Gas emission from active fault zones around the Jilantai faulted depression basin and its implications for fault activities

Soil gases from fault zones are good indicators of tectonic and seismic activities, to which many seismologists and geochemists have been paid much attention. Five measuring sections for soil gas and one for earth resistivity were designed on the four active faults around the Jilantai basin, northwestern China. The data of earth resistivity, concentration and flux of soil gases Rn, Hg and CO₂ were attained, and the chemical compositions of soil were analyzed in all sections and the relative index K_Q of fault activity was calculated. All the results showed that soil gases CO₂ and Rn were blocked by sandy soil layers with low permeability and escaped along the hanging wall of the faults with broken structures, easily forming concentration peaks. High concentrations and fluxes of Rn, Hg and CO₂ were distributed in the southern margin of the Jilantai basin, which might be related to the migration of U and Ra in granites in southwestern margin of the basin and the decomposition of local carbonate rocks in south margin of the basin. The variation characteristics of relative index K_Q of fault activity in each section indicated normal and reverse faults with higher K_Q values than strike-slip faults. The maximum K_Q value was observed in the piedmont fault of Bayanwula mountains, probably indicating that this fault is of the strongest activity and is also a potential area of high seismic hazards.