RADON ANOMALY AT THE ANTUNG HOT SPRING BEFORE THE TAIWAN M6.8 CHENGKUNG EARTHQUAKE

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1 PROCEEDINGS, Thirty-First Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, California, January 30-February 1, 2006 SGP-TR-179 RADON ANOMALY AT THE ANTUNG HOT SPRING BEFORE THE TAIWAN M6.8 CHENGKUNG EARTHQUAKE T. Kuo 1, K. Fan 1, W. Chen 2, H. Kuochen 3, Y. Han 1, C. Wang 1, T. Chang 1, Y. Lee 4 1 Department of Mineral and Petroleum Engineering, National Cheng Kung University, Tainan, Taiwan 2 Department of Geosciences, National Taiwan University, Taipei, Taiwan 3 Central Weather Bureau, Taipei, Taiwan 4 Water Resources Agency, Ministry of Economic Affairs, Taipei, Taiwan mctkuobe@mail.ncku.edu.tw ABSTRACT The 2003 Chengkung earthquake of magnitude (M) 6.8 on December 10, 2003 was the strongest earthquake near the Chengkung area in eastern Taiwan since The radon-monitoring station at the Antung hot spring as located 20 km from the epicenter. Approximately 65 days prior to the 2003 Chengkung earthquake, precursory changes in the groundwater s radon concentration were observed. The radon anomaly was a decrease from a background level of 780 pci/l to a minimum of 330 pci/l. Observations at the Antung hot spring suggest that the groundwater radon, when observed under suitable geological conditions, can be a sensitive tracer for strain changes in the crust preceding an earthquake. The Antung hot spring is situated in a tuffaceous-sandstone fractured block inside mudstone. Under such geological conditions, the dilation of rock masses was produced at a rate faster than the recharge rate of pore water and gas saturation developed in rock cracks preceding the earthquake. Meanwhile, the radon in groundwater volatilized and partitioned into the gas phase. Our understanding of geology and radon phase-behavior would help select radon-monitoring sites in the future. INTRODUCTION Variations in radon (Rn-222) content in groundwater observed prior to some earthquakes constitute an important precursor to earthquakes (Trique et al., 1999; Igarashi et al., 1995, 1993; Igarashi and Wakita, 1990; Wakita et. al., 1991; Liu et al., 1985; Hauksson and Goddard, 1981; Noguchi and Wakita, 1977; Shapiro et al., 1981; Teng, 1980; Wakita et al., 1980). According to a worldwide survey (Hauksson, 1981), most radon (Rn-222) anomalies showed increases in radon content of groundwater. Contrarily, few anomalies manifested decreases in radon content of groundwater. The radon anomaly prior to the 2003 Chengkung earthquake was a decrease from a background level of 780 pci/l to a minimum of 330 pci/l. Geological conditions near the Antung radon-monitoring station were investigated to identify mechanisms causing the phenomena precursory to the 2003 Chengkung earthquake in eastern Taiwan. Radon-222 is a radioactive nuclide with a half-life of approximately 3.8 days. The radon concentration in groundwater is proportional to the uranium concentration in adjacent rocks in an aquifer. Radon is chemically inert. Transport behavior of radon in geological environments can be described on the basis of physical processes such as fluid advection, diffusion, partition between liquid and gas phases, and radioactive decay. Because of radon s short recoil length (3 *10-8 cm), only atoms produced at the surface of rock grains are released to the surrounding groundwater. Thus, the radon concentration in groundwater is largely dependent on the surface area of rocks (Torgersen et al.,1990). Before earthquake occurrence, when regional stress increases, formation of microcracks in rock masses could cause an increase in the surface area of rocks. As a result, radon concentration rises in the groundwater (Igarashi et al., 1995; Teng, 1980). However, mechanisms and geological conditions for interpreting anomalous decreases in the radon concentration of groundwater prior to earthquakes are seldom discussed in the literature yet. To accumulate data on groundwater radon concentration, we began to study the Antung hot spring in eastern Taiwan about 3 km southeast of the Chihshang fault in July 2003 (Figure 1). The Chihshang fault is the most active segment of the Longitudinal Valley fault which is the present-day plate suture between the Eurasian and the Philippine Sea plates. The Chihshang fault (Hsu, 1962) ruptured during two 1951 earthquakes of magnitude (M) 6.2 and (M) 7.0. The annual survey of geodetic and GPS measurements consistently revealed the active creeping Chihshang fault moving at a rapid steady rate, about 2-3 cm/yr during the past 20 years

