What causes earthquakes?
Short answer
The Earth's crust is broken into tectonic plates that move slowly on the mantle beneath. Where plates meet, friction can lock them together and pressure builds up. When the pressure is suddenly released, the rocks slip and send out seismic waves, which we feel as an earthquake. The point underground where this happens is the focus; the point on the surface above it is the epicentre.
Step by step
- Convection currents in the mantle move the tectonic plates a few centimetres each year.
- At plate boundaries, plates move towards, away from or past each other.
- Friction stops the plates moving smoothly and stress builds up in the rock.
- The rock suddenly fractures or slips, releasing energy as seismic waves.
- The waves are strongest near the epicentre and weaken with distance.
Worked example
A case study to learn
- Japan lies where several plates meet, including the Pacific and Philippine Sea plates
- In 2011, a magnitude 9.0 earthquake off the coast caused a tsunami
- Use it to explain both the cause and the effects
Common mistakes
- Confusing the focus with the epicentre.
- Saying earthquakes only happen at destructive boundaries. They happen at all types of boundary.
- Forgetting to explain why the plates move.
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