A senior U.S. official on Tuesday disclosed additional seismic information that he said supports an allegation of an underground nuclear test at China's Lop Nor test grounds on June 22, 2020.
Assistant Secretary of State Christopher Yeaw, speaking at an event hosted by the Hudson Institute in Washington, said data from a remote seismic monitoring station in Kazakhstan - identified as PS23 - registered an "explosion" of magnitude 2.75. The station is located about 450 miles (720 km) from the Lop Nor testing area in western China, according to Yeaw.
"I’ve looked at additional data since then. There is very little possibility I would say that it is anything but an explosion, a singular explosion," Yeaw said, asserting that the readings were inconsistent with mining blasts. He added the data were "entirely not consistent with an earthquake." Yeaw, who previously served as an intelligence analyst and defense official and holds a doctorate in nuclear engineering, said the pattern of signals was "what you would expect with a nuclear explosive test."
The Comprehensive Test Ban Treaty Organization (CTBTO), which operates a global network to detect nuclear test explosions, said the available record does not allow it to confirm Yeaw’s allegation with confidence. CTBTO Executive Secretary Robert Floyd noted that the PS23 station recorded "two very small seismic events" on June 22, 2020, separated by 12 seconds.
Floyd said the CTBTO’s system is capable of detecting events consistent with nuclear test explosions with yields of 551 tons (500 metric tons) of TNT or greater, but that the two events recorded on June 22 were "far below that level." "As a result, with this data alone, it is not possible to assess the cause of these events with confidence," he said.
In his remarks, Yeaw suggested that China used a technique known as decoupling to intentionally dampen the seismic signature by detonating a device inside a large underground chamber. Yeaw said that, in his assessment, the data did not match the characteristics of either typical mining blasts or natural earthquakes.
China has denied setting off an underground nuclear test blast following the initial U.S. allegation made earlier this month. The country signed, but has not ratified, the 1996 international treaty banning nuclear testing, and its last officially acknowledged underground nuclear test took place in 1996.
The Chinese embassy in Washington did not immediately respond to a request for comment on Yeaw’s additional statements.
Yeaw’s allegation arrives against the backdrop of renewed pressure by U.S. President Donald Trump for China to join the United States and Russia in negotiations to replace New START, the bilateral strategic nuclear arms limitation agreement between Washington and Moscow that expired on February 5. The treaty’s expiration, the U.S. administration has warned, has raised concerns about a potential acceleration of a nuclear arms buildup.
China has rejected the call for three-way negotiations, arguing that its strategic nuclear arsenal is much smaller than those of the United States and Russia. Meanwhile, the Pentagon has reported that China has more than 600 operational warheads and is undertaking a significant expansion of its strategic nuclear forces, projecting that China could field more than 1,000 warheads by 2030.
The United States, like China, has signed but not ratified the 1996 test ban treaty; under international law both countries are obliged to uphold the pact's provisions. The U.S. conducted its last underground nuclear test in 1992 and has since relied on a multibillion-dollar program using advanced diagnostic tools and supercomputer simulations to certify that its nuclear warheads remain reliable without explosive testing.
With competing claims over the interpretation of low-magnitude seismic signals and differing assessments of strategic arsenals, the episode highlights enduring tensions over monitoring, verification and arms control. The CTBTO's caution that the small events recorded were below their confident detection threshold underscores the technical limits that can complicate definitive public assessments of such incidents.
As officials and monitoring bodies weigh the available evidence, the diplomatic debate over nuclear testing, treaty obligations and future arms control arrangements between major powers is likely to remain central to discussions about global security and strategic stability.