Summary of the 'hazardous environments' topic for 9696 CIE A-Level Geography
The summary mostly follows the syllabus and includes:
9.1 Hazards Resulting from tectonic processes, such as earthquakes, volcanoes, tsunamis and their hazards and management
9.2 Hazardous environments resulting from m...
Hazardous environments
9.1 Hazards Resulting from tectonic processes
Global distribution of tectonic hazards
● Include seismic activities, volcanoes and tsunamis
● Most of the world’s earthquakes occur in clearly defined linear patterns, which
usually follow plate boundaries
Earthquakes
● Broad belts of earthquakes are usually associated with subduction zones,
where dense oceanic plates subduct a less dense continental plate
● Narrow belts of earthquakes are associated with constructive (divergent)
plate margins, where new materials are formed and plates move apart
● Collision boundaries (convergent), such as the Himalayas are associated with
broad belts of earthquakes
● There appear to be occurrences of earthquakes related to the isolated plumes
of tectonic activity, known as hotspots
● Earthquakes are series of seismic waves that originate from the focus, the
point at which plates release their tension or compression suddenly
● The epicentre marks the point on the surface, immediately above the focus
● Large have foreshocks and aftershocks
● Aftershocks can be particularly damaging as the building has already been
weakened by the main shock
● Seismic waves are able to travel along the surface of the Earth and also
through the body of the Earth
● Two types of body waves occur(Earth’s interior) P-waves and transverse
S-waves
● P-waves travel by compression and expansion and are able to pass through
rocks, liquids and gases
● S-waves travel in a side-to-side motion and are able to pass through solids
but not liquids and gases as they have no rigidity to support sideways motions
● Seismic activity close to the epicentre showed slow-travelling P and S-waves,
by contrast, those further away from the shock showed faster-moving waves
● These shocks are reflected or refracted when they hit rocks with different
densities
● If the shock waves pass through denser rocks, they speed up
● If they pass through less dense rocks, they slow down
● When P and S-waves travel to the surface, they become surface waves
● Love waves cause the waves to go sideways
, ● Rayleigh waves cause the Earth to go up and down
● Surface waves often cause the most damage
● The nature of the rocks and sediment in the ground influenced the patterns of
shocks and vibration
● Unconsolidated sediment, such as sand, shakes in a less predictable way
than solid rocks, which makes the damage of the earthquake greater
● P-waves from earthquakes can turn solid sediment into fluids like quicksand,
disrupting sub-surface water conditions
● This is known as liquefaction and can wreck the foundations of large buildings
and other structures
Secondary hazards
● Ground failure and soil liquefaction
● Landslides and rockfalls
● Debris flow and mudflow
● Tsunamis
Impacts
● Loss of lives
● Loss of livelihood
● Total or partial destruction of building structures
● Interruption of water supplies
● Breakage of sewage disposal systems
● Loss of public utilities such as electricity and gas
● Floods from collapsed dams
● Release of hazardous materials
● Fires
● Spread of chronic illness
Hazards
● Most earthquakes occur with little warnings in advance
● Some places like Japan and California have earthquake action plans and
information programmes to increase public awareness about what to do in an
earthquake
● Most problems are associated with damage to buildings, structures and
transport systems
, ● The collapse of the building causes many injuries and deaths and reduces the
effectiveness of emergency services
● Aftershocks shake the already weakened structures and they are longer
lasting and more frequent than the main tremors
● Some earthquakes involve surface displacements, which leads to the facture
of pipes and causes damage to the lines of communication
● The cost of repairing such fractures is considerable
● Earthquakes might cause other geomorphological hazards such as landslides,
liquefaction and tsunamis
Earthquakes and plate boundaries
● The movement of oceanic crust into the subduction zone creates some of the
deepest earthquakes recorded from 700 km below the ground
● When the cool oceanic crust slides into the hot fluid mantle, it takes time to
warm up
● As the oceanic crust sinks into the hot, fluid mantle, it bends, breaks and
causes earthquakes along the way
● However, by the time the crust has cracked, it has slipped several hundred of
km down into the mantle
● The chances of an earthquake increase over time if no earthquakes have
occurred in a while
● Plates move at a rate of 1.5 and 7.5 cm a year, however, a large earthquake
can move the plates by a few metres
● Many earthquakes are created due to the pressure created by moving plates
● This increases the stress on the rocks, the rocks deform and eventually give
up and snap
● The snapping releases energy and form an earthquake
● The size of an earthquake depends on the thickness of the descending slab
and the rate of the movement
● The strength of earthquakes is measured by the Richter Scale and the
Mercalli Scale
Richter scale:
● A numerical scale used to measure the magnitude of earthquakes
● It quantifies the amounts of seismic energy released by an earthquake by
measuring the amplitude of seismic waves recorded by seismographs
Mercalli scale:
● A subjective scale that measures the intensity of an earthquake based on
observed effects and damage at specific location s
● It provides a descriptive assessment of an earthquake’s impact on people,
buildings and the environment ranging from 1 (rarely felt) to 12 (total damage)
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