What geological occurrence creates an 'fault line'?

erosion

earthquake

volcanic activity

Generally, an earthquake is the geological occurrence that creates a fault line.

Explanation:

A fault line occurs when the tensional stress in the rocks transcends the elastic capacity of the rocks and a fracture is caused. For a fault line to occur, there has to be a tensional pressure. When a tremor or an earthquake invigorates the tensional pressure from the rock to be released, a rock cracks up in two parts and a fault line appears on the Earth's surface.

Most of the time is earthquakes.