Transport Properties Of Wenchuan Earthquake Fault Zone-Implications For Coseismic Slip Weakening Processes

With relation to the strength of an earthquake fault and its stress release process, pore pressure within the shear zone has always received much attention. The permeability structure of fault zones is of great importance to understand fluid’s active state, frictional strength and stability of active faults, as well as the coseismic thermal pressurization process. This thesis reviews previous studies on fault zone permeability, especially on the seismogenic fault zones. To perform measurements of transport properties on fault rocks, a novel fluid-flow system was recently developed. Choosing two typical surface exposures on the Yingxiu-Beichuan fault zone as the objects, this work analyzed internal structure of the fault zone, and mineral composition and grain size characteristics of the fault rocks; and measured the gas permeability of fault rocks collected from both exposures. Lastly, utilizing the fluid-flow apparatus, this work carried out integrated measurements of permeability, porosity and specific porosity of fault rocks. Based on these lab-derived data, this work conducted numerical modeling of the coseismic thermal pressurization processes during the 2008 Wenchuan great earthquake.First, this thesis introduces the details of the principles, basic structure and performance of a newly developed fluid-flow system. Typical measurement results and error analyses are given, including procedures for integrated measurements of permeability, porosity and specific storage. By using a specially-designed microvolumeter, the absolute porosity of porous rock was for the first time measured under an intra-vessel condition, using water as the pore fluid. Employing the pore pressure oscillation method, permeability as low as 10-22m2 can be measured.Particular size distribution (PSD) of fault rocks has been widely used to investigate the fracturing mechanism, frictional property and energetic partitioning of earthquakes. Combining sieve-weighting and laser diffraction analysis, this work performed measurements on natural fault rocks collected from the two surface exposures studied, with particle size measured ranging from 0.2 μm to 16 mm. These measurements apparently show the following results:1) There exists a critical particular size dc (0.95-1.90 μm), and particles with larger and smaller size than dc follow different power law relations between particle number (Ad) and particle size (d), indicating that the PSD of the fault rocks studied are non-self-similar.2) The fractal dimensions (D-value) obtained from the particles with d> dc, are well relevant to the fault rock types. This means that the D-values obtained by sieve-weighting method and by laser diffraction analysis are consistent with one another. The D-values of fault rocks analyzed show an increase trend towards the slip surface, from 2.6 for fracturing breccia, to 3.0 for crushed breccia, and to almost 3.5 for fresh fault gouges. By contrast, particles with d< dc have low D-values of 1.7-2.1. This work infers that variation in a fractal dimension for different particle size intervals reflects the change in cataclasis mechanisms. The critical particle size dc, probably corresponds to the grand limit of the gouge material.3) From the PSD results, the surface fracture energy calculated is 0.63 MJ/m2 for the Wenchuan earthquake fault gouge.Cross-fault permeability measurement was conducted on the Wenchuan earthquake fault. The results show that this seismogenic fault consists of fault core with low permeability (2.4×10-19-3.8×10-16 m2), highly permeable fracture zone (3.7×10-16-3.0×10-15 m2) and the fractured protolith (6.0×10-18-4.3×10-13 m2) (40 MPa effective pressure), among which fresh gouges of the Wenchuan earthquake are the most impermeable. Impeded by the impermeable protolith and fault gouge, cross-fault fluid flow was inhibited and fluid activity was confined within the fracture zone. This recognition is consistent with microstructure and grain size distribution of fault rocks. Condition of co-seismic thermal pressurization was calculated and the results show that at depth greater than 2 km, the principal slip zone of the Wenchuan earthquake has the characteristics of low permeability for thermal pressurization to occur.This thesis also focuses on the dynamic weakening processes associated with the 2008 Wenchuan earthquake. Experiments have been performed to examine the frictional and transport properties of fault rocks collected from a surface exposure. The water-dampened gouges show high-velocity frictional behavior characterized by a rapid stress drop at the start of slip, and by intermittent jumps after attaining steady state, suggesting operation of thermal pressurization (TP). A novel fluid-flow system, allowing for parallel measurements of permeability, porosity and specific storage has been developed. Strong pore fluid pressurization induced by elevated confining pressure was observed during the porosity measurements. Analogical analysis of this compaction-induced pressurization succeeded in predicting the pore pressure build-up for a faulting process. The measurements revealed that the fault zone consists of low-permeability fault gouges (2.6×10-20 m2 at 165 MPa) and high-permeability damaged-zone rocks. The fault gouges and intact country rocks act as barriers to fluid flow across the fault, whereas the damaged zone acts as a fluid conduit, hence the fault zone manifests a "conduit/barrier" hydrological structure. With the lab data as input, this work performed numerical modeling of coseismic slip weakening including TP and mineral decomposition. The results indicate that fluid pressurization played an important role during the Wenchuan earthquake at the exposure site, where dynamic stress reduction was strongly enhanced by increase of pore pressure due to frictional heating and smectite dehydration. The modeling further suggests less importance of high-velocity weakening compared with weakening due to pore fluid pressurization. In a nutshell, the experimental and modeling results as well as the microstructure observed, all suggest that thermochemical pressurization has been an important slip-weakening mechanism during the Wenchuan earthquake rupture. The dramatic weakening predicted may explain the large coseismic displacements and rupture accelerations associated with earthquake rupture at the study site.