A study of variation in soil gas concentration associated with earthquakes near Indo-Burma Subduction zone
© The Author(s). 2016
Received: 25 May 2016
Accepted: 11 November 2016
Published: 22 November 2016
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.
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, France. During the investigation period, the radon concentration varied from 163.27 Bq/m3 to 2557.82 Bq/m3 with an average and standard deviation of 1116.15 Bq/m3 and 591.76 Bq/m3 respectively.
Certain anomalies observed in radon concentration have been correlated to the earthquakes within the range of magnitudes 4.7 ≤ M ≤ 5.5, while some other anomalies are due to the influence of meteorological parameters.
KeywordsSoil-gas Radon LR-115 films Correlations Meteorological parameters Earthquake
Migration of carrier gas by bubbles is considered to be an important transport mechanism governing distribution of carrier (CO2 and CH4) and trace (Rn, He) gases over wide areas on the earth surface. Soil-gas anomalies and chemical changes in groundwater, observed during seismic events may be attributed to gas carrier dynamics (Etiope and Martinelli, 2002). During the last several decades, analysis of earthquake precursory phenomena reveals that significant changes in geophysical and geochemical process may occur prior to intermediate and large earthquake. The behavior of the gas concentration anomalies has been quite variable. Several investigators have reported increase in gas concentrations before the occurrence of seismic events (Cai et al., 1984; Nersesov, 1984; Kawabe, 1985). Besides these, declines in radon concentration or concentration ratio immediately and prior to seismic events have also been reported (King et al., 1981; Barsukov et al., 1985; Sugisaki and Sugiura, 1986). In some cases, anomalies have also occurred contemporaneously with or after the events (Birchard and Libby, 1980; King, 1985; Thomas et al., 1986). Soil-gas concentrations are not sensitive to hydrologic changes as they are extremely susceptible to a number of other environmental effects. However, many authors in the past suggest that spatial and temporal variations in soil-gas concentrations are most intensively influenced by meteorological interferences (Kraner et al., 1964; Klusman, 1981; Fleischer, 1983; Robinson and Whitehead, 1986; Guedalia et al., 1970).
Radon emanation and earthquake
Seismicity of the study area
Experimental techniques and methods
Lists of earthquakes that occurred around the investigation area during the observation period (source: www.imd.gov.in)
Date of event
Date of anomaly observed
Epicenter distance (km)
Precursor/postcursor time (Days)
Results and Discussions
Now the CDF value of each Radon value is used for calculating the expected radon values and Z- score at each radon value by the following formulae.
NORMSINV (CDF at each radon value) for Z-score, and NORMINV (CDF at each radon value, mean, standard deviation) for expected values.
Effects of meteorological parameters on radon concentration
Descriptive statistics of radon and the meteorological parameters
Standard deviation (σ)
% Variation coefficient (σ/Avg.)
Correlation of radon concentration with seismic events
According to the characteristics trends of radon concentration as illustrated in Fig. 6, there are three positive peaks and three negative peaks recorded during the given time period. The first radon peak (negative anomaly) was observed on 9/6/2013 followed by an event of 4.8 M which occurred on 9/7/2013. Since the observed peak do not crosses the X-2σ limit, therefore it seems necessary to investigate the behavior of meteorological parameters carefully. During this period the relative humidity and rainnfall, which shows positive correlation with radon was quite low. Therefore, this decline in radon concentration is attributed and/or have caused by variation in meteorological parameters and not by seismic events. The second radon peak (negative) was recorded on 4/10/2013. During this time period a fair amount of rainfall was received and the temperature and humidity which shows positive correlation with radon were quite high indicating that this decline in radon concentration is caused by some other geophysical process which was not mature enough to produce an earthquake (Walia et al., 2009). Three consecutive positive radon peaks were recorded on 10/11/2013 and 10/25/2013 crossing the X + 2σ limit while the third peak on 11/8/2013 just exceeding X + 1σ level followed by two seismic events of 4.7 M and 5.5 M recorded on 10/29/2013 and 11/6/2013 with an epicenter distances of 176 km and 320 km from the measuring site. These positive anomalies may be due to the combining effects of these two earthquakes. The positive radon anomalies can be explained by the Dilatancy-diffusion model (Mjachkin et al., 1975) where the increase in radon content prior to earthquakes is connected with the amount of cracking of rocks and therefore is sharply increased and then flattens out due to relaxation of stress. Another sharp fall in radon concentration was observed on 11/22/2013 but no seismic events occurred during this period. Besides, it is quite difficult to explain such a large radon decrease by environmental parameters. This abrupt decrease in radon concentration may be either due to additional compression closing cracks and pores (Singh et al., 1991; Ramola et al., 2008) or from expansion causing under saturation of the pore volume (Whitcomb, 1983).
In the present study, the radon data generated during the mentioned time period have been analyzed with seismic events and meteorological parameters. Some considerable positive radon anomalies have been observed crossing the limits of X + 2σ before and after the earthquake of 4.7 and 5.5 magnitude. Such variation in radon concentration could be due to crustal deformation along Indo-Myanmar subduction zone during these two seismic events. Besides these, few abnormal declines in radon data having negative correlation with seismicity were also recorded. It can be concluded that these changes may be either because of meteorological parameters influencing radon concentration or due to the complexity of its transport mechanism from deeper soil. However, for better correlation and to pinpoint the seismic event with anomaly, longer periods of data collection along with measurements of other carrier and trace gases (like thoron).
This work was funded by the Ministry of Earth Sciences (MoES), Govt. of India, New Delhi; in the form of Major project vide Sanction Order No. MoES/P.O.(Seismo)/1(167)/2013 Dated 10.12.2013.
SS collected the data and drafted the manuscript. HPJ helped to collect the data and performed the statistical analysis. RPT helped to draft the manuscript and site selection. RCT helped in the experimental design, participated in the extensive revision and overall supervision. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
Open AccessThis 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.
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