Study on active tectonic faults using soil radon gas method in Viet Nam

27 Vietnam Journal of Earth Sciences, 39(1), 27-46, DOI: 10.15625/0866-7187/39/1/9182 (VAST) Vietnam Academy of Science and Technology Vietnam Journal of Earth Sciences Study on active tectonic faults using soil radon gas method in Viet Nam Pham Tich Xuan, Nguyen Van Pho, Vu Van Chinh, Pham Thanh Dang, Nguyen Thi Lien, Doan Thu Tra, Hoang Tuyet Nga, Bui Van Quynh, Nguyen Van Luan, Nguyen Xuan Qua Institute of Geological Sciences, Vietnam Academy of Science and Technology Received 23 December 2015. Accepted 20 January 2017 ABSTRACT This paper presents the results of soil radon gas measurement in three areas, including Thac Ba and Song Tranh 2 hydropower plants, and the planned locations of the nuclear power plants Ninh Thuan 1&2 using solid-state nuclear track detectors (SSNTD) with the aim of clarifying the activity of tectonic faults in these areas. The activity of tecton- ic faults was assessed through radon activity index KRn (the ratio between anomaly and threshold), which was divid- ed into 5 levels as follows ultra-high (KRn> 10), high (10≥KRn> 5), high (5≥KRn> 3), medium (3≥KRn> 2) and low (KRn≤2). Soil radon gas measurement results showed that in the radon gas concentrations in the Thac Ba hydropower plant area ranged from 72 Bq/m3 to 273.133 Bq/m3 and maximum radon activity index KRn reached 9.75 (high level). High KRn indexes show Chay River fault active in recent time and the sub-meridian distribution of Rn anomalies sug- gested a right-slip motion of the fault. Rn concentrations in the Ninh Thuan 1&2 areas ranged from 6 Bq/m3 to 52.627 Bq/m3, however, the KRn indexes were mostly low (KRn≤3) and the highest value was only 3.42, suggesting that expression of activity of the tectonic faults in this region is not clear, even no expression of fault activity. In the Song Tranh 2 hydropower plant and adjacent areas, radon concentrations ranged from 29 Bq/m3 to 77.729 Bq/m3 and maximum KRn index was 20.16 (ultra-high level). The faults having clearer activity expression are Hung Nhuong - Ta Vy, Song Tra Bong and some high order faults, especially the northwest - southeast segments of these faults or their intersections with the northwest - southeast faults. In addition, the high values KRn in the mentioned intersections can be evidenced for the activeness of northwest - southeast faults at the present time. The studies on active faults using soil radon gas method were performed in areas with very different geological and structural features, but the results are well consistent with the results of previous investigations obtained by other methods. It confirmed the effective- ness and capability of soil radon gas geochemistry applying to study active tectonic faults. Keywords: Radon, Active Fault, Radon activity index, Thac Ba, Song Tranh 2, Ninh Thuan. ©2017 Vietnam Academy of Science and Technology 1. Introduction The elevation of radon (Rn) concentration in soil gas is known to be associated with *Corresponding author, Email: Email: tichxuan@gmail.com seismic activity and the movement of tectonic faults. The relationship between the change in concentration of Rn in soil gas and activity of tectonic faults has been an object of interest of many geologists and used to detect, identify 28 Pham Tich Xuan, et al./Vietnam Journal of Earth Sciences 39 (2017) and locate active faults (King et al., 1996; King, 1978; Al-Hilal and Al-Ali, 2010; Am- ponsah et al., 2008; Asumadu-Sakyi et al., 2010; Baubron et al., 2002; Burton et al., 2004; Font et al., 2008; González-Díez et al., 2009; Haerudin et al, 2013; Ioannides et al., 2003; Israel and Bjornsson, 1967; Lombardi and Voltattorni, 2010; Papastefanou, 2010; Seminsky and Demberel, 2013; Swakon et al., 2004; Tanner, 1980; Utkin and Yurkov, 2010; Walia et al, 2008). Since then, soil radon gas geochemistry is considered as a quantitative method to study active faults. In addition, monitoring of radon concentration change is also widely used as a tool to predict earth- quakes (Wakita et al., 1980; Hauksson, 1981; Toutain and Baubron, 1999; Ghosh et al., 2009; Laskar, et al., 2011; Wang et al., 2014; Riggio et al., 2015). In Vietnam, the soil radon gas method has been effectively applied in the assessment of active faults in some areas such as Northwest and south- Central regions, Thac Ba, Son La (Nguyen Van Pho et al., 1996a, b, 1999, 2004; Tran Van Duong, Tran Trong Hue, 1996; Tran Trong Hue, 1996, 1999). More re- cently, Rn measurements had been carried out by authors in the expected sites of nuclear power plants in Ninh Thuan province (2012- 2013) and hydropower plant Song Tranh 2 ar- ea, Quang Nam province (2014). This paper presents some results of soil gas radon measurements in Ninh Thuan, Thac Ba and Song Tranh 2 hydropower plants. 