In the afternoon on 22 June 2020, a seismic station in eastern Kazakhstan registered two small earthquakes 12 seconds apart near China’s Lop Nur nuclear test site. Closely spaced jolts can arise from underground explosions followed by a cavity collapse, or simply from earthquakes. But U.S. officials this month asserted the shaking was from a clandestine nuclear detonation—an accusation that could sound the starting gun for a new global arms race.
Weeks earlier, U.S. President Donald Trump ordered the Pentagon to resume nuclear weapons tests “on an equal basis” with other nations. “We’re not going to play on a nonlevel field anymore,” Christopher Yeaw, assistant secretary for the Bureau of Arms Control and Nonproliferation at the Department of State, said at a public forum on 18 February. Such tests could contravene the Comprehensive Nuclear-Test-Ban Treaty (CTBT), which the major nuclear powers have adhered to, even though China and the United States have not ratified it and Russia rescinded ratification in 2023. Renewed testing would also underscore the limitations of the CTBT and its International Monitoring System (IMS), a global network of instruments that can spot blatant treaty violations but is not equipped to distinguish low-yield tests from earthquakes or nonnuclear blasts.
Incentives for testing are strong. The U.S. is developing a new submarine-launched warhead, Russia is deploying hypersonic missiles nearly impossible to intercept, and China is ramping up its arsenal. All three powers need to ensure the reliability of new or existing warheads, and they may calculate that insights gleaned from low-yield tests outweigh the risks of adversaries following suit. Heightening concerns is the lapse this month of the New Strategic Arms Reduction Treaty (New START), which capped U.S. and Russian deployed nuclear warheads at 1550 each.
“There’s a real danger that we’re on the verge of a new nuclear arms race,” says John Holdren, a Harvard University physicist who led arms control studies under former President Bill Clinton.
Thomas DiNanno, the State Department’s undersecretary for arms control and international security, made the accusations against China at a disarmament meeting in Geneva on 6 February. At the Hudson Institute last week, Yeaw, a nuclear engineer, added that detections at the IMS seismic station in Makanchi, Kazakhstan—several hundred kilometers northwest of Lop Nur—are clear evidence for a 2020 test. “There is very little possibility that it is anything other than an explosion,” he asserted. A Chinese official dismissed the allegations as “completely groundless.”
China conducted fewer than four dozen nuclear tests at Lop Nur before signing the CTBT in 1996, compared with about 700 Soviet tests and more than 1000 by the U.S. “China needs [testing] much more than the U.S. does,” says Julien de Troullioud de Lanversin, a nuclear engineer at the Hong Kong University of Science and Technology. Low-yield tests would help China refine weapons designs and probe plutonium properties as it expands from a stockpile of about 600 nuclear weapons to what the Pentagon projects will be roughly 1000 by 2030. China, de Troullioud de Lanversin says, is “preparing for a world where it can resume nuclear weapons tests.”
Last year, satellite images circulated of what appears to be a new laser fusion complex in Sichuan province akin to the U.S. National Ignition Facility, which weapons scientists use to simulate the intense temperatures and pressures of a thermonuclear explosion. Other imagery has spotted the excavation of three tunnels and 30-meter-tall rigs for drilling shafts at Lop Nur. Renny Babiarz, a geospatial analyst with AllSource Analysis, believes two shafts dug in the past few years are “most likely” for yield-producing nuclear tests. In Geneva, DiNanno asserted China is preparing tests “with designated yields in the hundreds of tons.”
But did China jump the gun and conceal a nuclear test in 2020? The pair of quakes near Lop Nur registered on the IMS network, which includes more than 300 seismic, hydroacoustic, infrasound, and radionuclide stations worldwide. But the IMS can distinguish nuclear explosions from other events only at yields greater than roughly 500 tons of TNT, says Robert Floyd, executive director of the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO), which runs the IMS. “With this data alone, it is not possible to assess the cause of these events with confidence.”
The detected shaking “could feasibly come from a chemical explosion of only a few to several tons,” says Christopher Wright, an independent nuclear analyst formerly at the University of New South Wales. “Nothing in the information so far revealed about the seismic signal is uniquely explosive as opposed to natural, uniquely nuclear if nonnatural, or even uniquely from the Chinese nuclear test site.”
DiNanno claimed China used a method called decoupling to make a nuclear blast hard to detect. Decades ago, the U.S. pioneered the technique, which tamps down seismic waves from a test by conducting it in a cavern or in a diving bell–like containment vessel. Other clues could unmask low-yield tests: images of a vessel before installation, eavesdropped chatter among Chinese weapons scientists, or detection of telltale radionuclides wafting from Lop Nur by the U.S.’s Constant Phoenix aircraft. U.S. officials have not revealed whether any such evidence exists for the 2020 event.
Finer grained data from more seismic stations could settle the matter, argues Paul Richards, a geophysicist at Columbia University. The China Earthquake Administration runs a large seismometer network that is largely off-limits to the outside world. “Seismologists in China surely have such data sets,” Richards says. “Let’s encourage China to point us toward relevant data.”
If the CTBT were to come into force, investigators could also do on-site inspections after suspected tests. For example, sensors that detect changes in gravity and electrical resistivity at the surface could detect underground cavities and structural damage. Arrays of active seismometers, which act as receivers for artificially generated seismic noise that illuminates the subsurface, could also reveal new voids in the crust. CTBTO plans to test such instruments this year in a simulation exercise in Namibia. But prospects for the CTBT entering into force are remote.
In Geneva, DiNanno said that instead of a new arms race, the demise of New START could set the stage for a new chapter in arms control—one that involves China. “As we sit here today, China’s entire nuclear arsenal has no limits, no transparency, no declarations, and no controls,” he said. “The next era of arms control … will require the participation of more than just Russia.” On that point, Holdren agrees: “China will have to become involved if we are to have meaningful restraints again.”
