Hypocenter

"Ground zero" redirects here. For other uses, seeGround zero (disambiguation).

Ahypocenterorhypocentre(fromAncient Greekὑπόκεντρον(hupókentron)'below the center'), also calledground zeroorsurface zero,is the point on theEarth's surface directly below anuclear explosion,meteor air burst,or other mid-air explosion. Inseismology,the hypocenter of anearthquakeis its point of origin below ground; a synonym is thefocusof an earthquake.

Generally, the termsground zeroandsurface zeroare also used in relation toepidemics,and otherdisastersto mark the point of the most severe damage or destruction. The term is distinguished from the termzero pointin that the latter can also be located in the air, underground, or underwater.

Trinity, Hiroshima and Nagasaki

In mapping the effects of an atomic bomb, such as on the city ofHiroshimahere, 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".

Monument marking the hypocenter, or ground zero, of the atomic bomb explosion overHiroshima.

The term "ground zero" originally referred to the hypocenter of theTrinity testinJornada del Muerto desertnearSocorro, New Mexico,and theatomic bombings of Hiroshima and NagasakiinJapan.TheUnited States Strategic Bombing Surveyof the atomic attacks, released in June 1946, used the term liberally, defining it as:

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.'

William Laurence,anembedded reporterwith theManhattan Project,reported that "Zero" was "the code name given to the spot chosen for the [Trinity] test" in 1945.

TheOxford English Dictionary,citing the use of the term in a 1946New York Timesreport on the destroyed city ofHiroshima,definesground zeroas "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 firstnuclear weaponwas at "point zero", and moved into general use very shortly after the end ofWorld War II.At Hiroshima, the hypocenter of the attack wasShima Hospital,approximately 800 ft (240 m) away from the intended aiming point atAioi Bridge.

The Pentagon

The former hot dog stand nicknamed Cafe Ground Zeroin the Pentagon's center courtyard.

During theCold War,the Pentagon(headquarters ofUnited States Department of DefenseinArlington County, Virginia) was an assured target in the event ofnuclear war.The open space in the center of the Pentagon became known informally as ground zero. Asnack barthat used to be located at the center of this open space was nicknamed "Cafe Ground Zero".

World Trade Center

Main article:World Trade Center site

Aerial view of theWorld Trade Center sitein September 2001.

During theSeptember 11 attacksin 2001, two aircraftwere hijackedby 10al-Qaedaterroristsand were flown into the Twin Towers of theWorld Trade CenterinNew York City,causing massive damage and starting fires thatcaused the weakened 110-story skyscrapers to collapse.The destroyedWorld Trade Center sitesoon 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 newOne World Trade Centerand theNational September 11 Memorial & Museumwere well under way, the term was still frequently used to refer to the site, as when opponents of thePark51project 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 mayorMichael Bloombergurged that the "ground zero" moniker be retired, saying, "…the time has come to call those 16 acres [6.5 hectares] what they are: The World Trade Center and the National September 11th Memorial and Museum."

Meteor air bursts

The hypocenter of ameteor air burst,anasteroidorcometthat explodes in the atmosphere rather than strike the surface, is the closest point on the surface to the explosion. TheTunguska eventoccurred inSiberiain 1908 and flattened an estimated 80 million trees over an area of 2,150 km²(830 sq mi) 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 2013Chelyabinsk meteor's hypocenter inRussiawas more populated than that of Tunguska, resulting in civil damage and injury, mostly from flying glass shards from broken windows.

Earthquakes

Hypocenter (Focus) and epicenter of an earthquake

An earthquake's hypocenter or focus is the position where thestrainenergy stored in the rock is first released, marking the point where thefaultbegins to rupture.This occurs directly beneath theepicenter,at a distance known as thehypocentral depthorfocal depth.

The focal depth can be calculated from measurements based onseismic wavephenomena. As with allwavephenomena inphysics,there is uncertainty in such measurements that grows with thewavelengthso 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 ofleast squareswhich 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

External links

• The dictionary definition ofhypocenterat Wiktionary
• Media related toHypocentersat Wikimedia Commons