Earthquakes (SL IB Geography)

Earthquake characteristics

• An earthquake is the sudden, violent shaking of the ground
• Earthquakes are the result of pressure building when tectonic plates move
• The violent shaking of the ground is the release of this pressure as energy travelling through the crust
• The epicentre is the point on the Earth's surface directly above the focus
• 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

Earthquake features

• The magnitude (amount of energy released) by earthquakes is measured on the Moment Magnitude Scale, which replaced the Richter scale
• Seismometers are used to measure the magnitude
• The damage caused by earthquakes is measured on the Mercalli Scale

Seismic waves

• The movement felt during an earthquake is the result of seismic waves
• 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 waves Fastest Reach the surface first Travel through liquids and solids Cause backwards and forwards shaking Least damaging
Secondary - S Waves	Body waves Slower than P waves Only travel through solids Cause a sideways motion More damaging
Love - L Waves	Surface waves  Slowest Cause a side-to-side motion Larger and energy is focussed on the surface Most damaging Also known as Q waves

• The differences in the seismic waves can be seen in the effect they have on the crust

Seismic waves and their effect on the crust

• Earthquakes can occur anywhere but mostly occur at or near plate boundaries
• Earthquakes happen at all plate boundaries: constructive, destructive, collision zones and conservative 
• Plate movement leads to the build-up and then release of pressure

Constructive plate boundary

• At a constructive plate boundary, earthquakes are weaker as the plates are moving apart

A constructive plate boundary

Destructive plate boundary

• At destructive, collision and conservative plate boundaries, earthquakes are stronger
• At destructive boundaries, the narrow area where earthquakes tend to occur in the subduction zone is known as the Benioff Zone

Destructive plate boundary

Collision plate boundary

Collision boundary

Conservative plate boundary

Conservative plate boundary – note that conservative plate boundaries can also move in the same direction but at different speeds

• Human activity may trigger earthquakes
• Evidence suggests that dam building and resource extraction may trigger earthquakes

Dam building

• The building of dams leads to the formation of a reservoir
• The added weight of the water in the reservoir adds stress to fault lines, causing them to fracture:
• The weight of water in the Three Gorges Dam is 84 trillion pounds
• Additionally, water seeps into cracks along the fault line, changing the pressure levels
• Scientists believe that the Sichuan earthquake was in part caused by the construction of the Zipingpu Dam:
• The reservoir at Zipingpu contains water weighing 315 million metric tons
• The added weight weakened the fault and increased the stress 
• The effect of this was 25 times that of a year’s worth of natural stress 
• The earthquake in 2008 killed over 80,000 people

Resource extraction

• The injection of water at high pressure is used to crack rock formation in hydraulic fracturing or fracking
• This allows gas to be extracted
• This was the cause of the 2.9-magnitude Lancashire earthquake in the UK in 2019 

Tsunami

• When an earthquake occurs beneath the sea bed this can lead to a tsunami:
• As the sea bed jolts due to the release of pressure, water is displaced and forced upwards, creating a wave
• As the wave approaches the land it slows and the wavelength becomes compressed:
• This leads to an increase in wave height; they frequently reach 5–10 metres, but can reach as high as 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

• Other causes of tsunami include:
• Landslides, which may be due to earthquake and volcanic eruptions displacing the water
• Underwater volcanic eruptions
• Rarely, they can be caused by a meteor strike 
• Tsunami usually occur close to plate boundaries and are most common in the area surrounding the Pacific Ocean – the “Ring of Fire”

Liquefaction 

• Liquefaction occurs when the shaking causes particles in the ground to move further apart, causing it to act like a liquid rather than a solid
• The process occurs when saturated, unconsolidated soil is affected by the s-waves in an earthquake
• This causes the water-filled pore spaces to collapse
• It increases the water pressure and the soil particles can move more freely
• This causes the soil to behave like a liquid and buildings to collapse

Liquefaction