The present work is proposed to investigate the spatiotemporal variations in soil gas Rn and CO 2 across the active faults in the capital area of China in order to understand fault activities and assess seismic hazard. A total of 342 soil gas sampling sites were measured twice in 2011 and 2012 along seven profiles and across four faults. The results of soil gas surveys show that, in each profile, due to the variation in gas emission rate, the concentrations of Rn and CO 2 changed in the

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... of faults. Spatial distributions of Rn and CO 2 in the study areas were different from each other, which was attributed to soil types affecting the existence of Rn and CO 2 . Compared with the measurement result of 2011, the increasing amplitude of average concentration value of Rn and CO 2 in profiles in 2012 ranged from 30.2 to 123.4 % and 66.3 to 131.7 %, respectively, which were coincident with the enhancement of seismic activities in the capital area of China. Our results indicate that special attention with regard to seismic monitoring should be paid to the Xinbaoan-Shacheng Fault and the northeastern segment of the Tangshan Fault in the future.