● The tsunami reached up to 40 m, travelling inland for several km
● The tsunami caused widespread destruction, including the destruction of
coastal communities, the displacement of buildings, infrastructure damage
and the loss of thousands of lives
● A nuclear power plant, located near the coast, suffered significant damage
and released radioactive materials
● The tsunami killed 15000 people with thousands more injured or missing
● It resulted in the displacement of coastal communities and highlighted the
importance of disaster preparedness, early warning systems and effective
response strategies in mitigating the impact
Boxing Day Tsunami, Indian Ocean 2004
● The focus of the earthquake that triggered the tsunami was located under the
oceanic crust in the Indian Ocean
● It was a magnitude 9.0 on the Richter Scale
● The earthquake occurred along the subduction zone, where the Indian plate
sank under the Eurasian Plate
● The earthquake caused a sudden uplift of the seafloor, displacing a massive
amount of water and generating a tsunami
● The tsunami generated waves that were 30m and higher
● It was seen as a global disaster, killing people from nearly 30 countries
(including tourists)
● Between 180 000 and 280 000 people were killed
● The Indonesian island of Sumatra, the closest area to the epicentre,
experienced devastation
● 70% of the inhabitants died
● In Sri Lanka too, at least 31 000 people died
9.2 Hazardous environments resulting from mass
movements
Causes of mass movements
Mass movement is any large-scale movement of the Earth’s surface that is not
accompanied by a moving agent
● Mass movements are common natural events in unstable, steep areas
● They can lead to a loss of life, disruption of transport and communications and
damage to property and infrastructure
● The most important factors that determine mass movements are gravity, slope
angle and pore pressure
● Increases in shear stress or a decrease in shear resistance trigger mass
movements
, ● Gravity moves the materials downslope (slide component) and acts to stick
the particles to the slope (stick component)
● The downslope movement is proportionate to the weight of the particles and
the slope angle
● Water lubricates particles and in some cases fills the spaces between the
particles, which forces them apart under pressure
● Pore pressure will greatly increase the ability of the materials to move
● This factor is particularly important in the movements of wet materials on
low-angle slopes
Factors contributing to shear stress
● Removal of lateral support through undercutting or slope steepening (erosion
by rivers, wave actions, faulting, previous rock falls)
● Removal of underlying support (undercutting by rivers and waves, subsurface
solutions, loss of strength by exposure of sediment)
● Loading slope (weight of water, vegetation, accumulation of debris)
● Lateral pressure (water in cracks, freezing in cracks, swelling, pressure
release)
● Transient stresses (earthquakes, movement of trees in wind)
Factors contributing to reduced shear strength
● Weathering effect (disintegration of granular rocks, hydration of clay minerals,
solution of cementing minerals in rock or soil)
● Changes in pore-water content (saturation, softening of materials)
● Changes of structure (creation of cracks in clays, remoulding of sands and
clays)
Human activities increase the risk of mass movements
● Increasing the slope angle by undercutting through high ground - slope
instability increases with slope angles
● Placing extra weight on a slope like new buildings, adds to the stress on the
slope
● Removing vegetation - roots bind the soil together and interception by leaves
reduced rainfall compaction
● Exposing rock joints and bedding planes, can accelerate the rate of
weathering
Landslides
● Common natural events in unstable, steep areas
