hypocenter
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A hypocenter or hypocentre (), also called ground zero or surface zero, is the point on the
Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...
's surface directly below a nuclear explosion, meteor air burst, or other mid-air explosion. In
seismology Seismology (; from Ancient Greek σεισμός (''seismós'') meaning "earthquake" and -λογία (''-logía'') meaning "study of") is the scientific study of earthquakes (or generally, quakes) and the generation and propagation of elastic ...
, the hypocenter of an
earthquake An earthquakealso called a quake, tremor, or tembloris the shaking of the Earth's surface resulting from a sudden release of energy in the lithosphere that creates seismic waves. Earthquakes can range in intensity, from those so weak they ...
is its point of origin below ground; a synonym is the focus of an earthquake. Generally, the terms ''ground zero'' and ''surface zero'' are also used in relation to epidemics, and other
disaster A disaster is an event that causes serious harm to people, buildings, economies, or the environment, and the affected community cannot handle it alone. '' Natural disasters'' like avalanches, floods, earthquakes, and wildfires are caused by na ...
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.


Trinity, Hiroshima and Nagasaki

The term "ground zero" originally referred to the hypocenter of the Trinity test in Jornada del Muerto desert near
Socorro, New Mexico Socorro (, ''Help:Pronunciation respelling key, sə-KOR-oh'') is a city in Socorro County, New Mexico, Socorro County in the U.S. state of New Mexico. It is in the Middle Rio Grande Valley AVA, Rio Grande Valley at an elevation of . At the 2020 ...
, and the
atomic bombings of Hiroshima and Nagasaki On 6 and 9 August 1945, the United States detonated two atomic bombs over the Japanese cities of Hiroshima and Nagasaki, respectively, during World War II. The aerial bombings killed between 150,000 and 246,000 people, most of whom were civili ...
in
Japan Japan is an island country in East Asia. Located in the Pacific Ocean off the northeast coast of the Asia, Asian mainland, it is bordered on the west by the Sea of Japan and extends from the Sea of Okhotsk in the north to the East China Sea ...
. The United States Strategic Bombing Survey of the atomic attacks, released in June 1946, used the term liberally, defining it as: William Laurence, an embedded reporter with the
Manhattan Project The Manhattan Project was a research and development program undertaken during World War II to produce the first nuclear weapons. It was led by the United States in collaboration with the United Kingdom and Canada. From 1942 to 1946, the ...
, reported that "Zero" was "the code name given to the spot chosen for the rinitytest" in 1945. The ''
Oxford English Dictionary The ''Oxford English Dictionary'' (''OED'') is the principal historical dictionary of the English language, published by Oxford University Press (OUP), a University of Oxford publishing house. The dictionary, which published its first editio ...
'', citing the use of the term in a 1946 ''
New York Times ''The New York Times'' (''NYT'') is an American daily newspaper based in New York City. ''The New York Times'' covers domestic, national, and international news, and publishes opinion pieces, investigative reports, and reviews. As one of ...
'' report on the destroyed city of
Hiroshima is the capital of Hiroshima Prefecture in Japan. , the city had an estimated population of 1,199,391. The gross domestic product (GDP) in Greater Hiroshima, Hiroshima Urban Employment Area, was US$61.3 billion as of 2010. Kazumi Matsui has b ...
, 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 A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission (fission or atomic bomb) or a combination of fission and fusion reactions (thermonuclear weapon), producing a nuclear exp ...
was at "point zero", and moved into general use very shortly after the end of
World War II World War II or the Second World War (1 September 1939 – 2 September 1945) was a World war, global conflict between two coalitions: the Allies of World War II, Allies and the Axis powers. World War II by country, Nearly all of the wo ...
. At Hiroshima, the hypocenter of the attack was Shima Hospital, approximately away from the intended aiming point at Aioi Bridge.


The Pentagon

During the
Cold War The Cold War was a period of global Geopolitics, geopolitical rivalry between the United States (US) and the Soviet Union (USSR) and their respective allies, the capitalist Western Bloc and communist Eastern Bloc, which lasted from 1947 unt ...
,
the Pentagon The Pentagon is the headquarters building of the United States Department of Defense, in Arlington County, Virginia, across the Potomac River from Washington, D.C. The building was constructed on an accelerated schedule during World War II. As ...
(headquarters of
United States Department of Defense The United States Department of Defense (DoD, USDOD, or DOD) is an United States federal executive departments, executive department of the federal government of the United States, U.S. federal government charged with coordinating and superv ...
in
Arlington County, Virginia Arlington County, or simply Arlington, is a County (United States), county in the U.S. state of Virginia. The county is located in Northern Virginia on the southwestern bank of the Potomac River directly across from Washington, D.C., the nati ...
) was an assured target in the event of
nuclear war Nuclear warfare, also known as atomic warfare, is a War, military conflict or prepared Policy, political strategy that deploys nuclear weaponry. Nuclear weapons are Weapon of mass destruction, weapons of mass destruction; in contrast to conven ...
. 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".


