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A study of variation in soil gas concentration associated with earthquakes near Indo-Burma Subduction zone

A study of variation in soil gas concentration associated with earthquakes near Indo-Burma... Background: In the recent past, several efforts have been made by a number of researchers to measure anomalous emanations of geo-gases in seismic prone regions of the world and radon has been the most preferred geo-gas as possible earthquake precursor since it is easily detectable. Results: In the present investigation, continuous measurements of radon concentration at 80 cm inside the soil has been carried out at Chite Fault (23.73°N, 92.73°E), Aizawl, Mizoram situated in the seismic zone V in North Eastern part of India near Indo-Burma subduction zone, using LR-115 Type-II nuclear track detectors manufactured by Kodak Pathe, 3 3 France. During the investigation period, the radon concentration varied from 163.27 Bq/m to 2557.82 Bq/m with an 3 3 average and standard deviation of 1116.15 Bq/m and 591.76 Bq/m respectively. Conclusion: Certain anomalies observed in radon concentration have been correlated to the earthquakes within the range of magnitudes 4.7 ≤ M ≤ 5.5, whilesomeother anomaliesare duetothe influenceofmeteorological parameters. Keywords: Soil-gas, Radon, LR-115 films, Correlations, Meteorological parameters, Earthquake Background Barsukov et al., 1985; Sugisaki and Sugiura, 1986). In some Migration of carrier gas by bubbles is considered to be cases, anomalies have also occurred contemporaneously an important transport mechanism governing distribu- with or after the events (Birchard and Libby, 1980; King, tion of carrier (CO and CH ) and trace (Rn, He) gases 1985; Thomas et al., 1986). Soil-gas concentrations are 2 4 over wide areas on the earth surface. Soil-gas anomalies not sensitive to hydrologic changes as they are extremely and chemical changes in groundwater, observed during susceptible to a number of other environmental effects. seismic events may be attributed to gas carrier dynamics However, many authors in the past suggest that spatial (Etiope and Martinelli, 2002). During the last several de- and temporal variations in soil-gas concentrations are cades, analysis of earthquake precursory phenomena re- most intensively influenced by meteorological interfer- veals that significant changes in geophysical and ences (Kraner et al., 1964; Klusman, 1981; Fleischer, 1983; geochemical process may occur prior to intermediate and Robinson and Whitehead, 1986; Guedalia et al., 1970). large earthquake. The behavior of the gas concentration anomalies has been quite variable. Several investigators Radon emanation and earthquake have reported increase in gas concentrations before the Radon concentration in the soil-gas increases with depth occurrence of seismic events (Cai et al., 1984; Nersesov, (Jonsson, 1995; Kristiansson and Malmqvist, 1984) until 1984; Kawabe, 1985). Besides these, declines in radon con- a certain depth is reached which depend on the soil’s centration or concentration ratio immediately and prior to properties and moisture content. Radon act as an indica- seismic events have also been reported (King et al., 1981; tor for changes in the gas streams. The most sensitive depth to detect such changes is between 0.5 to 1 m (Friedmann, 2012). Since 1971 much effort has been de- * Correspondence: ramesh_mzu@rediffmail.com 1 voted to explain earthquake on the basis of Dilatancy Department of Physics, Mizoram University, Aizawl 796004, India Full list of author information is available at the end of the article and fluid flow (Scholz et al., 1973). Dilatancy means an © The Author(s). 2016 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Singh et al. Geoenvironmental Disasters (2016) 3:22 Page 2 of 8 Fig. 1 Dilatancy: increasing stress causes cracks in the rock, which enlarges the material perpendicular to the main axis of stress. This causes an effective increase in volume (after Friedmann, 2012) elastic increase in volume under stress as shown in Fig. 1. mechanisms which are consequences of the redistri- According to the Dilatancy theory, a substantial change bution of water in the earth’s crust can take into ef- (increase in the perpendicular direction to the main axis fect (Imme and Morelli, 2012). of the stress) in the rock properties will occur shortly before earthquake leading to the possibility of water Seismicity of the study area penetration in the cracks and/or the number of cracks Earthquake most probably occurs due to movements increases exponentially. As a result, significant masses along the faults that have evolved through geological are moved which will cause the movement of sub- and tectonic processes. Northeast India is considered surface soil-gas towards the earth surface (Friedmann, one of the six most seismically active regions of the 2012). Consequently opening of new cracks, widening world. The Tectonic Map of Northeast India is shown in or closing of old cracks or redistribution of open and Fig. 2. The region has experienced 18 large earthquakes closed cracks can happen. In dry rocks opening or clos- (M ≥ 7) during the last hundred years and several hun- ing of cracks will lead to significant changes of the dif- dred small or micro earthquakes. The high seismicity in fusion coefficient of radon. Volumetric changes in the the region is attributed to the collision tectonics between rock will also lead to a subsurface gas flow and there- the Indian plate and Eurasian plate in the north and sub- fore to an additional radon transport. If the new open duction tectonics along the Indo-Myanmar range in the cracks are filled with water, the increased water-rock east (Kayal, 1998; Sarmah, 1999). There has been a phe- interface leads to an increase in the transfer of radon nomenal increase in the population density and develop- from the rock matrix to the water. If water filled cracks ment programmes in the northeast India. Besides, the close, the