Real time drilling mud gas records the asymmetric damaging process associated with the 2008 Ms 8.0 Wenchuan earthquake

Abstract: A considerable part of energy released during an earthquake is used to create fractures within the surrounding rocks. The real-time drilling mud gas from WFSD-1 shows that the fractures generated during the Wenchuan Earthquake coincide with large fluid peaks, and hence they are favorable passages for underground fluid migrations. The drilling mud gas distributes asymmetrically around the principal slip zone (PSZ), with the major anomaly zone emerging 120 m below the PSZ. Gas concentrations are much higher and vary more intensively than the upper 120 m. Additionally, the geophysical logs also prove that this zone is highly fractured and associated with serious water inrush, suggesting that the 2008 Wenchuan earthquake caused more damage to the footwall than to the hanging wall. The difference in rock strength between the volcanic hanging wall of Pengguan complex and the sedimentary footwall of Xujiahe Formation and the asymmetric stress during the rupture propagation might have contributed to the damage process.