Detection of atmospheric radon concentration anomalies and their potential for earthquake prediction using Random Forest analysis.

Various anomalies occurring before earthquakes are currently being studied to predict seismic events, with one of them being the radioactive element radon (222Rn). Radon concentrations in the soil, water, and atmosphere fluctuate in response to crustal movement. Recent research has statistically detected anomalies by analyzing the fluctuations in radon concentrations before earthquakes and conducting quantitative evaluations of radon. However, the method used to determine the parameters in the analysis was problematic. Therefore, in this study, we compared observed atmospheric radon concentration data with predicted values based on typical annual patterns using Random Forest analysis. We conducted a more objective analysis by employing this method and statistically determining anomalies using thresholds. This analysis was conducted using atmospheric radon concentration observation data obtained at Kobe Pharmaceutical University (KPU) before the 1995 Kobe Earthquake, and ionization currents emitted when radon decays were obtained at Fukushima Medical University (FMU) before the 2011 Tohoku-oki Earthquake. Consequently, before the major earthquakes occurred at both locations, the difference between the predicted and observed values exceeded the standard deviation by a factor of three. These results indicate the potential of Random Forest analysis to identify anomalies in atmospheric radon concentrations before earthquakes occur.

Calibration experiments for radon in drinking water measurements using portable-type electrostatic-collection radon monitors.

Portable-type electrostatic-collection radon monitors (RAD7) are often used for in-situ measurements of radon in water. In this study, we evaluated the calibration factors and their uncertainties for two RAD7 monitors based on comparative measurements with the liquid scintillation counting method. In the first experiment, we found that both RAD7 monitors had relatively large uncertainties due to leakage of radon gas that bubbled from the gaps between the lids of the desiccant container and the glass vial. Therefore, for the second experiment, these gaps were closed as much as possible using parafilm and clay, respectively. As a result, the relative uncertainties for both RAD7 monitors were significantly decreased. Furthermore, we collected spring water samples to confirm the reliability of radon concentrations. After closing the leakage point, the uncertainty of radon concentrations in spring water we measured using the typical protocol of the RAD7 were significantly lower, which improves the measurement.

222Rn and 226Ra Concentrations in Spring Water and Their Dose Assessment Due to Ingestion Intake.

222Rn and 226Ra concentrations of less than a few to several thousands of Bq L-1 have been observed in several underground bodies of water around the world. Although regulations for these concentrations in water have been implemented internationally, there are currently no regulations in place in Japan. However, concentrations that exceed these internationally recognized regulatory values have also been observed in Japan. In this study, concentrations in spring water in the northern part of Japan were measured and the effective dose from intake of the water was evaluated. 222Rn concentrations were measured using a liquid scintillation counter, and 226Ra concentrations were measured using a high purity germanium detector after chemical preparation. The measured 222Rn concentrations (=12.7 ± 6.1 Bq L-1) and 226Ra concentrations (<0.019-0.022 Bq L-1) did not exceed the reference values set by international and European/American organizations. A conservative estimate of the annual effective ingestion dose of 8 µSv for 222Rn and 226Ra obtained in this study is much smaller than the estimated overall annual effective dose of 2.2 mSv from natural radiation to the Japanese population. However, this dosage accounts for 8% of the WHO individual dosing criteria of 0.1 mSv/year for drinking water.

Preseismic atmospheric radon anomaly associated with 2018 Northern Osaka earthquake.

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.

Radon degassing triggered by tidal loading before an earthquake.

The presence of anomalous geochemical changes related to earthquakes has been controversial despite widespread, long time challenges for earthquake prediction. Establishing a quantitative relationship among geochemical changes and geodetical and seismological changes can clarify their hidden connection. Here we determined the response of atmospheric radon (222Rn) to diurnal tidal (K1 constituent) loading in the reported 11-year-long variation in the atmospheric radon concentration, including its anomalous evolution for 2 months before the devastating 1995 Kobe earthquake in Japan. The response to the tidal loading had been identified for 5 years before the occurrence of the earthquake. Comparison between these radon responses relative to crustal strain revealed that the response efficiency for the diurnal K1 tide was larger than that for the earthquake by a factor of 21-33, implying the involvement of crustal fluid movement. The radon responses occurred when compressional crustal stress decreased or changed to extension. These findings suggest that changes in radon exhaled from the ground were induced by ascent flow of soil gas acting as a radon carrier and degassed from mantle-derived crustal fluid upwelling due to modulation of the crustal stress regime.

