The point inside Earth where an earthquake rupture begins is called the hypocenter.
The hypocenter, also known as the focus, is the underground starting point of an earthquake’s rupture. From there, the rupture can spread along a fault, releasing stored strain energy as seismic waves. Its location is described by latitude, longitude, and depth.
The epicenter is often confused with the hypocenter. The epicenter is the point on Earth’s surface directly above the hypocenter; it is not where the rupture begins. Seismologists estimate both locations by comparing the arrival times of different seismic waves at monitoring stations.
Depth matters because earthquakes at different depths can affect the surface in different ways. Shallow earthquakes often produce especially strong shaking nearby, while deeper events may be felt across a broader region. The term “focus” is also a fully established synonym for hypocenter.