2 (Angelier et al., 2000; Lee et al., 2003; Yu et al., 1990, 2001). METHODS Radon determination. Radon was partitioned selectively into a mineral-oil scintillation cocktail immiscible with the water sample (Noguchi, 1964). After the sample was dark-adapted and equilibrated, it was measured in a liquid scintillation counter using a region or window of the energy spectrum optimal for radon alpha particles. A calibration factor for the LSC measurements of 7.1 ± 0.1 cpm/pci was calculated using an aqueous Ra-226 calibration solution, which is in secular equilibrium with Rn-222 progeny. For a count time of 50 min and background less than 6 cpm, a detection limit below 18 pci/l was achieved using the sample volume of 15-ml (Prichard et al., 1992). Radon phase-behavior. The purpose of radonpartitioning experiments was to determine Henry s coefficient for radon at formation temperature (60 ) using formation brine from the Antung hot spring. Six levels of gas saturation were investigated (i.e., Sg = 0 %, 5 %, 10 %, 15 %, 20 %, and 25 %). Triplicate experiments were conducted for each level of gas saturation. Every experiment employed 40 ml of formation brine and six levels of headspace volume (i.e., 0 ml, 2 ml, 4 ml, 6 ml, 8 ml, and 10 ml). Twophase equilibrium was achieved for each experiment in 30 minutes at formation temperature (60 ). RESULTS A 10-month observation started in July 2003 at a well (D1) located in the Antung hot spring (Figure 1). The observation well (D1) is located roughly 20 km north of the hypocenter of the magnitude (M) 6.8 earthquake that occurred at 4:38 am December 10, 2003 (UT). On the basis of the distribution of the aftershocks, the faulting generated by the earthquake extended close to the observation well. Discrete samples of geothermal water have been collected from a well at the Antung hot spring for analysis of radon (Rn-222) content. The production interval ranges from 167 m to 187 m below ground surface. The well was pumped more or less continuously. The sampling frequency was twice per week. Liquid scintillation method was adopted to determine of the activity concentration of radon-222 in groundwater (Noguchi, 1964; Prichard et al., 1992). Anomalous changes in radon concentration were observed to precede the earthquake of magnitude (M) 6.8 that occurred on December 10, 2003 near Chengkung in eastern Taiwan. The radon concentration in ground-water was fairly stable (780 pci/l in average) in the period from July 2003 to September 2003 (Figure 2). Sixty-five days before the magnitude (M) 6.8 earthquake (December 10, 2003), the radon concentration of ground-water started to decrease for 45 days. Twenty days prior to the earthquake, the radon concentration reached a minimum value of 330 pci/l and started to increase. Before the earthquake (December 10, 2003), the radon concentration recovered to the previous background level of 780 pci/l. The main shock also produced a sharp coseismic anomalous increase (~300 pci/l). Since the earthquake, some irregular variations were observed, which we interpret as an indication that the strain release by the main shock was not complete and that some accumulation and release of strain continued in the region. DISCUSSION Environmental records such as temperature and rainfall were examined to check if the radon anomaly could be attributed to these environmental factors. The temperature of the groundwater was very stable and its variation was less than 0.2 during the observation period. There was no heavy rainfall responsible for the radon anomaly. It is also difficult to explain such a large radon decrease by mixing of groundwater. The hypocentral distributions of aftershocks of the 2003 Chengkung earthquake reveal the close relationship between seismicity and the Chihshang fault. The magnitude (M) 6.8 mainshock occurred on the Chihshang fault which has a faulting surface extending about 30 km in depth and dips approximately 50 to southeast (Figure 1). Focal mechanism of the mainshock is a thrust with strike of N36 E and dip of 50 SE. The Antung hot spring is about 3 km southeast of the Chihshang fault. The anomalous decrease in radon concentration observed at the Antung hot spring suggests that the groundwater radon, when observed at suitable sites, can be a sensitive tracer for strain changes in crust associated with earthquake occurrences (Trique et al., 1999; Roeloffs, 1999; Silver and Wakita, 1996). A detailed investigation of nearby geological conditions would help understand the physical basis causing the groundwater radon anomaly and select sites for monitoring groundwater radon in the future. The studied region is in a unique tectonic setting located at the boundary between Eurasian and Philippine Sea plates. Figure 3 shows the geological map and cross section near the radon-monitoring well (D1) in the Antung hot spring located in the area of Coastal Range. The Coastal Range is recognized four stratigraphic units defined by their lithology, i.e., Tuluanshan, Fanshuliao and Paliwan Formations and Lichi mélange. The Tuluanshan Formation can be recognized three arc-related volcanic units as lava,