water will be compressed to another sub- region has witnessed a mushroom growth of unplanned surface volume where the emanation from the rock to urban centers in the previous two decades. This has re- the water may be different. All these effects result in sulted into increasing vulnerability of human population pressure and water level variations of the relevant and physical structures to the earthquakes. Thus, it aquifer. This may also lead to changes in the mixing becomes essential to assess the status of seismicity in the ratios for the water which can be observed at the northeastern region realistically. This will provide a earth’s surface. Finally gas flows can also move some sound database for earthquake disaster mitigation. groundwater and again all previously discussed Moreover, high seismic risk in the region calls for an Singh et al. Geoenvironmental Disasters (2016) 3:22 Page 3 of 8 Fig. 2 Seismo-tectonic map of North East India showing epi-centres of damaging earthquakes (after Jaishi et al., 2014c). Tectonic zones (zones A, B, C, D and E) and major thrusts (MBT-Main Boundary Thrust; MCT-Main Central Thrust; NT-Naga Thrust; DT-Disang Thrust) are also shown. Thick blue line represents the International boundary of North Eastern part of India bordering Myanmar and Bangladesh (not to scale) and red lines represents faults and thrusts urgent and sustained mitigation effort. Accordingly, radon measurement at weekly interval at Chite Fault there seems to be a real need for employing all available (Singh et al., 2014). The location of the study area is efforts including radon variations and seismicity records shown in Fig. 3. The main objective of this study is con- that may assist in reducing seismic risk in the area. tinuous monitoring of soil radon and their possible correl- Radon variations might seem to be a good precursor of ation with seismic events. The present paper reports crustal motion resulting in earthquakes. This fact encour- continuous soil radon measurements carried out at Chite ages researchers to find a way of using this phenomenon Fault from August 2013 to January 2014. for earthquakes prediction. However earthquakes are not always preceded by a radon anomaly and not every radon Experimental techniques and methods increase is followed by an earthquake. But the radon tech- Solid State Nuclear Track Detector (SSNTD) is one of the nique has been successfully used in several seismic areas most widely used devices for the last few decades for of the world for the purpose of earthquake precursory re- measuring radon concentration in earthquake precursory search. Based on the status of seismicity in the northeast- studies. In the present investigation, weekly measurements ern India, it seems very necessary to apply whatever were carried out using LR-115 Type-II SSNTD films man- available techniques which may help to understand the ufactured by M/S Kodak Pathe, France. The detectors behavior of soil-gas radon concentration and correlate (LR-115 Films) were cut into a size of 3 cm × 3 cm and with nearby seismic events. Keeping the above in view, loaded in a twin cup radon/thoron discriminating dosime- present authors began studies of radon variation as a pos- ters, designed and fabricated by Mayya and group (Mayya sible seismic precursor in 2011 at Mat Fault in Mizoram et al., 1998) at BARC, Mumbai (India). The experimental for the first time and the results of this work have been re- detail is discussed in Singh et al. (2014). The meteoro- ported (Jaishi et al., 2013; 2014a; 2014b;). Encouraged with logical parameters for the study area were obtained from the outcome and for future research, authors extended the IMD-Regional Meteorological Centre, Guwahati, Singh et al. Geoenvironmental Disasters (2016) 3:22 Page 4 of 8 Fig. 3 Geographical location of the study area Assam. Due to certain constraints integrated measure- details of earthquakes that occurred around the study ment for 7 days was taken for the soil gas instead of area fulfilling Eq. (1) is given in Table 1 and the distribu- daily recording. Total rainfall was calculated for 7 days. tion of the earthquakes that occurred around the study Temperature, pressure and relative humidity are the area is presented in Fig. 4. moving average of 7 days. Regarding selection of seis- mic activity that could correlated with radon data there Results and Discussions is no such general rule with respect to epicenter dis- Normality test tance from the measuring sites. Virk (1996) modified Before correlating the measured radon data set with me- the model proposed by Fleischer (1981) by considering teorological parameters, we wanted to check whether the 142 case studies in N-W Himalaya, India as radon data shows normal distribution, which is inevitable because the soil radon data obeying the fundamental laws 10 expðÞ 0:32M ;ðÞ 10 < D < 50 of geochemistry are usually normally distributed (Ahrens, 10 expðÞ 0:43M ;ðÞ 50 < D < 100 1954). To do this we performed normality test which is D ¼ ð1Þ 10 expðÞ 0:56M ;ðÞ 100 < D < 500 > used to determine whether a data set resembles the normal 10 expðÞ 0:63M ;ðÞ 500 < D < 1250 distribution. In the present work, authors have imple- mented normal probability plot to perform normality test Where, D is the epicenter distance in km and M is the in Microsoft excel, 2007. One characteristic that defines the magnitude of earthquake on the Richter scale. The normal distribution is that normally distributed data will Table 1 Lists of earthquakes that occurred around the investigation area during the observation period (source: www.imd.gov.in) Date of event Date of anomaly Latitude (°N) Longitude (°E) Depth (km) Magnitude Epicenter distance (km) Precursor/postcursor observed time (Days) 09/07/2013 09/06/2013 25.3°N 94.9°E 90 4.8 278 1 10/29/2013 10/25/2013 22.9°N 94.2°E 46 4.7 176 4 11/06/2013 10/25/2013 26.5°N 93.5°E 20 5.5 320 12 12/30/2013 12/06/2013 24.3°N 93.2°E 10 4.5 78 24 Singh et al. Geoenvironmental Disasters (2016) 3:22 Page 5 of 8 Fig. 