Epicenter of a nuclear explosion
The epicenter of a nuclear explosion is the point on the Earth's surface under which an underground or underwater nuclear explosion occurred or the one above which an aerial nuclear explosion was carried out [Shevchenko, Chumak, Yarun, p. 88].
Concerning nuclear explosions and other large bombs, ground zero (also called surface zero) is the point on the Earth's surface closest to a detonation. In the case of an explosion above the ground, ground zero is the point directly below the nuclear detonation. It is sometimes called the hypocenter (from Ancient Greek ὑπο- (hupo-) 'under-, sub-,' and center) [Wayback Machine].
Generally, ground zero and surface zero are also used for earthquakes, epidemics, and other disasters to mark the point of the most severe damage or destruction. The term is distinguished from the zero point because the latter can also be located in the air, underground, or underwater [U.S. DoD Terminology].
Shevchenko, L., Chumak, V., Yarun, G., Shevchenko, I., Bugakov, O., Bilonozhenko, V. (2010—2020). Dictionary of the Ukrainian language. Kyiv: Scientific opinion.
Nuclear Matters: A Practical Guide, Appendix B. Wayback Machine. Retrieved from: https://web.archive.org/web/20101201082850/http://www.acq.osd.mil/ncbdp/nm/nmbook/appendices/ap_B.htm
Zero point. U.S. DoD Terminology. Retrieved from: https://www.militaryfactory.com/dictionary/military-terms-defined.php?term_id=5818