3 volcanic breccia and tuffaceous sandstone. The lithofacies of Fanshuliao and Paliwan Formations consist of rhythmic sandstone and mudstone turbidites. The Lichi mélange occurs a highly deformed mudstone that is characterized by penetrative foliation visible in the outcrop. The Antung hot spring is situated at a tuffaceoussandstone exotic block which included within predominantly mudstone of the Paliwan Formation (Chen and Wang, 1996). Well-developed minor faults and joints are common in the block displaying intensively brittle deformation. It is possible that these fractures reflect the deformation and disruption by the Longitudinal Valley and Yongfeng faults. Based on the geological map, it suggests that the block displays intensively brittle deformation and has developed in a ductile-deformed mudstone strata. The hot ground-water is conducted from the fault zone and then diffused into the block along the minor fractures, because the hot spring and monitoring well (D1) are near the Yongfeng fault about several tens meters apart. According to the dilatancy mechanism for earthquake occurrence (Nur, 1972; Brace et al., 1966; Scholz et al., 1973), when regional stress increases, dilation of rock masses could be produced at a rate faster than the rate at which pore water can flow into the newly created pore volume. During this stage, gas saturation and two phases (vapor and liquid) develop in rock cracks and pores. Meanwhile, the radon in groundwater volatilizes and partitions into the gas phase. Therefore, the radon concentration in groundwater decreases during this stage. The sequence of events for radon data shown in Figure 2 can be interpreted in three stages. In the period from July 2003 to September 2003 (Stage 1), the radon concentration in groundwater was fairly stable (780 pci/l in average). Stage 1 consisted of the accumulation of tectonic strain, which produced a slow, steady increase of effective stress. Sixty-five days before the magnitude (M) 6.8 earthquake (December 10, 2003), the radon concentration of groundwater started to decrease and reached a minimum value of 330 pci/l twenty days before the earthquake. During the above 45-day period (Stage 2), the dilation of rock masses was produced at a rate faster than the recharge rate of pore water and gas saturation developed in rock cracks and pores. Stage 3 started at the point of minimum radon concentration; the water saturation in rock cracks and pores began to increase again. During Stage 3, the radon concentration in groundwater increased and recovered to the previous background level six days before the earthquake (December 10, 2003). Since the earthquake, some irregular variations were observed, which we interpret as an indication that the strain release by the main shock was not complete and that some accumulation and release of strain continued in the region. The Antung hot spring is situated under such geological conditions that dilation rate of fractured rocks could exceed the rate at which pore water can flow into the newly created pore volume preceding an earthquake. Figure 4 shows the vapor-liquid twophase equilibrium results for radon-partitioning experiments at formation temperature (60 ) using formation brine from the Antung hot spring. Experimental data in Figure 4 were regressed with the following two-phase partitioning model to determine Henry s coefficient. C0 Cw ( H Sg 1 ) where C0 is initial radon concentration in formation brine, pci/l; Cw is equilibrium radon concentration in liquid phase, pci/l; Sg is gas saturation, %; H is Henry s coefficient for radon, dimensionless. Figure 4 shows the regressed line with H 12.8 and R (regression coefficient). Figure 4 can be used to estimate the amount of gas saturation required for various decreases in groundwater radon concentration. According to Figure 4, the anomalous decrease of radon concentration from 780 pci/l to 330 pci/l required a gas saturation of 10 % developed in rock cracks and pores. Under suitable geological conditions (for example, the Antung hot spring), groundwater radon can be a sensitive tracer for strain changes in the crust associated with earthquake occurrences. ACKNOWLEDGEMENT Support by the National Science Council of Taiwan is appreciated. We thank Drs. C. Chen, S. Tasaka, and C. King for discussions of radon research; Mr. C. Lin, Mr. C. Lee, Mr. C. Chiang, and Mr. K. Chiang for field assistance in radon monitoring. REFFERENCES Angelier, J., Chu, H. T., Lee, J. C. and Hu, J. C. (2000), Active faulting and earthquake hazard: The case study of the Chihshang fault, Taiwan, J. Geodyn., 29, Brace, W. F., Paulding, B. W., Jr. and Scholz, C. (1966), Dilatancy in the fracture of crystalline rocks, J. Geophys. Res., 71(16), Chen, W. S. and Wang, Y. (1996), Geology of the Coastal Range, eastern Taiwan, Geology of Taiwan, 7. Hauksson, E. (1981), Radon content of groundwater as an earthquake precursor: Evaluation of worldwide data and physical basis, J. Geophys. Res., 86(B10), Hauksson, E. and Goddard, J. G. (1981), Radon earthquake precursor studies in Iceland, J. Geophys. Res., 86(B8),

