2 SIDLE A N D OCHIAI

[Brunsden and Prior, 1984; Eisbacherand Clague, 1984; Dikau etal, 1996]. However,
relatively little attention has been paid to the influence of natural factors and extensive
land management practices on slope stability, as well as the effects of landslides on
natural and human-affected environments. This book follows an earlier work by Sidle
et al [1985] that addressed interactions of land use and slope stability. Incorporated
herein are new concepts and findings related to hydrogeomorphic process, seismicity,
weathering, and vegetation influences; land use interactions and amelioration measures;
landslide hazard analysis, prediction techniques, and warning systems; damage assess­
ments; global landslide coverage, including many examples from Japan and Southeast
Asia; climate change scenarios; and interrelations among various hazard types.

OVERALL SIGNIFICANCE OF LANDSLIDES

Most parts of the world experience some level of landslide activity or more chronic mass
wasting. For example, landslides have occurred in all 50 states in the USA, and almost all
nations in the Western Hemisphere have experienced major socioeconomic impacts related
to landslides [Swanston and Schuster, 1989; Schuster and Highland, 2001]. Because land­
slides only attract widespread attention when people are killed or property is damaged, it
is difficult to ascertain the level of, or any increases in, landslide activity during the past
few centuries. Even the most devastating landslide in recent history, the December 1920
dry loess flow triggered by an earthquake in central China, was not known to the outside
world for nearly a year after its occurrence [Close and McCormick, 1922; Derbyshire et
al, 2000]. The relatively high level of landslide damage recorded in Japan over the past
century may somewhat reflect the better documentation of natural hazards compared with
developing nations [Takahashi, 1994; Nunamoto et al, 1999] (Table 1.1).
Very large landslides can sometimes be recognized in geologic archives [Hewitt, 1998;
Loope et al., 1999; Sukhija et al, 1999; Weidinger et al, 2002], however, smaller landslides
are usually obscured in sediment records [DeRose et al, 1993; Sidle et al, 2004a]. Although
it is difficult to sort out specific sequences of smaller landslide events from sediment depos­
its, it is clear that widespread land cover changes (particularly forest clearance and conver­
sion) have accelerated landslide erosion in the past few centuries [e.g., Kuruppuarachchi and
Wyrwoll, 1992; Lucbnan etal, 1999; Slaymaker, 2000]. Notable examples are New Zealand,
India, Nepal, and Western Australia. Additionally, residential development and engineering
earthworks in cities such as Hong Kong; San Francisco; San Paulo and Rio de Janeiro, Brazil;
Basilicata, Italy; Guatemala City; and Kobe, Sendai, and Niigata, Japan, have contributed to
numerous landslide occurrences [e.g., Nilsen et al, 1979; Gostelow et al, 1997; Smyth and
1
Royle, 2000; Yoshida et al, 2001 ]. Where human intervention has accelerated landslide
occurrence and hazards, the prior environmental conditions controlling slope stability may
have been in a state of tenuous equilibrium. By reducing vegetation rooting strength, over-
steepening and overloading natural slopes, altering hydrologic regime, and modifying soil
properties, land use can disrupt this tenuous equilibrium and increase landslides in both the
long- and short-term, as well as impacting humans, property, and the environment.

1
Kamai et al, 2004; Kwong et al, 2004; Sidle et al, 2005.
 -

TABLE 1.1. Examples of the most destructive landslides in the past century. Z
>-l
Failure E3
Location Date Failure type/ description Cause volume (m3) Major losses and damage

= >-l
Usoy, Tajikistan 1911 massive rock and debris earthquake (M 7.4) 2.0 billion town of Usoy destroyed;
�
-

Murgab River avalanche 54 people killed; unstable

sediment dam on Murjab �
River now affects 5 million �
people downstream 0

Kedri, east Java, 1919 lahars draining of Crater not reported 5160 deaths; 150 km2 of arable �
tTl
Indonesia Lake land devastated; 104 villages
-
Kelud Volcano destroyed �
tTl
Gansu Province, 16 December dry loess flows earthquake (M = 8.5) huge, but ""'180,000 people killed; 40 �
China; also 1920 unknown lakes formed by 675 dry loess 0
'-r:I
affected Ningxia & flows; 50,000 km2 of land t""'
Shaanxi Provinces affected
�
0
Alma-Ata, 1921 huge debris f low snowmelt not known 500 people killed til
t""'
-
Kazakhstan
0
Malaya tTl
Almaatinka River '"C

