A hypocenter or hypocentre, also called ground zero 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.
Trinity, Hiroshima and Nagasaki
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".]]
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 test in 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:
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.
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 away from the intended aiming point at Aioi Bridge. At Nagasaki, the hypocenter of the attack was above Urakami Valley, approximately away from the intended aiming point at the Mitsubishi Steel and Arms Works.
The Pentagon
[[File:PentagonHotDogStand.jpg|thumb|left|The former hot dog stand nicknamed Cafe Ground Zero
World Trade Center
thumb|right|Aerial view of the [[World Trade Center site in September 2001.]]
During the September 11 attacks in 2001, two aircraft were hijacked by 10 al-Qaeda terrorists and were flown into the Twin Towers of the World Trade Center in New York City, 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 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 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 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 30 million trees over a area. 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.
Earthquakes
thumb|240px|Hypocenter (Focus) and epicenter of an earthquake
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. This occurs directly beneath the epicenter, at a distance known as the hypocentral depth or focal depth. 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.