2. Study areas Radon measurements had been carried out in 3 areas, including Thac Ba and Song Tranh 2 hydropower plants, and planned sites of nuclear power plants Ninh Thuan 1 & 2 (Figure 1). 2.1. Thac Ba hydropower plant area Thac Ba hydropower reservoirs are on Chay River, located in the Yen Binh District, Yen Bai Province. The study area is located at the southeastern end of hydropower reser- voirs. Figure 1. Location of study areas Thac Ba hydropower area and adjacent are composed of Proterozoic - Cambrian meta- morphic rocks, Paleozoic terrigenous - car- bonate, Neogene terrigenous sediments and Quaternary unconsolidated sediments (Figure 2). The Proterozoic - Cambrian rocks include gneiss, amphibolite, schist, quartzite and mar- ble of Nui Con Voi (PR1 nv), Ngoi Chi (PR1 nc), Thac Ba (PR3-Є1 tb) and Ha Giang (Є2 hg) formations. Paleozoic terrigenous - car- bonate sediments include marble, sericite schist, sandy siltstone, shale and limestone of Phia Phuong (D1 pp), Song Mua (D1 sm), Ban Nguon (D1 bn), and Mia Le Formation (D1 ml). Neogene terrigenous sediments contain conglomerate, gritstone, quartz sandstone, siltstone and coal seams of Phan Luong (N1pl) and Co Phuc (N1 cp) formations. Quaternary sediments include pebble, gravel, sand, clay spread along rivers and streams. 29 Vietnam Journal of Earth Sciences, 39(1), 27-46 Figure 2. Geological map of Thac Ba hydropower plant area and adjacent (modified from Geological and Mineral Resources Map of Vietnam on 1:200,000, 2005) 1. Nui Con Voi formation: (PR1 nv); 2. Ngoi Chi formation (PR1 nc); 3. Thac Ba formation (PR3-Є1 tb); 4. Ha Giang formation (Є2 hg); 5. Phia Phuong formation (D1 pp); 6. Song Mua Formation (D1 sm); 7. Ban Nguon Formation (D1 bn); 8. Mia Le Formation (D1 ml); 9. Phan Luong Formation (N1 pl); 10. Co Phuc Formation (N1 cp); 11. Quaternary sediments; 12. Granite Song Chay Complex (γaD1sc); 13. Granite Ngan Son Complex (γaD1ns); 14. Phia Bioc Com- plex (γaT2npb); 15. Faults; 16. Lake; 17. Study area Intrusive rocks are mainly granitoids in- cluding granodiorite-gneiss, porphyritic gran- itogneiss, biotite granite, pegmatite, aplitic granite of Song Chay Complex (γaD1sc); bio- tite granite, two-mica granite, aplite, pegmatite of Ngan Son Complex (γaD1ns) and porphyrit- ic aluminium-rich biotite granite, medium- to fine-grained granite, leucocratic granite, aplite, pegmatite, quartz-tourmaline veins of Phia Bioc Complex (γaT2npb). The main fault systems in the area are Red River, Da River and Lo River Faults running parallel in the northwest - southeast direction along with their accompanying minor faults. 30 Pham Tich Xuan, et al./Vietnam Journal of Earth Sciences 39 (2017) These are deep zoning faults considered as ac- tive in recent time. Also, sub-meridian and northeast - southwest faults are also well de- veloped in the area. The main dams of hydro- power reservoirs are built within the Song Chay fault zone, dam No. 9, especially is lo- cated directly on the fault line (Figure 2). Kontum massive and compose of Proterozoic and Early Paleozoic metamorphic rocks, Qua- ternary sediments and numerous of intrusive rocks (Figure 3). The Proterozoic rocks in- clude gneiss and plagiogneiss, amphibolite, crystalline schist, graphite-bearing schist, oli- vine and dolomitic marble of Song Re (PR 2.2. Song Tranh 2 hydropower plant area In the Song Tranh 2 hydropower plant ar- ea, the studies had been carried out in the large area of Bac Tra My, Tien Phuoc and Hiep Duc districs (Quang Nam province) (Figure 1). Song Tranh 2 hydropower plant area and adjacent regions on the northern edge of the 1 sr), Tac Po (PR1 tp), Kham Duc (PR2-3 kđ). Early Paleozoic rocks consist of sericite schist, quartz-sericite schist, marble of Dak Long Formation (Є-S đlg). Along the rivers and streams occurred Quaternary unconsoli- dated sediments including pebbles, gravel, gravelly sand, clay, dough. ←Figure 3. Geological map of Song Tranh 2 hydropower plant area and adjacent (modified from Geological and Mineral Resources Map of Vietnam on 1:200,000, 2005) 1. Song Re Formation (PR1 sr); 2. Tac Po Formation (PR1 tp); 3. Kham Duc Formation (PR2-3 kđ); 4. Dak Long Formation (Є-S đlg); 5. Quaternary sediments; 6. Ta Vy Complex (νPR3 tv): Gabbroamphibolite; 7. Nam Nin Complex (γ-δPR3 nn): Plagiogranitogneiss; 8. Chu Lai Complex (γPR3 cl): granitogneiss; 9. Hiep Duc Complex (σPZ1 hđ): Ultramfic rocks; 10. Nui Ngoc Complex (μνPZ1 nn): Gabbro, gabbrodiabas; 11. Tra Bong Complex (δ-γδO-S tb): Diorite, granodiorite; 12. Dai Loc Complex (γaD1 đl): Gneissogranite, granite; 13. Ben Giang - Que Son Complex (γξPZ3bg-qs): Diorite, granodiorite; 14. Cha Val Complex (νaT3cv): Gabbropyroxenite; 15. Hai Van Complex (γaT3 hv): Granite; 16. Ba Na Complex (γK-E bn): Leucogranite, alaskite; 17. Faults; 18. Lake The intrusive rocks are