Temporal and Spatial Variation of Radon Concentrations in Environmental Water from Okinawa Island, Southwestern Part of Japan.

In this study, to get a better understanding in characterizing groundwater and ensure its effective management, the radon concentrations in water samples were measured through Ryukyu limestone in southern Okinawa Island, Japan. Water samples were collected from a limestone cave (Gyokusendo cave, dropping water) and two springs (Ukinju and Komesu, spring water), and the radon concentrations were measured by liquid scintillation counters. The radon concentrations in the samples from the Gyokusendo cave, and Ukinju and Komesu springs were 10 ± 1.3 Bq L-1, 3.2 ± 1.0 Bq L-1, and 3.1 ± 1.1 Bq L-1, respectively. The radon concentrations showed a gradually increasing trend from summer to autumn and decreased during winter. The variation of radon concentrations in the dripping water sample from the Gyokusendo cave showed a lagged response to precipitation changes by approximately 2-3 months. The estimated radon concentrations in the dripping water sample were calculated with the measured radon concentrations from the dripping water obtained during the study period. Based on our results, groundwater in the Gyokusendo cave system was estimated to percolate through the Ryukyu limestone in 7-10 days, and the residence time of groundwater in the soil above Gyokusendo cave was estimated to be approximately 50-80 days. This work makes a valuable contribution to the understanding of groundwater processes in limestone aquifers, which is essential for ensuring groundwater sustainability.

EVALUATION OF A RADON AIR MONITOR IN THE MEASUREMENT OF RADON CONCENTRATION IN WATER IN COMPARISON WITH A LIQUID SCINTILLATION COUNTER.

The World Health Organisation (WHO) recommends that the concentration of radon in water should be no more than 100 kBq m-3 (100 BqL-1) and the Codex Alimentarius Commission states that the limit of quantification (LOQ) of a method should be no more than one-fifth of this value. In this study, a degassing method with an RAD7 device was used to measure radon concentrations in water, compared to a liquid scintillation counter (LSC) method used as the reference, to investigate whether the numerical value of the LOQ of this method was more than 1/5 (20 kBq m-3) of 100 kBq m-3. The degassing method with leak prevention was shown to reach a target value of 20 kBq m-3 or less under a relative humidity of 6% or lower in the chamber of the RAD7 device. Accordingly, the RAD7 degassing method with leak prevention can be used to accurately measure radon concentrations in water within the guidance level set out by the WHO.

Non-parametric detection of atmospheric radon concentration anomalies related to earthquakes.

Anomalous phenomena related to earthquakes have been studied to aid in the forecasting of large earthquakes. Radon (222Rn) concentration changes are known to be one of those phenomena. Many studies have quantified radon anomalies to identify physical aspects of radon emanations related to earthquakes. Here, we apply singular spectrum transformation, non-parametric analysis to estimate change points in time series, to atmospheric radon concentration. From 10 years of data from continuous observation of the atmospheric radon concentration over northeastern Japan and Hokkaido, we identify anomalies in the atmospheric radon concentration related to the moment releases of large earthquakes. Compared with a conventional model-based method, the singular spectrum transformation method identifies more anomalies. Moreover, we also find that change points in the atmospheric radon concentration prior to the 2011 Tohoku-Oki earthquake (Mw 9.0; 11 Mar. 2011, N38.1°, E142.9°) coincided with periods of other anomalous precursory phenomena. Our results indicate that singular spectrum transformation can be used to detect anomalies in atmospheric radon concentration related to the occurrences of large earthquakes.

Accurate Measurement of Indoor Radon Concentration using a Low-Effective Volume Radon Monitor.

