Frictional and transport properties of the 2008 Wenchuan Earthquake fault zone: Implications for coseismic slip-weakening mechanisms

Abstract

Abstract This paper reports on the frictional and transport properties of fault rocks collected from a surface exposure associated with the 2008 Wenchuan earthquake. The water-dampened gouges showed 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. Our 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 displays a “conduit/barrier” hydrological structure. With our lab data as input, we 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. Our modeling further suggests less importance of high-velocity weakening compared with weakening due to pore fluid pressurization. Taken together, our 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.