The hypocenter of an earthquake is the subsurface location at which energy stored along a locked fault is first released. The epicenter is that location at the surface, which is closest to the hypocenter.
During an earthquake, rupture propagates from the hypocenter to both sides along the originally locked fault (see “Earthquake,” Fig. 1). The hypocenter is directly located beneath the epicenter, and the distance or depth of the hypocenter, respectively, is the focal depth. Seismic waves initially propagate spherically away from the hypocenter and will be reflected and refracted at, e.g., the boundaries of geological layers, which show a contrast in impedance, which is the product of seismic velocity and rock density. This leads to complicated travel paths of seismic waves arriving at seismic stations.
One of the most important inverse problems of seismology is to find the hypocenter, i.e., the location, where the earthquake nucleates, from the...