UNDERSTANDING
LANDSLIDES
Dr. Walter Hays,
Global Alliance For
Disaster Reduction
ESSENTIAL KNOWLEDGE FOR
SOCIETAL SUSTAINABILITY
A FRAMEWORK FOR
UNDERSTANDING THE IMPACTS
OF LANDSLIDES ON PEOPLE AND
THEIR COMMUNITIES
LANDSLIDES
NATURAL PHENOMENA THAT
OCCUR WITH OR WITHOUT
HUMAN ACTIVITY
FORCASTING FUTURE EVENTS
 The goal is
recording,
processing, and
interpreting
geologic
changes in time
frames of
seconds to
weeks; i.e., “realtime geology.”
FORCASTING FUTURE EVENTS
 Predicting a
large volume
landslide is
important for
society, but it
depends on
long-term
monitoring of
individual slides.
LANDSLIDES
Falls, Topples, Slides,
Spreads, Flows
LANDSLIDE HAZARD
 Landslides encompass all
categories of gravity-related
slope failures in Earth
materials.
SLOPES
 Slopes are the most common
landforms.
 Although they appear stable and
static, slopes are actually
dynamic, evolving systems.
SLOPES
 Material is constantly moving on
slopes at rates varying from
imperceptible creep to
thundering avalanches and rock
falls moving at high velocities.
LANDSLIDES
 Gravity slope failures are triggered
by earthquake ground shaking or
excess precipitation
 The slope does not need to be very
steep for a landslide to occur.
FORCASTING FUTURE EVENTS
 To predict when
an earthquake
will occur is
important for
society, but it is
not yet feasible
because
earthquake
prediction isn’t.
LAHAR
 A lahar is a special
“landslide” or mud flow
induced by an explosive
volcanic eruption.
FORCASTING FUTURE EVENTS
 Predicting when
a volcano will
erupt is
important for
society, but it
depends on
knowledge of
each volcano’s
eruption history.
MOUNT RANIER
 Mount Ranier has generated
more than 60 lahars during
the past 10,000 years.
 Many of these occurred
during periods of volcanic
quiescence.
MOUNT RANIER
 Today, Mount Ranier continues
to vent steam and convert its
andesitic summit into clay.
 A plan to evacuate 50,000 people
has been developed.
LANDSLIDE HAZARD: ALASKA
 THE 1964 PRINCE
WILLIAM SOUND
EARTHQUAKE
TRIGGERED A LARGE
VOLUME LANDSLIDE
AT TURNAGAIN
HEIGHTS.
 MILLIONS OF CUBIC
METERS OF SOIL AND
ROCK MOVED DOWN
SLOPE.
LANDSLIDE HAZARD: ALASKA
 Slope failure was
induced by
ground shaking
of “Quick Clay.”
LANDSLIDE HAZARD: ECUADOR
LANDSLIDE HAZARD: JAPAN
LANDSLIDE HAZARD:
WASHINGTON STATE
LATERAL SPREADING: JAPAN
LATERAL SPREAD: SAN
FRANCISCO