Editing Epicenter

{{short description|Point on the Earth's surface that is directly above the hypocentre or focus in an earthquake}}
{{Other uses}}
{{Use American English|date=August 2020}}
[[File:Epicenter Diagram.svg|thumb|right|The epicenter is directly above the [[earthquake]]'s [[hypocenter]] (also called the {{em|focus}}).]]
{{Earthquakes}}
The '''epicenter''' ({{IPAc-en|ˈ|ɛ|p|ɪ|ˌ|s|ɛ|n|t|ər}}), '''epicentre''', or '''epicentrum'''<ref>''Oxford English Dictionary'': "The point over the centre: applied in Seismol. to the outbreaking point of earthquake shocks."</ref> in [[seismology]] is the point on the [[Earth]]'s surface directly above a [[hypocenter|hypocenter or focus]], the point where an [[earthquake]] or an underground explosion originates.

==Determination==
[[Image:1906_loma_prieta.png|right|thumb|Modern and historic seismograms]]
The primary purpose of a [[seismometer]] is to locate the initiating points of earthquake epicenters. The secondary purpose, of determining the 'size' or [[Moment magnitude scale|magnitude]] must be calculated after the precise location is known.{{citation needed|date=October 2023}}

The earliest seismographs were designed to give a sense of the direction of the first motions from an earthquake. The Chinese frog seismograph<ref>{{cite web|url=http://engagetolearn.com/ETL/quakeimages/ChineseSeismograph.jpg|archive-url=https://web.archive.org/web/20190916022456/http://engagetolearn.com/ETL/quakeimages/ChineseSeismograph.jpg|archive-date=2019-09-16|title=Chinese Seismograph|format=jpg|access-date=2023-08-11}}</ref> would have dropped its ball in the general compass direction of the earthquake, assuming a strong positive pulse. We now know that first motions can be in almost any direction depending on the type of initiating rupture ([[focal mechanism]]).<ref>{{Cite web|url=http://quake.wr.usgs.gov/recenteqs/beachball.htm|title=USGS Earthquake Hazards Program|access-date=2023-09-06|archive-date=2005-12-22|archive-url=https://web.archive.org/web/20051222083702/http://quake.wr.usgs.gov/recenteqs/beachball.htm|url-status=dead}}</ref>

The first refinement that allowed a more precise determination of the location was the use of a [[time]] scale. Instead of merely noting, or recording, the absolute motions of a [[pendulum]], the [[displacement (vector)|displacement]]s were plotted on a moving graph, driven by a [[clock]] mechanism. This was the first [[seismogram]], which allowed precise timing of the first [[ground motion]], and an accurate plot of subsequent motions.

From the first seismograms, as seen in the figure, it was noticed that the trace was divided into two major portions. The first seismic wave to arrive was the [[P wave]], followed closely by the [[S wave]]. Knowing the relative 'velocities of propagation', it was a simple matter to calculate the distance of the earthquake.<ref>{{Cite web|url=http://www.geo.mtu.edu/UPSeis/locating.html|title=How Can I Locate the Earthquake Epicenter?}}</ref>

One seismograph would give the distance, but that could be plotted as a circle, with an infinite number of possibilities. Two seismographs would give two intersecting circles, with two possible locations. Only with a third seismograph would there be a precise location.

Modern earthquake location still requires a minimum of three seismometers. Most likely, there are many, forming a seismic array. The emphasis is on precision since much can be learned about the [[fault mechanics]] and [[seismic hazard]], if the locations can be determined to be within a kilometer or two, for small earthquakes. For this, computer programs use an iterative process, involving a 'guess and correction' algorithm.<ref>{{Cite web|url=http://quake.wr.usgs.gov/info/eqlocation|title=USGS Earthquake Hazards Program|access-date=2023-09-06|archive-date=2005-12-18|archive-url=https://web.archive.org/web/20051218055744/http://quake.wr.usgs.gov/info/eqlocation/|url-status=dead}}</ref> As well, a very good model of the local crustal [[seismic velocity|velocity]] structure is required: seismic velocities vary with the local geology. For P waves, the relation between velocity and [[bulk density]] of the medium has been quantified in [[Gardner's relation]].

