Ahypocenter orhypocentre (from Ancient Greekὑπόκεντρον (hupókentron)'below the center'), also calledground zero[1][2] orsurface zero, is the point on theEarth's surface directly below anuclear explosion,meteor air burst, or other mid-air explosion. Inseismology, the hypocenter of anearthquake is its point of origin below ground; a synonym is thefocus of an earthquake.[3]
Generally, the termsground zero andsurface zero are also used in relation toepidemics, and otherdisasters to mark the point of the most severe damage or destruction. The term is distinguished from the termzero point in that the latter can also be located in the air, underground, or underwater.[4]
In mapping the effects of an atomic bomb, such as on the city ofHiroshima 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".Monument marking the hypocenter, or ground zero, of the atomic bomb explosion overHiroshima.
TheOxford English Dictionary, citing the use of the term in a 1946New York Times report on the destroyed city ofHiroshima, definesground 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 firstnuclear weapon was 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.
In advance of the 10th anniversary of the attacks, New York City mayorMichael Bloomberg urged 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."[9]
The hypocenter of ameteor air burst, anasteroid orcomet that explodes in the atmosphere rather than strike the surface, is the closest point on the surface to the explosion. TheTunguska event occurred inSiberia in 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 inRussia was more populated than that of Tunguska, resulting in civil damage and injury, mostly from flying glass shards from broken windows.[10]
An earthquake's hypocenter or focus is the position where thestrain energy stored in the rock is first released, marking the point where thefault begins to rupture.[3] This occurs directly beneath theepicenter, at a distance known as thehypocentral depth orfocal depth.[3]
The focal depth can be calculated from measurements based onseismic wave phenomena. As with allwave phenomena inphysics, there is uncertainty in such measurements that grows with thewavelength 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.[11] 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.[12] 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 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.[12]
^Royal Air Force Common Core and Deployment Skills Aide-Memoire AP 3242B VOL 5, ABBREVIATIONS
^abcThe 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."Earthquake Glossary – hypocenter". United States Geological Survey.Archived from the original on 15 March 2010.
^abSmith, Steven Donald (September 20, 2006)."Pentagon Hot Dog Stand, Cold War Legend, to be Torn Down". United States Department of Defense. Archived fromthe original on 2015-09-24. Retrieved2010-05-06."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".
^Hamill, Denis (16 September 2001). "Rescue Workers Keep Up Quest for Signs of Life Ruin All Over, But Not One Unkind Word".Daily News (New York).
