Fluid Behaviors Within The Wenchuan Earthquake Fault Zone And The Accompanied Healing Process

The2008Mw7.9Wenchuanearthquake happened in the southeast margin of the Tibetan Plateau, creating two surface rupture zones along the Yinxiu-Beichuan fault and Anxian-Guanxian fault. It raises a number of questions unsettled even today.The role the underground fluid plays is critical to answer those problems. Basically, the relation between fluid activity and earthquake can be summarized as trigger before the earthquake, weakening during the earthquake and healing after the earthquake. In this paper, we use the real-time drilling mud gas data, which contains8gas species:He, Rn, Ar, CH4, CO2, H2, N2and O2, and the long term water level observation data in Wenchuan Earthquake Fault Scientific Drilling Project Hole-1(WFSD-1), to probe questions related to the fluid activitieswithin the Wenchuan earthquake fault zone.We place our emphases on post-earthquake fluid problems since all the data are actually collected after the earthquake, and mainly focus on four categories as below:1) thespatial fluid distribution within the Wenchuan earthquake fault zone;2) the distribution of passages created by the earthquake itself;3) the relation between real-time drilling mud and the Wenchuan earthquake aftershocks;4) the healing process happened within the fault zone after the main shock. Because the WFSD-1drill hole is the most rapid fault zone drilling after large earthquake, the fluid and water level data can provide timely information for the above four questions. For one hand, the newly formed fracture during the earthquake are highly permeable, the real time drilling mud gases distribution are not only able to clarify the fluid distribution in the fault zone but also the damage associated with the earthquake. For the other hand, the geological position of WFSD-1makes the water level data record extraordinary earth tides.It makes the earth tide analysis applicable to restore the fault zone healing process.The result shows the real-time drilling mud gases distribute unevenly in the whole drilling section. The volcanic hanging wall possesses higher Helium and Radon concentrations, with relatively low Methane. The sedimentary footwall however, contains higher Methane and Carbon Dioxide. We assume that the gas distributions are partly lithologycontrolled, since the volcanic rocks usually contain higher radiation elements and the sedimentary rocks enrich organic material. However, with a closer scrutiny, we find that the gases are more constrained by open fractures, which are highly possible products of the2008Wenchuan earthquake. That leads to our primary conclusion:the lithology controls the background value for all gas species, but the subtle changes are more controlled by fracture. Gases peak where fractures or large fracture density emerges. This is also supported by the geophysical logs, such as formation resistivity and seismic wave velocity.The gas anomaly zone usually coincides with low resistivity and P wave velocity, which indicates that gas invading happened via dissolving in water.The real time drilling mud gas implies that the2008Wenchuan earthquake damaged the footwall more than the hanging wall, since the gas anomaly zone mainly distributes below the principal slip zone. The anomaly zone is200m wide in string depth and the vertical width is117m. The asymmetric damage pattern may be controlled by the mechanic difference between the above volcanic rocks and the underlying sedimentary rocks. Anyway, it is the first time using the real-time drilling mud gas to discuss the damage process accompanied with a single large earthquake, and our results show that the damage could be asymmetric. Moreover, we prove that the damage zone for a large earthquake could be as large as117m.The average fault zone transmissibility1.5to6.3years after the Wenchuan earthquake is1.83X10"2m2s"1calculated from the phase shift between the water level and earth tide strain. The average fault zone permeability is as high as10-l5m2.It is unusually higher than the results from laboratory experiment and also one magnitude larger than the normal Xujiahe Formation. It indicates that major fluid circulation probably happened in the Wenchuan earthquake fault zone, and the fluid activity during the earthquake could be serious. The fault zone hydrological parameters display clear descending trend during the whole observation time and indicating that fast fault zone healing happened immediately after the Wenchuanearhtuake. The last two years hydraulic diffusivity is1.48x10-2m2s-1comparing with the beginning two years2.4x10-2m2s-1, the Wenchuan earthquake fault zone may have been healed by over50%during the last4years. Interestingly, the healing process is not smooth at all; the local or distant earthquake with large seismic energy or surface amplitude disturbs the healing signals occasionally, they always lead higher fault zone conductivities. Similarly, we also observe that earthquakes with large surface strain could also cause the real-time drilling mud fluxes.