Previous studies have shown elevated levels of radon in the atmosphere prior to the mainshock of a huge inland earthquake. However, in a new study, researchers revealed an anomaly in this phenomenon as they found that the concentration of the radioactive element decreased.
"For the first time, we found a decrease in the atmospheric radon associated with seismic quiescence before the mainshock of an inland earthquake," said lead author Jun Muto, a professor from the Graduate School of Science at Tohoku University.
Through the analysis of data before and after the Northern Osaka earthquake in 2018, the researchers found that the atmospheric radon concentration decreased. There was a decline around a year before the quake-- a trend that continued until June 2020.
Seismic activity in the vicinity of the monitoring area also decreased prior to the tremblor, along with a reduced seismic activity following the mainshock for the entire Kansai region, excluding the areas of aftershock.
All the observations indicate that radon levels did not increase following the quake and created a more complicated picture of the radon and fluid movements. These can cause radon exhalation in the subsurface when large tremors happen.
"The discovery revealed that more processes are occurring before earthquakes than previously thought," Muto said.
"Further analysis of other earthquakes will lead to a better understanding of the physiochemical processes at play and help us use atmospheric radon concentrations to clarify various crustal movements correlated with major earthquakes."
The group has established monitoring networks for atmospheric radon concentration measurement at radioisotope facilities across Japan. Expansion of the network will help clarify the area and timing of anomalies, the geological attributes that cause such anomalies, and the relationship with various earthquake types. This will ultimately help in a radon-based earthquake prediction system.
Image: 2018 North Osaka earthquake shake map. Credit: USGS
"Preseismic atmospheric radon anomaly associated with 2018 Northern Osaka earthquake" - Muto, J., et al. - Scientific Reports - DOI: 10.1038/s41598-021-86777-z
Despite the challenges in identifying earthquake precursors in intraplate (inland) earthquakes, various hydrological and geochemical measurements have been conducted to establish a possible link to seismic activities. Anomalous increases in radon (222Rn) concentration in soil, groundwater, and atmosphere have been reported prior to large earthquakes. Although the radon concentration in the atmosphere is lower than that in groundwater and soils, a recent statistical analysis has suggested that the average atmospheric concentration over a relatively wide area reflects crustal deformation. However, no study has sought to determine the underlying physico-chemical relationships between crustal deformation and anomalous atmospheric radon concentrations. Here, we show a significant decrease in the atmospheric radon concentration temporally linked to the seismic quiescence before the 2018 Northern Osaka earthquake occurring at a hidden fault with complex rupture dynamics. During seismic quiescence, deep-seated sedimentary layers in Osaka Basin, which might be the main sources of radon, become less damaged and fractured. The reduction in damage leads to a decrease in radon exhalation to the atmosphere near the fault, causing the preseismic radon decrease in the atmosphere. Herein, we highlight the necessity of continuous monitoring of the atmospheric radon concentration, combined with statistical anomaly detection method, to evaluate future seismic risks.
Featured image credit: USGS