Earthquakes Revision

Earthquakes Revision

The nature, causes and effects of earthquakes

• Causes: Earthquakes are caused by movements in the Earth's crust along a fault line. This can occur when two plates collide, or as two plates slide past each other. • A well-known example of two plates sliding past each other is the San Andreas fault in California. This is called a passive plate boundary.

Nature of earthquakes •

The exact place where earthquakes start below the Earth's surface is called the focus.

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The point on the Earth's surface directly above the focus is called the epicentre of the earthquake.

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From its starting point at the focus, a series of shock waves pass through the rocks of the Earth's crust. They move out in different directions like ripples in a pond. (Imagine a large stone thrown into a pond. The point at which it hits the water could be compared to the focus of an earthquake. The ripples are strongest closest to the stone's entry point, and diminish in power further away).

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In the same way, the power of the shockwaves get less the further they move from the focus.

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These shock waves cause the ground to shake, buildings and bridges to collapse, roads to crack, burst water and gas pipes and collapsing power lines. This often leads to death and destruction.

Nature of earthquakes • 'Shallow' earthquakes do the most damage, because they are close to the earth's surface. Here the focus is usually less than 40 kms below the earth's surface, resulting in powerful shockwaves reaching the surface. • The strength of an earthquake is recorded on a seismograph, and given a value of 1-10 on the Richter scale. This scale allows for comparison of the strength and severity of earthquakes.

Effects of earthquakes • Primary or immediate effects: collapsing buildings, bridges, roads, burst gas mains or water pipes. The collapsing buildings may cause immediate deaths. • Secondary or after-effects: These are the delayed results of earthquakes. • Fires: caused by burst gas pipes and collapsed live electricity cables. Many lives may be lost as a result of fires breaking out in the chaotic aftermath of an earthquake. • Landslides: In steep areas earthquakes destabilise the underlying sand and rock, resulting in landslides • Tsunamis: When earthquakes occur under the sea, the shockwaves can cause huge tidal waves called tsunamis. • Disease: Often water supplies are damaged during earthquakes and no fresh water is available. After severe earthquakes people may be housed at temporary shelters, which are overcrowded. This leads to the spreading of diseases such as cholera.

Human responses to the earthquake hazard • The best way for people to cope with the threat of earthquakes is preparation and predication. • Preparation in anticipation of an earthquake:

Human responses to the earthquake hazard

Prediction of earthquakes to allow for evacuation: Using sensitive instruments to monitor the build up of pressure below the earth's surface. This allows scientists to predict the location of an earthquake, which allows enough time for evacuation. At present this is not reliable enough to predict earthquakes.

Tectonic hazards • Tectonic hazards are those disasters caused by earth movement.

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Dangers of tectonic hazards include earthquakes and volcanoes destruction of buildings loss of life destruction of farmland fires

• economic losses

Tectonic hazards Reasons why people choose to stay in these high risk areas

• Inertia: reluctance to move once settled • Volcanic soil is fertile • Volcanic areas can utilise steam and hot water for geothermal production • Lack of farming space • Tourist attraction • Mineral deposits

A comparison of the damage done by these tectonic hazards in urban and rural areas

• urban areas have a higher density of people and buildings than rural areas, and thus usually suffer more loss of life and a higher economic cost • bridges collapse, the economy is affected

A comparison of the effects of tectonic hazards in MEDCs and LEDCs

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MEDCs cope better with natural disasters because

• they have better monitoring equipment to predict impending disasters • of stricter building controls • of more effective relief measure • of better infrastructure • there is access to trauma counselling

A comparison of the effects of tectonic hazards in MEDCs and LEDCs

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LEDCs cope badly with natural disasters because

• they have little or no monitoring equipment to predict impending disasters • few building controls • of poor quality building materials • of ineffective relief measure • of poor infrastructure • there is access to trauma counselling