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Could an underground nuclear test’s seismic signature be obscured by the tremors of a natural earthquake?
New insights suggest this may be the case, according to a recent review published in the Bulletin of the Seismological Society of America, challenging long-held beliefs about the phenomenon known as explosion “masking.”
The study, led by Joshua Carmichael and his team at Los Alamos National Laboratory, reveals that state-of-the-art signal detection technology can accurately identify a 1.7-ton underground explosion 97% of the time. However, when seismic signals from an explosion overlap with those from an earthquake occurring within 100 seconds and roughly 250 kilometers away, this success rate plummets to just 37%.
Carmichael emphasized that the intertwined waveforms of an explosion and an earthquake complicate even the most advanced digital detectors, obscuring their ability to discern the explosion signal.
This research invites a reassessment of a 2012 report, which asserted that earthquake signals could not effectively mask those of an explosion. The potential for natural seismic events to conceal explosion signals raises significant concerns for scientists involved in global nuclear test monitoring.
In the context of North Korea, which has conducted six nuclear tests over the last two decades, the proliferation of seismic instruments in the area indicates an uptick in low-magnitude seismic activity near test sites, an observation that Carmichael remarked was greater than previously understood.
The results point to a troubling reality: “Background seismicity—even in areas with minor seismic activity—can considerably diminish the chances of detecting signals from underground explosions,” he added.
Furthermore, the researchers found that signals from earthquake swarms or recurring seismic events can also become masked by overlapping waveforms, resulting in a significant drop in detection rates from 92% to a mere 16%.
This suggests a potential underestimation of low-magnitude seismic events originating from swarms or aftershock sequences, noted Carmichael. “In essence, we may be greatly undercounting the number of earthquakes occurring during these swarms or aftershock phases.”
Studying explosion masking has proven challenging due to the limited number of explosions available for analysis and the scarcity of datasets that include both explosion and natural seismic signals.
One approach to investigate this phenomenon involves simulating explosion data; however, Carmichael pointed out that the complexities of high-frequency explosion signals present too many uncertainties for accurate seismogram creation reflecting small-scale explosions at a distance.
Instead, Carmichael and his colleagues employed techniques utilizing data gathered from explosions and natural seismicity at the Nevada National Security Site. They devised a method to reduce the amplitude of explosion data to simulate the waveforms from smaller explosions, which were then combined with earthquake signals to evaluate the efficacy of advanced multi-channel correlation detectors in identifying explosion signals.
This methodology yielded a comprehensive dataset, allowing researchers to rigorously test the premise that natural seismicity cannot obscure explosion signals.
In addition to seismic signals, researchers consider a variety of indicators in nuclear test monitoring, including the detection of specific radionuclides in the atmosphere. While it is unlikely that an earthquake would entirely mask an explosion, the findings of the recent study provide “at least a recipe” to estimate the probability of explosion detection through seismic signals. This information can be integrated with other monitoring tools to enhance overall assessment strategies, according to Carmichael.
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