Editing Hypocenter

{{short description|Point of origin of an earthquake or the point below a nuclear explosion or meteor air burst}}
{{redirect|Ground zero|other uses}}
{{Use American English|date=August 2020}}
{{Earthquakes}}
A '''hypocenter''' or '''hypocentre''' ({{etymology|grc|''{{Wikt-lang|grc|ὑπόκεντρον}}'' ({{grc-transl|ὑπόκεντρον}})|below the center}}), also called '''ground zero'''<ref name="CollinsDict">{{cite web | url=https://www.collinsdictionary.com/dictionary/english/hypocenter | title=hypocenter —- Definitions | work=Collins English Dictionary | access-date=12 July 2024}}</ref><ref>Royal Air Force Common Core and Deployment Skills Aide-Memoire AP 3242B VOL 5, ABBREVIATIONS</ref> or '''surface zero''', is the point on the [[Earth]]'s surface directly below a [[nuclear explosion]], [[meteor air burst]], or other mid-air explosion. In [[seismology]], the hypocenter of an [[earthquake]] is its point of origin below ground; a synonym is the '''focus''' of an earthquake.<ref name="USGS" />

Generally, the terms ''ground zero'' and ''surface zero'' are also used in relation to [[epidemic]]s, and other [[disaster]]s to mark the point of the most severe damage or destruction. The term is distinguished from the term '''zero point''' in that the latter can also be located in the air, underground, or underwater.<ref>{{cite web |url=http://www.militaryfactory.com/dictionary/military-terms-defined.asp?term_id=5818 |title=U.S. DoD Terminology: zero point |access-date=28 November 2016}}</ref>

==Trinity, Hiroshima and Nagasaki==
[[File:Hiroshima Damage Map.gif|right|thumb|In mapping the effects of an atomic bomb, such as on the city of [[Hiroshima]] here, concentric circles are drawn centered on the point below the detonation and numbered at radial distances of 1,000 feet (305 meters). This point below the detonation is called "Ground Zero".]]
[[File:Hiroshima-Hypocenter.jpg|upright|thumb|Monument marking the hypocenter, or ground zero, of the atomic bomb explosion over [[Hiroshima]].]]
The term "ground zero" originally referred to the hypocenter of the [[Trinity (nuclear test)|Trinity test]] in [[Jornada del Muerto|Jornada del Muerto desert]] near [[Socorro, New Mexico]], and the [[atomic bombings of Hiroshima and Nagasaki]] in [[Japan]]. The [[United States Strategic Bombing Survey]] of the atomic attacks, released in June 1946, used the term liberally, defining it as:

{{blockquote|For convenience, the term 'ground zero' will be used to designate the point on the ground directly beneath the point of detonation, or 'air zero.'<ref>[http://www.trumanlibrary.org/whistlestop/study_collections/bomb/large/documents/index.php?pagenumber=10&documentid=65&documentdate=1946-06-19&studycollectionid=abomb&groupid= U.S. Strategic Bombing Survey: The Effects of the Atomic Bombings of Hiroshima and Nagasaki] {{Webarchive|url=https://web.archive.org/web/20110607124802/http://www.trumanlibrary.org/whistlestop/study_collections/bomb/large/documents/index.php?pagenumber=10&documentid=65&documentdate=1946-06-19&studycollectionid=abomb&groupid= |date=2011-06-07 }}. June 19, 1946. President's Secretary's File, Truman Papers. Page 5.</ref>}}

[[William Laurence]], an [[embedded reporter]] with the [[Manhattan Project]], reported that "Zero" was "the code name given to the spot chosen for the [Trinity] test" in 1945.<ref>[[William L. Laurence]], ''Dawn over Zero'' (London: Museum Press, 1947), 4.</ref>

The ''[[Oxford English Dictionary]]'', citing the use of the term in a 1946 ''[[New York Times]]'' report on the destroyed city of [[Hiroshima]], defines ''ground zero'' as "that part of the ground situated immediately under an exploding bomb, especially an atomic one."
The term was military slang, used at the Trinity site where the weapon tower for the first [[nuclear weapon]] was at "point zero", and moved into general use very shortly after the end of [[World War II]]. At Hiroshima, the hypocenter of the attack was [[Shima Hospital]], approximately {{convert|800|ft|m|abbr=on}} away from the intended aiming point at [[Aioi Bridge]].