widely spread and varied, including gabbroamphibolite of Ta Vy Complex (νPR3 tv); plagiogranitogneiss, to- nalitogneiss of Nam Nin Complex (γ-δPR3 nn); granite-gneiss, migmatite granite, garnet- biotite granite, two-mica gneiss-granite, peg- matite, aplite of Chu Lai Complex (γPR3 cl); serpentinized, carbonatized and talcifized oli- vinite, harzburgite, wehrlite, pyroxenite of Hiep Duc Complex (σPZ1 hđ); gabbro, gab- brodiabase of Nui Ngoc Complex (μνPZ1 nn); diorite, quartz-diorite, granodiorite, tonalite 31 Vietnam Journal of Earth Sciences, 39(1), 27-46 2 and granite of Tra Bong Complex (δ-γδO-S tb); medium-grained porphyritic two-mica gneissogranite of marginal facies, melanocrat- melanocratic coarse- to medium-grained two- mica gneissogranite of central facies, fine- to medium-grained two-mica gneissogranite, granite, pegmatite bearing big scales of mica, aplite granite of Dai Loc Complex (γaD1 đl); gabbrodiorite, diorite, quartz -biotite- hornblende diorite, horblende-biotite granodiorite, porphyritic hornblende granodiorite, hornblende-biotite granite, spessartite, porphyritic diorite of Ben Giang - Que Son Complex (γξPZ3 bg-qs); gabbro, melanocratic coarse- to medium-grained gabbropyroxenite of Cha Val Complex (νaT3 cv); biotite granite, two-mica granite, granite aplite, tourmaline- and garnet-bearing pegmatite of Hai Van Complex (γaT3 hv); biotite granite, leucocratic medium- to coarse- grained quartz-rich two-mica granite, two- mica granite, fine-grained leucocratic quartz- rich alaskite granite of Ba Na Complex (γK-E bn). Within a radius of 30 km from the center of the main dam of Song Tranh 2 hydropower plant, tectonic faults are developed in different directions, including sub-parallel, northwest - southeast and northeast - southwest fault systems. The main faults include first order Tam Ky - Phuoc Son fault; of second order is Hung Nhuong - Ta Vy fault; and third order is Tra Bong fault; and of fourth order includes Suoi Nha Nga, Song Gia, Song Nuoc Trang, Song Nuoc Xa faults and higher order faults (Figure 3) 2.3. Planned sites of nuclear power plants This region is located in Late Mesozoic Da Lat active continental margin (Tran Van Tri, Vu Khuc et al., 2009). The study area is composed mostly of granitoids of Dinh Quan, Deo Ca and Ca Na complexes, in wich the granitoids of the Deo Ca and Dinh Quan complexes occupied most of the area (Figure 4). The Late Jurassic Dinh Quan complex (γJ3 đq) is composed of granodiorite, diorite, fine- to medium grained quartz diorite. The Cretaceous Deo Ca complex (γK đc) is comprised of biotite granodiorite, fine- to medium -grained quartz monzodiorite, granite, medium- to coarse-grained, sometime porphyritic biotite (hornblend) granosyenite, fine-grained granite and dykes of porphyritic granite, granosyenit, aplite and pegmaite. The Late Creaseous Ca Na complex (γK2 cn) has small distributive area and is consisted of biotite-muscovite granite, alaskite and dykes of porphyritic granite, aplite and pegmatite. Also, present are a series of dykes consisting of porphyritic granite, granosyenite of Neogene Phan Rang complex (γE pr). Notably, the mentioned granitoids are penetrated by series of dykes of diabase, gabbrodiabase, gabbrodiorite of Cu Mong complex (E cm). Interspersedly there are terrigenous sediments of sandstone, sandy siltstone, shale, horns rocks of La Nga formation (J2 ln), intermediate volcanic, sub- volcanic rocks of Deo Bao Loc formation (J3 đbl), acidic eruptive rocks of Nha Trang (K nt) and Don Duong (K2 đd) formations (Figure 4). Neogene sediments of Maviek formation (N 2mv) include lime gritstone and sandstone. The Middle Pleistocene sediments 2 Ninh Thuan 1&2 of Phan Thiet formation (Q1 pt) are mainly Areas planned to build nuclear power plants Ninh Thuan 1&2 are located in the Thuan Nam and Vinh Hai districts (Ninh Thuan province) (Figure 1). red, yellowish brown fine quartz sand. The middle - upper Pleistocene sediments consist of sand, pebbles, gravel, grit, silt and clay and Holocene sediments are sand, silt and clay (Figure 4). 32 Pham Tich Xuan, et al./Vietnam Journal of Earth Sciences 39 (2017) ←Figure 4. Geologiacal map of Ninh Thuan 1&2 area and adjacent (modified from Geological and Mineral Resources Map of Vietnam on 1:200,000, 2005) 1. La Nga formation (J2 ln); 2. Deo Bao Loc formation (J3 đbl); 3. Nha Trang for- mation (K nt); 4. Don Duong formation 2(K2 đd); 5. Maviek formation (N2 2 mv); 6. Phan Thiet formation (Q1 pt); 7. Midle According to the geological map of 1: 200,000 scale (Nha Trang and Dalat sheets) the study area has a number of northeast - southwest faults. But the latest research results reported by Vu Van Chinh in the framework of a national project "Study and evaluation of active faults serving the site selection to build nuclear power plants in Ninh Thuan province", code 01/2012, show that only 03 small faults had been detected in neighborhood of expected site of nuclear power plants Ninh Thuan 2, named Nui Ba Duong, VL109-110 and VL111 faults (see Figure 9). In the neighborhood of Ninh Thuan 1 site have been revealed fracture zones Suoi Mia, Suoi Bau Ngu, Suoi Mot, Bau Ngu - Son Hai, Van Lam - Tu Thien, Vinh Truong (see Figure 10), and in the neighborhood of Ninh Thuan 2 site have been detected East Hon Gio, Nui Ong Kinh, Da Hang, VL111, VL110, Deo Dinh Ba, VL241, VL100 and Mui Thi fracture zones (Figure 11). 