Earthquakes (DP IB Geography)

Earthquakes

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

• 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

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

Causes of an Earthquake

• 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

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

Collision plate boundary

Conservative plate boundary

Human Triggers of Earthquakes

• 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 

Secondary Hazards of Earthquakes

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

• 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