially for Blocks No.1 and 2. Footpaths need to beconstructed in the forested areas; they will be used for repeated
investigations and regular field observation of surface
deformation.
Fig. 7 Oblique airphoto of the Inca citadel, indicating
delineation of exsting and potential landslides.

７．GEOLOGICAL BORINGS AND
INSTALLATION OF MONITORING EQUIPMENT
Several borings of 100-200 m deeps will be necessary atcrossing the central flat area of the citadel is expressed as a

Block No. 2 to better understand landslides at immediate risk todotted line. Block No. 5 including "Intiwatana," the highest

the Inca Citadel. Borings also are needed in Block No.1 topoint of the citadel is surrounded by a dotted line. There is also

protect the access road and hotel, though these borings would be

the possibility that the entire block, including the citadel, may

for the ordinary type of investigation for reactivated landslides.

slide along the rock joint in the same way as the sliding ② in

An inclinometer casing should be inserted into each borehole in

Block No. 1, or previous landsliding ① in Block No. 2. All

order to periodically monitor movement along the sliding

dotted lines are still hypothetical, and need additional study.

surface. A vertical extensometer for continual record of
deformation should be placed in the same borehole.

５．INVESTIGATION, AND MONITORING AND
POSSIBLE COUNTERMEASURES

Surface extensometers are sensitive and reliable monitoring
instruments. At least several extensometers should be installed.

The possibility of landslides that may destroy this invaluable

A series of extensometers should be positioned crossing theWorld Heritage site has been manifested by this study.

possible shear zone of the "zone of broken structure" shown in

However, this investigation was not supported by quantitative

Fig. 1, and also on the head scarps of landslides (Blocks No. 4,

data. Therefore, by itself it is not enough to allow any5, 6) on the Back Slope to check retrogressive movement at theconvincing conclusions as the basis for extensive investment

citadel.

into protection measures. Landslide-risk mitigation requires a

initiate detailed geological investigations and precise monitoring

８．INSTALLATION OF EXTENSOMETERS IN
MACHU PICCHU CITADEL AND
PRELIMINARY MONITORING RESULTS

of slope movement in preparation for a reliable landslide-risk（1）Non-recording type extensometers andevaluation. The investigation should be done step by step, but

selfrecording type extensometerslarge budget for such large-scale landslide activity. However,
we hope that the results of this study are interesting enough tothe first steps should be initiated immediately so that it may not

For preliminary monitoring of potential landslide blocks in

be too late. This study can serve as the frontier for scientific

Machu Picchu citadel, authors have developed 10 sets ofresearch in the field of prediction of large-scale landslides in thesimplified, non-recording type extensometer system, which

precursor stage, as well as for landslide-hazard assessment andconsists of super-invar wire, weights, pulley, dial and pointerrisk mitigation. It will necessitate development of a reliable and(Fig. 8, left). These were designed with purposes of (1) simple

practical monitoring system, as well as long-distance datamechanism with no power supply, (2) enough resolution andtransfer in a developing country and a location that is not easy toprecision up to 0.2 mm, and (3) easy maintenance and highaccess.

durability in the field. However, this cannot record on any
media by itself, so the Peruvian operator must record its readings

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Landslide Risk at Inca's World Heritage in Machu Picchu, Peru

Fig. 8 Two types of extensometers installed in Machu Picchu
citadel.Left:Simplified type extensometer (#1 in Fig. 9)
which was developed for monitoring of active / potential
landslides in Machu Picchu citadel.Right:Self-recording
type extensometers (S-1 in Fig. 9).

Fig. 9 Location of extensometers in the Front Slope of
Machu Picchu citadel.

periodically and manually.
Besides, two sets of self-recording type extensometers (Fig.
8, right) were brought to Machu Picchu citadel to be installed atmost apparent active landslide blocks. These extensometers areof most popular type in Japan and can record continuous data onrecording sheet on rotating drum which is connected with its
pulley. Movement of super-invar wire is exaggerated by 5 times
on the recording sheet by the pulley and drum.

Fig.10 Location of the foue sets of simplified type extensometers(N-5,6,7 &8) installed in the Back Slope beneath the
Intiwatana at risk of retrogressing landslide.

（2）Installation of extensometers and their monitoring
results in November - December 2000
During 9 - 14 November 2000, two of the authors (Fukuoka
and Kamai) visited Machu Picchu citadel again for geologicalinvestigation and installation of the extensometers. They
installed the extensometers of both types in four active /potential landslide blocks of Machu Picchu citadel. Location ofeach extensometer in the Front Slope and Back Slope is shownin Figs. 9, 10, and 11. INC and INRENA have started
monitoring of the 12 extensometers in November 2000.
Preliminary results by the extensometers for about one month
between 13 November and 16 December 2000 are shown infollowing sections.

