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Earthquakes are the unexpected uncontrollable shaking of the ground.

• The plates in the Earth's crust are slowly moving on top of the mantle.
• Sometimes, plates try to move but become stuck together due to friction.
• Pressure builds up because the plates are still trying to move. When the pressure is released suddenly, huge amounts of energy are released causing earthquakes.

The point inside the Earth's crust where the earthquake begins is called the focus. The earthquake's energy is released in seismic waves which spread out in all directions from the focus.

• Primary or longitudinal waves move the ground up and down.
• Secondary or transverse waves cause side to side movement.

The seismic waves are most powerful at the epicentre. This is the point on the Earth's surface directly above the focus. The focus can be shallow or deep inside the crust. The shallower the focus the stronger the earthquake effects will be at the surface.