What characteristic defines a fault line?

A fault line is fundamentally characterized as a fracture in the Earth's crust where rocks on either side have moved relative to one another. This movement can occur as a result of tectonic forces and leads to various geological phenomena, including earthquakes. Fault lines can be identified by the displacement of geological features across them and can vary in length, depth, and activity. Their study is crucial in the field of geology as they play significant roles in determining seismic activity and understanding the structure of the Earth's crust.

The other options refer to different geological concepts—sedimentary rock pertains to types of rock formed from sediment, tools used in geological surveys are instruments for measuring and examining geological features, and measures of geological time relate to the dating of events in Earth's history. None of these options capture the defining aspect of a fault line as accurately as recognizing it as a fracture in the Earth's crust.