We investigate the drilling mud gas from 70 m to 1200 m borehole depth of the Wenchuan earthquake Fault Scientific Drilling Hole-1 (WFSD-1) in the context of rapid response fault zone drilling to large earthquakes. Complete depth profiles are accomplished for CH4, He, H2, CO2, N2, Ar, O2 and N2/Ar. 222Rn is also available for a limited depth. The volcanic hanging wall generates high Rn and He, whereas the CH4 is low. The sedimentary footwall yields higher CH4 and CO2, with CH4 peaks up to 2.5 vol.% in the intensely fractured shale and siltstone from 640 m to 715 m.

The drilling mud gases distribute asymmetrically across the co-seismic slip plane at 589.2 m, with the bellow 117 m true depth manifests as a prominent gas anomaly zone. Gas concentrations in this zone are much higher and vary more violently than the upper 200 m. The low P-wave velocity and resistivity imply that the strata are highly fractured, and gases intruded the borehole mainly by dissolving in water. We suggest that the gas distributions are partially lithology affected, but mostly fracture constrained according to the lithology variations, subsidiary fault zone and fracture distribution retrieved from the core. Considering the low deformation rate and seismicity in this region before the earthquake, together with the ductile behavior of the clay-rich fault gouge under slow tectonic loading, and the decreasing gas anomalies away from the slipping plane, we propose that the fractures were mostly generated during the Wenchuan earthquake. It indicates that the Wenchuan earthquake damaged the footwall more than the hanging wall, and the 117 m depth marks the width of the fracturing zone. The differential mechanic property between the hanging wall and the footwall, and the asymmetric stress during the rupturing may contribute to the asymmetric fracture distribution.