Radiation Measurements

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1 Radiation Measurements 8 () e7 Contents lists available at SciVerse ScienceDirect Radiation Measurements journal homepage: Concurrent concentration declines in groundwater-dissolved radon, methane and ethane precursory to M W. Chimei earthquake T. Kuo a, *, C. Liu a,c.su a, C. Chang b, W. Chen c, Y. Chen a, C. Lin a, H. Kuochen d,y.hsu a, Y. Lin a, Y. Huang a, H. Lin e a Department of Mineral and Petroleum Engineering, National Cheng Kung University, University Ave., Tainan 7, Taiwan b Central Weather Bureau, Taipei, Taiwan c Department of Geosciences, National Taiwan University, Taipei, Taiwan d Institute of Geophysics, National Central University, Jhongli, Taiwan e Sinotech, Taipei, Taiwan highlights In situ radon volatilization has been corroborated by methane and ethane. Radon is the best-choice precursor among the groundwater-dissolved gases. Radon precursors can be consistently caught under suitable geological conditions. article info abstract Article history: Received August Received in revised form July Accepted 8 April Keywords: Earthquake precursors Radon Methane Ethane Groundwater Radon volatilization mechanism into the gas phase was hypothesized to explain the anomalous declines in groundwater radon precursory to three major earthquakes e () M W ¼ 6.8 Chengkung, () 6 M W ¼ 6. Taitung, and () 8 M W ¼. Antung in Taiwan. The epicenters were located km, km, and km from the Antung radon-monitoring well D, respectively. To verify the mechanism of in situ volatilization, we monitored groundwater-dissolved ethane in addition to radon and methane at well D in the Antung hot spring since November,. The mechanism of in situ radon volatilization has been corroborated by the simultaneous concentration declines in groundwater-dissolved radon, methane, and ethane precursory to the M W. Chimei earthquake. The epicenter was located km from the Antung radon-monitoring well D. Observations at the Antung hot spring also suggest that radon is the best-choice tracer among the groundwater-dissolved gases for strain changes in the crust preceding an earthquake. On the southern segment of the Chihshang fault, the observed radon minima decrease as the earthquake magnitude increases. Ó Elsevier Ltd. All rights reserved.. Introduction We have been monitoring groundwater radon since July at well D in the Antung hot spring in eastern Taiwan (Fig. ). The Antung hot spring is located near the Chihshang fault that is part of the eastern boundary of the present-day plate suture between the Eurasia and the Philippine Sea plates. The Chihshang fault ruptured during two 9 earthquakes of magnitudes M ¼ 6. and M ¼ 7. (Hsu, 96). Since July, recurrent anomalous decreases in radon concentration were observed to precede the M W ¼ 6.8 * Corresponding author. Tel.: þ x687; fax: þ address: mctkuobe@mail.ncku.edu.tw (T. Kuo). Chengkung, 6 M W ¼ 6. Taitung, and 8 M W ¼. Antung earthquakes that occurred on December,, April, 6, and February 7, 8, respectively. The observation well (D) is located only km, km, and km from the epicenters of the M W ¼ 6.8, 6 M W ¼ 6., and 8 M W ¼. earthquakes, respectively. Prior to the three major earthquakes, radon decreased from background levels of 9..7, 8.., and 7..8 Bq/L to minima of..,.7., and Bq/L, respectively (Kuo et al., a). Mechanisms and geological conditions were first discussed for interpreting the anomalous decline in radon prior to the Chengkung earthquake (Kuo et al., 6). The anomalous radon decreases prior to the three mainshocks clearly progress in a sequence of three stages (Fig. ). Stage is buildup of elastic strain. During Stage, the radon concentration in groundwater was fairly -87/$ e see front matter Ó Elsevier Ltd. All rights reserved.