4 Spatial distribution of earthquakes (open star) around the study area during the investigation period have the same amount of area of normal curve between NORMSINV (CDF at each radon value) for Z-score, each point. The area under the normal curve between each and NORMINV (CDF at each radon value, mean, stand- point can be determined by cumulative distribution func- ard deviation) for expected values. tion (CDF) using the following Excel formula. A plot of expected radon values versus Z-score will be a straight line. We now observed the actual radon data CDF ¼ NORMDISTðÞ Radon value; mean; standard deviation; TRUE compared to the expected radon data for normally distrib- uted data having the same mean and standard deviation Now the CDF value of each Radon value is used for and observed that the actual radon data maps closely to calculating the expected radon values and Z- score at the expected radon values (Fig. 5). So it may be concluded each radon value by the following formulae. that the data is derived from a normally distributed Fig. 5 Normal probability plot comparing actual radon value with expected radon value Singh et al. Geoenvironmental Disasters (2016) 3:22 Page 6 of 8 population. Also, the skew factor of the radon data was et al., 2005; Yang et al., 2005; Walia et al., 2005) for iden- calculated and it was found to be 0.49 indicating that the tifying possible threshold values of the anomalous radon radon value is slightly skewed towards the right. In order concentrations. A very common approach could be to to check the significance of skew factor we have used the examine the difference between radon peak and the Excel formula viz. if skew > 2*sqrt(6/count) then the skew mean value of the radon concentration for a few months factor is significant (i.e. the distribution is not normal) and or a year. In our case, the average value of radon con- if skew < 2*sqrt(6/count) then it is non-significant (i.e. the centration (X) is taken as the background level and the distribution is normal). In our case the skew factor was value crossing X ± 2σ (mean ± 2 standard deviation) is found to be non- significant (i.e., 0.49 < 0.94). considered as anomalous. In cases when the radon max- imum increases or decreases by ±1σ from its mean, a Effects of meteorological parameters on radon possible influence of the meteorological parameters is concentration carefully examined and accordingly a radon anomaly is Analysis of radon concentration in soil gas along with assumed. The variation of radon concentration together meteorological parameters viz. temperature, rainfall, with the meteorological parameters for the given time relative humidity and pressure provide useful informa- period is presented in Fig. 6. tion about the dependence of these parameters on radon According to the characteristics trends of radon con- emanation. The average concentration of radon in soil centration as illustrated in Fig. 6, there are three positive gas at Chite fault for this time window is reported to be peaks and three negative peaks recorded during the 3 3 1116.15 Bq/m with a standard deviation of 591.76 Bq/m . given time period. The first radon peak (negative anom- The percentage variation co-efficient (σ/Avg.) of radon is aly) was observed on 9/6/2013 followed by an event of 53.02% (Table 2). From Table 2 it is clear that the mea- 4.8 M which occurred on 9/7/2013. Since the observed sured radon shows a very low positive correlation with peak do not crosses the X-2σ limit, therefore it seems temperature and rainfall i.e. the value of radon concentra- necessary to investigate the behavior of meteorological tion increases with increase in these parameters and vice parameters carefully. During this period the relative hu- versa. The reason may be due to the capping effect of wet midity and rainnfall, which shows positive correlation soil layers at the surface which prevents radon from escap- with radon was quite low. Therefore, this decline in ing in to the atmosphere (Virk et al., 2000). As a result the radon concentration is attributed and/or have caused by radon values initially falls and then start rising over a variation in meteorological parameters and not by seis- period of time. A moderate positive correlation coefficient mic events. The second radon peak (negative) was re- of 0.31 was found between radon and relative humidity. corded on 4/10/2013. During this time period a fair The percentage variation coefficient was found to be amount of rainfall was received and the temperature and 6.96%. This demonstrates that the variation in radon emis- humidity which shows positive correlation with radon sion was much more influenced by relative humidity ra- were quite high indicating that this decline in radon ther than temperature and rainfall. Therefore, increase in concentration is caused by some other geophysical soil moisture may increase the fraction of radon produced process which was not mature enough to produce an in rocks to migrate into pore fluids, thus increasing the earthquake (Walia et al., 2009). Three consecutive posi- radon content of soil gas (Tanner, 1964; Fleischer, 1983). tive radon peaks were recorded on 10/11/2013 and 10/ A very low negative correlation coefficient (−0.005) was 25/2013 crossing the X + 2σ limit while the third peak found between radon and pressure suggesting that pres- on 11/8/2013 just exceeding X + 1σ level followed by sure have a non-significant influence on the measured two seismic events of 4.7 M and 5.5 M recorded on 10/ radon concentration during the investigation period. 