Fig.11 Plan map of Machu Picchu sitadel showing active /potential landslide blocks and location of extensometers.
"N-3" shows exetnsometer No.3 of non-recording type.
"S-2" shows extensometer No.2 of self-recording type.

1）Forecourt and back slope of the Machu Picchu
Sanctuary Lodge
The open cafe in the forecourt of the Machu Picchu Sanctuary

settled at its head scarp to monitor retrogressive movement of
the block.

Lodge is suffering from ground deformation for years. There

Observed data of the non-recording extensometers are shown

are new cracks on the ground, which imply this site is head scarp

in Fig. 12. The extensometer at the open cafe showed almostof a small landslide. Every day a lot of tourist take rest at thiscumulative extensive movement of about 15 mm in this period.

cafe and the instability of its foundation must be a serious

Extension was expected at this site because the extensometer

problem. One set of non-recording type extensometer (N-1) andspan was installed at the head of a small landslide scarp. The

one set of self-recording type extensometer (S-1) were installed

extension of the first week (about 5 mm) might be caused bycrossing the cracks. The back slope of the Machu Picchu

stretching of super-invar wire itself due to loading by weight.

Sanctuary Lodge showed an old landslide topography and two

However, about 10 mm of further cumulative movement stillsets of non-recording type extensometers (N-9, N-10) were

continued thereafter. This movement might be interpreted as a42

こうえいフォーラム第１０号／２００２. １

from the helicopter in March 2000, a debris flow was observedon the Back Slope just beneath the peak of the Intiwatana("Head scarp of debris flow" in Fig. 10). The head scarp of thisdebris flow has obviously retrogressed and it has become visible
from the peak of the Intiwatana in 2000. It is certain that thenext retrogressive failure will give damage to the steps of theback slope of Intiwatana. These steps might be restored after
Bingham's discovery by the Peruvian government. This slide ismost significant among all of the potential landslides in Machu
Picchu citadel. Four sets of non-recording type extensometers
Fig.12 Monitoring record of the simplified extensometers before the open
cafe and back slope of the Machu Picchu Sancetuary Lodge.

were installed on the back slope of Intiwatana for monitoring thereactivation of debris flow as well as the possible instability of
the lower steps on the back slope (N-5,6,7&8 in Fig. 10). Twoof the spans connected from inside of the debris flow head scarp
to the artificial step beneath Intiwatana. Other spans connected
lower step to higher ones. If the instability of the steps become
obvious, urgent remedial work would be necessary.
Fig. 14 is the monitoring result of the extensometers N-5,6,7& 8. Three of the four extensometers show cumulative
compression of 4 - 8 mm in about one month. On the other
hand, N-8 extensometer shows almost no change. These results
strongly imply that the lower steps of the back slope ofIntiwatana is unstable and already starts downslope movement.

Fig.13 Cracks progressing in the wall and their temporal
reinforcement by timbers inside the Luggage Checkbeneath the open cafe of Machu Picchu Sanctuary Lobge

3）Sub-block 2-1 and imminent rock fall sites in Block 4
In order to monitor the activity of the Sub-Block 2-1, one setof self-recording type extensometer and one set of non-recording

result of landslide activity because the 10 mm movement is

type extensometer were installed in-line in the slope direction.

larger than scattering of 3 - 5 mm. In Machu Picchu district, theIn Block 4, two of large rocks show serious cracks which show

rainy season is from December to March, however, precipitation

recent progressive opening. Two sets of non-recording type

record is not yet obtained and cannot be compared at thisextensometers are settled between ground and the imminent rockmoment. Cracks are obvious in the beam of the cafe and theyfall mass. Monitored records of Sub-Block 2-1 shows larger

are progressing inside the walls of the Luggage Check justscatter of 10 mm and no cumulative movement. Other two

beneath the cafe (Fig. 13, left). The hotel made temporal
reinforcement by timber wood (Fig. 13, right). This deformation
due to the ground subsidence are caused by landslide activity
just before the cafe. On the other hand, N-9 extensometer
showed exceptionally large extension of about 15 mm in the firstweek and in turn, gradual compression motion of about 4 mm in
the next 3 weeks. The N-10 extensometer showed almost nodisplacement. These two extensometers need further monitoring
to judge the instability of this block.
2）Back slope of Intiwatana
At the peak of Intiwatana, there is a ancient solarium made of
intact rock on top of the steep hill and the most popular tourist
spot in the Machu Picchu citadel. According to the observation

Fig.14 Monitoring record of the four extensometers installed on
the bacslope of Intiwatana.