2 T. Kuo et al. / Radiation Measurements 8 () e7 Fig.. Map of the epicenters of the earthquakes that occurred in December,, April and, 6, February 7, 8, July, near the Antung hot spring. (a) Geographical location of Taiwan. (b) Study area near the Antung hot spring (filled stars: mainshocks, filled triangle: radon-monitoring well, : Chihshang, or, Longitudinal Valley Fault, : Yongfeng Fault). (a) Stage M w = 6.8 Chengkung Mainshock 6 days days /8/ /9/ // // // - Strain change (ppm) (b) M w = 6. Taitung Mainshock 6 days 7 days Stage - // // 6// 6// 6// 6// Strain change (ppm) (c) days days Stage 7// 7// 7// 8// 8// 8 M w =. Antung Mainshock - Strain change (ppm) Fig.. Observed radon anomalies and calculated crustal-strain transients prior to (a) Chengkung, (b) 6 Taitung, and (c) 8 Antung earthquakes. Stage is buildup of elastic strain. Stage is development of cracks. Stage is influx of groundwater (solid circles: observed radon concentration; open triangles: calculated crustal-strain).

3 T. Kuo et al. / Radiation Measurements 8 () e7 stable. During Stage, the development of new cracks in aquifer rock occurred at a rate faster than the recharge of pore water. Gas saturation and two phases (vapor and liquid) developed in the rock cracks. The radon in groundwater volatilized into the gas phase and the radon concentration in groundwater decreased. Stage is influx of groundwater. During Stage, the radon concentration in groundwater increased and possibly recovered to the previous background level before the mainshock. Fig. shows that the Antung hot spring is a low-porosity fractured isolated aquifer situated in an andesitic block and surrounded by a ductile mudstone of the Lichi mélange (Chen and Wang, 996). Under these geological conditions, the development of new cracks in aquifer rock could occur at a rate faster than the recharge of pore water. Gas saturation and two phases (vapor and liquid) developed in the rock cracks (Brace et al., 966; Nur, 97; Scholz et al., 97). The mechanism of in situ radon volatilization was also substantiated by the simultaneous anomalous declines in groundwaterdissolved radon and methane precursory to the 8 M W ¼. Antung earthquake (Kuo et al., b). The composition of dissolved gases taken from a separator flow test on December 6, 6 consists of 6.8% of nitrogen, 6.7% of methane, and.% of ethane by volume. To further corroborate the in situ volatilization mechanism, we initiated the monitoring of groundwater-dissolved ethane in addition to radon and methane at well D in the Antung hot spring since November,. This paper presents the monitoring results of the simultaneous concentration declines in groundwater radon, methane, and ethane precursory to the M W. Chimei earthquake.. Monitoring methods Since November 7, discrete samples of groundwater have been collected from well (D) about twice per week for analysis of both radon (Rn-) and methane. The production interval ranges from 67 m to 87 m below ground surface. Every sampling starts with flushing the stagnant water in the monitoring well and in the screen zone. An insufficiently pumped volume represents a major source of error, because the water sample would contain a mixture of stagnant water from the monitoring well, pore water from the filter gravel and groundwater influenced by the natural emanation rate of the aquifer. A minimum of five well-bored volumes were pumped before taking samples for radon, methane, and ethane measurements. The liquid scintillation method was adopted to determine the activity concentration of Fig.. Geological map and cross section near the radon-monitoring well D in the area of Antung hot spring (B: tuffaceous andesitic blocks; D: radon-monitoring well; : Chihshang, or, Longitudinal Valley Fault, : Yongfeng Fault).