==Surface damage==
Before the instrumental period of earthquake observation, the epicenter was thought to be the location where the greatest damage occurred,<ref>{{cite book|title=The Geology of Earthquakes|first1=R. S.|last1=Yeats|first2=K. E.|last2=Sieh|authorlink2=Kerry Sieh|first3=C. R.|last3=Allen |authorlink3=Clarence Allen (geologist)|year=1997|isbn=978-0-19-507827-5|publisher=[[Oxford University Press]]|page=64}}</ref> but the subsurface [[Fault (geology)|fault]] rupture may be long and spread surface damage across the entire rupture zone. As an example, in the magnitude 7.9 [[2002 Denali earthquake|Denali earthquake of 2002]] in [[Alaska]], the epicenter was at the western end of the rupture, but the greatest damage was about {{cvt|330|km|||}} away at the eastern end.<ref>{{cite web|url=http://pubs.usgs.gov/fs/2003/fs014-03/|title=Rupture in South-Central Alaska – The Denali Fault Earthquake of 2002|publisher=United States Geological Survey|first=Gary|last=Fuis|author2=Wald, Lisa|access-date=2008-04-20}}</ref> Focal depths of earthquakes occurring in continental crust mostly range from {{convert|2|to|20|km|sp=us}}.<ref name="JordanGrotzinger2012">{{Cite book|title=The essential Earth|last1=Jordan|first1=Thomas H.|date=2012|publisher=W. H. Freeman|last2=Grotzinger|first2=John P.|isbn=9781429255240|edition=2nd|location=New York|page=429|oclc=798410008}}</ref> Continental earthquakes below {{cvt|20|km|||}} are rare whereas in [[subduction]] zone earthquakes can originate at depths deeper than {{cvt|600|km|||}}.<ref name="JordanGrotzinger2012"/>

==Epicentral distance==
{{Main|Epicentral distance}}
During an earthquake, [[seismic wave]]s propagates in all directions from the hypocenter. [[Seismic shadowing]] occurs on the opposite side of the Earth from the earthquake epicenter because the planet's [[Earth's outer core|liquid outer core]] [[refraction|refracts]] the [[longitudinal waves|longitudinal]] or compressional ([[P wave]]s) while it absorbs the [[transverse wave|transverse]] or shear waves ([[S wave]]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 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 [[Degree (angle)|°]] (degrees) and denoted as Δ (delta) in seismology. The [[Václav Láska (mathematician)|Láska's empirical rule]] provides an approximation of epicentral distance in the range of 2,000−10,000&nbsp;km.

Once distances from the epicenter have been calculated from at least three seismographic measuring stations, the point can be located, using [[trilateration]].

Epicentral distance is also used in calculating [[Seismic scale#Local magnitude scale and related scales|seismic magnitudes as developed by Richter and Gutenberg]].<ref>{{cite web|publisher=United States Geological Survey|author=Tyler M. Schau|url=http://www.johnmartin.com/earthquakes/eqsafs/safs_693.htm|title=The Richter Scale (ML)|year=1991|access-date=2008-09-14|archive-date=2016-04-25| archive-url=https://web.archive.org/web/20160425121745/http://www.johnmartin.com/earthquakes/eqsafs/safs_693.htm|url-status=dead}}</ref><ref>{{cite web|publisher=United States Geological Survey|author=William L. Ellsworth|url=http://www.johnmartin.com/earthquakes/eqsafs/safs_694.htm|title=Surface-Wave Magnitude (M<sub>s</sub>) and Body-Wave Magnitude (mb)|year=1991|access-date=2008-09-14|archive-date=2009-02-02| archive-url=https://web.archive.org/web/20090202043713/http://www.johnmartin.com/earthquakes/eqsafs/safs_694.htm|url-status=dead}}</ref>

