A radon-thoron isotope pair as a reliable earthquake precursor

Abnormal increases in radon (222Rn, half-life = 3.82 days) activity have occasionally been observed in underground environments before major earthquakes. However, 222Rn alone could not be used to forecast earthquakes since it can also be increased due to diffusive inputs over its lifetime. Here, we show that a very short-lived isotope, thoron (220Rn, half-life = 55.6 s; mean life = 80 s), in a cave can record earthquake signals without interference from other environmental effects. We monitored 220Rn together with 222Rn in air of a limestone-cave in Korea for one year. Unusually large 220Rn peaks were observed only in February 2011, preceding the 2011 M9.0 Tohoku-Oki Earthquake, Japan, while large 222Rn peaks were observed in both February 2011 and the summer. Based on our analyses, we suggest that the anomalous peaks of 222Rn and 220Rn activities observed in February were precursory signals related to the Tohoku-Oki Earthquake. Thus, the 220Rn-222Rn combined isotope pair method can present new opportunities for earthquake forecasting if the technique is extensively employed in earthquake monitoring networks around the world.

, and stable isotope ratios (δ 2 H and δ 18 O) have been monitored in numerous fault and volcanic zones for earthquake prediction [1][2][3][4][5][6] . Such precursor signals are anticipated by the formation of micro cracks in fault zones or by groundwater mixing due to crustal dilation 1,6 . Stresses that develop prior to an earthquake are thought to be responsible for the release and accumulation of certain constituents that may be useful as tracers or precursors of these tectonic forces. However, these potential precursors have not been extensively used to forecast tectonic or volcanic activities because abnormal increases of these components also occur due to other environmental processes, including changing meteorological conditions. Among the potential earthquake precursors, 222 Rn in soil and groundwater has shown the highest sensitivity because of its radioactive nature and origin in the subsurface [7][8][9][10][11][12] . However, 222 Rn still suffers from interferences by meteorological phenomena and tidal forces 10,13,14 .
In order to test the concept of a dual isotopic tracer, we monitored both 222 Rn and 220 Rn every hour in a limestone cave (Seongryu Cave, Korea) from May 18, 2010 to June 17, 2011 (see Methods). To our knowledge, this represents the first study to evaluate this 222 Rn-220 Rn isotope pair in an underground environment as a precursor of earthquakes. Seongryu cave, which is ~250 Ma in age, is located in Seonyu Mountain (elevation: 199 m) in the eastern part of Korea (Fig. 1). The cave is at 20 m above the mean sea level and is ~330 m in length, 1-13 m in height, and has an entrance of ~1 m 2 . The main cave contains many branches, including three lakes, of which the two located near the entrance are affected by an outside stream 15 . More detailed description of this cave is available in Oh and Kim 16 .
For the 220 Rn measurements, the position of the air-inlet above the cave floor is critical because 220 Rn decays almost immediately (~5 minutes for 97% decay) after emanation from the source. Since 220 Rn in the soil air should be in equilibrium with its parent 224 Ra (even in the skin layer) due to its short half-life, variations in the 220 Rn activity of soil air, including that of the porewater in rocks and soils would not be useful. Therefore, the air-inlet must be positioned at a height where the inputs of 220 Rn from general environmental processes are minimal, but also where large inputs from earthquakes are detectable. In addition, the monitoring location should be isolated from the outside air, which can also generate 220 Rn anomalies due to wind-driven skin flow through rocks and soils. The atmosphere in the first 130 m of Seongryu Cave is influenced by the outside air, while the atmosphere in the inner cave is almost stagnant 15 . Thus, in this study we chose an air-inlet position 0.2 m above the cave floor and 180 m from the entrance ( Fig. 1). At this position, the noise from general meteorological driving forces was minimal and the meteorological conditions were relatively constant for both 220 Rn and 222 Rn. The activity of 220 Rn was not detectable when the air-let was positioned 1.5 m above the cave floor due to its short half-life (see Supplementary Fig. S1). The activities of 220 Rn were higher at the entrance site than at the monitoring site due to the influence of wind-driven skin flow (see Supplementary Fig. S2). At the monitoring site, 222 Rn was both significantly stable and enriched relative to the entrance site (see Supplementary Fig. S2). Furthermore, since the activity ratios of 222 Rn to 220 Rn are high in the normal natural environment, there is a possibility of counts from 222 Rn spilling over to 220 Rn (see Methods). Previous study 16 has shown that without a correction for this spillover, one could observe erroneous positive correlations between 222 Rn and 220 Rn. Over the monitoring period, 222 Rn activity was on average higher during the summer (avg. 645 ± 194 Bq m −3 ) than during the winter (avg. 140 ± 172 Bq m −3 ) (Fig. 2a), which is typical of 222 Rn observations in caves around the world 17,18 . This seasonal variation in 222 Rn activity is known to be due to the difference in air ventilation intensity. In contrast, 220 Rn activity was on average higher in the winter (avg. 9.7 ± 10.1 Bq m −3 ) than in the summer (avg. 1.2 ± 3.7 Bq m −3 ) (Fig. 2b). In particular, we noted that both 222 Rn and 220 Rn showed high peaks in February 2011 that are decoupled from the general seasonal patterns (Fig. 3). Outside weather parameters (temperature, relative humidity, and pressure) showed large variations (− 13.8-35.2 °C , 7.5-97.9%, and 990.9-1033.6 mbar, respectively) relative to the inside air (10.7-15.7 °C , 94-99.9%, and 999.0-1034.8 mbar, respectively).