Sichuan, China 1933 several large landslides & Deixi earthquake not reported 6800 deaths from landslides;
E3
to
Min River landslide dam failed (M = 7.5) 2500 people drowned when t""'
tTl
landslide dam failed �
til
Khait, Tajikistan 1949 rock slide transformed to earthquake (M = 7.5) not reported about 18,000 people killed
debris avalanche; many other as boulders buried Khait and
landslides other villages and farms to
depths up to 150 m

Kanogawa River September landslide and debris flows typhoon storm not reported 1094 deaths; 19,754 homes
Valley, Shizuoka, 1958 destroyed
Japan

w
 .j>.
TABLE 1.1. Examples of the most destructive landslides in the past century. cont.

Failure
Location Date Failure type/ description Cause volume (m3) Major losses and damage

Rupanco region, May 1960 widespread landslides M = 7.5 earthquake not reported 210 deaths; many buildings,
Chile preceded by heavy port facilities, roads and
rain agricultural fields destroyed

Mt. Huascaran, 10 January debris and rock avalanche ice and rock 13 million 4000-5000 casualties; town
Peru 1962 avalanche triggered of Ranrahirca destroyed;
debris avalanche US$200 million in damages

Vajont Gorge 9 October rockslide in limestone groundwater 300 million 1899 deaths by downstream
near Longarone, 1963 weakened by fractures, old accretion during flood that destroyed or
Italy (Vajont Dam erosion & faulting heavy seasonal rains heavily damaged towns
disaster) & rising of lake; slide of Castellavazzo, Pirago,
entered reservoir Villanova, Forance, Fae, &
Codissago; cost US$200
million

Anchorage, 27 March liquefaction failures & major earthquake not reported 9 deaths; 215 homes destroyed,
Alaska, USA 1964 landslides in quick, glacial (M=9.2) 157 commercial properties
clay deposit damaged; lifelines damaged;
cost"" US $200 million

Rio de Janero, January 1966 widespread debris slides, heavy rainfall in not reported about 1000 casualties from
Brazil avalanches, and flows areas excavated & landslides and resultant
cleared of vegetation flooding in and around Rio de
CI'l
Janeiro ....
0
SW of Rio de 23 January thousands debris slides, heavy and intense >10 million about 1200 deaths attributed t""'
ttl
Janeiro, Brazil 1967 avalanches and flows rain to landslides; > 100 homes :>
(Serra das Araras) destroyed; damage to lifelines Z
0
Los Angeles area, Jan.-Feb. shallow landslides & resultant heavy rain in mid not reported 18 deaths; 175 homes and 0
(")
California, USA 1969 debris flows Jan. followed by rain major highway damaged: cost :I:
....
in Feb. >$40 million :>
....
TABLE 1.1. Examples of the most destructive landslides in the past century. cont. Z
>-l
Failure �
Location Date Failure type/ description Cause volume (m3) Major losses and damage 0
C
(")
Mt. Huascaran, 31 May 1970 debris avalanche/debris flow earthquake (M = 7.7) 30-50 destroyed Yungay and partly >-l
-
Peru million destroyed Ranrahirca; 18,000
dead; many homeless
�
>
Mantaro Valley, 25 April 1974 rock/debris avalanche probably rainfall and 1.6 billion resulting flooding and debris Z
0
Peru Mayunmarca dammed Mantaro River; dam fluvial undercutting from breach of landslide dam
landslide breached suddenly killed 450 downstream; many �
tTl
farms, roads, and bridges
destroyed -
�
tTl
Guatemala City 4 February >10,000 debris slides, rock earthquake (M = 7.5) unknown :::::250 deaths; >500 homes �
region, Guatemala 1976 falls, slumps & debris damaged; rail lines damaged; �
avalanches several landslide-dammed
lakes breached !;:
Z
0
Yungjing County, August 1979 numerous landslides in loess heavy rainfall unknown ::::: 800 people killed or injured; Vl
r
-
Gansu Province, deposits >200 ha of farmland damaged
0
China tTl
"'C
Mount St. Helens, 18 May 1980 debris avalanche/rockslide volcanic blast 2.8 billion 53 deaths; US$l.l billion losses
Washington, USA preceded by M = 5.1 to civil works, agriculture/
�
OJ
earthquake forests, plus personal property r
tTl
loss, clean-up cost, and loss of �
Vl
tourism and wildlife