29/2013 and 11/6/2013 with an epicenter distances of 176 km and 320 km from the measuring site. These Correlation of radon concentration with seismic events positive anomalies may be due to the combining effects Various statistical methods have been used by different of these two earthquakes. The positive radon anomalies authors in the past (Guerra and Lombardi, 2001; Fu can be explained by the Dilatancy-diffusion model Table 2 Descriptive statistics of radon and the meteorological parameters Parameter Average (Avg.) Standard deviation (σ) % Variation coefficient (σ/Avg.) Correlation coefficient Radon (Bq/m ) 1116.15 591.76 53.02 - Temperature (°C) 22.24 3.83 17.2 0.16 Rainfall(mm) 40.03 51.92 129.7 0.16 Humidity (%) 81.80 5.63 6.88 0.31 Pressure (mbar) 960.81 5.59 0.58 −0.01 Singh et al. Geoenvironmental Disasters (2016) 3:22 Page 7 of 8 Fig. 6 Plot showing the variation of a Radon concentration Rn, b Temperature T, c Relative humidity RH, d Rainfall RF, e Pressure P during the observation period. The vertical lines represent earthquakes along with their magnitude. The solid horizontal line represents the average value of radon concentration (X) and the dotted lines represent the deviation (σ) from the average value (Mjachkin et al., 1975) where the increase in radon con- earthquake of 4.7 and 5.5 magnitude. Such variation in tent prior to earthquakes is connected with the amount radon concentration could be due to crustal deformation of cracking of rocks and therefore is sharply increased along Indo-Myanmar subduction zone during these two and then flattens out due to relaxation of stress. Another seismic events. Besides these, few abnormal declines in sharp fall in radon concentration was observed on 11/ radon data having negative correlation with seismicity 22/2013 but no seismic events occurred during this were also recorded. It can be concluded that these period. Besides, it is quite difficult to explain such a changes may be either because of meteorological param- large radon decrease by environmental parameters. This eters influencing radon concentration or due to the abrupt decrease in radon concentration may be either complexity of its transport mechanism from deeper soil. due to additional compression closing cracks and pores However, for better correlation and to pinpoint the (Singh et al., 1991; Ramola et al., 2008) or from expan- seismic event with anomaly, longer periods of data col- sion causing under saturation of the pore volume lection along with measurements of other carrier and (Whitcomb, 1983). trace gases (like thoron). Acknowledgements Conclusion This work was funded by the Ministry of Earth Sciences (MoES), Govt. of In the present study, the radon data generated during India, New Delhi; in the form of Major project vide Sanction Order No. MoES/ P.O.(Seismo)/1(167)/2013 Dated 10.12.2013. the mentioned time period have been analyzed with seis- mic events and meteorological parameters. Some consid- Author’s contributions erable positive radon anomalies have been observed SS collected the data and drafted the manuscript. HPJ helped to collect the crossing the limits of X + 2σ before and after the data and performed the statistical analysis. RPT helped to draft the manuscript Singh et al. Geoenvironmental Disasters (2016) 3:22 Page 8 of 8 and site selection. RCT helped in the experimental design, participated in the King, C.Y., W.C. Evans, T. Presser, and R.H. Husk. 1981. Anomalous chemical extensive revision and overall supervision. All authors read and approved the changes in well water and possible relation to earthquakes. Geophysical final manuscript. Research Letters 8(5): 425–428. Klusman, R.W. 1981. Variations in mercury and radon emission at an aseismic site. Geophysical Research Letters 8(5): 461–464. Kraner, H.W., G.L. Schroeder, and R.D. Evans. 1964. 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Two models for earthquake forerunners. Pure and Applied Geophysics 113: 169–181. References Nersesov IL. 1984. Development of earthquake prediction in the USSR. Ahrens, L.H. 1954. The lognormal distribution of the elements (a fundamental law International symposium on continental seismicity and earthquake of geochemistry and its subsidiary). Geochimica et Cosmochimica Acta 5: 49–73. prediction, 373–383. Beijing (China): Seismological Press. Barsukov, V.L., G.M. Varshal, and N.S. Zamokina. 1985. Recent results of Ramola, R.C., Y. Prasad, G. Prasad, S. Kumar, and V.M. Choubey. 2008. Soil-gas hydrogeochemical studies for earthquake prediction in the USSR. radon as seismotectonic indicator in Garhwal Himalaya. Applied Radiation Pure and Applied Geophysics 122: 143–156. and Isotopes 66(10): 1523–1530. Birchard, G.F., and W.F. Libby. 1980. Soil radon concentration changes preceding Robinson, R., and N.E. Whitehead. 1986. Radon variations in the Wellington and following four magnitude 4.2–4.7 earthquakes on the San Jacinto Fault region, New Zealand, and their relation to earthquakes. Earthquake Prediction in southern California. Journal of Geophysical Research 85: 3100–3106. Research 4: 69–82. Cai Z, Shi H, Zhang W, Luo GEX, Shi X, Yang H. 1984. Some applications of fluid- Sarmah, S.K. 1999. The probability of occurrence of a high magnitude earthquake geochemical methods to earthquake prediction in China: International in Northeast India. Journal of Geophysics 20(3): 129–135. symposium on continental seismology and earthquake prediction, 384–395. Scholz, C.H., L.R. Sykes, and Y.P. Aggarwal. 1973. Earthquake prediction: a physical Beijing (China): Seismological Press. basis. Science 181: 803–810. Etiope, G., and G. Martinelli. 2002. Migration of carrier and trace gases in the Singh M, Ramola RC, Singh B, Singh S, Virk HS (1991) Subsurface soil gas radon geosphere: an overview. 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Seismicity of Northeast India and surroundings: Development over the past 100 years. Journal of Geophysics 19(1): 9–34. King, C.Y. 1985. Impulsive radon emanation on a creeping segment of the San Andreas Fault, California. Pure and Applied Geophysics 122(2–4): 340–352. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Geoenvironmental Disasters Springer Journals

A study of variation in soil gas concentration associated with earthquakes near Indo-Burma Subduction zone

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Springer Journals
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Copyright © 2016 by The Author(s).
