On June 20, 2025, a 5.1-magnitude earthquake struck northern Iran’s Semnan province, approximately 27 kilometers southwest of Semnan city, at a depth of 10 kilometers, according to the U.S. Geological Survey (USGS) and Tasnim News Agency. The quake, felt as far as Tehran, occurred near the Semnan Space Center and Missile Complex, key military facilities, prompting speculation on social media about a possible underground nuclear test by Iran, especially amid ongoing conflict with Israel.
However, seismic data analyzed by experts, including the USGS and the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO), indicates the event was a natural earthquake. Nuclear explosions typically produce distinct seismic signatures, dominated by compressional P-waves, whereas natural earthquakes generate both P-waves and shear S-waves. The waveform and depth of this quake align with natural tectonic activity, consistent with Iran’s location on the Alpine-Himalayan seismic belt, where it experiences over 2,000 earthquakes annually, including 15–16 above magnitude 5.0.
Speculation was fueled by the quake’s proximity to nuclear-related sites and recent Israeli airstrikes targeting Iran’s nuclear infrastructure, including Natanz, Isfahan, and Fordow. Social media posts on X suggested a nuclear test, with some claiming Iran may have conducted a boosted fission weapon test or built warheads, but these lack credible evidence. Others, including seismologists and analysts, dismissed these theories, noting the quake’s characteristics match natural events and that digging 10 kilometers deep for a covert test is highly impractical.
A Johns Hopkins University study on a similar 4.5-magnitude quake in Semnan on October 5, 2024, also debunked nuclear test claims, highlighting how misinformation spreads rapidly during geopolitical tensions. Iran’s nuclear program remains under scrutiny, with reports indicating it is close to weapons-grade uranium enrichment, but no confirmed evidence supports nuclear testing. The quake caused no reported casualties or significant damage, per Iran’s IRNA news agency.