==Fault rupture==
{{Main|Earthquake rupture}}
The point at which fault slipping begins is referred to as the focus of the earthquake.<ref name="JordanGrotzinger2012"/> 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.<ref name="JordanGrotzinger2012"/> The magnitude of an earthquake is related to the total area of its fault rupture.<ref name="JordanGrotzinger2012"/> 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.<ref name="JordanGrotzinger2012"/> Fault ruptures in large earthquakes can extend for more than {{cvt|100|km|||}}.<ref name="JordanGrotzinger2012"/> 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.<ref>{{Cite web|url=https://earthquake.usgs.gov/data/rupture/directivity.php|title=What is Directivity?|publisher=United States Geological Survey|access-date=2018-07-01}}</ref>

==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.<ref>{{cite book|last1=Musson|first1=R. M. W.|last2=Cecić|first2=I.|chapter=49 Macroseismology |title=International Geophysics|date=1 January 2002|volume=81|pages=807–822|chapter-url=https://www.sciencedirect.com/science/article/abs/pii/S007461420280256X|publisher=Academic Press|doi=10.1016/S0074-6142(02)80256-X|isbn=9780124406520}}</ref>

==Etymology==
The word is derived from the [[Neo-Latin]] noun ''epicentrum'',<ref>{{Cite web|publisher=Merriam-Webster Dictionary|title=epicenter|year=2009|url=http://www.merriam-webster.com/dictionary/epicenter|access-date=2009-10-19}}</ref> the [[Latinisation (literature)|latinisation]] of the [[ancient Greek]] adjective ἐπίκεντρος ({{transliteration|grc|epikentros}}), "occupying a cardinal point, situated on a centre",<ref>[https://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0057%3Aentry%3De%29pi%2Fkentros ἐπίκεντρος], Henry George Liddell, Robert Scott, ''A Greek-English Lexicon'', on Perseus</ref> from ἐπί (''epi'') "on, upon, at"<ref>[https://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0057%3Aentry%3De%29pi%2F2 ἐπί], Henry George Liddell, Robert Scott, ''A Greek-English Lexicon'', on Perseus</ref> and κέντρον (''kentron'') "[[wikt:center|centre]]".<ref>[https://web.archive.org/web/20100925151005/http://oxforddictionaries.com/view/entry/m_en_gb0269590#m_en_gb0269590 epicentre], on Oxford Dictionaries</ref> The term was coined by Irish [[seismologist]] [[Robert Mallet]].<ref>{{cite book|last=Filiatrault|first=A.|title=Elements of Earthquake Engineering and Structural Dynamics|publisher=Presses inter Polytechnique|date=2002|edition=2nd|page=1|isbn=978-2-553-01021-7|url=https://books.google.com/books?id=zEqHNr4YHkgC&pg=PR7}}</ref>

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."<ref>{{cite book|author=Rick Thompson|title=Writing for Broadcast Journalists|url=https://books.google.com/books?id=u_fiOtsJL8UC|year=2004|publisher=Routledge|isbn=978-1-134-36915-7|page=160}}</ref><ref>{{cite book|last=Oltermann|first=P.|title=How to Write|url=https://books.google.com/books?id=LMueSHGvPjwC&pg=PA246|year=2009|publisher=Random House|isbn=978-0-85265-138-4|page=246}}</ref> ''[[Garner's Modern American Usage]]'' gives several examples of use in which "epicenter" is used to mean "center". [[Bryan A. Garner|Garner]] also refers to a [[William Safire]] 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".<ref>{{Cite news|last=Safire|first=William|date=2001-05-06|title=On Language|page=22|work=The New York Times Magazine|url=https://www.nytimes.com/2001/05/06/magazine/the-way-we-live-now-5-6-01-on-language-fulminations.html|access-date=2022-10-17|archive-url=https://web.archive.org/web/20221017100755/https://www.nytimes.com/2001/05/06/magazine/the-way-we-live-now-5-6-01-on-language-fulminations.html?searchResultPosition=3|archive-date=2022-10-17}}</ref> Garner has speculated that these misuses may just be "metaphorical descriptions of focal points of unstable and potentially destructive environments."<ref>{{cite book|url=https://books.google.com/books?id=FwmQpyibKkAC&pg=PA310|last=Garner|first=B. A.|title=Garner's Modern American Usage|publisher=Oxford University Press|year=2009|page=310|isbn=9780199888771}}</ref>

==References==
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[[Category:Seismology]]
[[Category:Geometric centers]]
[[Category:Geographic position]]