Why do earthquakes happen along faults?

A fault is a fracture in Earth’s crust where blocks of rock move relative to one another. As tectonic plates push, pull, or slide past each other, friction can lock the fault so stress accumulates over years or centuries. When the rocks finally break free, the sudden movement releases energy as seismic waves, producing an earthquake. The point where the rupture starts is called the focus or hypocenter, and the point directly above it at the surface is the epicenter. Major fault systems, such as the San Andreas Fault in California, are closely watched because they can generate large damaging earthquakes.