Subject
Environment; Environment, general; Earth Sciences, general; Geography, general; Geoecology/Natural Processes; Natural Hazards; Environmental Science and Engineering
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2197-8670
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10.1186/s40677-016-0055-8
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Abstract

Background: In the recent past, several efforts have been made by a number of researchers to measure anomalous emanations of geo-gases in seismic prone regions of the world and radon has been the most preferred geo-gas as possible earthquake precursor since it is easily detectable. Results: In the present investigation, continuous measurements of radon concentration at 80 cm inside the soil has been carried out at Chite Fault (23.73°N, 92.73°E), Aizawl, Mizoram situated in the seismic zone V in North Eastern part of India near Indo-Burma subduction zone, using LR-115 Type-II nuclear track detectors manufactured by Kodak Pathe, 3 3 France. During the investigation period, the radon concentration varied from 163.27 Bq/m to 2557.82 Bq/m with an 3 3 average and standard deviation of 1116.15 Bq/m and 591.76 Bq/m respectively. Conclusion: Certain anomalies observed in radon concentration have been correlated to the earthquakes within the range of magnitudes 4.7 ≤ M ≤ 5.5, whilesomeother anomaliesare duetothe influenceofmeteorological parameters. Keywords: Soil-gas, Radon, LR-115 films, Correlations, Meteorological parameters, Earthquake Background Barsukov et al., 1985; Sugisaki and Sugiura, 1986). In some Migration of carrier gas by bubbles is considered to be cases, anomalies have also occurred contemporaneously an important transport mechanism governing distribu- with or after the events (Birchard and Libby, 1980; King, tion of carrier (CO and CH ) and trace (Rn, He) gases 1985; Thomas et al., 1986). Soil-gas concentrations are 2 4 over wide areas on the earth surface. Soil-gas anomalies not sensitive to hydrologic changes as they are extremely and chemical changes in groundwater, observed during susceptible to a number of other environmental effects. seismic events may be attributed to gas carrier dynamics However, many authors in the past suggest that spatial (Etiope and Martinelli, 2002). During the last several de- and temporal variations in soil-gas concentrations are cades, analysis of earthquake precursory phenomena re- most intensively influenced by meteorological interfer- veals that significant changes in geophysical and ences (Kraner et al., 1964; Klusman, 1981; Fleischer, 1983; geochemical process may occur prior to intermediate and Robinson and Whitehead, 1986; Guedalia et al., 1970). large earthquake. The behavior of the gas concentration anomalies has been quite variable. Several investigators Radon emanation and earthquake have reported increase in gas concentrations before the Radon concentration in the soil-gas increases with depth occurrence of seismic events (Cai et al., 1984; Nersesov, (Jonsson, 1995; Kristiansson and Malmqvist, 1984) until 1984; Kawabe, 1985). Besides these, declines in radon con- a certain depth is reached which depend on the soil’s centration or concentration ratio immediately and prior to properties and moisture content. Radon act as an indica- seismic events have also been reported (King et al., 1981; tor for changes in the gas streams. The most sensitive depth to detect such changes is between 0.5 to 1 m (Friedmann, 2012). Since 1971 much effort has been de- * Correspondence: ramesh_mzu@rediffmail.com 1 voted to explain earthquake on the basis of Dilatancy Department of Physics, Mizoram University, Aizawl 796004, India Full list of author information is available at the end of the article and fluid flow (Scholz et al., 1973). Dilatancy means an © The Author(s). 2016 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Singh et al. Geoenvironmental Disasters (2016) 3:22 Page 2 of 8 Fig. 1 Dilatancy: increasing stress causes cracks in the rock, which enlarges the material perpendicular to the main axis of stress. This causes an effective increase in volume (after Friedmann, 2012) elastic increase in volume under stress as shown in Fig. 1. mechanisms which are consequences of the redistri- According to the Dilatancy theory, a substantial change bution of water in the earth’s crust can take into ef- (increase in the perpendicular direction to the main axis fect (Imme and Morelli, 2012). of the stress) in the rock properties will occur shortly before earthquake leading to the possibility of water Seismicity of the study area penetration in the cracks and/or the number of cracks Earthquake most probably occurs due to movements increases exponentially. As a result, significant masses along the faults that have evolved through geological are moved which will cause the movement of sub- and tectonic processes. Northeast India is considered surface soil-gas towards the earth surface (Friedmann, one of the six most seismically active regions of the 2012). Consequently opening of new cracks, widening world. The Tectonic Map of Northeast India is shown in or closing of old cracks or redistribution of open and Fig. 2. The region has experienced 18 large earthquakes closed cracks can happen. In dry rocks opening or clos- (M ≥ 7) during the last hundred years and several hun- ing of cracks will lead to significant changes of the