Soil radon emissions along active fault zones in the 2001 bhuj earthquake epicentral region, Kachchh, India: implications for environmental radiation and human health risk.

The release of naturally occurring radioactive gases such as radon (222Rn) and thoron (22⁰Rn) from the subsurface is significantly enhanced in tectonically disturbed regions and may contribute to environmental radiation exposure. In this study, soil gas radon and thoron concentrations were investigated in the seismically active Kachchh region of western India to assess their spatial distribution, tectonic controls, and potential environmental implications. The study area hosts several active faults and experienced intense seismic activity, including the devastating 2001 Bhuj earthquake (M 7.7). In-situ soil gas measurements were carried out at 277 locations covering an area of approximately 8,250 km2. The measured soil radon concentrations range from 11.3 to 15,319 Bq/m3 with an average value of 1,331 Bq/m3, whereas soil thoron concentrations vary from 0 to 24,100 Bq/m3 with a mean value of 4,120 Bq/m3. Spatial analysis reveals that elevated concentrations of both gases are predominantly associated with major active fault zones in the region. Significantly, higher emissions are observed along the Kachchh Mainland Fault, where shallow basement depth and intense crustal fracturing facilitate enhanced migration of subsurface gases. Elevated concentrations are also detected near the epicentral zone of the 2001 Bhuj earthquake, suggesting a strong structural control on soil gas emanation. The relatively higher thoron concentrations compared to radon are attributed to the greater abundance of thorium (232Th) relative to radium (22⁶Ra) in the soils and the rapid decay characteristics of thoron. Statistical analyses further indicate distinct emission patterns between tectonically disturbed zones and relatively stable areas. Lithology strongly controls soil gas behavior: U-Th-rich volcanic rocks yield higher radon and thoron emissions, while clay-rich, low-permeability sediments show lower emissions. The estimated annual effective dose due to radon inhalation ranges from 0.0001 to 0.14 mSv/yr, with an average value of 0.012 mSv/yr. Although the maximum values slightly exceed the recommended limit of 0.1 mSv/yr suggested by the World Health Organization, they remain below the reference limit of 1 mSv/yr recommended by the International Commission on Radiological Protection. These results highlight the strong influence of tectonic structures/formations on soil gas emissions and provide important insights into environmental radiation levels in seismically active regions.