3. Method of measuring Rn concentration in soil gas Concentrations of Rn in soil gas were - Upper Pleistocene sediments; 8. Holo- cene sediments; 9. Dinh Quan complex (γJ3 đq): Diorite, granodiorite; 10. Deo Ca complex (γK đc): Granodiorite,granite; 11. Ca Na complex (γK2 cn): Granite, alaskite; 12. Porphyric granite Dykes of Rang complex (γE pr); 13. Diabas, gab- brodiabas dykes of Cu Mong complex (E cm) measured along profiles, which were identified in collaboration with geologists to their crossing the fault lines or fracture zones. However, the setting of profiles also depends on field conditions (relief, water bodies, river and stream set, characteristics of ground and rocks, etc...), so the most of the profiles are not a straight line. Distances between measuring points in a profile were chosen in the range of 25 to 30 m. The distance was determined based on migration ability of Rn in soil gas (Barnet et al., 2008). The positions of measuring points were located by a Garmin GPSmap 60CSx GPS. Rn concentrations in soil gas were measured using solid-state nuclear track detectors (SSNTDs). This method shows many advantages compared to Alpha scintillation detectors (Nguyen Van Pho, 1996). Film-detectors DOSIRAD LR115 (type 2P, serial number 5058180) were used. The film-detectors were glued to the bottom of the plastic cups, which then had been covered with PE film to prevent direct penetration of water. At each measuring point, the plastic cup containing film-detector was placed face-down in the bottom of the drag pit 33 Vietnam Journal of Earth Sciences, 39(1), 27-46 of 25 cm diameter and 40-50 cm deep (Figure 5). The average measuring time was 5 days, and the time of placing and collecting of de- tectors was carefully recorded with accuracy in a minute. Collected film-detectors were carefully preserved in plastic bags for further treatment. At each setpoint of the detector, the pit bottom radiation intensity was measured using radiometer СRP 88Н (Russia) to elimi- nate the anomalies caused by the accumula- tion of radioactive elements. Etching, track counting and radon concentration calculating were completed at the Institute for Nuclear Science and Technology (Hanoi). Figure 5. Setting of detector in the pit Data were analyzed by the statistical meth- od, ranges of background values, thresholds and anomalies were determined using boxplot method (Reimann et al., 2005). 4. Results of soil radon gas measurement 4.1. Thac Ba area Measured profiles were focused in the southeastern area of the lake, where the main dams are located, including dam N.9 in the south and main dam and hydropower plant in the southeast. Two profiles were performed in the dam N.9 area and 9 remaining profiles were in the east of the main dam and around the hydropower plant (Figure 6). Figure 6. Distribution of radon measurement profiles in Thac Ba area Total 452 data points were obtained. The results showed that concentrations of Rn in soil gas varied from 72 Bq/m3 to 273.133 Bq/m3, mean = 12.840 Bq/m3. The statistical parameters are shown in Table 1. Boxplot di- agram suggests the background range from 72 Bq/m3 to 28.139 Bq/m3, and the threshold value is 28.139 Bq/m3 (Figure 7). So 46 of to- tal 452 measured points have anomalous con- centrations, and highest anomaly value reached 273.133 Bq/m3. 34 Pham Tich Xuan, et al./Vietnam Journal of Earth Sciences 39 (2017) ←Figure 7. Boxplot diagram of radon concentration in Thac Ba area 4.2. Song Tranh 2 area In the hydropower plant Song Tranh 2 and adjacent area, radon measurements were per- formed along 25 profiles with 782 data points obtained. Distribution of profiles is shown in Figure 8. The concentrations of radon varied from 29 Bq/m3 to 77.729 Bq/m3, mean x = 2225 Bq/m3. The statistical parameters are shown in Table 1. LEGEND Figure 8. Distribution of radon measurement profiles in Song Tranh 2 hydropower plant and adjacent 35 Vietnam Journal of Earth Sciences, 39(1), 27-46 3 Table 1. Radon concentrations in Thac Ba, Ninh Thuan andSongTranh2areas Site (n) Concentrations (Bq/m ) with 401 data points (Figure 10); while in the area of Ninh Thuan 2 Rn measurements were conducted along 15 profiles with 479 data SD Mean(x ) Min Max Thac Ba 452 12.840 72 27.3133 21.750 Ninh Thuan 880 5.515 6 52.627 5.980 Song Tranh 2 780 2.225 29 77.729 4.871 Boxplot diagram shows the background range from 29 Bq/m3 to 3.855 Bq/m3, the threshold value is 3.855 Bq/m3 (Figure 9). Thus, there are 116 points having anomaly concentrations of radon from total 782 meas- ured points, including the highest value of 77.729 Bq/m3. 