Earthquakes happen primarily due to the sudden release of built-up stress in the Earth's crust along fractures called faults. The Earth's crust is made up of tectonic plates that constantly move, but they can get stuck at their edges due to friction. Over time, stress builds up from this movement until the rocks suddenly slip past each other, releasing energy in the form of seismic waves that cause the ground to shake. This process is known as the elastic rebound theory. Most earthquakes occur along the boundaries of tectonic plates where these stresses are greatest, but they can also happen inside plates due to reactivation of old faults or other geological processes.

In summary, earthquakes occur because:

• The Earth's crust is fractured into tectonic plates.
• These plates move slowly but can get stuck due to friction.
• Stress accumulates at these stuck points.
• When the stress exceeds the strength of the rocks, they slip suddenly.
• This slip releases energy as seismic waves, shaking the ground.

Other causes can include volcanic activity, landslides, or human activities like fracking, but tectonic plate movements along faults remain the primary cause of earthquakes.