With the exception of the anomalous peaks (red circles in Fig. 3), the daily average of 222 Rn activities showed a significant positive correlation (n = 234, r 2 = 0.66) with the daily average of temperature (density) difference (the inside temperature is subtracted from the outside temperature) (Fig. 3a), which is typical of cave air behavior 17,18 . In general, the 222 Rn activities in the outside air are approximately two orders of magnitude lower than those in cave airs. Thus, the lower 222 Rn activities in winter could be due to greater ventilation of the denser outside air. In contrast, during the summer, 222 Rn is trapped inside the cave due to the atmospheric stratification. In addition to the density difference, in some regions, the land surface humidity affects the activity of 222 Rn in the underground air because pore space can be affected by water vapor condensation 19 . As such, we observed a positive correlation (n = 396, r 2 = 0.57) between 222 Rn activity and the relative humidity of the outside air. The temperature and relative humidity of the inside air remained fairly constant, and they correlated weakly with 222 Rn activities (r 2 = 0.27 and r 2 = 0.32, respectively). Precipitation and outside pressure showed no significant correlations with 222 Rn activity (r 2 < 0.1 and r 2 = 0.12, respectively). Thus, the variations in 222 Rn activities in the cave seem to be predominantly affected by variations in the outside temperature and humidity. In contrast to 222 Rn, the daily average of 220 Rn activities, except for anomalous peaks, showed a negative correlation (n = 234, r 2 = 0.51) with temperature difference (Fig. 3b) and relative humidity (r 2 = 0.41). However, poor correlations were observed between the activities of 220 Rn and outside pressure and precipitation (r 2 = 0.15 and r 2 < 0.1, respectively). 220 Rn activities in the winter were higher than those in the summer, resulting from higher ventilation which results in rapid advection of the pore air (with 220 Rn already in equilibrium with 224 Ra) in the cave, in the cold season.
The significant 222 Rn and 220 Rn anomalies observed in February 2011 (Fig. 3a-c) cannot be explained by normal meteorological variations, including episodic precipitation events (Fig. 3d). Thus, we consider that these anomalies may have been precursors of the Tohoku-Oki Earthquake, which occurred approximately one month later (Fig. 2). A recent study 20 showed that the Tohoku-Oki Earthquake was preceded Rn and 220 Rn activities. Numbers in parentheses in (d) denote distance (km) from the monitoring site. Due to a lack of a precise calibration for 220 Rn, activities are presented in 'arbitrary units' .
by a series of small earthquakes that started on 13 February 2011. Our results showed that the 222 Rn alone could not distinguish the February anomalies from the summer peaks (Fig. 3a), however, there are clear anomalous signals based on 220 Rn alone or the combined 222 Rn vs. 220 Rn plots (Fig. 3b,c).
In general, carrier gases (CO 2 , CH 4 , Ar, and He) play a critical role in controlling the migration and transport of trace gases (e.g., 222 Rn) towards the surface 21,22 . From our results, we assume that the degassing of carrier gases peaked on February 12, 2011, reduced continuously until March 1, 2011, and then almost stopped on the day of the Tohoku-Oki Earthquake. Many studies have reported that carrier gases anomalies occurred days-weeks before earthquakes 23 .