dif- dred small or micro earthquakes. The high seismicity in fusion coefficient of radon. Volumetric changes in the the region is attributed to the collision tectonics between rock will also lead to a subsurface gas flow and there- the Indian plate and Eurasian plate in the north and sub- fore to an additional radon transport. If the new open duction tectonics along the Indo-Myanmar range in the cracks are filled with water, the increased water-rock east (Kayal, 1998; Sarmah, 1999). There has been a phe- interface leads to an increase in the transfer of radon nomenal increase in the population density and develop- from the rock matrix to the water. If water filled cracks ment programmes in the northeast India. Besides, the close, the water will be compressed to another sub- region has witnessed a mushroom growth of unplanned surface volume where the emanation from the rock to urban centers in the previous two decades. This has re- the water may be different. All these effects result in sulted into increasing vulnerability of human population pressure and water level variations of the relevant and physical structures to the earthquakes. Thus, it aquifer. This may also lead to changes in the mixing becomes essential to assess the status of seismicity in the ratios for the water which can be observed at the northeastern region realistically. This will provide a earth’s surface. Finally gas flows can also move some sound database for earthquake disaster mitigation. groundwater and again all previously discussed Moreover, high seismic risk in the region calls for an Singh et al. Geoenvironmental Disasters (2016) 3:22 Page 3 of 8 Fig. 2 Seismo-tectonic map of North East India showing epi-centres of damaging earthquakes (after Jaishi et al., 2014c). Tectonic zones (zones A, B, C, D and E) and major thrusts (MBT-Main Boundary Thrust; MCT-Main Central Thrust; NT-Naga Thrust; DT-Disang Thrust) are also shown. Thick blue line represents the International boundary of North Eastern part of India bordering Myanmar and Bangladesh (not to scale) and red lines represents faults and thrusts urgent and sustained mitigation effort. Accordingly, radon measurement at weekly interval at Chite Fault there seems to be a real need for employing all available (Singh et al., 2014). The location of the study area is efforts including radon variations and seismicity records shown in Fig. 3. The main objective of this study is con- that may assist in reducing seismic risk in the area. tinuous monitoring of soil radon and their possible correl- Radon variations might seem to be a good precursor of ation with seismic events. The present paper reports crustal motion resulting in earthquakes. This fact encour- continuous soil radon measurements carried out at Chite ages researchers to find a way of using this phenomenon Fault from August 2013 to January 2014. for earthquakes prediction. However earthquakes are not always preceded by a radon anomaly and not every radon Experimental techniques and methods increase is followed by an earthquake. But the radon tech- Solid State Nuclear Track Detector (SSNTD) is one of the nique has been successfully used in several seismic areas most widely used devices for the last few decades for of the world for the purpose of earthquake precursory re- measuring radon concentration in earthquake precursory search. Based on the status of seismicity in the northeast- studies. In the present investigation, weekly measurements ern India, it seems very necessary to apply whatever were carried out using LR-115 Type-II SSNTD films man- available techniques which may help to understand the ufactured by M/S Kodak Pathe, France. The detectors behavior of soil-gas radon concentration and correlate (LR-115 Films) were cut into a size of 3 cm × 3 cm and with nearby seismic events. Keeping the above in view, loaded in a twin cup radon/thoron discriminating dosime- present authors began studies of radon variation as a pos- ters, designed and fabricated by Mayya and group (Mayya sible seismic precursor in 2011 at Mat Fault in Mizoram et al., 1998) at BARC, Mumbai (India). The experimental for the first time and the results of this work have been re- detail is discussed in Singh et al. (2014). The meteoro- ported (Jaishi et al., 2013; 2014a; 2014b;). Encouraged with logical parameters for the study area were obtained from the outcome and for future research, authors extended the IMD-Regional Meteorological Centre, Guwahati, Singh et al. Geoenvironmental Disasters (2016) 3:22 Page 4 of 8 Fig. 3 Geographical location of the study area Assam. Due to certain constraints integrated measure- details of earthquakes that occurred around the study ment for 7 days was taken for the soil gas instead of area fulfilling Eq. (1) is given in Table 1 and the distribu- daily recording. Total rainfall was calculated for 7 days. tion of the earthquakes that occurred around the study Temperature, pressure and relative humidity are the area is presented in Fig. 4. moving average of 7 days. Regarding selection of seis- mic activity that could correlated with radon data there Results and Discussions is no such general rule with respect to epicenter dis- Normality test tance from the measuring sites. Virk (1996) modified Before correlating the measured radon data set with me- the model proposed by Fleischer (1981) by considering teorological parameters, we wanted to check