Gansu Province, August 1983 deep landslide in loess prolonged rainfall 35 million 227 deaths; 22 injuries; 2 km2
China (Saleshan deposits followed by large of farmland overwhelmed; 4
Landslide) storm villages destroyed

Spanish Fork 13-17 April translation slide in old rain-on-snow 21 million dammed river; flooded
Canyon, Utah, 1983 earthflow and debris flow (El Nino event) Thistle; destroyed railroad
USA (Thistle) deposits and highway; 1330 jobs lost;
total cost >US$400 million
Ul
(direct + indirect)
 0\
TABLE 1.1. Examples of the most destructive landslides in the past century. cont.

Failure
Location Date Failure type/ description Cause volume (m3) Major losses and damage

Jizukiyama, 26 July 1985 deep-seated slump-earthflow >1 month of high 3.5 million 25 deaths; 4 injuries; 64
Nagano City, Japan rain; intense rainfall homes damaged; nursing
prior to failure home, roads, farms, forests,
& waterworks damaged; cost
US$40 million

Nevado del Ruiz/ 13 November lahars rapid melting of 60 million 23,000 people killed; 5000
Armero, Colombia 1985 snow & ice caused by itgured; >5000 homes
volcanic eruption & destroyed; destroyed Amero
pyroclastic f low town

Napo Province, 5 March 1987 rock & debris slides, two earthquakes 75-110 =1000 people killed; 70 km
Ecuador avalanches & flows in wet (M= 6.1 & M=6.9) million of oil pipe line and major
(Reventador soils highway destroyed; much
Volcano) damage to hydroelectric
development and agriculture;
direct losses = US$1 billion

Kaiapit, Morobe 6 September large landslides-debris flows likely moderate 1.8 billion 74 people killed; 800 people
Province, Papua 1988 earthquake that affected; 3 villages destroyed;
New Guinea destabilized rather 4 landslide lakes formed, 3 of
dry regolith which breached

. Gilan Province, 20 June 1990 >76 large landslides, many Manjil earthquake huge, but =300 deaths; roads blocked,
types, including liquefaction (M = 7.7) unknown preventing rescuers from til
.north Iran -

accessing victims; buildings, 0

t'"
infrastructure & farms t%1
damaged
�
0
Toluk, Suusamyr 19 August many landslides; large rock Suusamyr earthquake =1 million =35 deaths; extensive damage
0
Valley, Kyrgyzstan 1992 avalanche (M=7.3) (rock slide) to the Bishkek-Osh highway ("l
::I:
-

>
-
TABLE 1.1. Examples of the most destructive landslides in the past century. cont. Z
>-l
Failure 13
Location Date Failure type/ description Cause volume (m3) Major losses and damage 10
c:::
(')
Kagoshima Bay July- many debris avalanches & seasonal heavy rain not reported 105 deaths; heavy damage >-l
-
area, Japan September flows (0.8-1.2 m in July) to housing, roads and public
1993 facilities
�
>
Harihara River, 10 June 1997 deep landslide that triggered a 4 days of heavy rain 160,000 21 deaths; 13 injuries; 15 Z
10
Izumi, Kagoshima, debris flow homes destroyed
Japan �
tr1
Malpa, northern 17 August large rockfall/debris avalanche 4 days of heavy rain "large" 207 deaths; 5.2 million rupees
-
�
India 1998 in other direct costs & 0.5 tr1
million rupees indirect costs �
0
Nishigo, 26-31 August >1000 landslides and debris 5 days of heavy rain not reported 9 people killed; many homes "'J'j
t""'
Shirakawa, and 1998 flows (max. = 1.2 m) and buildings destroyed >
Nasu, Japan Z
10
r:/1
North coast of 14-16 widespread shallow landslides nearly 1 m of rain unknown =30,000 deaths, 8000 t""'
-
Venezuela near December and debris flows along a in less than a 3-day residences & 700 apartments
Caracas 1999 40-km coastal strip period destroyed; extensive
1il
"'tl
infrastructure damage; all
losses include flood damages
13
bj
t""'
Bajo Caliente, 27 June 2000 debris avalanche & subsequent possibly fracturing & 12 million 9 deaths; 2.5 km2 of cropland tr1
�
Costa Rica block slide faulting of bedrock & 9 homes destroyed; US$O.4 r:/1
(Arancibia million damage
landslide)

K ara Taryk 20 April 2003 large landslide in Soviet-era rain-on-snow 1.5 million 38 deaths; 13 homes
(Osh region), uranium mining area destroyed; potential pollution
Kyrgyzstan of a river

Southwest Guizhou 11 May 2003 road-related landslide heavy rain and road not reported 35 road workers killed and 2
Province, P.R. construction buildings and road destroyed
China -..l
 00
TABLE 1.1. Examples of the most destructive landslides in the past century. cont.