==The Pentagon==
[[File:PentagonHotDogStand.jpg|thumb|left|The former hot dog stand nicknamed Cafe Ground Zero<ref name="Pentagon" /> in the Pentagon's center courtyard.]]
During the [[Cold War]], [[the Pentagon]] (headquarters of [[United States Department of Defense]] in [[Arlington County, Virginia]]) was an assured target in the event of [[nuclear war]]. The open space in the center of the Pentagon became known informally as ground zero. A [[snack bar]] that used to be located at the center of this open space was nicknamed "Cafe Ground Zero".<ref name="Pentagon" >{{cite web|url=http://archive.defense.gov/news/newsarticle.aspx?id=1049|title=Pentagon Hot Dog Stand, Cold War Legend, to be Torn Down|quote= "It's rumored that a portion of their (Soviet) nuclear arsenal was directed at that building, the Pentagon hot dog stand", tour guides tell visitors as they pass the stand. "This is where the building earned the nickname Cafe Ground Zero, the deadliest hot dog stand in the world". |date=September 20, 2006|publisher=United States Department of Defense |first1=Steven Donald |last1=Smith |access-date=2010-05-06|archive-date=2015-09-24|archive-url=https://web.archive.org/web/20150924002708/http://archive.defense.gov/news/newsarticle.aspx?id=1049|url-status=dead}}</ref>

==World Trade Center==
{{Main article|World Trade Center site}}
[[File:Wtc-photo.jpg|thumb|right|Aerial view of the [[World Trade Center site]] in September 2001.]]
During the [[September 11 attacks]] in 2001, two aircraft [[Aircraft hijacking|were hijacked]] by 10 [[al-Qaeda]] [[Terrorism|terrorists]] and were flown into the Twin Towers of the [[World Trade Center (1973–2001)|World Trade Center]] in [[New York City]], causing massive damage and starting fires that [[Collapse of the World Trade Center|caused the weakened 110-story skyscrapers to collapse]]. The destroyed [[World Trade Center site]] soon became known as "ground zero". Rescue workers also used the term "The Big Momma!", referring to the pile of rubble that was left after the buildings collapsed.<ref>{{cite news|last=Hamill | first= Denis|title=Rescue Workers Keep Up Quest for Signs of Life Ruin All Over, But Not One Unkind Word|work=[[Daily News (New York)]]|date=16 September 2001}}</ref>

Even after the site was cleaned up and construction on the new [[One World Trade Center]] and the [[National September 11 Memorial & Museum]] were well under way, the term was still frequently used to refer to the site, as when opponents of the [[Park51]] project that was to be located two blocks away from the site labeled it the "[[Ground Zero mosque]]".

In advance of the 10th anniversary of the attacks, New York City mayor [[Michael Bloomberg]] urged that the "ground zero" moniker be retired, saying, "…the time has come to call those {{convert|16|acre|ha|abbr=off|disp=sqbr|round=0.5}} what they are: The World Trade Center and the National September 11th Memorial and Museum."<ref>{{cite web |url=https://www.bbc.co.uk/news/magazine-14827694 |title=Is it time to retire 'Ground zero'? |publisher=BBC |author=Geoghegan, Tom |date=2011-09-07 |access-date=2011-09-10}}</ref>

==Meteor air bursts==
The hypocenter of a [[meteor air burst]], an [[asteroid]] or [[comet]] that explodes in the atmosphere rather than strike the surface, is the closest point on the surface to the explosion. The [[Tunguska event]] occurred in [[Siberia]] in 1908 and flattened an estimated 80 million trees over an area of {{convert|2150|km2||abbr=on}} of forest. The trees at the hypocenter of the blast were left standing, but all their limbs had been blown off by the shockwave. The 2013 [[Chelyabinsk meteor]]'s hypocenter in [[Russia]] was more populated than that of Tunguska, resulting in civil damage and injury, mostly from flying glass shards from broken windows.<ref>{{Cite web |date=16 February 2013 |title=Пострадавшую при падении метеорита перевезут из Челябинска в Москву |url=https://ria.ru/20130216/923200576.html |url-status=live |website=RIA |access-date=15 February 2023 |archive-date=15 February 2023 |archive-url=https://web.archive.org/web/20230215220924/https://ria.ru/20130216/923200576.html |language=ru }}</ref>

