Hypocenter

The hypocenter, also known as the focus, is the point within the Earth where an earthquake rupture begins. It is the source from which seismic waves originate, and it is distinct from the epicenter, which is the point on the Earth’s surface directly above the hypocenter. The hypocenter is defined by geographic coordinates (latitude and longitude) and a depth beneath the surface.

Locating the hypocenter involves analyzing seismic waves recorded by networks of seismometers. By comparing the arrival times of P-waves and S-waves at multiple stations and applying a model of how waves travel through the Earth, seismologists solve an inverse problem to estimate the hypocentral coordinates and the origin time. The accuracy of the location depends on the density and distribution of stations and the quality of the Earth velocity model used.

Depth estimates are often more uncertain than horizontal coordinates because small errors in the velocity model can translate into larger depth errors. Earthquakes are typically categorized by depth as shallow (0–70 km), intermediate (70–300 km), and deep (300–700 km), with deeper events commonly occurring in subduction zones.

The hypocenter is a key parameter in understanding rupture dynamics, ground shaking, and seismic hazard. It also helps in delineating the fault plane and estimating the potential rupture area. In addition to natural earthquakes, hypocenters are used to locate artificial explosions, such as for monitoring compliance with treaties.