Nature and Causes of Seismic Hazards (AQA A Level Geography)

• 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

Global distribution of earthquakes

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

The focus and epicentre of an earthquake

• 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

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	Body wave Fastest Reach the surface first Travel through liquids and solids Cause backwards and forwards shaking Least damaging
Secondary - S waves	Body wave Slower than P waves Only travel through solids Cause a sideways motion More damaging
Love - L waves	Surface wave Slowest Cause a side-to-side motion Larger and energy is focused on the surface Most damaging

Seismic 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

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

Landslides triggered by the Gorkha earthquake

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

Tsunami formation as a result of an earthquake

• 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