Causes and main characteristics of an earthquake

STRESS EXCEEDS STRENGTH

1. Earthquakes are causes by the tension that builds up at all three types of plate margins
2. When these plates move past each other it sends out seismic waves, these vibrations are the earthquake
3. The seismic waves spread out from the focus (the place in the atmosphere where the earthquake starts)
4. The epicentre is the point on the Earth's surface where the earthquake is first felt (straight above the focus) 

Other causes of earthquakes:

• Reactivation of old fault lines
• Subsidence as a result of deep mining
• Pressure on surface rocks from water in large reservoirs

There are three types of seismic waves:

• P waves: can travel through solids and liquids, fastest, push and pull.
• S waves: can travel through only solids, waves move the earth at 90' to the direction of travel.
• Surface waves: Travel near the earths crust in either Love waves, through liquids and solids, side to side, or Rayleigh waves, through liquids and solids in a rolling motion

Measurements: 

• Richter scale: measures the magnitude of an earthquake, 1-9+
• Mercalli scale: measures the impacts of an earthquake, 1-12