4.3. Ninh Thuan area In the area of Ninh Thuan 1, radon meas- urements were carried out along 11 profiles points (Figure 11). Figure 9. Boxplot diagram of radon concentrations in Song Tranh 2 area and adjacent Figure 10. Distribution of radon measurement profiles in Ninh Thuan 1 area 36 Pham Tich Xuan, et al./Vietnam Journal of Earth Sciences 39 (2017) The results showed that concentrations of radon varied from 6 Bq/m3 to 52.627 Bq/m3, and mean = 5515 Bq/m3. The statistical pa- rameters are shown in Table 1. Boxplot dia- gram showed that the background concentra- tions fall in range 6 Bq/m3 to 15.410 Bq/m3, the threshold value is 15.410 Bq/m3 (Figure 12). So 58 of total 880 measured points have anomalous concentrations, and highest anom- aly value reached 52.627 Bq/m3. Notably, the results of radon measurements showed the difference in radon concentrations in the Ninh Thuan 1 and Ninh Thuan 2 areas. Generally, Ninh Thuan 2 area had higher con- centrations of radon compared with Ninh Thuan 1. While in the Ninh Thuan 1 area the maximum value of radon concentrations was just only 16.513 Bq/m3 and slightly exceeded the threshold, in the Ninh Thuan 2 area the maximum value of radon concentrations reached 52.627 Bq/m3. LEGEND Figure 11. Distribution of radon measurement profiles in Ninh Thuan 2 area 37 Vietnam Journal of Earth Sciences, 39(1), 27-46 Figure 12. Boxplot diagram of radon concentrations in Ninh Thuan area 5. Discussion 5.1. Relationship between radon concentration and fault activity The concentration of radon in soil gas de- pends on many factors, but mostly on geolog- ical features, lithologies and tectonic activities in a given region. Therefore, the absolute val- ue of radon in soil gas in different regions can be very different. For this reason, a soil radon gas mapping may have various different clas- sifications of radon concentrations. In Germa- ny, Kemski et al. (2001) divided radon con- centrations into 4 categories such as: a) low (<10 kBq/m3); b) medium (10-100 kBq/m3; c) increased (100 - 500kB/m3) and d) high (>500 kBq/m3). While in Hong Kong used is 3 category scale as follows a) low (<10 kBq/m3); b) medium (10 - 100 kBq/m3) and c) high (>100 kBq/m3) (Tung et al., 2013). Classifications of concentration levels using absolute values of concentration mentioned above were based on criteria for human health and ecological environment safety and only had implication in soil radon gas mapping with environmental protection purpose. Many studies (King et al., 1996; Moussa and Arabi, 2003; Ciotoli et al., 1999, 2007) have shown that the radon anomalous concen- trations are sensitive to fault activity. The val- ue of anomalous radon concentrations can many times, even dozens of times higher than background depending on the activity level of a fault (AI-Tamimi and Abumurad, 2001; Wang et al., 2014, Seminsky et al., 2013). The maximum radon concentrations and back- ground values in some faults are shown in Ta- ble 2 (Richon et al., 2010), suggesting that maximum concentrations and background levels are very different for different faults, but in general, the maximum values are many folds greater than background values (high ra- tio of Max/BG). Table 2. Maximum concentration and mean background level of radon in soil gas at some faults (Richon et al., 2010) Location Fault Max Rn (Bq/m3) Mean background Ratio of Max/BGlevel(Bq/m3) Calaveras Fault, Hollister, California Strike-Slip 30000 3000 10 San Andreas Fault, Point Reyes, California Strike-Slip 30000 5000 6 Johnson Valley Fault, California Strike-Slip 4000 400 10 Dead Sea, Israel Normal >500000 26000 >19 Dead Sea Transform, Wadi Araba, Jordan Strike-Slip 1800 1000 1.8 Dead Sea Transform, Jordan Strike-Slip 3000 3 North Anatolian Fault, Turkey Strike-Slip 9800 6000 2 Shan-Chiao Fault, Taiwan Normal 28000 6000 5 Crati graben, Italy Normal 39000 9100 4 Norht and Northwestern Greece Normal 13000 2000 6.5 Jaut Pass, Pyrenees, France Normal 70000 10000 7 Bad Nauheim Fault, Aachen, Germany Normal >1000000 30000 >33 Neuwied Basin, Rhine Graben, Germany Normal 140000 14 Main Central Thrust, Nepal Thrust 60000 4400 14 38 Pham Tich Xuan, et al./Vietnam Journal of Earth Sciences 39 (2017) Seminsky and Demberel (2013), when studying the relationship between the concen- tration of radon in soil gas and activity of faults in Central Mongolia proposed to use Radon Activity Index KQ - the ratio between the highest radon concentration (Qmax) and con- centration of radon outside the fault