The underground point where an earthquake’s fault rupture begins is called the focus.
The focus is also known as the hypocenter. It marks the initial rupture point within Earth’s crust or upper mantle. From there, the rupture may spread along a fault, releasing stored elastic strain as seismic waves.
The epicenter is the focus’s position projected vertically onto Earth’s surface. These terms describe different locations, although news reports sometimes use “epicenter” loosely to mean the general source area. Earthquake depth is measured from the surface down to the focus.
Most earthquakes occur in the shallow crust, but earthquakes in subducting oceanic plates can begin much deeper. The depth of the focus affects how strongly shaking is felt at the surface: shallow earthquakes often produce intense nearby motion, while deeper events may be felt across a wider region.