Representative image (AP)

A senior US official revealed what he described as new details about an alleged underground nuclear test that China conducted in June 2020, citing seismic data recorded in Central Asia, a claim Beijing has strongly denied.Speaking at an event hosted by the Hudson Institute in Washington on Tuesday, Assistant Secretary of State Christopher Yeaw said a remote seismic station in Kazakhstan detected an “explosion” of magnitude 2.75 on June 22, 2020.

The signal was traced to China’s Lop Nor test grounds in the country’s west, about 450 miles (720 km) away.“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, adding that the data were not consistent with mining blasts, as quoted by Reuters.“It’s also entirely not consistent with an earthquake,” Yeaw said. “It is … what you would expect with a nuclear explosive test.”The seismic station, known as PS23, is part of a global monitoring network run by the Comprehensive Nuclear-Test-Ban Treaty Organization, which is tasked with detecting nuclear test explosions. The agency said it lacked sufficient data to confirm the allegation with confidence.CTBTO Executive Secretary Robert Floyd said the station recorded “two very small seismic events” 12 seconds apart on June 22, 2020. He noted that the organization’s system can detect “events” consistent with nuclear test explosions with yields of 551 tons (500 metric tons) of TNT or greater.“These two events 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,” Floyd said.China rejected the allegation. A spokesperson, cited by Reuters, for the Chinese embassy in Washington, Liu Pengyu, said the claim was “entirely unfounded” and an attempt “to fabricate excuses for resuming” US nuclear testing.“This is political manipulation aimed at pursuing nuclear hegemony and evading its own nuclear disarmament responsibilities,” Liu said in an emailed statement. “China urges the US to reaffirm the five nuclear-weapon states’ commitment on refraining from nuclear tests, uphold the global consensus against nuclear tests, and take concrete steps to safeguard the international nuclear disarmament and non-proliferation regime.”

What is decoupling?

Yeaw said China may have tried to conceal the alleged test using “decoupling”—a method in which a device is detonated inside a large underground chamber to reduce the shockwaves recorded by seismic sensors.If a nuclear explosion is set off inside a large cavity deep underground, the seismic waves it produces can be significantly weaker. This evasion technique is known as decoupling, and the extent to which the seismic signal is reduced is called the decoupling factor, according to National Academies of Sciences, Engineering, and Medicine.In a fully decoupled underground blast, most of the explosive energy goes into sharply increasing the gas pressure inside the cavity—potentially to around 100 times atmospheric pressure—rather than being transmitted into the surrounding rock. By contrast, in a normal “well-coupled” underground explosion, much of the energy melts and deforms nearby rock and generates stronger seismic waves that are easier to detect.China has signed but not ratified the 1996 treaty banning nuclear tests and denies conducting any underground nuclear blast since 1996. The United States has also signed but not ratified the pact and last carried out an underground nuclear test in 1992.The allegation comes amid rising concern over nuclear arms control after the expiration of the New START treaty on February 5. US President Donald Trump has urged China to join Washington and Moscow in negotiating a replacement agreement, a proposal Beijing has rejected, saying its nuclear arsenal is far smaller than those of the United States and Russia.The Pentagon says China has more than 600 operational warheads and is expanding its strategic nuclear forces, projecting the country could field more than 1,000 warheads by 2030.