zone (Qmin) for classification of activity of radon. Accordingly, for the Middle Mongolian the faults are characterized by levels of Radon Ac- tivity Index as follows: ultra-high (KQ > 10), high (10 > KQ > 5), increased (5> KQ > 3), medium (3 2) and low (KQ < 2). How- ever, comparing the maximum anomalies with the lowest concentration outside the fault zone will lead to more uncertainty, cite, for example, what is value outside the fault zone, what posi- tion is considered to be outside the faults zone. Hence the comparison value must be calculated based on a statistical basis, such as comparing the maximum value to the threshold or the av- erage value of the background. Wang et al., 2014 identified anomalous concentrations and the threshold of background for a number of faults in the Tangshan area (North China). If using the method for calculating Radon Activi- ty Index as the ratio between the maximum value and the threshold of the background can be seen the all of active faults have quite large indexes, ranging from 6.6 to 18, 4 (Table 3). Table 3. Maximum concentration and background level of radon at some faults in China (Wang et al., 2014) Rn concentration (Bq/m3) Standard Location Deviation Ratio of Max/BG Min Max Backgroundvalue(BG) (SD) Tangshan Lhasa, Tibet 411.8 38470.6 4730.4 4992.6 8.1 757.6 87402.6 7634.9 11.5 Yanhuai basin, Hebei 1302.0 57812.0 8105.78 5937.4 7.1 Haiyuan, Ningxia 1000.0 38300.0 5800.0 6.6 Jixian Mountain, Tianjing 52.0 58646.0 3188.0 4598.0 18.4 Using arguments described above, we sug- gest using the ratio between the anomalous concentration of radon and the threshold value as index of radon activity (KRn) and use scheme of classification provided by Seminsky and Demberel (2013) to classify radon activity un- der this index. So we will have 5 levels of ac- tivity of radon corresponding to the Radon ac- tivity Indexes KRn as follows ultra-high (KRn > 10), high (10 > KRn > 5), increased (5 ≥ KRn > 3), medium (3 ≥ KRn > 2) and low (KRn ≤ 2). Corresponding to 5 levels of Radon Activity Indexes KRn, we have 5 levels of fault activity expression (Table 4). Table 4. Classification of radon activity index and correlative activity expression of fault Radonactivityindex Expression of fault activityKRn Level KRn>10 Ultra-high Fault having strong activity expression 10≥ KRn>5 High Fault having clear activity expression 5≥ KRn>3 Increased Fault having activity expression or existence of strong tectonic fracture zone 3≥ KRn>2 Medium Fault having not clear activity expression or existence of fracture zone KRn≤2 Low Fault having no activity expression or no existence of fault Radon Activity Index KRn allows identifing the existence as well as the activity expression of tectonic faults, although its activity rates need further studies. 5.2. Fault activities in the Thac Ba area The concentrations of soil radon gas meas- urements show 46 out of 425 data points are anomalous. The anomalous points mainly dis- tribute in the main dam and hydropower plant area with 43 points having radon concentra- tions ranging from 28.139 Bq/m3 to 273.133 Bq/m3. In the dam N.9 area there are only 03 of total 81 data points having anomalous con- centrations ranging from 28.481 Bq/m3 to 35.988 Bq/m3. Radon Activity Index KRn var- 39 Vietnam Journal of Earth Sciences, 39(1), 27-46 ied from 1 to 9.75, including TB2-22 and TB2-49 points (Profile N.2) having high KRn. Anomalous radon concentrations and high Radon Activity Index indicated fault activities in this area. Notably, anomalous points are distributed in the sub-meridian direction coin- ciding with the direction of the regional ex- tension structure and reflecting strike-slip tec- tonic stress field with sub-meridian compres- sion in recent time (Figure 13). This observa- tion is consistent with the Chay River fault re- ported elsewhere (Nguyen Trong Yem, 1996, Nguyen Dang Tuc, 2000). In summary, the results of soil radon gas measurements rec- orded the activity of Chay River fault and neighboring areas suggest the right strike-slip mechanism at the present time. Figure 13. Map showing sub-meridian distribution of ra- don anomalies in Thac Ba ares: 1 - 3: Radon concentra- tions: 1 - 10,000-20,000 Bq/m3, 2 - 20,000-30,000 Bq/m3, 3 - >30,000 Bq/m3; 4. Radon measurement profiles; 5. Fault; 6. Dam; 7. Roads; 8. Lake; 9. Relief 5.3. Fault activities in the Song Tranh 2 area In the Song Tranh 2 hydropower plant and adjacent area, 114 out of 782 measurements were identified as anomalous with KRn rang- ing from 1.00 to 20.16, including 23 points having KRn at increased or higher levels. In the northwest - southeast segment of Hung Nhuong - Ta Vy fault, soil radon gas was measured in 3 profiles. In a total of 98 measurements, 19 were recognized as anoma- lous, with 3 points having KRn at a high level and 3 others at