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temporal lag after the occurrence of a triggering event.
Figure 12.13 illustrates this complexity in landslide
initiation. Ultimately, failure will eventuate when the
shear strength of the soil is exceeded by a critical shear
stress. The critical shear stress can fluctuate slowly or
dramatically over time, while the shear strength of the
material can oscillate because of seasonal factors, or
decrease slowly because of any of the above conditions.
Slope failure may happen at a number of points in
time. If stress dramatically increases, for instance
because of heavy rainfall, and exceeds soil strength,
then the cause of the landslide is obvious. However,
soil strength could be decreasing, or the stress on a
slope increasing slowly and imperceptibly over time, to
the point that the cause of the landslide is not defin-
able. Thus, with any landslide, there is randomness as
to the timing and cause of the failure.
vulnerable to typhoon-generated slides near urban
areas. Kobe was hit in 1939 by rain-induced landslides
that killed 461 people and damaged 100 000 homes.
In 1945, comparable landslides killed 1154 people in
Kure. In 1958, Tokyo was struck by a typhoon that
generated over 1000 landslides and killed 61 people.
In underdeveloped, heavily vegetated regions,
especially in the tropics, mega-landslides play a major
role in the downslope movement of material. In 1935,
New Guinean landslides cleared 130 km
2
of vegetated
slopes. The Bialla earthquake of 10 May 1985, on the
island of New Britain in Papua New Guinea, triggered
a mega-landslide in the Nakanai Range that dramati-
cally infilled the Bairaman River (Figure 12.14). About
12 per cent of slopes in New Guinea are subject to
landslide denudation every century. Elsewhere in the
tropics, an area of 54 km
2
was cleared on slopes in
Panama in 1976. Tropical Cyclone Wally, which struck
Fiji in April 1980, and Cyclone Namu, which passed
over the Solomon Islands in 1986, both generated
hundreds of landslides with an average volume of
material moved in excess of 500 m
3
per hectare. Such
landslides had not been witnessed in these areas in the
previous 50 years. The cyclones effectively destabilized
the landscape enough to ensure continued mass
movements triggered by less severe rainfalls for years
to come.
Landslide disasters
(Bolt et al., 1975; Cornell, 1976; Whittow, 1980; Coates,
1985; Blong & Johnson, 1986)
Landslide disasters are difficult to separate from other
types of land instability, and from some of the larger
disasters that trigger the event. For instance, tropical
cyclones, apart from drowning people, usually bring
very heavy rainfalls; this not only increases pore-water
pressure in potentially unstable material but, through
added weight, also increases stress in slope deposits.
Some of the worst natural disasters have occurred
because of landslides triggered by earthquakes. The
Chinese earthquakes in Shensi province in 1556, and
at Kansu in 1920, caused failure in loess deposits,
collapsing homes dug into the cliffs. Urban areas seem
to be most affected by landslides. In Rio de Janeiro in
1966, record-breaking rainfall in January and March
caused catastrophic landslides that struck shantytowns
around the mountains in the city. Over 500 people
were killed in the slides and another 4 000 000
were affected by disrupted transportation and
communications. In the following two-year period,
over 2700 people were killed in the Rio de Janeiro area
by landslides and other slope instability events that
afflicted over 170 km
2
. In Hong Kong, a tropical
cyclone in 1976 dropped 500 mm of rain in two days,
triggering landslides on steep slopes and killing 22
people. Similar slides there in 1966 killed 64 people.
Many of the slides occurred where dense urban sprawl
encroached upon steep slopes and undermined the
toe of unstable sediments. Japan is also particularly
Mega-landslide on the northern slope of the Nakanai
Range, New Britain, Papua New Guinea. This slide was
triggered by the magnitude 7.0 Bialla earthquake on
10 May 1985. The slide occurred in Miocene limestone
and extensively infilled the Bairaman River Valley
(photograph by Dr Peter Lowenstein, courtesy of
C.O. McKee, Principal Government Volcanologist, Rabaul
Volcanology Observatory, Department of Minerals and
Energy, Papua New Guinea Geological Survey).
Fig. 12.14
