Here's why:

* Earthquakes are caused by the movement of tectonic plates: Earthquakes occur when the massive plates that make up the Earth's outer layer (the lithosphere) move, interact, and cause stress.

* Foci mark the source of the earthquake: The focus (or hypocenter) of an earthquake is the point underground where the rupture starts. This point is usually located along the fault line, the zone of weakness where the plates are moving.

* Fault lines define plate boundaries: Plate boundaries are the areas where tectonic plates interact. These interactions cause the formation of fault lines, which are often the sites of earthquakes.

By mapping the locations of earthquake foci, geologists can identify the boundaries of tectonic plates. These boundaries are characterized by increased seismic activity and are often associated with other geological features like volcanoes and mountain ranges.