1. 4 The Geography of Earthquakes
Earthquake activity – how, why and where it occurs
How earthquakes and their effects may be predicted
The effects of earthquake activity on different worldwide examples

2. Chapter 4: The Geography of Earthquakes
What is an earthquake?
Occur due to the movement of tectonic plates
Shaking or vibration of the Earth’s crust
Sudden release of energy along a fault line
Seismic waves/tremors are known as shocks
Three types of shocks:
Fore shock
Main shock
After shock

3. Chapter 4: The Geography of Earthquakes
What are the causes of earthquakes?
Plate movement fuelled by convection currents
Sometimes the plates get stuck
Stress and pressure builds up in the rocks of the plates
Rocks bend under this pressure
Plates slip along the fault line
Energy is suddenly released
Shock waves sent to the Earth’s surface resulting in an earthquake

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The Richter scale
Used to measure the intensity of an earthquake
Point scale, measuring 1 to 10
Each point on the Richter scale is a 10-fold increase in power

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The Mercalli scale
Measures earthquake damage
Scale of 12 points
1 means no damage
12 means complete devastation

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Categories of earthquake
Shallow earthquakes
Intermediate earthquakes
Deep earthquakes

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Shallow earthquakes
Focus is close to the surface
Most common type
Great loss of life and destruction
Occur at plate boundaries
Result of a plate sinking becoming stuck and then releasing

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2. Intermediate earthquakes
Lower depths
Associated with melting plates

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3. Deep earthquakes
Focus is deep in the Earth
Associated with changes in chemical or mineral composition

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Where do earthquakes occur?
Commonly associated with destructive plate boundaries.
They occur:
when two continental plates collide
when a continental and oceanic plate collide

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Three types of fault are associated with earthquake activity:
Normal fault
Thrust fault
Transform fault

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1. Normal faults
Crust of the Earth has vertical movement
Rocks are stretched and pulled apart
Area of crust moves downwards along the fault line
e.g. Great African Rift Valley

13. Chapter 4: The Geography of Earthquakes
2. Thrust faults
Occur where rocks of the Earth’s crust push over one another
Forms an area of block mountains
e.g. Ox mountains, Sligo

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3. Transform faults
Plates slide past each other horizontally
e.g. San Andreas Fault, California, USA

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Seismic waves
The waves that shake the Earth’s crust. There are three types:
P-waves
Primary waves and are the first to be recorded
Fast-moving waves
Compress and expand the material resulting in vibrations forward and backward

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Seismic waves (continued)
2. S-waves
Secondary waves and are second to arrive
Slower-moving waves that move through rock
Causes a right-angled movement/vibration

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Seismic waves (continued)
3. Surface waves
Travel close to the surface and are the last waves
Slowest waves
Arrival is noted by vibrations of the Earth’s crust in an upward and sideways motion

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Earthquake prediction
Seismologists study the history of earthquake activity
Patterns of earthquake activity along a fault line
Greater the time since the last earthquake striking, the more imminent is the next

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Seismic gaps
The time gap between earthquakes
Period of inactivity
Tiltmeters record changes that occur to levels of land
Tell-tale signs, e.g. animal behaviour

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Damage caused by earthquake
Depends on a number of factors:
Time of the earthquake
Magnitude of the earthquake
Depth of the focus
Distance from the epicentre
Duration of the earthquake
Type of rocks and soils
Population within the affected region
Building structures
Utilities of the region

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The effects of earthquakes
Loss of life
Damage to property
Gas leaks
Power cuts
Liquefaction
Fires
Tsunami
Damage to sewage system
Landslides

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Ways to limit earthquake damage
Linked to level of development
Developed economies more earthquake-proof
Less developed or developing economies are most affected
Earthquake-proof buildings
Made of materials that can withstand the pressure of earthquake activity

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Ways to limit earthquake damage (continued)
Columns which are flexible to allow for movement of the building
Rollers to allow for movement from side to side
Flexible materials
Fire-proofing
Build on stable ground
Liquefaction
Emergency plans in place

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Indian Ocean Earthquake, 2004
Occurred under the sea at 00:58 (UTC) on December
Recorded as 9.1 on the Richter scale
Epicentre was off the west coast of Sumatra, Indonesia
Caused by subduction
For approximately 1600 km the fault slipped about 15 m along the subduction zone where the Indian Plate subducts under the Burma Plate

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Indian Ocean Earthquake, 2004
Resulted in a rise of the ocean floor and the displacement of large volumes of water, resulting in the tsunami striking the Indian Ocean coastlines
Generated tsunami waves of some 30 m in height
Affected many countries in south-east Asia and further afield, including Indonesia, Sri Lanka, India, Thailand, the Maldives and Somalia