Hypocenter

Adapted from the English Wikipedia article Hypocenter, available under CC BY-SA 4.0Updated 7 August 2025

A hypocenter or hypocentre (), also known as ground zero or surface zero, refers to the location on the Earth's surface directly beneath a nuclear explosion, meteor air burst, or other aerial detonation. In the field of seismology, an earthquake's hypocenter denotes its subterranean point of origin, commonly called the focus of the earthquake.

The expressions ground zero and surface zero are additionally applied to epidemic outbreaks and various disaster scenarios to indicate the area experiencing the most intense destruction. This is distinct from zero point, which can be situated in the atmosphere, underground, or underwater.

Trinity, Hiroshima and Nagasaki

The term "ground zero" was initially associated with the hypocenter of the Trinity test conducted in the Jornada del Muerto desert near Socorro, New Mexico, as well as the atomic bombings of Hiroshima and Nagasaki in Japan. According to the United States Strategic Bombing Survey (June 1946):

William Laurence, a journalist embedded with the Manhattan Project, described "Zero" as "the code name assigned to the location selected for the [Trinity] test" in 1945.

The Oxford English Dictionary, referencing a 1946 New York Times article about Hiroshima, defines ground zero as "the portion of ground directly beneath an exploding bomb, particularly an atomic one." Originating as military jargon at Trinity—where the initial nuclear device was detonated at "point zero"—the term entered widespread usage shortly after World War II. In Hiroshima, the hypocenter was situated close to Shima Hospital, approximately from the intended target at Aioi Bridge.

The Pentagon

Throughout the Cold War, the Pentagon, headquarters of the United States Department of Defense located in Arlington County, Virginia, was considered a prime target in hypothetical nuclear conflict scenarios. The building’s open central courtyard informally became known as ground zero. A snack bar inside this courtyard gained the nickname "Cafe Ground Zero" due to its symbolic exposure.

World Trade Center

In the course of the September 11 attacks of 2001, two hijacked airplanes crashed into the Twin Towers of the World Trade Center in New York City, resulting in massive destruction and fires that caused both 110-story buildings to collapse. The devastated site was termed "ground zero," with rescue personnel also referring to the debris field as "The Big Momma."

Following cleanup efforts and ongoing development of the new One World Trade Center alongside the National September 11 Memorial & Museum, this designation persisted. Opponents of the nearby Park51 development notably used "Ground Zero mosque" to refer to that project.

Prior to the tenth anniversary, New York City mayor Michael Bloomberg called for retiring the phrase "ground zero," encouraging acknowledgment of the location as The World Trade Center and the National September 11th Memorial and Museum.

Meteor air bursts

The hypocenter of a meteor air burst refers to the point on the Earth's surface directly beneath an asteroid or comet explosion occurring in the atmosphere, rather than a direct impact on the ground. During the 1908 Tunguska event in Siberia, approximately 80 million trees were leveled; however, those located at the hypocenter remained upright but were stripped of all branches due to the intense shockwave. In contrast, the 2013 Chelyabinsk meteor event in Russia had its hypocenter over a more densely inhabited area, resulting in property damage and numerous injuries primarily caused by shattered glass fragments propelled by the blast.

Earthquakes

The hypocenter or focus of an earthquake designates the initial location where accumulated stress within rocks is released, initiating the rupture along a fault. This point lies directly beneath the epicenter at a specific depth known as the hypocentral or focal depth. Determining this depth relies on analyzing seismic wave data; however, precision decreases with longer wavelength waves typical of larger earthquakes, as these originate from more extensive volumes of rock.

By charting the hypocenters of foreshocks, main shocks, and aftershocks, geologists can construct three-dimensional representations of fault activity. The seismic waves travel at velocities of several kilometers per second and are recorded by surface monitoring stations to triangulate the hypocenter’s position. To account for changes in wave speeds caused by varying geological materials, an iterative process solving linear equations through least squares methods is employed, enhancing the accuracy of hypocenter location estimates within the limits imposed by velocity model uncertainties.

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