Epicenter

Theepicenter(/ˈɛpɪˌsɛntər/),epicentre,orepicentrum^[1]inseismologyis the point on theEarth's surface directly above ahypocenter or focus,the point where anearthquakeor an underground explosion originates.

Determination[edit]

The primary purpose of aseismometeris to locate the initiating points of earthquake epicenters. The secondary purpose, of determining the 'size' ormagnitudemust be calculated after the precise location is known.^{[citation needed]}

The earliest seismographs were designed to give a sense of the direction of the first motions from an earthquake. The Chinese frog seismograph^[2]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).^[3]

The first refinement that allowed a more precise determination of the location was the use of atimescale. Instead of merely noting, or recording, the absolute motions of apendulum,thedisplacementswere plotted on a moving graph, driven by aclockmechanism. This was the firstseismogram,which allowed precise timing of the firstground 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 theP-wave,followed closely by theS-wave.Knowing the relative 'velocities of propagation', it was a simple matter to calculate the distance of the earthquake.^[4]

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 thefault mechanicsandseismic 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.^[5]As well, a very good model of the local crustalvelocitystructure is required: seismic velocities vary with the local geology. For P-waves, the relation between velocity andbulk densityof the medium has been quantified inGardner's relation.

Surface damage[edit]

Before the instrumental period of earthquake observation, the epicenter was thought to be the location where the greatest damage occurred,^[6]but the subsurface fault rupture may be long and spread surface damage across the entire rupture zone. As an example, in the magnitude 7.9Denali earthquake of 2002inAlaska,the epicenter was at the western end of the rupture, but the greatest damage was about 330 km (210 mi) away at the eastern end.^[7]Focal depths of earthquakes occurring in continental crust mostly range from 2 to 20 kilometers (1.2 to 12.4 mi).^[8]Continental earthquakes below 20 km (12 mi) are rare whereas insubduction zoneearthquakes can originate at depths deeper than 600 km (370 mi).^[8]

Epicentral distance[edit]

During an earthquake,seismic wavespropagates in all directions from the hypocenter.Seismic shadowingoccurs on the opposite side of the Earth from the earthquake epicenter because the planet'sliquid outer core refractsthelongitudinalor compressional (P-waves) while it absorbs thetransverseor shear waves (S-waves). 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 theepicentral distance,commonly measured in°(degrees) and denoted as Δ (delta) in seismology. TheLáska's empirical ruleprovides 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, usingtrilateration.

Epicentral distance is also used in calculatingseismic magnitudes as developed by Richter and Gutenberg.^[9]^[10]

Fault rupture[edit]

The point at which fault slipping begins is referred to as the focus of the earthquake.^[8]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.^[8]The magnitude of an earthquake is related to the total area of its fault rupture.^[8]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.^[8]Fault ruptures in large earthquakes can extend for more than 100 km (62 mi).^[8]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.^[11]

Macroseismic epicenter[edit]

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.^[12]

Etymology[edit]

The word is derived from theNeo-Latinnounepicentrum,^[13]thelatinisationof theancient Greekadjective ἐπίκεντρος (epikentros), "occupying a cardinal point, situated on a centre",^[14]from ἐπί (epi) "on, upon, at"^[15]and κέντρον (kentron) "centre".^[16]The term was coined by Irishseismologist Robert Mallet.^[17]

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."^[18]^[19]Garner's Modern American Usagegives several examples of use in which "epicenter" is used to mean "center".Garneralso refers to aWilliam Safirearticle 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".^[20]Garner has speculated that these misuses may just be "metaphorical descriptions of focal points of unstable and potentially destructive environments."^[21]

References[edit]

