The epicenter, epicentre () or epicentrum in

seismology Seismology (; from Ancient Greek σεισμός (''seismós'') meaning "earthquake" and -λογία (''-logía'') meaning "study of") is the scientific study of earthquakes and the propagation of elastic waves through the Earth or through other ...

is the point on the

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's sur ...

's surface directly above a hypocenter or focus, the point where an

earthquake An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, fr ...

or an underground explosion originates.

Surface damage

Before the instrumental period of earthquake observation, the epicenter was thought to be the location where the greatest damage occurred, but the subsurface fault rupture may be long and spread surface damage across the entire rupture zone. As an example, in the magnitude 7.9 Denali earthquake of 2002 in

Alaska Alaska ( ; russian: Аляска, Alyaska; ale, Alax̂sxax̂; ; ems, Alas'kaaq; Yup'ik: ''Alaskaq''; tli, Anáaski) is a state located in the Western United States on the northwest extremity of North America. A semi-exclave of the U ...

, the epicenter was at the western end of the rupture, but the greatest damage was about away at the eastern end. Focal depths of earthquakes occurring in continental crust mostly range from . Continental earthquakes below are rare whereas in

subduction zone Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the ...

earthquakes can originate at depths deeper than .

Epicentral distance

During an earthquake,

seismic wave A seismic wave is a wave of acoustic energy that travels through the Earth. It can result from an earthquake, volcanic eruption, magma movement, a large landslide, and a large man-made explosion that produces low-frequency acoustic ener ...

s propagate in all directions from the hypocenter. Seismic shadowing occurs on the opposite side of the Earth from the earthquake epicenter because the planet's liquid outer core

refracts In physics, refraction is the redirection of a wave as it passes from one medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commonly observed phenomeno ...

the

longitudinal Longitudinal is a geometric term of location which may refer to: * Longitude ** Line of longitude, also called a meridian * Longitudinal engine, an internal combustion engine in which the crankshaft is oriented along the long axis of the vehicle, ...

or compressional (

P-wave A P wave (primary wave or pressure wave) is one of the two main types of elastic body waves, called seismic waves in seismology. P waves travel faster than other seismic waves and hence are the first signal from an earthquake to arrive at any ...

s) while it absorbs the

transverse Transverse may refer to: *Transverse engine, an engine in which the crankshaft is oriented side-to-side relative to the wheels of the vehicle * Transverse flute, a flute that is held horizontally * Transverse force (or ''Euler force''), the tange ...

or shear waves (

S-wave __NOTOC__ In seismology and other areas involving elastic waves, S waves, secondary waves, or shear waves (sometimes called elastic S waves) are a type of elastic wave and are one of the two main types of elastic body waves, so named because th ...

s). Outside the seismic shadow zone, both types of wave can be detected but, because of their different velocities and paths through the Earth, they arrive at different times. By measuring the time difference on any

seismograph A seismometer is an instrument that responds to ground noises and shaking such as caused by earthquakes, volcanic eruptions, and explosions. They are usually combined with a timing device and a recording device to form a seismograph. The outpu ...

and the distance on a travel-time graph on which the P-wave and S-wave have the same separation, geologists can calculate the distance to the quake's epicenter. This distance is called the ''epicentral distance'', commonly measured in ° (degrees) and denoted as Δ (delta) in seismology. The Láska's empirical rule provides an approximation of epicentral distance in the range of 2 000 − 10 000 km. Once distances from the epicenter have been calculated from at least three seismographic measuring stations, the point can be located, using

trilateration Trilateration is the use of distances (or "ranges") for determining the unknown position coordinates of a point of interest, often around Earth (geopositioning). When more than three distances are involved, it may be called multilateration, for e ...

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

Fault rupture

The point at which fault slipping begins is referred to as the focus of the earthquake. The fault rupture begins at the focus and then expands along the fault surface. The rupture stops where the stresses become insufficient to continue breaking the fault (because the rocks are stronger) or where the rupture enters ductile material. The magnitude of an earthquake is related to the total area of its fault rupture. Most earthquakes are small, with rupture dimensions less than the depth of the focus so the rupture doesn't break the surface, but in high magnitude, destructive earthquakes, surface breaks are common. Fault ruptures in large earthquakes can extend for more than . When a fault ruptures unilaterally (with the epicenter at or near the end of the fault break) the waves are stronger in one direction along the fault.

Macroseismic epicenter

The macroseismic epicenter is the best estimate of the location of the epicenter derived without instrumental data. This may be estimated using intensity data, information about foreshocks and aftershocks, knowledge of local fault systems or extrapolations from data regarding similar earthquakes. For historical earthquakes that have not been instrumentally recorded, only a macroseismic epicenter can be given.

Derivation and usage

The word is derived from the

New Latin New Latin (also called Neo-Latin or Modern Latin) is the revival of Literary Latin used in original, scholarly, and scientific works since about 1500. Modern scholarly and technical nomenclature, such as in zoological and botanical taxonomy ...

noun ''epicentrum'', the latinisation of the

ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Dark Ages (), the Archaic pe ...

adjective ἐπίκεντρος (), "occupying a cardinal point, situated on a centre", from ἐπί (''epi'') "on, upon, at" and κέντρον (''kentron'') "

centre Center or centre may refer to: Mathematics *Center (geometry), the middle of an object * Center (algebra), used in various contexts ** Center (group theory) ** Center (ring theory) * Graph center, the set of all vertices of minimum eccentricity ...

". The term was coined by the Irish

seismologist Seismology (; from Ancient Greek σεισμός (''seismós'') meaning "earthquake" and -λογία (''-logía'') meaning "study of") is the scientific study of earthquakes and the propagation of elastic waves through the Earth or through other ...

Robert Mallet Robert Mallet (3 June 1810 – 5 November 1881) was an Irish geophysicist, civil engineer, and inventor who distinguished himself in research on earthquakes and is sometimes called the father of seismology. His son, Frederick Richard Mallet was ...

. It is also used to mean "center of activity", as in "Travel is restricted in the Chinese province thought to be the epicentre of the SARS outbreak." ''

Garner's Modern American Usage ''Garner's Modern English Usage'' (''GMEU''), written by Bryan A. Garner and published by Oxford University Press, is a usage dictionary and style guide (or ' prescriptive dictionary') for contemporary Modern English. It was first published in 1 ...

'' gives several examples of use in which "epicenter" is used to mean "center". Garner also refers to a

William Safire William Lewis Safire (; Safir; December 17, 1929 – September 27, 2009Safire, William (1986). ''Take My Word for It: More on Language.'' Times Books. . p. 185.) was an American author, columnist, journalist, and presidential speechwriter. He ...

article in which Safire quotes a geophysicist as attributing the use of the term to "spurious erudition on the part of writers combined with scientific illiteracy on the part of copy editors". Garner has speculated that these misuses may just be "metaphorical descriptions of focal points of unstable and potentially destructive environments."

References

{{Authority control Seismology Geometric centers