IGCSE Geography / Hazardous environments
Why earthquakes happen
Edexcel International GCSE Geography (4GE1), chapter 3: Hazardous environments, specification point 3.1c. Free lesson.
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Earthquakes happen when friction locks moving plates, pressure builds in the rock and the rock suddenly snaps, releasing energy as seismic waves. Knowing where the earthquake starts and which hazards follow helps you explain why some earthquakes cause more damage than others.
How pressure builds into an earthquake
Plates are pushed slowly by convection currents in the mantle, moving a few centimetres a year. Friction between rough plate edges can lock them together. The plates keep being pushed, so the rock bends and stores energy. When pressure overcomes friction, the rock snaps and the plates jolt forward. The stored energy travels through the ground as seismic waves. An earthquake is the sudden release of energy stored in rock. Plates do not need to crash into each other: they move slowly, stick, then suddenly slip.
Where an earthquake starts
The focus is underground, where the rock breaks and the energy starts. The epicentre is on the surface directly above the focus, where shaking is usually strongest. Seismic waves spread out from the focus and lose energy as they travel. A shallow focus is close to the surface, so the waves lose less energy before reaching it. In Nepal in 2015, the focus was only about 8 km deep. The waves lost little energy before reaching the surface.
Earthquakes at different plate boundaries
Locking and slipping can happen wherever plates meet, so earthquakes occur at all four types of boundary. The size of the earthquake depends partly on how much rock locks before slipping. Destructive: one plate is forced under another. A huge area of rock can lock, so these produce the largest earthquakes. Collision: two continental plates push together. Nepal in 2015 was a magnitude 7.8 earthquake at a collision boundary. Conservative: plates slide past each other, lock and then jolt, as along the San Andreas Fault. Constructive: plates move apart. Earthquakes here are…
Primary and secondary earthquake hazards
A primary hazard is caused directly by the earthquake. A secondary hazard is set off by the shaking. Hazards are dangers caused by the event. Collapsed buildings and deaths are impacts, so they are not hazards. Ground shaking is the primary hazard. It is caused by seismic waves and is strongest near the epicentre. Landslides are secondary hazards. Shaking loosens rock and soil on steep slopes. In Nepal in 2015, an avalanche hit Everest Base Camp. Liquefaction happens when shaking makes wet, loose ground behave like a liquid. Buildings can sink or tilt. Fires can start…
Why a shallow focus can cause more damage
A shallow focus is close to the surface. The seismic waves therefore travel a shorter distance and lose less energy before they arrive. This makes the ground shaking stronger, so more buildings can collapse. For a three-mark explanation, link the shallow focus to the short wave journey, then to less energy lost and stronger shaking.
More lessons in Hazardous environments
- Natural hazards: types and where they happen
- How tropical cyclones form
- Plates and plate boundaries
- Volcanoes and hotspots
- Measuring hazards
- Why some countries are more vulnerable
- Case study: the Nepal earthquake, 2015
- Case study: the central Italy earthquakes, 2016
- Case studies: Mount Merapi and Mount Ontake
- Why people live with the risk
- Case study: Hurricane Katrina, 2005
- Case study: Typhoon Haiyan, 2013
- Preparing for earthquakes: warning, drills and mapping
- Earthquake-resistant buildings
- After the earthquake: short-term response and relief
- After the earthquake: longer-term planning
- Case study: managing the earthquake in Nepal
- Case study: managing earthquakes in Italy
- Case study: preparing for earthquakes in India
- Hazard fieldwork: collecting the data
- Hazard fieldwork: results, conclusions and evaluation