Based on our results, we present three points of evidence to indicate that the 220 Rn-222 Rn isotope pair may be an excellent precursor of earthquakes: (1) 220 Rn peaks during the anomalous period were much higher than those during normal periods over the year. The observed anomalies cannot be explained by any normal environmental conditions during the monitoring period (Figs 2 and 3); (2) A positive correlation was observed between 220 Rn and 222 Rn during the anomalous period, perhaps due to the venting of carrier-gases (e.g., CO 2 ) from the sub-surface. On the other hand, negative correlations were observed more generally (Fig. 3c); (3) The peak hours of the 220 Rn and 222 Rn anomalies were episodic and decoupled from normal diurnal patterns (see Supplementary Fig. S3). In general, 222 Rn showed a diurnal fluctuation pattern, particularly in spring and fall when temperature differences between day and night were largest, although this pattern is not seen for the short-lived 220 Rn.
Although the monitoring site in this study is ~1200 km distant from the epicenter of the Tohoku-Oki Earthquake, the earthquake impacted the Korean Peninsula in a number of ways. The Korean Peninsula is located on the Eurasian tectonic plate, which extends to Japan. As a result of the Tohoku-Oki Earthquake it was estimated to have moved eastward by 1.2-5.6 cm 24 . In addition, 46 out of 320 monitoring wells in Korea showed changes in water level, temperature, and electrical conductivity as a result of the earthquake 25,26 . Therefore, it is not surprising that the radon isotope anomalies observed in Seongryu Cave may represent precursors of this extremely large earthquake.
On the basis of our observations of 222 Rn and 220 Rn, we suggest that a network of 222 Rn-220 Rn monitoring stations should be constructed to further verify the potential of this method for forecasting the locations and strengths of pending earthquakes. In order to filter out other environmental forcing factors, meteorological parameters and potential carrier gases (CO 2 , CH 4 ) should also be monitored. Most importantly, we have to carefully select suitable natural or artificial cave systems for these stations. We can easily develop more sensitive 220 Rn monitoring systems, data transmission setups to remote laboratories, and institute a canary program to automatically detect potential earthquake signals.

Methods
A detailed description of the RAD7 radon monitor is available in Burnett et al. 27 and Lane-Smith et al. 28 . Briefly, the RAD7 uses a silicon alpha detector to determine the daughters of 222 Rn and 220 Rn, 218 Po (t 1/2 = 3.05 min; 6.00 MeV), 214 Po (t 1/2 = 164 μ s; 7.67 MeV), and 216 Po (t 1/2 = 0.15 s; 6.78 MeV). The surface of the detector uses electrostatic attraction to capture Po + ions using an electric potential of 2000 to 2500 volts, and the alpha detector counts 218 Po, 216 Po, and 214 Po alpha decays. We used both 214 Po and 218 Po peaks for the 222 Rn measurements. An air filter is used at the entrance of the RAD7 to prevent dust particles and charged ions from entering the radon chamber. The internal air pump of the RAD7 (flow rate: 1 L min -1 ) was activated for 1 minute every 5 minutes to reduce maintenance labor in the humid cave air. In order to maintain relative humidity of < 10%, which is necessary for a constant detection efficiency of the RAD7, a desiccant column and a passive moisture exchanger (DRYSTIK, Durridge Co.) were coupled to the air path of the RAD7. During the measurement period, a new desiccant column was replaced every 3 or 4 weeks.
In order to obtain accurate 220 Rn activity data in the presence of extremely high 222 Rn levels, we corrected for the 'spillover effect' of 222 Rn to 220 Rn by using the method of Chanyotha et al. 29 . Briefly, we assume that the efficiency of 220 Rn detection is a quarter of that for 222 Rn to account for thoron decay sample in the intake system (volume of sample tube + drying unit) and internal cell of the RAD7. For the correction of 'spillover effect' , the spill factor is assumed to be 0.015, which is an average value calibrated and measured by Durridge. The spillover from one of the radon channels (C: 214 Po) into the thoron channel (B: 216 Po) can be corrected by the analysis software (Capture) provided by Durridge. Due to a lack of a precise calibration for 220 Rn, the activity data are presented in 'arbitrary units' . While there is uncertainty in the calibration of absolute 220 Rn activities, it does not affect the interpretation of our results since the same procedures and conditions were held constant during the measurement period. The atmospheric parameters (temperature, relative humidity, and pressure) in the cave were measured hourly using external sensors (MSR145, MSR electronics) and stored in a data logger. Outside weather parameters were obtained from the Korea Meteorological Administration (KMA).