increased level. Notably, in both profiles ST-1 and ST-3 the anomalies with high KRn had been occurred, including ST101 and ST102 points in profile ST-1 with KRn = 5.94 and 7.67 respectively, point ST317 in profile ST-3 with KRn = 7.44 (Figure 14). In profile ST-2 there are 2 anomalous points with KRn = 1.07 and 2.3 (medium and low levels), but these points are quite far from fault. Thus, the existence of anomalies with radon activity index at the high level indicates that Hung Nhuong - Ta Vy fault shows clear activity expression in recent time at least in the northwest - southeast segment. In the Tra Bong fault area measurements had been carried out in 4 profiles (St-4, ST-6, ST-7 and ST-8), and 23 of total 136 measured points show anomalous radon concentrations, including 2 points having KRn at increased level and 01 point (ST613, profile ST-6) hav- ing highest radon concentration (77727 Bq/m3) with KRn at ultra-high level (KRn = 20.16). The presence of anomalies with KRn at increased or higher levels may be the evi- dence of activity of Tra Bong fault in recent time. However, it should be added that most of the anomalous points with KRn at increased or higher level are located at intersections of Tra Bong with northwest - southeast faults. For example, point ST613 with KRn = 20.16 is located at the intersection with Song Tra No fault, point ST722 (profile ST-7) - at the inter- section with Song Gia fault, or the anomalies in the profile ST-4 also (Figure 15), while in profile ST-8 outside the junctions of this type only encountered anomalies with KRn at me- dium level. Therefore, high radon activity in- dex KRn at the fault intersections may be evi- 40 Pham Tich Xuan, et al./Vietnam Journal of Earth Sciences 39 (2017) denced not only by the activity of Tra Bong fault but also for northwest - southeast faults. Although in the place where profile ST-14 crossed Song Tra No fault was recognized an anomaly, but KRn = 2.61 is only at the medium level, but due to execution conditions of pro- file ST-14 were very bad, so the results may not fully reflect characteristics of radon concentrations here. Nevertheless, combining with measurement results in profile ST-6 described above it can be considered that Song Tra No fault also may be active. Figure 14. Combined graphic of radon concentrations at Hung Nhuong - Ta Vy fault Figure 15. Combined graphic of radon concentrations at Song Tra Bong fault In the segment of Tam Ky - Phuoc Son, that crosses study area and has northeast - southwest direction, were reported 23 anoma- lies of total 123 measured points. However most of these anomalous points have KRn at medium or low levels, only 3 points have slightly increased KRn (KRn varied from 3.1 to 3.5 near the threshold value) (Figure 16). It is noteworthy that anomalous points in this fault are concentrated in its junctions with high or- der faults. So, according to the results of ra- don measurement Tam Ky - Phuoc Son fault have activity expression but not really clear in recent time. 41 Vietnam Journal of Earth Sciences, 39(1), 27-46 Figure 16. Combined graphic of radon concentrations at Tam Ky - Phuoc Son fault In some sub-parallel high order faults, there observed anomalies with KRn at high and in- creased levels. In the Song Nuoc Trang fault, in the profile ST-11 and ST-12 were recognized anomalous points with KRn at increased level, proving activity expression of the fault (Figure 17). In Ho Khanh - Trung Dang fault, the point ST2037 (profile ST-20) lied directly in the fault line has anomalous radon concen- tration with KRn = 13.52 at ultra-high level and in addition, in adjacent of this point there are some anomalies with KRn at high level, so the Ho Khanh - Trung Dang fault had clear expres- sion of activity in recent time. Figure 17. Combined graphic of radon concentration at Song Nuoc Trang faul On the profile ST-18 that crossed northeast - southwest fault Suoi Deo Luu had met some anomalies, but indexes KRn are only at medi- um or low levels, except one point having KRn at increased level but very slightly (KRn = 3.08), so activity expression of this fault is not clear. In the profile ST-19 crossed another northeast - southwest fault Phu Ninh also had been recognized anomalies with KRn at me- dium to increased levels, but the points in the 42 Pham Tich Xuan, et al./Vietnam Journal of Earth Sciences 39 (2017) fault line did not discover any anomalous concentration of radon, so it is difficult to conclude about it activeness. 