whether the 142 case studies in N-W Himalaya, India as radon data shows normal distribution, which is inevitable because the soil radon data obeying the fundamental laws 10 expðÞ 0:32M ;ðÞ 10 < D < 50 of geochemistry are usually normally distributed (Ahrens, 10 expðÞ 0:43M ;ðÞ 50 < D < 100 1954). To do this we performed normality test which is D ¼ ð1Þ 10 expðÞ 0:56M ;ðÞ 100 < D < 500 > used to determine whether a data set resembles the normal 10 expðÞ 0:63M ;ðÞ 500 < D < 1250 distribution. In the present work, authors have imple- mented normal probability plot to perform normality test Where, D is the epicenter distance in km and M is the in Microsoft excel, 2007. One characteristic that defines the magnitude of earthquake on the Richter scale. The normal distribution is that normally distributed data will Table 1 Lists of earthquakes that occurred around the investigation area during the observation period (source: www.imd.gov.in) Date of event Date of anomaly Latitude (°N) Longitude (°E) Depth (km) Magnitude Epicenter distance (km) Precursor/postcursor observed time (Days) 09/07/2013 09/06/2013 25.3°N 94.9°E 90 4.8 278 1 10/29/2013 10/25/2013 22.9°N 94.2°E 46 4.7 176 4 11/06/2013 10/25/2013 26.5°N 93.5°E 20 5.5 320 12 12/30/2013 12/06/2013 24.3°N 93.2°E 10 4.5 78 24 Singh et al. Geoenvironmental Disasters (2016) 3:22 Page 5 of 8 Fig. 4 Spatial distribution of earthquakes (open star) around the study area during the investigation period have the same amount of area of normal curve between NORMSINV (CDF at each radon value) for Z-score, each point. The area under the normal curve between each and NORMINV (CDF at each radon value, mean, stand- point can be determined by cumulative distribution func- ard deviation) for expected values. tion (CDF) using the following Excel formula. A plot of expected radon values versus Z-score will be a straight line. We now observed the actual radon data CDF ¼ NORMDISTðÞ Radon value; mean; standard deviation; TRUE compared to the expected radon data for normally distrib- uted data having the same mean and standard deviation Now the CDF value of each Radon value is used for and observed that the actual radon data maps closely to calculating the expected radon values and Z- score at the expected radon values (Fig. 5). So it may be concluded each radon value by the following formulae. that the data is derived from a normally distributed Fig. 5 Normal probability plot comparing actual radon value with expected radon value Singh et al. Geoenvironmental Disasters (2016) 3:22 Page 6 of 8 population. Also, the skew factor of the radon data was et al., 2005; Yang et al., 2005; Walia et al., 2005) for iden- calculated and it was found to be 0.49 indicating that the tifying possible threshold values of the anomalous radon radon value is slightly skewed towards the right. In order concentrations. A very common approach could be to to check the significance of skew factor we have used the examine the difference between radon peak and the Excel formula viz. if skew > 2*sqrt(6/count) then the skew mean value of the radon concentration for a few months factor is significant (i.e. the distribution is not normal) and or a year. In our case, the average value of radon con- if skew < 2*sqrt(6/count) then it is non-significant (i.e. the centration (X) is taken as the background level and the distribution is normal). In our case the skew factor was value crossing X ± 2σ (mean ± 2 standard deviation) is found to be non- significant (i.e., 0.49 < 0.94). considered as anomalous. In cases when the radon max- imum increases or decreases by ±1σ from its mean, a Effects of meteorological parameters on radon possible influence of the meteorological parameters is concentration carefully examined and accordingly a radon anomaly is Analysis of radon concentration in soil gas along with assumed. The variation of radon concentration together meteorological parameters viz. temperature, rainfall, with the meteorological parameters for the given time relative humidity and pressure provide useful informa- period is presented in Fig. 6. tion about the dependence of these parameters on radon According to the characteristics trends of radon con- emanation. The average concentration of radon in soil centration as illustrated in Fig. 6, there are three positive gas at Chite fault for this time window is reported to be peaks and three negative peaks recorded during the 3 3 1116.15 Bq/m with a standard deviation of 591.76 Bq/m . given time period. The first radon peak (negative anom- The percentage variation co-efficient (σ/Avg.) of radon is aly) was observed on 9/6/2013 followed by an event of 53.02% (Table 2). From Table 2 it is clear that the mea- 4.8 M which occurred on 9/7/2013. Since the observed sured radon shows a very low positive correlation with peak do not crosses the X-2σ limit, therefore it seems temperature and rainfall i.e. the value of radon concentra- necessary to investigate the behavior of meteorological tion increases with increase in these parameters and vice parameters carefully. During this period the relative hu- versa. The reason may be due to the capping effect of wet midity and rainnfall, which shows positive correlation soil layers at the surface which prevents radon from escap- with radon was quite low. Therefore, this decline in ing in to the atmosphere (Virk et al., 2000). As a result the radon concentration