Epicenter

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Epicenter

The epicenter or epicentre is the point on the Earth

Earth

Earth is the third planet from the Sun. Earth is the largest of the terrestrial planets in the Solar System in diameter, mass and density. It is also referred to as the World and Wiktionary:Terra.Note that by International Astronomical Union convention, the term "Terra" is used for naming extensive land masses, rather...
's surface that is directly above the hypocenter

Hypocenter

The hypocenter or hypocentre , refers to the site of an earthquake or to that of a nuclear explosion. In the former, it is a synonym of the focus; in the latter, of ground zero....
or focus, the point where an earthquake

Earthquake

An earthquake is the result of a sudden release of energy in the Earth's crust that creates seismic waves. Earthquakes are recorded with a seismometer, also known as a seismograph....
or underground explosion originates. The word derives from the Greek

Greek language

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The epicenter or epicentre is the point on the Earth

Earth

Hypocenter

Earthquake

Greek language

Greek is an Indo-European languages native to the southern Balkan peninsula, the language of the Greek people. It forms an independent branch within Indo-European....
?p??e?t??? (epikentron), "occupying a cardinal point", from ?p? (epi), "on, upon, at" + ???t??? (kentron), "centre".

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 earthquake

2002 Denali earthquake

The 2002 Denali earthquake occurred at 22:12:41 UTC November 3, 2002, with an epicenter 66 km ESE of Denali National Park, Alaska. This 7.9 Moment magnitude scale earthquake was the largest recorded in the interior of the United States for more than 150 years....
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.

Epicentral distance

During an earthquake, Seismic wave

Seismic wave

Seismic waves are waves that travel through the Earth or other elastic body, for example as the result of an earthquake, explosion, or some other process that imparts forces to the body....
s propagate spherically out from the hypocenter. Seismic shadowing

Seismic shadowing

Seismic shadowing is a global effect of an earthquake. The seismic waves generated by an earthquake pass through the body of the Earth, but between 104? and 140? from the hypocenter of an earthquake, little or no seismic waves can be detected....
occurs on the opposite side of the Earth from the earthquake epicenter because the liquid outer core refracts

Refraction

Refraction is the change in direction of a wave due to a change in its speed. This is most commonly observed when a wave passes from one optical medium to another....
the longitudinal or compressional (P-wave

P-wave

P-waves are type of elastic wave, also called seismic waves, that can travel through gases , elastic solids and liquids, including the Earth....
s) while it absorbs the transverse

Transverse wave

A transverse wave is a moving wave that consists of oscillations occurring perpendicular to the direction of energy transfer. If a transverse wave is moving in the positive x-direction, its oscillations are in up and down directions that lie in the y-z plane....
or shear waves (S-wave

S-wave

A type of seismic wave, the S-wave, secondary wave, or shear wave is one of the two main types of elastic body wave s, so named because they move through the body of an object, unlike surface waves....
s). 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

Seismometer

Seismometers are instruments that measure and record motions of the ground, including those of seismic waves generated by earthquakes, nuclear explosions, and other seismic sources....
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 °

Degree (angle)

A degree , usually denoted by ? , is a measurement of plane angle, representing 1/360 of a Turn ; one degree is equivalent to p/180 radians....
(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

Trilateration

Trilateration is a method for determining the intersections of three sphere surfaces given the centers and radii of the three spheres....
.

Epicentral distance is also used in calculating seismic magnitudes developed by Richter and Gutenberg

Seismic scale

A seismic scale is used to measure and compare the severity of earthquakes. Two fundamentally different but equally important types of scales are commonly used by seismologists to describe earthquakes....
.