5.4. Fault activities in the Ninh Thuan area In the Ninh Thuan area, there are 63 anomalous out of total 880 measured points. Most of the anomalous points concentrated in the Ninh Thuan 2 area (62 points), while in the Ninh Thuan 1 area there was only one anomalous point. However, most of the anomanlous points have low radon activity indexes (KRn<3), only 2 points VH1-42 (pro- file VH1) and VH4-9 (profile VH4) in Vinh Hai have KRn = 3.42 and 3.18, respectively, and at increased level. Thus, in the Ninh Thu- an 1&2 areas, expressions of activity of faults are not clear, even there are not expressions of activity of faults. Radon anomalies indicate the presence of fracture zones (increasing em- anation) rather than expressions of active faults. The existence of tectonic fracture zone through the increase of radon concentrations was recognized in many profiles. In Ninh Thuan 1 and in Tam Lang fracture zone, alt- hough all measured radon concentrations fall within the range of background values, but in the profiles PD7, PD8, PD9 had been recog- nized some points with increased radon con- centrations (Figure 18). In the Ninh Thuan 2 area, many points in the profiles performed in Thai An and Tram Bang showed high concen- trations of radon suggesting the existence of series of fracture zones (Figure 19, 20). Com- parison of radon activity indexes KRn ob- tained for the above areas shows that KRn in- dexes are different in the Thac Ba, Song Tranh 2 and Ninh Thuan areas, where the KRn in the Song Tranh 2 reached ultra-high level (KRn = 20.16), high level (KRn = 9.75) was recorded in the Thac Ba area, whereas Ninh Thuan 1 and 2 areas show medium or low levels, except for only 2 values reached in- creased level (KRn max = 3.42) (Figure 21). The difference of active radon indexes in ob- served areas reflects the different levels of fault activity expression in this area, in which in Thac Ba and Song Tranh 2 the activity ex- pression of faults is clear, while in Ninh Thu- an area activity expression is not clear, even no activity expression. Figure 18. Combined graphic of radon concentrations at tectonic frature zone Suoi Tam Lang, showing increased radon concentrations within fracture zone in profiles PD7, PD8 and PD9 43 Vietnam Journal of Earth Sciences, 39(1), 27-46 Figure 19. Combined graphic of radon concentration in Thai An area. Increasing of radon concentrations in profiles VH4, VH5, VH9, BS1 and BS2 recognized presence of series of fracture zones (see text for details) Figure 20. Combined graphic of radon concentrations in Tram Bang area. Increasing of radon concentrations in profiles VH1, BS3, BS4 and BS5 recognized presence of series of fracture zones (see text for details) Figure 21. Radon activity indexes (KRn) of of radon anomalous points Thac Ba, Ninh Thuan and Song Tranh 2 areas 44 Pham Tich Xuan, et al./Vietnam Journal of Earth Sciences 39 (2017) 6. Conclusions The results of soil radon gas measurements in 3 different areas showed differences in expression levels of fault activity as follows: In the Thac Ba hydropower plant area, Song Chay fault shows clear activity expression and right slip motion in recent time with maximum KRn = 9.75 (high level). In the Song Tranh 2 hydropower plant area, faults showing clear activity expression in recent time are Hung Nhuong - Ta Vy, Song Tra Bong and some high order faults with maximum KRn reached 20.16 (ultra-high level). Distribution of anomalous points with KRn at increased or higher levels characterized by their focusing or in the segments, where these sub-parallel faults changed to northwest - southeast direction, or in their intersections with higher order northwest - southeast faults. Such distribution of anomalies also showed that the northwest - southeast faults are active. In the areas where nuclear power plants Ninh Thuan 1&2 planned to build, the expressions of fault activity are not clear, although it is assumed that fault activity expression ranging between medium to low levels of KRn indexes (KRn ≤3). These results are consistent with previously reported results acquired for the mentioned faults by other methods. The studies of activity of tectonic faults using soil radon gas method had been carried out in different tectonic structure areas having different geological characteristics, but the obtained results are consistent with the results of previous studies. It confirms the effectiveness and capability of soil radon gas geochemistry in studying active tectonic faults and should be further developed and applied for other faults. Acknowledgements Soil radon gas data of the areas expected to build nuclear power plants Ninh Thuan 1&2 and hydropower plant Song Tranh 2 area were obtained by the authors in framework of National Projects "Study and evaluation of active faults for the approval of construction planning for nuclear power plant in Ninh Thuan province", coded 01/2012 and “Study on impact of seismotectonics to the stability of Song Tranh 2 hydropower plant, Bac Tra My region, Quang Nam province”, code 02 - 2012/HĐ - ĐTĐL. References Al-Hilal M., Al-Ali A., 2010. 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