The first results of studies of temporary variations in soil-radon activity of faults in Western Pribaikalie

Abstract

Radon concentrations in soil air are variable depending on factors that are considered external (planetary) and internal (geodynamic) relative to the Earth. In active fault zones, variations of gas emanations are most intense. A permanent monitoring station was established near Tyrgan settlement in Western Pribaikalie to study temporal variations of soil radon concentration, Q, in the faults of the Baikal rift, East Siberia. This station is located in the zone of the Primorsky normal fault that is the largest in the region. The station is equipped with radon radiometer PPA01M03 that records Q values every 85 minutes and also monitors a number of meteorological parameters, including atmospheric pressure, humidity, and air temperature. We analysed records of two measurement sessions (148 and 66 days) covering a part of the year during which field measurement of Q are possible in the cold climate conditions of the area under study. According to the available monitoring data, variations of radon concentrations in the Primorsky fault zone may vary by more than one order of magnitude through a springsummerautumn period, and such variations are oscillatory. Significant changes of permeability in time occur due to intensive changes in the state of stresses of the rock massives under the impacts of the planetary and geodynamic factors. The influence of the first group of factors, i.e. planetary ones, is manifested by synchronous oscillations of radon concentrations and atmospheric pressure, which phases of occurrence are opposed. Domination of daily and fourday periods gives evidence that the state of stresses of the rock massives is impacted by the lunar tides and cyclonic phenomena associated with the interaction between the Earth and the Sun. The influence of the second group of factors, i.e. geodynamic ones, is suggested by an evident relation between radon emanations and seismic events, including the catastrophic earthquake in Japan (March 11, 2011, M=9.0). Tectonophysics The external and internal factors are acting together, but their roles are different with regard to variations of radon concentrations in different periods of time. In the monitoring periods, radon emanation variations were mainly controlled by the planetary factors. Radon exhalation increases and decreases according to periodic variations in atmospheric pressure, which, in additional to ‘pumping’ effects, may lead to opening/closure of pores and cracks in the rocks. While external pressures are reduced, internal stresses are released by relatively weak earthquakes. The guiding influence of atmospheric pressure on the yield of radon is disturbed when internal stresses are in excess of a certain level due to intensive movements along faults in the Baikal rift or displacements of plates in neighbouring active zones (for example, due to the strongest earthquake in Japan). In such relatively short periods of time, when seismic activity is increased, the influence of tectonic stresses on permeability of rocks and radon emanations becomes dominant. Based on our analysis of the measurements of soil radon concentrations obtained on the local site in the Primorsky fault zone through the monitoring period, it became possible, for the first time for Pribaikalie, to reveal and theoretically model the principal specific features of variation of soil radon concentrations, Q, in time and the dependence of such variations on the external and internal factors. Prospects of these studies are related to installation of a network of monitoring stations in the territory of the Baikal rift and assurance of longterm monitoring sessions.