Failure
Location Date Failure type/ description Cause volume (m3) Major losses and damage

Ratnapura and May 2003 many landslides and debris continual heavy rains not reported >260 deaths (some due to
Hambantota flows (extensive associated floods) >24,000 homes &
Districts, south- flooding) schools destroyed; ::::: 180,000
central Sri Lanka families homeless

Minamata and 20 July 2003 debris avalanches & debris heavy and intense 37,000 at 25 deaths (23 at Minamata
Hishikari, southern flows; many small landslides rainfall Minamata debris flow; 2 at Hishikari
Kyushu, Japan landslide landslide); 7 homes destroyed;
roads, power & hot spring
lines damaged

Miyagawa area, October 2004 numerous landslides and heavy and intense unknown 17 deaths, 9 injuries; 87
Mie Prefecture, debris flows rainfall homes damaged/destroyed;
Japan extensive forest road damage

Niigata Prefecture, 23 October thousands of landslides on three successive and unknown 2 deaths; Yamakoshi village
Japan 2004 natural slopes, roads, paddy large earthquakes isolated; 2 landslide dams
fields, and fills caused flooding; many homes
and roads damaged by fill
failures

La Conchita, 10 January landslide-debris f low in a 378 mm of rain in the 0.2 million 10 deaths; 30 homes seriously
California, USA 2005 slump-earthflow deposit from preceding 2 weeks damaged or destroyed
1995
tI}
-
Guinsahugon 17 February large, deep-seated landslide, 685 mm of rain "large" 1800 deaths; 375 homes and
village, Leyte, The 2006 debris flow in about 2 weeks; a school destroyed; 40 ha of �
tTl
Philippines possibly affected by a land buried >
very small (M = 2.3) Z
t)
earthquake
0
n
::I:
-

>
-
INTRODUCTION AND OVERVIEW OF LANDSLIDE PROBLEMS 9

DISTRIBUTION OF LANDSLIDE HAZARDS WORLDWIDE

Because of the variable reliability of reporting and documentation of landslides in differ­

ent nations, it is difficult to accurately quantify the global distribution of landslide hazards
and related damages. While recent advances in remote sensing and geographic information
systems techniques can provide contemporary estimates of large landslides in remote and
poorer regions of the world [e.g., Lewi et al, 1992; Dhakal et al, 2000; Perotto-Baldiviezo
et al, 2004], comprehensive longer term landslide data from such areas, as well as damage
estimates, remain elusive. Nevertheless, certain generalizations related to the regional extent
of landslide hazards can be drawn based on geomorphic, tectonic, and climatic environments
together with land use patterns and past records of landslides.
The circum-Pacific region is naturally susceptible to landslides because of a combi­
nation of high and intense rainfall, mountainous terrain, active tectonics, volcanism,
geological history, and lithology/pedology conditions. During the past several hundred
years, the widespread land cover change throughout most areas around the Pacific Rim
has contributed to increased rates of landslide activity. Additionally, landslide risk in
the region has substantially increased due to population growth and the concentration
of people in high hazard areas; such demographic patterns have been most evident in
the past 30 yr [e.g., Harp et al., 1981; Elmhirst, 1999; Templeton andScherr, 1999]. The
circum-Pacific region is characterized by actively converging margins of lithosphere
plates; thus, volcanism and intense seismic activity at shallow, intermediate, and deep
focal levels are common features. This general convergent tectonic pattern has caused
major compression and uplift of deep crustal metamorphic and igneous rocks as well
as soft clay-rich sedimentary rocks. Large vertical (tens of kilometers) and horizontal
(hundreds of kilometers) displacements along major faults, in combination with crustal
compression, have produced extensive melange areas of extreme crustal deformation.
Triple junctions among tectonic plates near Japan, Papua New Guinea, and Taiwan make
these regions particularly susceptible to earthquake-triggered landslides. Many of the
coastal areas along the margins of the Pacific Rim experience high annual rainfall with
distinct seasonal distributions. Typhoons or monsoons, which often deliver both large
amounts and intense rainfall, are common along the Asian continental and equatorial
margins of the Pacific. In contrast to western North America, South America, and New
Zealand, where recent settlement and land cover change has exacerbated landslide
occurrence, much of the Asian margins of the Pacific Rim have been inhabited for mil­
lennia; however, recent demographic shifts to mega-cities and exploitation of natural
forests have increased landslide risk in Asia. Particularly in Southeast Asia and some
developing parts of East Asia, land cover changes promoted by overpopulation, internal
politics, transmigration, the drug trade, and economic investments from industrialized
nations whose natural resources have already been depleted or are protected from such
exploitation have contributed to landslide hazards and risk [Templeton and Scherr, 1999;
Verburg et al, 1999; Derbyshire et al, 2000; Sidle et al, 2004a].
Many other regions of the world experience significant landslide activity, includ­
ing the European Alps; Intermountain region, Rocky Mountains, and Appalachian
Mountains of USA and Canada; parts of the Middle East; mountains of the Caribbean
10 SIDLE AND OCHIAI

