Measuring How Stressed Rocks 'Sigh' Before Breaking Could Help Predict Geohazards
Too much stress can make even a rock crack. But before rocks reach their breaking point, they "sigh" a chemical warning by releasing nuclides, a type of atom defined by the number of neutrons as well as protons in the nucleus. Scientists have studied these naturally occurring geochemical emissions for more than half a century, but struggled to link nuclide release to the timing of rock breakage.
Now, an international team of scientists from universities in China (led by Xin Luo at Hong Kong University and Yifeng Chen at Wuhan University) and the United States (led by Michael Manga at the University of California, Berkeley) has cracked that mystery, by creating a model to connect nuclide signal fluctuations to progressive changes in rock structure that lead to critical failure.
When rocks deform and break, they can trigger landslides and avalanches, and may intensify damage from volcanic eruptions and earthquakes. The new findings, published April 9 in the journal Proceedings of the National Academy of Sciences, could help experts prepare for geohazards caused by rocks under stress.
"We explicitly link these structural changes to measurable features of nuclide signals," says Rong Mao, co-first author of the study and a postdoctoral research associate at New Jersey Institute of Technology's Center for Natural Resources. "To our knowledge, this is the first study to establish a quantitative theory for diagnosing rock rupture using naturally occurring nuclide signals," he says.
Anomalous Signals
As rocks weaken, their minerals release nuclides such as radon, helium and thoron into the rock's pores and cracks. Fissures in failing rocks widen, spread and connect to each other; as these spaces change, so do nuclides' release and transmission. This creates anomalous geochemical signals that scientists can measure.
Prior research hinted at a connection between shifts in nuclide signals and rock rupture. In lab experiments, other researchers "have consistently demonstrated that rock cracking and deformation can trigger measurable changes in nuclide emissions," Mao says.
Study co-authors (from right to left) Fushuo Gan, Rong Mao (co-first author), Xin Luo (co-corresponding author) and Jia-Qing Zhou (co-first author); and researchers Haibing Yu, Zhi-Jie Che and Shuai Miao, photographed during work on a geohazards monitoring system in the Three Gorges Reservoir Region in China. Photo provided by Jia-Qing Zhou.
Observations in natural settings have also linked nuclide release to environmental changes that weaken rocks. In 1995, scientists in Kobe, Japan observed an uptick in radon emissions in rocks about nine days prior to a magnitude 7.2 earthquake. Near reservoirs in the French Alps, researchers have noted periodic radon bursts, which they attributed to fluctuations in water levels that over time reshape rocks.
Because nuclide signals often originate in buried rocks but are detectable on the surface, they could provide a critical early warning for imminent geohazards. But despite decades of observations, scientists had not directly connected nuclide anomalies to progressive changes in rocks' physical properties, which limited practical applications for monitoring nuclide emissions.
"Our work addresses this gap by providing a theoretical foundation for interpreting these signals, paving the way toward nuclide-based prediction and improved early warning of geohazards and rock engineering management," Mao explains.
Stress Test
Mao and his co-authors analyzed two prior long-term observations of nuclide release in stressed rocks. One report, a laboratory experiment described in the journal Earth and Planetary Science Letters, monitored radon emissions in a granite cylinder over one month as it weakened and eventually broke. In the other paper, published in the journal Nature, scientists spent three years tracking radon emissions from a bedrock hillside near a reservoir in the French Alps. For the new study, the researchers reviewed the data from these observations, then constructed a model to analyze changes in the signals over time and couple them to progressive structural changes in rocks.
"Our model shows how nuclide signals evolve as rock rupture progresses through four stages: crack initiation, crack opening, crack dilation, and crack propagation," says Mao. "These stages correspond to distinct signal characteristics that can be quantitatively interpreted."
A monitoring site for radon emissions and hillslope deformation inside a tunnel in the Three Gorges Reservoir Region. This world-class geohazard observation platform was excavated directly inside a landslide to enable in-situ scientific investigation by researchers at the China University of Geosciences. Photo provided by Jia-Qing Zhou.
The model accurately reproduced radon signals across all stages of rock weakening and rupture in laboratory experiments, the study authors reported. Even in field applications — involving natural systems that are far more complex than those in controlled experiments — the model explained signals captured by directly monitoring bedrock. Their work offers promising applications for forecasting geohazards like earthquakes, and could help researchers monitor landscapes near reservoirs, where fluctuating water levels can affect rock stability and lead to landslides, according to Mao.
"In such settings, nuclide signals provide a sensitive and potentially real-time indicator of subsurface structural changes, offering valuable information for early warning and risk management."
However, the field results also highlighted the impact of external factors in natural settings that can affect nuclide signals.
"For example, deep fluids, such as thermal waters or brines, often have higher salinity or temperature, which can enhance nuclide release and transmission, leading to amplified signals," Mao explains. "When rock rupture connects to these deep fluid pathways, the observed signals may reflect both structural changes and fluid mixing processes. Incorporating these effects into the model will be an important direction for future work."
Future fine-tuning of the model will aim to improve how quickly it can interpret changing nuclide signals to identify when rocks are about to fail.
"While our model begins to quantify the timescales of signal genesis and transmission, this aspect has not yet been fully validated in field conditions," Mao says. "Addressing this gap will be critical for translating our framework into practical geohazard early warning systems."
The team has already established radon observation stations at three sites in China: the Huangtupo landslide in the Three Gorges Reservoir area, the reservoir-bank landslide near Xiluodu Hydropower Station, and the Po Shan Road slope in Hong Kong, says co-first author Jia-Qing Zhou, an associate professor at Wuhan University in China.
"These facilities are deployed to capture hydrogeochemical precursors of potential geohazards, to further validate and refine our theory," Zhou says. "Our research journey is far from over."