4 T. Kuo et al. / Radiation Measurements 8 () e7 radon- in groundwater (Noguchi, 96). Methane and ethane in groundwater were determined using the head space method with a gas chromatograph (Shimadzu GC-A), an HP-Plot/Q, m,. mm i.d. capillary column, and a flame ionization detector (FID). The concentrations of methane and ethane measured in the head space were converted to groundwater concentrations using Henry s constants of. and.9, respectively, for methane and ethane at a room temperature of 7 C(Katz, 99; Clever, 979).. Results and discussion Fig. aec shows the observed concentration anomalies for radon, methane, and ethane, respectively. The concentration errors are standard deviation after simple averaging of triplicates. Prior to the M W. Chimei earthquake, radon, methane, and ethane decreased from background levels of Bq/L, 8..8 mg/ L, and.7 mg/l to minima of 6..7 Bq/L,.8. mg/l, and.6 8 mg/l, respectively. The concurrent concentration declines in groundwater-dissolved radon, methane, and ethane were observed precursory to the M W. Chimei earthquake. The above observation supports the hypothesis of in situ volatilization (Kuo et al., 6). Under geological conditions such as those of the Antung hot spring, the sequence of events for the observed concentration anomalies (Fig. aec) can be interpreted in three stages (Kuo et al., 6). Stage is buildup of elastic strain. During Stage, the concentrations of radon, methane, and ethane in groundwater were fairly stable. During Stage, the development of new cracks in aquifer rock occurred at a rate faster than the recharge of pore Table Observed duration of the three stages at well D prior to () M W ¼ 6.8 Chengkung, () 6 M W ¼ 6. Taitung, () 8 M W ¼. Antung, and () M W ¼. Chimei earthquakes. Earthquake M W Duration (day) Precursory Stage Stage Stage time (day) Chengkung Taitung Antung. 6 6 Chimei water. Gas saturation and two phases (vapor and liquid) developed in the rock cracks. The radon, methane, and ethane in groundwater volatilized into the gas phase and the concentrations of radon, methane, and ethane in groundwater decreased. Stage is influx of groundwater. During Stage, the concentrations of radon, methane, and ethane in groundwater increased and possibly recovered to the previous background levels before the mainshock. Table summarizes the duration of the three stages observed at well D prior to () M W ¼ 6.8 Chengkung, () 6 M W ¼ 6. Taitung, and () 8 M W ¼. Antung, and () M W ¼. Chimei earthquakes. The precursor time for radon is defined as the time interval between the moment when the trend of the data starts to decline and the time of occurrence of the earthquake. As shown in Table, the precursor time for radon anomalies increases as the magnitude of earthquakes increases. Environmental records such as atmospheric temperature, barometric pressure, and rainfall were examined to check whether the radon anomaly could be (a) 7. M w =. Chimei Mainshock (b) days days M w =. Chimei Mainshock 9.6. Methane concentration (mg/l) 8 6 Stage // // // /6/ /7/ Stage // // // /6/ /7/ (c) M w =. Chimei Mainshock days. Ethane concentration (mg/l).. Stage // // // /6/ /7/ Year/Month/Day Fig.. Observed concentration anomalies prior to M W. Chimei earthquake (a) radon, (b) methane, and (c) ethane. Stage is buildup of elastic strain. Stage is development of cracks. Stage is influx of groundwater.

5 T. Kuo et al. / Radiation Measurements 8 () e7 Pressure (hpa) Temperature ( o C ) Rainfall (mm) // // /7/ // // Year/Month/Day Fig.. Comparison of radon concentration, atmospherical temperature, and rainfall data (arrow: mainshock; earthquake magnitude M W shown beside arrow).. attributed to these environmental factors (Fig. ). It is difficult to explain such a large radon decrease by these environmental factors. There was also no heavy rainfall responsible for the radon anomaly. Fig. 6aec shows the concentrations of groundwater-dissolved radon, methane, and ethane observed at the Antung D monitoring station since November,, respectively. The concentration errors are standard deviation after simple averaging of triplicates. The anomalous decreases in radon, methane, and ethane precursory to the M W ¼. Chimei earthquake are marked in yellow in Fig. 6aec, respectively. Among the three groundwater-dissolved gases monitored at the Antung hot spring, the background variation or noise in radon data is the lowest. The anomalous minima in methane and ethane recorded precursory to the M W ¼. Chimei earthquake can be easily masked by the noisy background. On the other hand, the radon anomalous minimum recorded precursory to the M W ¼. Chimei earthquake can be clearly distinguished from the background noise. The above observations suggest that radon is the best tracer for monitoring strain changes in the crust precursory to large earthquakes. The concurrent concentration changes in groundwater-dissolved radon, methane, and ethane shown in Fig. 6aec support the hypothesis of in situ volatilization (Kuo et al., 6). The major components of geo-gases are methane and nitrogen. Occasionally, carrier gases (methane and nitrogen) may assume a role in transport of radon released from deep sources at the Antung hot spring. Geo-gases might cause the noisy background in both methane and ethane. The in situ radon-volatilization mechanism was applied to correlate the concentration of groundwater-dissolved radon with the gas saturation in a low-porosity fractured aquifer (Kuo et al., 6). When the development of new cracks in aquifer rock occurred at a rate faster than the recharge of pore water, gas saturation developed in the crust. The gas saturation is defined here as the additional pore volume of the new cracks relative to the Methane concentration (mg/l).. // // /7/ // // // // /7/ // //. 8 6 Ethane concentration (mg/l).... // // /7/ // // Fig. 6. Concurrent concentration changes in groundwater-dissolved (a) radon, (b) methane, and (c) ethane observed at the Antung D monitoring station since November, (arrows: mainshocks; earthquake magnitude M W shown beside arrows).

