An earthquake is a sudden and violent shaking of the ground caused by movements within the Earth's crust or volcanic action.

Here are some key points about earthquakes:

• Cause: Earthquakes are primarily caused by the release of energy when rocks along a fault line suddenly break or slip. This energy is released in the form of seismic waves.
  
• Fault Lines: Most earthquakes occur along fault lines, which are cracks in the Earth's crust where tectonic plates meet.
  
• Seismic Waves: These are vibrations that travel through the Earth, carrying the energy released during an earthquake. There are several types of seismic waves, including P-waves (primary waves), S-waves (secondary waves), and surface waves.
  
• Magnitude: The size of an earthquake is measured using the Richter scale or the moment magnitude scale. These scales quantify the energy released by the earthquake. Each whole number increase on the magnitude scale represents a tenfold increase in amplitude and approximately a 31.6 times increase in energy.
  
• Effects: Earthquakes can cause a variety of effects, including ground shaking, tsunamis (if the earthquake occurs under the ocean), landslides, and damage to buildings and infrastructure.
  

For more detailed information, you can refer to these resources:

• USGS Earthquakes
  
• Ready.gov Earthquakes