Fault systems are geological structures formed by the movement of the Earth's crust. They occur when stress builds up in rocks, causing them to break and slide past each other along a fault line. These movements can lead to earthquakes, which are sudden releases of energy that occur when the stress exceeds the strength of the rocks. Fault systems can vary in size and complexity, with some extending for hundreds of kilometers. They are often classified into different types, such as normal faults, reverse faults, and strike-slip faults, based on the direction of movement. Understanding these systems is crucial for assessing earthquake risks and planning construction in affected areas.