6 6 T. Kuo et al. / Radiation Measurements 8 () e7 saturated pore volume. The in situ radon-volatilization model can be expressed as follows (Kuo et al., 6). C ;Rn ¼ C w;rn HRn S g þ () where C,Rn is the observed background radon concentration in groundwater when the gas saturation remains at zero, Bq/L; C w,rn is the observed radon concentration remaining in groundwater during the process of rock dilatancy, Bq/L; S g is gas saturation, %; H Rn is Henry s coefficient for radon, dimensionless. Likewise, the in situ volatilization model for groundwaterdissolved methane and ethane can be expressed in equations () and () as follows. based on the anomalous declines of groundwater-dissolved radon, methane, and ethane, respectively. Fig. 6aec indicates that radon is the best-choice tracer among groundwater-dissolved gases for monitoring strain changes in the crust precursory to large earthquakes. The maximum gas saturation at 8.6%, which was estimated from the radon decline, best represents the maximum gas saturation developed near well D in the Antung hot spring preceding the M W. Chimei earthquake. Recurrent groundwater radon anomalies were observed at the Antung D monitoring well in eastern Taiwan prior to the five earthquakes of magnitude M W 6.8, M W 6., M W.9, M W., and M W. that occurred on December,, April, 6, April, 6, February 7, 8, and July,, respectively (Fig. ). C ;Me ¼ C w;me HMe S g þ () C ;Et ¼ C w;et HEt S g þ () where C,Me is the observed background methane concentration in groundwater when the gas saturation remains at zero, mg/l; C w,me is the observed methane concentration remaining in groundwater during the process of rock dilatancy, mg/l; S g is gas saturation, %; H Me is Henry s coefficient for methane, dimensionless; C,Et is the observed background ethane concentration in groundwater when the gas saturation remains at zero, mg/l; C w,et is the observed ethane concentration remaining in groundwater during the process of rock dilatancy, mg/l; S g is gas saturation, %; H Et is Henry s coefficient for ethane, dimensionless. The concentrations of groundwater-dissolved radon, methane and ethane are affected by two key parameters, Henry s constant and the gas saturation developed in a low-porosity fractured aquifer during the process of rock dilatancy. The Henry s coefficients at 6 C are 7.9, 7.6, and 8. for radon, methane, and ethane, respectively (Clever, 979; Katz, 99). Specifically, radon, methane, and ethane decreased from background levels of Bq/L, 8..8 mg/l, and.7 mg/l to minima of 6..7 Bq/L,.8. mg/l, and.6 8 mg/l, respectively, precursory to the M W. Chimei earthquake. Using equations ()e(), the maximum gas saturation developed in newly created cracks preceding the M W. Chimei earthquake was estimated at 8.8%,.% and.9% Fig. 7. The spatial patterns and focal mechanisms of the five earthquakes that occurred in December,, April and, 6, February 7, 8, July, near the Antung hot spring. Fig. 8. The temporal patterns of the five earthquakes that occurred in December,, April and, 6, February 7, 8, July, near the Antung hot spring.