^Oxford English Dictionary:"The point over the centre: applied in Seismol. to the outbreaking point of earthquake shocks."
^"Chinese Seismograph".Archived fromthe original(jpg)on 2019-09-16.Retrieved2023-08-11.
^"USGS Earthquake Hazards Program".Archived fromthe originalon 2005-12-22.Retrieved2023-09-06.
^"How Can I Locate the Earthquake Epicenter?".
^"USGS Earthquake Hazards Program".Archived fromthe originalon 2005-12-18.Retrieved2023-09-06.
^Yeats, R. S.;Sieh, K. E.;Allen, C. R.(1997).The Geology of Earthquakes.Oxford University Press.p. 64.ISBN 978-0-19-507827-5.
^Fuis, Gary; Wald, Lisa."Rupture in South-Central Alaska – The Denali Fault Earthquake of 2002".USGS.Retrieved2008-04-20.
^^a ^b ^c ^d ^e ^f ^gJordan, Thomas H.; Grotzinger, John P. (2012).The essential Earth(2nd ed.). New York: W. H. Freeman. p. 429.ISBN 9781429255240.OCLC 798410008.
^Tyler M. Schau (1991)."The Richter Scale (ML)".USGS. Archived fromthe originalon 2016-04-25.Retrieved2008-09-14.
^William L. Ellsworth (1991)."Surface-Wave Magnitude (M_s) and Body-Wave Magnitude (mb) ".USGS. Archived fromthe originalon 2009-02-02.Retrieved2008-09-14.
^"What is Directivity?".earthquake.usgs.gov.Retrieved2018-07-01.
^Musson, R. M. W.; Cecić, I. (1 January 2002)."49 - Macroseismology".International Geophysics.81.Academic Press: 807–822.doi:10.1016/S0074-6142(02)80256-X.ISBN 9780124406520.
^"epicenter".Merriam-Webster Online Dictionary.2009.Retrieved2009-10-19.
^ἐπίκεντρος,Henry George Liddell, Robert Scott,A Greek-English Lexicon,on Perseus
^ἐπί,Henry George Liddell, Robert Scott,A Greek-English Lexicon,on Perseus
^epicentre,on Oxford Dictionaries
^Filiatrault, A. (2002).Elements of Earthquake Engineering and Structural Dynamics(2nd ed.). Presses inter Polytechnique. p. 1.ISBN 978-2-553-01021-7.
^Rick Thompson (2004).Writing for Broadcast Journalists.Routledge. p. 160.ISBN 978-1-134-36915-7.
^Oltermann, P. (2009).How to Write.Random House. p. 246.ISBN 978-0-85265-138-4.
^Safire, William (2001-05-06)."On Language".The New York Times Magazine.p. 22. Archived fromthe originalon 2022-10-17.Retrieved2022-10-17.
^Garner, B. A. (2009).Garner's Modern American Usage.Oxford University Press. p. 310.ISBN 9780199888771.

[1] Oxford English Dictionary:"The point over the centre: applied in Seismol. to the outbreaking point of earthquake shocks."

[2] "Chinese Seismograph".Archived fromthe original(jpg)on 2019-09-16.Retrieved2023-08-11.

[3] "USGS Earthquake Hazards Program".Archived fromthe originalon 2005-12-22.Retrieved2023-09-06.

[4] "How Can I Locate the Earthquake Epicenter?".

[5] "USGS Earthquake Hazards Program".Archived fromthe originalon 2005-12-18.Retrieved2023-09-06.

[6] Yeats, R. S.;Sieh, K. E.;Allen, C. R.(1997).The Geology of Earthquakes.Oxford University Press.p. 64.ISBN 978-0-19-507827-5.

[7] Fuis, Gary; Wald, Lisa."Rupture in South-Central Alaska – The Denali Fault Earthquake of 2002".USGS.Retrieved2008-04-20.

[:0-8] ^^a ^b ^c ^d ^e ^f ^gJordan, Thomas H.; Grotzinger, John P. (2012).The essential Earth(2nd ed.). New York: W. H. Freeman. p. 429.ISBN 9781429255240.OCLC 798410008.

[9] Tyler M. Schau (1991)."The Richter Scale (ML)".USGS. Archived fromthe originalon 2016-04-25.Retrieved2008-09-14.

[10] William L. Ellsworth (1991)."Surface-Wave Magnitude (M_s) and Body-Wave Magnitude (mb) ".USGS. Archived fromthe originalon 2009-02-02.Retrieved2008-09-14.

[11] "What is Directivity?".earthquake.usgs.gov.Retrieved2018-07-01.

[12] Musson, R. M. W.; Cecić, I. (1 January 2002)."49 - Macroseismology".International Geophysics.81.Academic Press: 807–822.doi:10.1016/S0074-6142(02)80256-X.ISBN 9780124406520.

[13] "epicenter".Merriam-Webster Online Dictionary.2009.Retrieved2009-10-19.

[14] ἐπίκεντρος,Henry George Liddell, Robert Scott,A Greek-English Lexicon,on Perseus

[15] ἐπί,Henry George Liddell, Robert Scott,A Greek-English Lexicon,on Perseus

[16] tre,on Oxford Dictionaries

[17] Filiatrault, A. (2002).Elements of Earthquake Engineering and Structural Dynamics(2nd ed.). Presses inter Polytechnique. p. 1.ISBN 978-2-553-01021-7.

[18] Rick Thompson (2004).Writing for Broadcast Journalists.Routledge. p. 160.ISBN 978-1-134-36915-7.

[19] Oltermann, P. (2009).How to Write.Random House. p. 246.ISBN 978-0-85265-138-4.

[20] Safire, William (2001-05-06)."On Language".The New York Times Magazine.p. 22. Archived fromthe originalon 2022-10-17.Retrieved2022-10-17.

[21] Garner, B. A. (2009).Garner's Modern American Usage.Oxford University Press. p. 310.ISBN 9780199888771.

[1]

[2]

[3]

[4]

[5]

[6]

[7]

[8]

[9]

[10]

[11]

[12]

[13]

[14]

[15]

[16]

[17]

[18]

[19]

[20]

[21]