World Trade Center

During the
September 11 attacks The September 11 attacks, also known as 9/11, were four coordinated Islamist terrorist suicide attacks by al-Qaeda against the United States in 2001. Nineteen terrorists hijacked four commercial airliners, crashing the first two into ...
in 2001, two aircraft were hijacked by 10
al-Qaeda , image = Flag of Jihad.svg , caption = Jihadist flag, Flag used by various al-Qaeda factions , founder = Osama bin Laden{{Assassinated, Killing of Osama bin Laden , leaders = {{Plainlist, * Osama bin Lad ...
terrorists and were flown into the Twin Towers of the World Trade Center in
New York City New York, often called New York City (NYC), is the most populous city in the United States, located at the southern tip of New York State on one of the world's largest natural harbors. The city comprises five boroughs, each coextensive w ...
, causing massive damage and starting fires that 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. Even after the site was cleaned up and construction on the new
One World Trade Center One World Trade Center, also known as One WTC and as the Freedom Tower, is the main building of the rebuilt World Trade Center complex in Lower Manhattan, New York City. Designed by David Childs of Skidmore, Owings & Merrill, One World Tr ...
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 Michael Rubens Bloomberg (born February 14, 1942) is an American businessman and politician. He is the majority owner and co-founder of Bloomberg L.P., and was its CEO from 1981 to 2001 and again from 2014 to 2023. He served as the 108th mayo ...
urged that the "ground zero" moniker be retired, saying, "…the time has come to call those what they are: The World Trade Center and the National September 11th Memorial and Museum."


Meteor air bursts

The hypocenter of a meteor air burst, an
asteroid An asteroid is a minor planet—an object larger than a meteoroid that is neither a planet nor an identified comet—that orbits within the Solar System#Inner Solar System, inner Solar System or is co-orbital with Jupiter (Trojan asteroids). As ...
or
comet A comet is an icy, small Solar System body that warms and begins to release gases when passing close to the Sun, a process called outgassing. This produces an extended, gravitationally unbound atmosphere or Coma (cometary), coma surrounding ...
that explodes in the atmosphere rather than strike the surface, is the closest point on the surface to the explosion. The
Tunguska event The Tunguska event was a large explosion of between 3 and 50 TNT equivalent, megatons that occurred near the Podkamennaya Tunguska River in Yeniseysk Governorate (now Krasnoyarsk Krai), Russia, on the morning of 30 June 1908. The explosion over ...
occurred in
Siberia Siberia ( ; , ) is an extensive geographical region comprising all of North Asia, from the Ural Mountains in the west to the Pacific Ocean in the east. It has formed a part of the sovereign territory of Russia and its predecessor states ...
in 1908 and flattened an estimated 80 million trees over an area of 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 Russia, or the Russian Federation, is a country spanning Eastern Europe and North Asia. It is the list of countries and dependencies by area, largest country in the world, and extends across Time in Russia, eleven time zones, sharing Borders ...
was more populated than that of Tunguska, resulting in civil damage and injury, mostly from flying glass shards from broken windows.


Earthquakes

An earthquake's hypocenter or focus is the position where the strain energy stored in the rock is first released, marking the point where the fault begins to rupture.''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.'' This occurs directly beneath the epicenter, at a distance known as the ''hypocentral depth'' or '' focal depth''. The focal depth can be calculated from measurements based on
seismic wave A seismic wave is a mechanical wave of acoustic energy that travels through the Earth or another planetary body. It can result from an earthquake (or generally, a quake), volcanic eruption, magma movement, a large landslide and a large ma ...
phenomena. As with all
wave In physics, mathematics, engineering, and related fields, a wave is a propagating dynamic disturbance (change from List of types of equilibrium, equilibrium) of one or more quantities. ''Periodic waves'' oscillate repeatedly about an equilibrium ...
phenomena in
physics Physics is the scientific study of matter, its Elementary particle, fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge whi ...
, there is uncertainty in such measurements that grows with the
wavelength In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats. In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...
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. 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. 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.


See also

* List of meteor air bursts * List of nuclear weapon explosion sites *'' Tenet'', a 2020 film that includes a sub-surface nuclear "hypocenter" in its storyline


References


External links

* * {{World Trade Center Seismology Geometric centers Atomic bombings of Hiroshima and Nagasaki Cold War terminology Metaphors referring to objects Metaphors referring to places Metaphors referring to war and violence Military slang and jargon September 11 attacks