2.3 Distribution of earthquakes, volcanoes and tectonic hazards

Question 1

How are earthquakes distributed?

Answer 1

• occur along all types of plate boundaries.
• largest concentration of earthquakes is at the Pacific Ring of Fire.
• At plate boundaries, plates are pushed by tectonic forces, stress builds up and energy is eventually released.
  —————
  Examples:
• Broad belt of earthquakes along convergent plate boundaries in the Pacific Ring of Fire.
• Narrow belt of earthquakes along the divergent plate boundary at the Mid-Atlantic Ridge.
• Narrow belt of earthquakes along the transform plate boundary at the San Andreas Fault.
• Earthquakes occur more commonly along convergent plate boundaries at subduction zones as more stress is built up during subduction.
• Exception: Some earthquakes may occur away from plate boundaries, such as within the Eurasian plate.

Question 2

Pacific ring of fire

Answer 2

• Broad belt around the Pacific Ocean where most earthquakes and active volcanoes occur.
• Comprise all three types of plate boundaries.
  —————
  Examples:
• Convergent: Pacific plate moving towards the Philippine plate. (PP)
• Divergent: Pacific plate moving away from the Nasca plate. (PN)
• Transform : Pacific plate sliding past the North American plate at the San Andreas Fault. (NAPS)

Question 3

How are volcanoes distributed

Answer 3

• Volcanoes are generally located near convergent and divergent plate boundaries.
• At these plate boundaries, magma rises up to the surface, forming volcanoes.
• Belts of volcanoes may be observed along:
  Convergent plate boundaries: A belt of volcanoes along subduction zones in the Pacific Ring of Fire.
  Divergent plate boundaries: A belt of volcanoes along divergent plate boundaries between the North American and Eurasian plates. (NEA)
• Unlike earthquakes, volcanoes are not found near transform plate boundaries & CC-CC convergent plate boundaries.
• At these boundaries, magma does not rise to the Earth’s surface to form volcanoes.
• Hot spot volcanoes are exceptions that can be found away from plate boundaries.
• Example: The Piton de la Fournaise is a hot spot volcano located on the island of Réunion in the Indian Ocean, away from plate boundaries.

Question 4

Earthquake hazards

Answer 4

1. Ground Shaking
2. Soil liquefaction
3. Landslides
4. Tsunamis
   —————
   - Earthquake hazards, except tsunamis, are often localised.
   - They are generally located within the same geographic region as the earthquake.
   - As earthquakes are found along plate boundaries, their hazards are also found near plate boundaries.
   - However, not all earthquake hazards are localised.
   - tsunamis spread beyond the geographic regions where earthquakes are located.
   - Tsunami waves can travel long distances without great loss of energy.
   Example:
   - The 2004 Indian Ocean tsunami travelled almost 5000km to Africa, with sufficient energy to cause deaths and destruction.

Question 5

Volcanic hazards

Answer 5

• tephra
• volcanic gases
• lava flows
• pyroclastic flows
• lahars
• volcanic landslides
  —————
• volcanic hazards such as volcanic gases, lava flows, pyroclastic flows, lahars and volcanic landslides are often found around the volcanoes.
• Since volcanoes are found along convergent and divergent plate boundaries, these hazards are also generally found along these plate boundaries.
• However, not all tectonic hazards are localised.
• volcanic ash may spread beyond the geographic regions volcanoes are located.
  —————
  Example:
• During the eruption of Mount Pinatubo in 1991, ash spread across Southeast Asia.
• However, whether the dispersal of ash is localised or spreads far beyond the geographic region of the volcano is dependent on wind conditions as well as the volume of ash erupted and the height the ash is thrown into the atmosphere.