Natural Hazards


Mass Movement

  • Mass Movement is the downhill transfer of slope materials as a coherant body, it is the balance between driving force and resisting force
  • Mass movement is more likely to occur when both physical and human factors disturb the equilibrium of a slope.
  • Mass movement has a range of environmental and social impacts on the areas affected, which create a range of human responses to the hazard.
  • strengthening and undercutting at the base of the slope increases the driving force.This can occur from natural erosion (a valley sided sliope undercut by a river) or human activity (a road cutting)
  • Mechanical weathering such as  freeze-thaw  occurs when water gets into cracks in rocks and freezes. This then expands by around 10%. The repeated freeze thaw action puts pressure on the rocks until they eventually crack and break the rock.
  • loading the slope increases the driving force. most common cause is heavy precipitation which adds to the mass of material on the slope.
  • Heavy precipitation lubricates slope material and decreses the resisting force by increasing water pressure within the pores of mineral materials, reducing their coherence.
  • Deforestation removes the binding effect of tree roots on slopes and increases the amount of water absorbed by slope material. 
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Mass Movement types


  • Earth flows tend to be faster than slow creep but less sporadic. The flow itself is elongate and usually occurs in fine-grained materials or clay-bearing rocks on moderate slopes and under saturated conditions. However, dry flows of granular material are also possible.


  • More mobile, saturated flows consisting of clay and silt-sized particles. more rapid than earthflows. common in areas of sparse vegetation. active volcanic areas loose ash is easily transported by run off, mudflows are known as lahars.


  • corse regolith (boulders). associated wuth heavy rainfall. they produce alluvian fans (lobe deposits).


  • A common cause of slumping is erosion at the base of a slope. For example, coastal storm waves erode cliff bases, removing supporting material. Slope failure occurs. The slump block rotates downward, producing a scarp (cliff) at the top of the slope.

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Types of mass movement

SLIDES - They are sudden, fast movements of a cohesive mass of soil, rock or regolith. TWO TYPES - 1- translational slide - These involve movement along a flat surface parallel to the angle of the slope in a stepped-liked pattern, with no rotation. 2 -  rotational slide and is the movement of surface material along a concave surface. Both types of landslides can be moist, but they are not normally saturated with water.

FLOWS - fast -   the material within them is normally saturated with moisture. Mudflows for example are a type of flow that can occur quickly after heavy precipitation saturates a surface. Earthflows are another type of flow that occur in this category, but unlike mudflows, they are not usually saturated with moisture and move  slower.

CREEP - gradual but persistent movements of dry surface soil.  soil particles are lifted and moved by cycles of moistness and dryness, temperature variations and grazing livestock. Freeze and thaw cycles in soil moisture also contribute to creep through frost heaving. When soil moisture freezes, it causes soil particles to expand out. When it melts, the soil particles move back down, causing the slope to become unstable.

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Mass Movement CASE STUDIES


CAUSES - slow moving cold front deposited heavy rain in the coastal mountains. GEOMORPHOLOGY - seaward facing slopes are very steep, runoff is rapid and streams have high energy. The igneous rocks that form the mountains are have been deeply weathered, the mountains are mantled in easily eroded clay that feeds large amounts of sediment to debris flow. Deforestation has further increased runoff and erosion.

IMPACTS - Debris was added to alluvian planes which coastal towns are sited, high risk from flood and debris hazards. over 15000 people died. economic impact - 20000 houses destroyed leaving people homeless. 

HUMAN RESPONSE - short term - re-settling of victims away from the coast. Long term - flood channels could be built on some alluvial fans to allow the passage of large scale debris flow. monitoring early warning of exceptional rainfall and runoff events in the mountains. land use controls in mountain catchments.

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Mass Movement CASE STUDIES


CAUSES - terrential rain and deforestation. heavy rainfall associated with a La Nina event, loading slopes and weakening slope materials.widespread deforestaion during the past 70 years due to a rapid population growth and the demand for fuel and new land from cultivation increased slope instability, ultimate trigger was a small earthquake measuring 2.6

IMPACTS - many villages sited at the foot of steep slopes occupy the run out zone of mudslides. killed over 1000 people, buried farmland and destroyed 300 houses. poor countries.

