The hypocenter, also called the focus, is the point within the Earth where an earthquake rupture initiates. Its depth below the surface is a critical parameter that strongly affects the intensity and distribution of ground shaking at the surface.
The hypocenter, also known as the focus, is the point within the Earth at which an earthquake rupture nucleates. It is defined by three coordinates β latitude, longitude, and depth β and represents the initiation point of the rupture, not necessarily the center of energy release. For large earthquakes, the rupture propagates over tens to hundreds of kilometers from the hypocenter, and the centroid of the rupture (the point of maximum energy release) may be far from the hypocenter itself.
Hypocentral depth is one of the most important parameters in earthquake engineering. Shallow earthquakes (depths of 0β20 km) concentrate their energy near the surface and can produce very strong shaking over a localized area, while deeper earthquakes (depths > 70 km) distribute their energy over a broader area but generally produce lower peak intensities. The 1994 Northridge earthquake (depth ~18 km) and the 2003 Bam earthquake in Iran (depth ~7 km) are examples of shallow events that caused severe localized damage. In contrast, the intermediate-depth earthquakes beneath the Zagros Mountains of Iran produce broader but less intense shaking at the surface.
Hypocenters are located by inverting seismic arrival times from multiple stations, typically using a velocity model of the Earth's crust. The precision of hypocentral determination depends on the number and geometry of stations, the accuracy of the velocity model, and the quality of phase picks. Modern networks achieve location uncertainties of 1β5 km for well-recorded events, but uncertainties can be much larger for offshore or deep events. The hypocenter is distinct from the epicenter (the surface projection of the hypocenter) and from the centroid moment tensor (CMT) location, which represents the centroid of moment release over the entire rupture. For moderate-to-large earthquakes, these three locations often differ, and using the wrong one in hazard analysis can introduce significant errors.