AlphaGUARD is a low-effective volume detector and one of the most popular portable radon monitors which is currently available. This study investigated whether AlphaGUARD can accurately measure the variable indoor radon levels. The consistency of the radon-concentration data obtained by AlphaGUARD is evaluated against simultaneous measurements by two other monitors (each ~10 times more sensitive than AlphaGUARD). When accurately measuring radon concentration with AlphaGUARD, we found that the net counts of the AlphaGUARD were required of at least 500 counts, <25% of the relative percent difference. AlphaGUARD can provide accurate measurements of radon concentration for the world average level (~50 Bq m-3) and the reference level of workplace (1000 Bq m-3), using integrated data over at least 3 h and 10 min, respectively.

Anomalous changes in atmospheric radon concentration before and after the 2011 northern Wakayama Earthquake (Mj 5.5).

A significant increase in atmospheric radon concentration was observed in the area around the epicentre before and after the occurrence of the shallow inland earthquake in the northern Wakayama Prefecture on 5 July 2011 (Mj 5.5, depth 7 km) in Japan. The seismic activity in the sampling site was evaluated to identify that this earthquake was the largest near the sampling site during the observation period. To determine whether this was an anomalous change, the atmospheric daily minimum radon concentration measured for a 13-year period was analysed. When the residual radon concentration values without the seasonal radon variation and the linear trend was > 3 standard deviations of the residual radon variation corresponding to the normal period, the values were deemed as anomalous. As a result, an anomalous increase in radon concentration was determined before and after the earthquake. In conclusion, anomalous change related to earthquakes with at least Mj 5.5 can be detected by monitoring atmospheric radon near the epicentre.

Annual variation in the atmospheric radon concentration in Japan.

Anomalous atmospheric variations in radon related to earthquakes have been observed in hourly exhaust-monitoring data from radioisotope institutes in Japan. The extraction of seismic anomalous radon variations would be greatly aided by understanding the normal pattern of variation in radon concentrations. Using atmospheric daily minimum radon concentration data from five sampling sites, we show that a sinusoidal regression curve can be fitted to the data. In addition, we identify areas where the atmospheric radon variation is significantly affected by the variation in atmospheric turbulence and the onshore-offshore pattern of Asian monsoons. Furthermore, by comparing the sinusoidal regression curve for the normal annual (seasonal) variations at the five sites to the sinusoidal regression curve for a previously published dataset of radon values at the five Japanese prefectures, we can estimate the normal annual variation pattern. By fitting sinusoidal regression curves to the previously published dataset containing sites in all Japanese prefectures, we find that 72% of the Japanese prefectures satisfy the requirements of the sinusoidal regression curve pattern. Using the normal annual variation pattern of atmospheric daily minimum radon concentration data, these prefectures are suitable areas for obtaining anomalous radon variations related to earthquakes.

Normal seasonal variations for atmospheric radon concentration: a sinusoidal model.

Anomalous radon readings in air have been reported before an earthquake activity. However, careful measurements of atmospheric radon concentrations during a normal period are required to identify anomalous variations in a precursor period. In this study, we obtained radon concentration data for 5 years (2003-2007) that can be considered a normal period and compared it with data from the precursory period of 2008 until March 2011, when the 2011 Tohoku-Oki Earthquake occurred. Then, we established a model for seasonal variation by fitting a sinusoidal model to the radon concentration data during the normal period, considering that the seasonal variation was affected by atmospheric turbulence. By determining the amplitude in the sinusoidal model, the normal variation of the radon concentration can be estimated. Thus, the results of this method can be applied to identify anomalous radon variations before an earthquake.

Simultaneous measurements of radon and thoron exhalation rates and comparison with values calculated by UNSCEAR equation.

We simultaneously measured radon and thoron exhalation rates in areas of weathered acid rocks in Japan to investigate the relationship between them and between measured and calculated radon exhalation rates. To calculate the radon exhalation rate, we used an equation proposed by UNSCEAR. Our analysis showed that both the radon and thoron exhalation rates measured at one location in a natural environment fluctuated widely, even within one day. We found a strong correlation between the radon and thoron exhalation rates. The measured radon exhalation rates tended to be lower than the calculated values: the mean ratio of the measured exhalation rate to the calculated rate was 0.65. We concluded that the UNSCEAR equation is applicable under a controlled environment (temperature, humidity, etc.), but not where there are artificial underground structures that may influence the transportation of radon in soil.