Earthquakes happen because of the sudden release of built-up stress in the Earth's crust. The Earth's crust is made of large plates called tectonic plates that are constantly moving but often get stuck at their edges due to friction. When the stress from the plates moving against each other overcomes the friction, the rocks suddenly slip past one another along fractures called faults. This sudden movement releases energy in the form of seismic waves that cause the ground to shake, which is experienced as an earthquake.

Why Earthquakes Occur

• The Earth's crust contains many fractures (faults) where rocks can stick due to friction.
• Tectonic plates move slowly, causing stress to build up where they are stuck.
• When the stress exceeds the strength of the rocks, they snap and slip suddenly.
• This sudden slip releases energy that travels as seismic waves, shaking the ground.

Key Terms

• Fault : A crack in the Earth's crust where rocks slip.
• Focus (hypocenter) : The underground point where the earthquake starts.
• Epicenter : The point on the surface directly above the focus.

Causes of Stress and Movement

• Plate tectonics driving the plates together, apart, or sliding past each other.
• Other natural processes like volcanic activity or landslides can also cause localized seismic waves.
• Human activities such as mining, reservoir filling, or fluid injection can induce small earthquakes.

Thus, earthquakes mainly result from the movement and interaction of tectonic plates and sudden energy release along faults.