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Landslide Risk at Inca's World Heritage in Machu Picchu, Peru

extensometers for rock fall monitoring show comparatively
small scatter and no cumulative movement in the monitoring
period.

CONCLUSIONS
Extensive landslide investigations and risk-mitigation
measures require much larger budgets, as well as high levels ofscientific and technical knowledge. However, the great expense（3）Plan of further monitoringmight be acceptable if we consider the cultural and historicalIn the preliminary monitoring of the extensometers shows

value of this World Heritage site, and also its great economic

obvious cumulative movement in the two blocks. This suggests

value in attracting almost one million tourists per year. In

at least the two landslide blocks are active. For further detail,addition, the landslide-risk evaluation must be reliable andprecise and continuous monitoring, authors are planning toconvincing to decision makers. Taking into consideration the

install electronic extensometers and crack gauges in Machu

scientific, technical, and economic situation in Peru and the

Picchu citadel. Extensometer data will be stored in computer

global value of Machu Picchu, scientific, technical, and financial

and be transmitted to responsible organization for monitoring

support and cooperation from the world are very necessary. Insuch as INRENA, INC, IGP and DPRI-KU through handy-

this regard, the IGCP-425 group has taken a very positive

phone and Internet.

attitude toward the investigation of landslides at Machu Picchu.
This work to predict a potential large-scale landslide and to

９．RISK-MITIGATION WORKS.

mitigate the landslide risk is at the frontier in the field of

The Inca Citadel is at landslide risk in various ways: rock falls

landslide science and technology. The research at Machu Picchu

and rock topples on the Back Slope, a large-scale translationalmust contribute to the progress of natural-disaster reduction andlandslide and retrogressive landslides (slumps) from the toe andprotection of cultural and natural heritage sites worldwide.

also from the border between Blocks No. 1 and 2 on the Front
Slope, and reactivated landslides in Block No. 1. The first step

ACKNOWLEDGEMENT

in mitigation will be a reliable landslide-hazard assessment and

The Japanese investigation team was well received by therisk evaluation. The second step will be cost estimation for risk-

related agencies in Peru and obtained significant support andmitigation works for each phenomenon. Step number three will

cooperation. We acknowledge the following institutes andbe decision making for risk mitigation works and their funding.

persons for their cooperation to this investigation: Dr. Mutsumi

This will be based on the significance of the economical andIshitsuka, Director of the Ancon Observatory, IGP, has

cultural value of this site and also on the significance ofcooperated in all aspects as the counterpart of this investigation

scientific and technical development of landslide-risk mitigationin Machu Picchu. The executive director of Instituto National

works, which are applicable in other areas. The landslide-riskde Cultura (INC), Gustavo Benza Pflucker, gave a special

mitigation works can include, A-- Indirect and relatively

permission to fly over the Machu Picchu Inca Citadel by

inexpensive measures: 1) Drainage of surface and ground water

chartered helicopter, and provided necessary supports to us. The

to decrease triggering factors and to reduce the rate of

Director General of Areas Naturales Protegidas y Fauna

weathering, 2) Prevention of toe erosion along the Urubamba

Silvestre, Luis Alfaro Lozano of Instituto National de Recursos

River. Heavy river erosion is a basic factor in triggering

Naturales (INRENA) and their staffs in Cusco and Machu

landslides, and slopes affected by landslides are "attacking slope

Picchu provided personnel necessary for field investigation. The

by river flow due to curvature of its course," 3) Stabilization ofvice director of IGP, Hernan Montes, and his staff cooperated inshallow landslides because they may increase instability of deep

importing monitoring apparatus, and Ms. Susana Kalafatovich,landslides, B-- Direct measures 1) to prevent rock topples and

PROEPTI, Cusco provided support in Cusco.

rock falls on the very steep Back Slope by means of rock anchor
or retaining frame works, and 2) to stabilize large-scale

REFERENCES

landslides by means of steel piles or shaft works (reinforced-

Sassa, K., H. Fukuoka & H. Shuzui. 2000. Field investigation of

concrete piles as much as a few meters in diameter), and any

the slope instability at Inca's World Heritage in Machu

new method to stabilize the very deep translational slide of

Picchu, Peru. Landslide News. No.13, pp.37-41.

Block 2, which is the most difficult and needs new development.

Carreno, R. and C. Bonnard. 1997. Rock slide at Machupicchu,
Peru. Landslide News, No.10, pp.15-17.

44