==Earthquakes==
{{Earthquakes}}
[[File:Epicenter Diagram.svg|thumb|240px|Hypocenter (Focus) and epicenter of an earthquake]]
An earthquake's hypocenter or focus is the position where the [[Strain (materials science)|strain]] energy stored in the rock is first released, marking the point where the [[fault (geology)|fault]] begins to rupture.<ref name="USGS">''The hypocenter is the point within the earth where an earthquake rupture starts. The epicenter is the point directly above it at the surface of the Earth. Also commonly termed the focus.'' {{Cite web|title=Earthquake Glossary – hypocenter|publisher=United States Geological Survey|url=https://earthquake.usgs.gov/learn/glossary/?term=hypocenter|archive-url=https://web.archive.org/web/20100315110912/http://earthquake.usgs.gov/learn/glossary/?term=hypocenter|archive-date=15 March 2010|url-status=live}}</ref> This occurs directly beneath the [[epicenter]], at a distance known as the ''hypocentral depth'' or ''[[focal depth]]''.<ref name="USGS" />

The focal depth can be calculated from measurements based on [[seismic wave]] phenomena. As with all [[wave]] phenomena in [[physics]], there is uncertainty in such measurements that grows with the [[wavelength]] so the focal depth of the source of these long-wavelength (low frequency) waves is difficult to determine exactly. Very strong earthquakes radiate a large fraction of their released energy in seismic waves with very long wavelengths and therefore a stronger earthquake involves the release of energy from a larger mass of rock.

Computing the hypocenters of foreshocks, main shock, and aftershocks of earthquakes allows the three-dimensional plotting of the fault along which movement is occurring.<ref>{{Cite web|title=Using GIS for Visualizing Earthquake Epicenters, Hypocenters, Faults and Lineaments in Montana|author1=Kennelly, Patrick J.  |author2=Stickney, Michael C.|year=2000|url=http://pubs.usgs.gov/of/2000/of00-325/kennelly.html|publisher=United States Geological Survey|id=USGS Open-File Report 00-325|work=Digital Mapping Techniques '00 -- Workshop Proceedings|archive-url=https://web.archive.org/web/20040323205317/http://pubs.usgs.gov/of/2000/of00-325/kennelly.html|archive-date=23 March 2004|url-status=live}}</ref> The expanding wavefront from the earthquake's rupture propagates at a speed of several kilometers per second; this seismic wave is what is measured at various surface points in order to geometrically determine an initial guess as to the hypocenter. The wave reaches each station based upon how far away it was from the hypocenter. A number of things need to be taken into account, most importantly variations in the waves speed based upon the materials that it is passing through.<ref name="How">{{Cite web|title=FAQs – Measuring Earthquakes: Q: How do seismologists locate an earthquake? |publisher=United States Geological Survey |department=Earthquake Hazrads Program |url=https://earthquake.usgs.gov/learn/faq/?categoryID=2&faqID=118/index.html |url-status=dead |archive-url=https://web.archive.org/web/20120311190548/http://earthquake.usgs.gov/learn/faq/?categoryID=2&faqID=118%2Findex.html |archive-date=2012-03-11 }}</ref> With adjustments for velocity changes, the initial estimate of the hypocenter is made, then a series of linear equations is set up, one for each station. The equations express the difference between the observed arrival times and those calculated from the initial estimated hypocenter.  These equations are solved by the method of [[least squares]] which minimizes the sum of the squares of the differences between the observed and calculated arrival times, and a new estimated hypocenter is computed.  The system iterates until the location is pinpointed within the margin of error for the velocity computations.<ref name="How" />

==See also==
*[[List of meteor air bursts]]
*[[List of nuclear weapon explosion sites]]
*''[[Tenet (film)|Tenet]]'', a 2020 film that includes a sub-surface nuclear "hypocenter" in its storyline

==References==
{{Reflist}}

==External links==
*{{Wiktionary-inline}}
*{{Commons category-inline|Hypocenters}}

{{Nuclear technology}}
{{United States Department of Defense}}
{{World Trade Center}}

[[Category:Seismology]]
[[Category:Geometric centers]]
[[Category:Atomic bombings of Hiroshima and Nagasaki]]
[[Category:Cold War terminology]]
[[Category:Metaphors referring to objects]]
[[Category:Metaphors referring to places]]
[[Category:Metaphors referring to war and violence]]
[[Category:Military slang and jargon]]
[[Category:September 11 attacks]]