Scientists at Los Alamos National Laboratory have made a discovery that could change how the world monitors nuclear tests. New research shows that underground nuclear explosions could be hidden within the seismic activity of natural earthquakes, making it harder to detect secret nuclear tests. The study, led by Joshua Carmichael, was published recently in the Bulletin of the Seismological Society of America.
Seismic experts have long believed that the powerful digital detectors used around the world could easily spot the difference between natural earthquakes and nuclear explosions.
However, the new study shows that if a nuclear explosion happens close in time and location to a natural earthquake, the seismic signals can overlap and confuse even the most advanced detectors.
Normally, detectors can identify a small, 1.7-ton underground explosion with a 97% success rate. But when an earthquake occurs within 100 seconds and about 250 kilometers of the explosion, that success rate drops sharply to just 37%.
Carmichael said that the overlapping seismic waves “obfuscate the ability of even the most sensitive digital signal detectors we have to identify that explosion.”
This challenges the earlier belief, based on a 2012 report, that earthquake signals could not hide explosion signals. Scientists now realize that natural seismic signals can interfere significantly, raising concerns about how reliably underground nuclear testing can be detected.
The idea that nuclear tests could be hidden within natural earthquakes is deeply worrying for those monitoring global security.
Under the 1996 Comprehensive Nuclear-Test-Ban Treaty (CTBT), all nuclear explosions are banned worldwide, whether for military or peaceful purposes. The international community relies heavily on seismic monitoring to ensure countries comply with this treaty.
If underground nuclear tests can go undetected because of overlapping earthquake signals, it could make monitoring much harder. Scientists fear that some countries might exploit this method to hide their tests from global view.
Carmichael explained that regions already active with seismicity, like North Korea’s test sites, could be even harder to monitor accurately. “There’s been a lot more low-magnitude seismicity in the vicinity of test sites than we initially realized,” he said. North Korea has conducted six nuclear tests in the past 20 years, and monitoring the region has always been a global priority.
Instead of creating new explosions to study, Carmichael and his team used real data from the Nevada National Security Site, where past nuclear tests were recorded. They took the seismic signals from explosions and reduced their strength to simulate smaller blasts. These were then mixed with natural earthquake data to see if current detectors could still tell the difference.
The results showed that when explosion and earthquake signals overlapped, it became extremely difficult for detectors to recognize the explosion. In cases of earthquake swarms—groups of small earthquakes—the detection rate for explosions dropped from 92% to just 16%.
Carmichael pointed out that not only could nuclear tests be hidden, but smaller earthquakes themselves might be undercounted during these seismic events. “We probably underestimate a lot of the low magnitude seismicity that is sourced during a swarm or an aftershock sequence,” he said.
While these findings are concerning, it is important to know that nuclear test detection is not based on seismic data alone. Experts also monitor for other signs, such as radionuclides released into the atmosphere, to confirm if a nuclear test has occurred. Therefore, hiding a nuclear explosion completely would still be difficult, but masking the seismic signal could delay or complicate detection efforts.
The study shines a light on weakness in current monitoring technology. It shows that while seismic detectors are very good, they are not perfect, especially when natural seismic activity is present at the same time as a nuclear explosion.