is attributed and/or have caused by radon values initially falls and then start rising over a variation in meteorological parameters and not by seis- period of time. A moderate positive correlation coefficient mic events. The second radon peak (negative) was re- of 0.31 was found between radon and relative humidity. corded on 4/10/2013. During this time period a fair The percentage variation coefficient was found to be amount of rainfall was received and the temperature and 6.96%. This demonstrates that the variation in radon emis- humidity which shows positive correlation with radon sion was much more influenced by relative humidity ra- were quite high indicating that this decline in radon ther than temperature and rainfall. Therefore, increase in concentration is caused by some other geophysical soil moisture may increase the fraction of radon produced process which was not mature enough to produce an in rocks to migrate into pore fluids, thus increasing the earthquake (Walia et al., 2009). Three consecutive posi- radon content of soil gas (Tanner, 1964; Fleischer, 1983). tive radon peaks were recorded on 10/11/2013 and 10/ A very low negative correlation coefficient (−0.005) was 25/2013 crossing the X + 2σ limit while the third peak found between radon and pressure suggesting that pres- on 11/8/2013 just exceeding X + 1σ level followed by sure have a non-significant influence on the measured two seismic events of 4.7 M and 5.5 M recorded on 10/ radon concentration during the investigation period. 29/2013 and 11/6/2013 with an epicenter distances of 176 km and 320 km from the measuring site. These Correlation of radon concentration with seismic events positive anomalies may be due to the combining effects Various statistical methods have been used by different of these two earthquakes. The positive radon anomalies authors in the past (Guerra and Lombardi, 2001; Fu can be explained by the Dilatancy-diffusion model Table 2 Descriptive statistics of radon and the meteorological parameters Parameter Average (Avg.) Standard deviation (σ) % Variation coefficient (σ/Avg.) Correlation coefficient Radon (Bq/m ) 1116.15 591.76 53.02 - Temperature (°C) 22.24 3.83 17.2 0.16 Rainfall(mm) 40.03 51.92 129.7 0.16 Humidity (%) 81.80 5.63 6.88 0.31 Pressure (mbar) 960.81 5.59 0.58 −0.01 Singh et al. Geoenvironmental Disasters (2016) 3:22 Page 7 of 8 Fig. 6 Plot showing the variation of a Radon concentration Rn, b Temperature T, c Relative humidity RH, d Rainfall RF, e Pressure P during the observation period. The vertical lines represent earthquakes along with their magnitude. The solid horizontal line represents the average value of radon concentration (X) and the dotted lines represent the deviation (σ) from the average value (Mjachkin et al., 1975) where the increase in radon con- earthquake of 4.7 and 5.5 magnitude. Such variation in tent prior to earthquakes is connected with the amount radon concentration could be due to crustal deformation of cracking of rocks and therefore is sharply increased along Indo-Myanmar subduction zone during these two and then flattens out due to relaxation of stress. Another seismic events. Besides these, few abnormal declines in sharp fall in radon concentration was observed on 11/ radon data having negative correlation with seismicity 22/2013 but no seismic events occurred during this were also recorded. It can be concluded that these period. Besides, it is quite difficult to explain such a changes may be either because of meteorological param- large radon decrease by environmental parameters. This eters influencing radon concentration or due to the abrupt decrease in radon concentration may be either complexity of its transport mechanism from deeper soil. due to additional compression closing cracks and pores However, for better correlation and to pinpoint the (Singh et al., 1991; Ramola et al., 2008) or from expan- seismic event with anomaly, longer periods of data col- sion causing under saturation of the pore volume lection along with measurements of other carrier and (Whitcomb, 1983). trace gases (like thoron). Acknowledgements Conclusion This work was funded by the Ministry of Earth Sciences (MoES), Govt. of In the present study, the radon data generated during India, New Delhi; in the form of Major project vide Sanction Order No. MoES/ P.O.(Seismo)/1(167)/2013 Dated 10.12.2013. the mentioned time period have been analyzed with seis- mic events and meteorological parameters. Some consid- Author’s contributions erable positive radon anomalies have been observed SS collected the data and drafted the manuscript. HPJ helped to collect the crossing the limits of X + 2σ before and after the data and performed the statistical analysis. RPT helped to draft the manuscript Singh et al. Geoenvironmental Disasters (2016) 3:22 Page 8 of 8 and site selection. RCT helped in the experimental design, participated in the King, C.Y., W.C. Evans, T. Presser, and R.H. Husk. 1981. Anomalous chemical extensive revision and overall supervision. All authors read and approved the changes in well water and possible relation to earthquakes. Geophysical final manuscript. Research Letters 8(5): 425–428. Klusman, R.W. 1981. Variations in mercury and radon emission at an aseismic site. Geophysical Research Letters 8(5): 461–464. Kraner, H.W., G.L. Schroeder, and R.D. Evans. 1964. 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