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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 New Latin noun epicentrum, the latinisation of the ancient Greek adjective ἐπίκεντρος (epikentros), "occupying a cardinal point, situated on a centre", from ἐπί (epi) "on, upon, at" and κέντρον (kentron) "centre".
In the case of earthquakes, the epicenter is directly above the point where the fault begins to rupture, and in most cases, it is the area of greatest damage. However, in larger events, the length of the fault rupture is much longer, and damage can be spread across the rupture zone. For example, in the magnitude 7.9, 2002 Denali 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.
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 velocities and 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, commonly measured in ° (degrees) and denoted as Δ (delta) in seismology.
Once epicentral distances have been calculated from at least three seismographic measuring stations, it is a simple matter to find out where the epicenter was located using trilateration.
Epicentral distance is also used in calculating seismic magnitudes developed by Richter and Gutenberg.