HUMAN RESPONSE - LONG TERM - evacuation centres were set up around st. bernard. emergency aid provided safe drinking water, sanitation and health services. southern leyte is remote and has poor communications that slowed disaster response times. international aid was provided by the red cross and red crescent and a appeal of US$1.6 million. LONG TERM -  the government commissioned a new $1.5 million geohazards survey and mapping of southern Leyte to identify areas most at risk.

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Earthquake CASE STUDY


TECTONIC PROCESS - most seismically active zones in the world, Japan is located at the meeting point of 3 tectonic plates, The Eurasian, Pacific and Philippines. It is a convergent DESTRUCTIVE boundary, with faults running off the boundary. Ocean trenches such as Japan trench and Sagami Trench mark plate boundaries where subduction occurs. The reason why the Tōhoku Earthquake happened was due to the build up in strain energy as the Pacific plate subducted under the Eurasian plate. This strain energy eventually overcame the frictional force holding the plates in place, and was released as earthquake waves of 6.8 magnitude. 

PRIMARY IMPACTS - more than 100,000 buildings were destroyed, collapse of the hanshin exoressway, fires that raged over large portions of the city. 500 people died.

SECONDARY IMPACTS - disruption of the electricity supply,  Between 3% and 5% of Japan’s industry is located in Kobe, damage of industry was severe. 300,000 people were made homeless.worst affected area was the centre. it was built on easily moving ground which LIQUIFIED, allowing building to collapse and sink. cost to rebuild was $100 billion.

HUMAN REACTION - SHORT TERM - evacuated and emergency rations provided.Rescue teams searched for survivors for 10 day. complacency - not had an earthquake in 400 years.

LONG TERM - thousands of seismometers and monitoring stations in Japan designed to give warning. jobs created in construction industry as part of a rebuilding programme. building regulations.

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Earthquake CASE STUDY


CAUSE - destructive plates of Eurasian and Indian plate. The plates moved towards each other and collided. 

IMPACT - magnitude of 7.6. 74,698 people died, hospitals, schools and resuce services including police and armed forces were paralysed. infrastructure and communications were badly affected. 3.3 million were made homeless.Massive landslides occurred burying villages and cutting remote areas off completely.The time of day meant that lots of children had just started their school day, many schools collapsed in the quake killing thousands of school children. SECONDARY - Diseases such as cholera and typhoid spread quickly from dirty water and dead bodies.The cold meant people caught pneunomia.People died as a result of the cold because they only had tents to live in.Cost of the damage was estimated to be $5 billion.

LEDC - relied on international aid - UN, WHO, British red cross. Builiding laws have been tightened on public buildings so that they should withstand future quakes.

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Mount Pinatubo 1991 - On the Island of Luzon in the Philippines, Located at the destructive plate boundary between to the Eurasian and Philippine Plate.chain of volcanoes known as the Luzon volcanic arc, which is the result of the Oceanic Philippine plate being subducted under the lighter Continental Eurasian plate. Oceanic plate is subducted it is melted and forced away as molten magma by the high pressures.then resurfaces as the density of the molten magma becomes lower than that of the rock, and so it pushes it up through the small cracks and explodes out through a volcano. StratavolcanoGlobal cooling caused by ash in the atmosphere of 0.5°C.

IMPACTS - 847 People Killed - 300 from collapsing roofs , 100 from the mud flows known as lahars, the rest from disease in the evacuation centres including measles.Electricity went off, water was contaminated, road links were destroyed, and telephone links were cut.1.2 million people lost their homes around the volcano and had to migrate to shanty towns in Manila.Farmland destroyed by falling ash and pumice, unusable for years, the 1991 harvest was destroyed and 650,000 people lost their jobs.

The PHIVOLCS and USGS (United States Geological Survey) put together the PVO (Pinatubo Volcano Observatory) Which produced a Hazard Map. 

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