Nature and Causes of Seismic Hazards (AQA A Level Geography)
Revision Note
Written by: Rhiannon Molyneux
Reviewed by: Bridgette Barrett
Distribution Of Earthquakes
The majority of earthquakes (about 95%) occur close to or at a plate margin
Many occur around the 'Ring of Fire' surrounding the Pacific Ocean
Earthquakes occur on all plate margins - constructive, destructive, collision and conservative
The most powerful earthquakes are usually associated with destructive or collision plate margins
Intra-plate earthquakes are those which do not happen at plate margins - these are often linked to hot spots or old fault lines
Forms of Seismic Hazards
Earthquakes
An earthquake is the sudden, violent shaking of the ground
When tectonic plates move, they can become locked together causing stress and pressure to build
Eventually, the stress becomes so great that the rocks fracture and the pressure is suddenly released
This causes intense ground shaking for seconds to minutes
The focus is the point at which the earthquake starts below the Earth's surface: the energy released by the earthquake travels out from the focus
The epicentre is the point on the Earth's surface directly above the focus
Earthquakes can occur anywhere but mostly occur at or near plate margins
Earthquakes happen at all plate margins: constructive, destructive, collision zones and conservative
At a constructive plate boundary, earthquakes tend to be weaker as the plates are moving apart
For example, Iceland is located at the Mid-Atlantic Ridge and experiences lots of earthquakes on a daily basis, though most of them have a magnitude of less than 3.0
At destructive, collision zone and conservative plate boundaries, earthquakes tend to be stronger
For example, the 2011 Tohoku earthquake in Japan which occurred on a destructive margin had a magnitude of 9.0-9.1
Examiner Tips and Tricks
Make sure you can explain how and why earthquakes happen by outlining each stage in turn.
For example:
Tectonic plates move due to convection currents and processes like slab pull and ridge push
Plates get stuck and become locked together due to friction
This causes rocks to deform and leads to a build-up of stress and pressure
When the strength of the rock is exceeded, it fractures (breaks)
This releases the energy in a process known as elastic rebound
The point where the rocks fracture is known as the focus
Seismic waves travel outward from the focus towards the Earth’s surface, causing the ground to shake
The epicentre is the point on the Earth’s surface directly above the focus
Shockwaves
The movement felt during an earthquake is the result of seismic shockwaves
These are the released energy radiating through the Earth
There are three types of seismic waves
Characteristics of Seismic Waves
Wave type | Characteristics |
|---|---|
Primary - P waves |
|
Secondary - S waves |
|
Love - L waves |
|
Liquefaction
Liquefaction occurs when the shaking causes loose or saturated soils to lose their strength
This causes them to act like a liquid rather than a solid and can result in significant damage to buildings and infrastructure
For example in the 2011 Christchurch earthquake in New Zealand, significant liquefaction caused damage to approximately 20,000 properties
Worked Example
Outline the process of liquefaction
[4 marks]
Remember, this answer is point marked with 1 mark for each valid point made with extra marks for developed points (d)
The command word is ‘outline’
The focus of the question is ‘liquefaction
You will gain marks for outlining what liquefaction is, how and where it happens, and what the impacts are
Answer:
Liquefaction occurs when compacted sediments lose strength and stiffness in response to an applied stress such as shaking during an earthquake (1). Material that is ordinarily a solid behaves like a liquid (1) (d). Liquefaction requires a degree of soil saturation to occur (1) (d).
The phenomenon is most often observed in saturated, loose (low density or uncompacted), sandy soils (1). This is because loose sand has a tendency to compress when a load is applied (1) (d). The loss of soil structure causes it to lose its strength (the ability to transfer shear stress), and it may be observed to flow like a liquid (1) (d).
Liquefaction can cause buildings and infrastructure to collapse as well as a significant risk to life as it acts like quick sand (1).
Landslides
A landslide is the downward movement of soil and rock on a slope
The intense shaking during an earthquake may trigger the collapse of material downhill
The risk of landslides is greater where soils are looser, slopes are steeper and where the shaking lasts longer or is particularly intense
For example, in the 2008 Sichuan earthquake in China, approximately 60,000 landslides were a major factor contributing to the devastation
The 2015 Gorka earthquake in Nepal also triggered more than 3000 avalanches and landslides
Tsunamis
When an earthquake occurs beneath the sea bed this can lead to a tsunami
As the sea bed jolts, water is displaced and forced upwards creating a wave
As the waves approach the land they slow and the wavelength becomes compressed
This leads to an increase in wave height: they frequently reach 5-10 metres, but can reach 30 metres
As the wave reaches the shore a vacuum is created and the water recedes rapidly out to sea leaving the sea bed exposed
Tsunamis usually occur close to plate boundaries and are most common in the area surrounding the Pacific Ocean - 'Ring of Fire'
For example, the 2011 Tohoku earthquake in Japan triggered tsunami waves of up to 40 metres which travelled inland for several kilometres in some areas
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