7 T. Kuo et al. / Radiation Measurements 8 () e7 7 Table Observed radon minima at well D prior to () M W ¼ 6.8 Chengkung, () 6 M W ¼ 6. Taitung, () 8 M W ¼. Antung, and () M W ¼. Chimei earthquakes. Earthquake M W Observed radon minimum (Bq/L) Chengkung Taitung Antung. 7.8 Chimei. 6. Figs. 7 and 8 show the spatial (with focal mechanisms) and temporal distributions of the sequences of the five earthquakes, respectively. Only event (April, 6) is strike-slip-faulting. All the other events are thrust-faulting. Only the largest earthquake (event ) was preceded with foreshocks. The number of aftershocks and the sequence duration increase as the local earthquake magnitude increases (Figs. 7 and 8). The observed radon minima decrease as the local earthquake magnitude increases (Table ). For earthquakes occurring on a given fault, the observed radon minima are correlated with local earthquake magnitude and crust strain. Monitoring precursory declines in groundwater radon at a suitable geological site can be a useful means of forecasting local disastrous earthquakes.. Conclusions The significance of our research can be summarized as follows.. The mechanism of in situ radon volatilization has been corroborated by the concurrent concentration declines in groundwater-dissolved radon, methane and ethane observed at the Antung D monitoring station prior to M W. Chimei earthquake. A low-porosity fractured aquifer isolated and surrounded by ductile formation in a seimotectonic environment is a suitable geological site to consistently detect precursory declines in groundwater radon and dissolved gases prior to local large earthquakes.. Radon anomalous declines in groundwater consistently recorded prior to local large earthquakes near the Antung hot spring in eastern Taiwan provide the reproducible evidence to detect radon precursors under suitable geological conditions. On the southern segment of the Chihshang fault, the observed radon minima decrease as the earthquake magnitude increases.. Observations at the Antung hot spring suggest that radon is the best-choice tracer among the groundwater-dissolved gases for strain changes in the crust preceding an earthquake. Acknowledgments Support by the National Science Council (NSC-96-6-M-6-, NSC-97-7-M-6-, NSC-98-6-M-6-6, NSC-99-6-M-6-9, NSC--9-M-6-7, and NSC--6-M- 6-) and Central Geological Surveys ( , , -69--, and ), Radiation Monitoring Center, and Institute of Earth Sciences at Academia Sinica of Taiwan are appreciated. The authors thank Drs. K. Fan and W. Cheng for their assistance and Dr. W. Chang of the National Science Council for his encouragement. References Brace, W.F., Paulding Jr., B.W., Scholz, C., 966. Dilatancy in the fracture of crystalline rocks. J. Geophys. Res. 7 (6), 99e9. Chen, W.S., Wang, Y., 996. Geology of the coastal range, eastern Taiwan. Geol. Taiwan 7,. Clever, H.L. (Ed.), 979. Krypton, Xenon and Radon e Gas Solubilities. Solubility Data Series, vol.. Pergamon Press, Oxford, UK. Hsu, T.L., 96. Recent faulting in the longitudinal valley of eastern Taiwan. Memoir Geol. Soc. China, 9e. Katz, D.L. (Ed.), 99. Handbook of Natural Gas Engineering. McGraw Hill, New York. Kuo, M.C. Tom, Fan, K., Kuochen, H., Chen, W., 6. A mechanism for anomalous decline in radon precursory to an earthquake. Ground Water (), 6e67. Kuo, T., Lin, C., Chang, G.., Fan, K., Cheng, W., Lewis, C., a. Estimation of aseimic crustal-strain using radon precursors of the M 6.8, 6 M 6., and 8 M. earthquakes in eastern Taiwan. Nat. Hazards, 9e8. Kuo, T., Cheng, W., Lin, C., Fan, K., Chang, G.., Yang, T., b. Simultaneous declines in radon and methane precursory to 8 M W. Antung earthquake: corroboration of in-situ volatilization. Nat. Hazards, 67e7. Noguchi, M., 96. New method of radon activity measurement with liquid scintillator. Radioisotopes (), 6e67. Nur, A., 97. Dilatancy, pore fluids, and premonitory variations of t s /t p travel times. Bull. Seismol. Soc. Amer. 6, 7e. Scholz, C.H., Sykes, L.R., Aggarwal, Y.P., 97. Earthquake prediction: a physical basis. Science 8 (), 8e8.