Islands; Brazil; Himalayas; and central Asia. China, India, and Nepal have long, but
incomplete histories of landslide occurrence and damage. Landslide initiation in East
Asia is exacerbated by previous and contemporary glaciation, tectonic uplift, frequent
earthquakes, high-intensity rainfall, and episodic snowmelt [Li, 1989; Froehlich et al,
1990; Sukhija et al, 1999; Gupta and Virdi, 2000]. Deep-seated landslides and slope
movements (i.e., slumps, earthflows, soil creep) are abundant in some areas where highly
weathered regoliths and certain clayey soils occur. Characteristics of these deep-seated
slope movements as well as other types of landslides are discussed in Chapter 2 along
with their geographic occurrence.

LOSS OF H U M A N LIFE

Statistics on loss of human lives from landslide disasters is complicated by highly vari­
able reporting amongst nations of differing economic status as well as a contemporary
bias due to the general trend of improving data collection in many nations. Additionally,
natural disaster statistics often combine landslide casualties and damages with those
of related hazards (e.g., floods, earthquakes) when they occur during the same disaster
[e.g., Yoon, 1991; Water Induced Disaster Prevention Technical Centre, 1993]. One of
the earliest landslide fatality records was the Wudu landslide in Wushan County, Gansu
Province, central China, which killed 760 people in 186 B.C. [Li, 1989]. The greatest
loss of life during a landslide disaster was triggered by the large (M = 8.5) Haiyuan
earthquake in Gansu, Shaanxi, and Ningxia provinces of China on 16 December 1920.
Thousands of landslides in mostly dry loess material occurred throughout a region of
2
50,000 k m , including more than 650 landslides that were >0.5 km wide; the subsequent
dry loess flows killed an estimated 180,000 people, dammed many streams and rivers,
and significantly changed the geomorphology of the region [Derbyshire et al, 2000].
Based on incomplete records compiled by UNESCO from 1971 to 1974, Varnes [1981]
estimated that 2378 people worldwide were killed by landslides, about 600 per year. Of
these casualties, 2126 (89%) occurred within the circum-Pacific region. Average annual
estimates of deaths attributed to landslides and debris flows for various nations are shown
in Table 1.2; these figures are derived from multiple sources of varying quality and do not
represent the same aggregate time periods; nevertheless, they give a general idea of casual­
ties in different geographic and socioeconomic regions of the world. The high death rates
in Nepal and China reflect the vulnerability of people in hazardous mountain areas where
few structural countermeasures exist and where little attention is paid to structures related
to potential landslide hazards. Italy, by far, has the highest number of landslide casualties of
all European nations, albeit much less than many Asian countries. While Japan represents
an extreme case for severe landslide hazards due to its topography, climate, tectonic activity,
and lithology, the high number of deaths may also be related to vulnerabilities of people who
have a false sense of security in protective measures [Sidle and Chigira, 2004]. In Japan,
many residences, infrastructures, and businesses are located in potential landslide hazard
areas, in some cases because these sites are virtually unavoidable, but in other cases because
there is a perception that the government will provide the necessary protective measures,
either via structures or warning systems. Oftentimes, the citizens would be better served by