Physical Processes &Tectonic Hazards (Edexcel A Level Geography)

• 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 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 focussed on the surface Most damaging

Seismic waves

Primary and secondary hazards

• Every earthquake is unique but they have common hazards which are either
  • Primary (a direct result of the earthquake)
  • Secondary (a result of the primary hazards)

Primary and Secondary Earthquake Hazards

Primary hazards	Secondary hazards
Ground shaking Crustal fracturing (when the movement causes the Earth's crust to crack)	Landslides and avalanches - the movement of the Earth may trigger the collapse of material down steep slopes Liquefaction - when the shaking causes particles in the ground to move further apart causing them to act like a liquid rather than a solid Flooding - caused by tsunami

Primary and secondary hazards

• Although they are all unique, volcanic eruptions have common hazards
  • Primary (a direct result of the eruption)
  • Secondary (a result of the primary hazards)

Primary and Secondary Volcano Hazards

Primary hazards	Secondary hazards
Pyroclastic flow - a mix of dense, hot, rock, ash and gases Lava flow - most move slowly enough that they are not a risk to human life but can reach over 1000oC Ash falls - can travel many km, causing injuries, damage, deaths and disruption to transport Gas eruptions - gases trapped in the magma are released during an eruption, they may form gas clouds which are hazardous to health	Lahars - a mixture of rocks, mud and water which flow down the volcano. They are fast-flowing and destroy everything in their path Jökulhlaups - floods caused by a sudden release of water and rocks when glacial ice is melted by the eruption

• 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 wave approaches the land they slow and the wavelength becomes compressed 
    • This leads to an increase in wave height: they frequently reach 5-10 meters, but can reach 30 meters 
  • 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 earthquakes 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 - 'Ring of Fire'