4 Hsu, T. L. (1962), Recent faulting in the Longitudinal Valley of eastern Taiwan, Mem. Geol. Soc. China, 1, Igarashi, G. et al. (1995), Ground-water radon anomaly before the Kobe earthquake in Japan, Science, 269, Igarashi, G., Tohjima, Y. and Wakita, H. (1993), Time-variable response characteristics of groundwater radon to earthquakes, Geophys. Res. Lett., 20(17), Igarashi, G. and Wakita, H. (1990), Groundwater radon anomalies associated with earthquakes, Tectonophysics, 180, Lee, J. C. et al. (2003), Active fault creep variations at Chihshang, Taiwan, revealed by creep meter monitoring, , J. Geophys. Res., 108(B11), Liu, K. K., Yui, T. F., Yeh, Y. H., Tsai, Y. B. and Teng, T. L. (1985), Variations of radon content in groundwaters and possible correlation with seismic activities in northern Taiwan, Pageoph, 122, Noguchi, M. and Wakita, H. (1977), A method for continuous measurement of radon in groundwater for earthquake prediction, J. Geophys. Res., 82(8), Noguchi, M. (1964), New method of radon activity measurement with liquid scintillator, Radioisotopes, 13(5), Nur, A. (1972), Dilatancy, pore fluid, and premonitory variations of ts / tp. travel times, Bulletin of the Seismological Society of America, 62(5), Silver, P. G. and Wakita, H. (1996), A search for earthquake precursors, Science, 273, Teng, T. L. (1980), Some recent studies on groundwater radon content as an earthquake precursor, J. Geophys. Res., 85(B6), Torgersen, T., Benoit, J. and Mackie, D. (1990), Controls on groundwater Rn-222 concentrations in fractured rock, Geophys. Res. Lett., 17(6), Trique, M., Richon, P., Perrier, F., Avouac, J. P. and Sabroux, J. C. (1999), Radon emanation and electric potential variations associated with transient deformation near reservoir lakes, Nature, 399, Wakita, H., Igarashi, G. and Notsu, K. (1991), An anomalous radon decrease in groundwater prior to an M6.0 earthquake: A possible precursor?, Geophys. Res. Lett., 18(4), Wakita, H., Nakamura, Y., Notsu, K., Noguchi, M. and Asada, T. (1980), Radon anomaly: A possible precursor of the 1978 Izu-Oshima-kinkai earthquake, Science, 207, Yu, S. B. and Kuo, L. C. (2001), Present-day crustal motion along the Longitudinal Valley Fault, eastern Taiwan, Tectonophysics, 333, Yu, S. B., Jackson, D. D., Yu, G. K. and Liu, C. C. (1990), Dislocation model for crustal deformation in the Longitudinal Valley area, eastern Taiwan, Tectonophysics, 183, AUTHOR INFORMATION The authors declare no competing financial interests. Correspondence and requests for materials should be addressed to T. Kuo (mctkuobe@mail.ncku.edu.tw). Prichard, H. M., Venso, E. A. and Dodson, C. L. (1992), Liquid-scintillation analysis of 222Rn in water by alpha-beta discrimination, Radioactivity and Radiochemistry, 3(1), Roeloffs, E. (1999), Radon and rock deformation, Nature, 399, Scholz, C. H., Sykes, L. R. and Aggarwal, Y. P. (1973), Earthquake prediction: A physical basis, Science, 181, Shapiro, M. H. et al. (1981), Relationship of the 1979 southern California radon anomaly to a possible regional strain event, J. Geophys. Res., 86(B3),

5 Figure. 1. Map of the epicentral and hypocentral distributions of the mainshock and aftershocks of the 2003 Chengkung earthquake. The open star is the 2003 mainshock; open circles denote 2003 aftershocks; filled stars are 1951 mainshocks; the filled triangle is the radon-monitoring station; is Longitudinal Valley Fault (Chihshang Fault); is Yongfeng Fault Figure. 3. Geological map and cross section near the radon-monitoring well in the area of Antung hot spring (Q: Holocene deposits, Lc: Lichi mélange, Plw: Paliwan Formation, Fsl: Fanshuliao Formation, Tls: Tuluanshan Formation, B: tuffaceous block, D1: radon-monitoring well) Radon concentration (pci/l) M 6.8 earthquake Stage Figure. 2. Radon concentration data at the monitoring well (D1) in the Antung hot spring. Radon-concentration errors are ±1 standard deviation after simple averaging of triplicates. Radon concentration in liquid phase (pci/l) Gas saturation (%) Figure. 4. Radon concentration partitioned in liquid phase as a function of gas saturation at 60 using formation brine from the Antung hot spring. Radon-concentration errors are ±1 standard deviation after simple averaging of triplicates.