Long-term radiometric and hydrological investigations at the Wettinquelle mineral spring in Bad Brambach demonstrated the existence of a fluidal connection to the currently most frequent earthquake-swarm hypocentre at Novy Kostel, 10 km east of Bad Brambach. The gas composition and δ¹³C_CH4 values of this mineral spring were monitored from May 2000 until October 2003, i.e., before, during and after the protracted swarm earthquake period from late August until late December 2000. About eight weeks after the beginning of the seismically active period, we observed an increase in the methane concentration (from ≈40 up to ≈250 ppmv) accompanied by a decrease in the methane δ¹³C values from ≈-50 to ≈-70‰. For more than two years, such periods of variations were repeatedly observed before returning to the "baseline" signature. It is assumed that this additional methane was microbially produced in the granite-enclosed aquifer using H₂, which was released (seismically triggered) from the fissured granite in which the Wettinquelle spring capture is located. The additional methane production might have started as a co-seismic event, with only the migration from the deep granite to the surface being responsible for the eight-week delay.