Epicenter

The epicenter or epicentre is the point on the Earth's surface that is directly above the hypocenter or focus, the point where an earthquake or underground explosion originates. The word derives from the Greek ἐπίκεντρον (epikentron), "occupying a cardinal point", from ἐπί {epi), "on, upon, at" + κέντρον (kentron), "centre"^[1].

The epicenter is usually the location of greatest damage. However, in some cases the epicenter is above the start of a much larger event. In these cases, damage may be spread across a larger area with the greatest damage possibly occurring some distance from the epicenter. For example, in the magnitude 7.9, 2002 Denali Fault Earthquake in Alaska, the epicenter was at the western end of the rupture, but the greatest damage occurred about 330 km away at the eastern end of the rupture zone.^[2]

During an earthquake, Seismic waves propagate spherically out from the hypocenter. Seismic shadowing occurs on the opposite side of the Earth from the earthquake epicenter because the liquid outer core refracts the longitudinal or compressional (P-waves) while it absorbs the transverse or shear waves (S-waves). Outside of the seismic shadow zone both types of wave can be detected, but due to their different paths through the Earth, they arrive at different times. By measuring the time difference on any seismograph as well as the distance on a travel-time graph at which the P-wave and S-wave have the same separation, geologists can calculate the distance to the earthquake's epicenter. This distance is called the epicentral distance. Once the epicentral distance has been calculated from at least three seismographic measuring stations, it is a simple matter to find out where the epicenter was located using trilateration.