[1] We determined the internal structure and mineral composition of the Yingxiu-Beichuan fault zone at the Zhaojiagou exposure and measured frictional and transport properties of the fault rocks collected to gain a better understanding of dynamic weakening mechanisms during seismic fault motion. This fault is a major fault in the Longmenshan fault system that caused the 2008 Wenchuan earthquake. The exposure studied is located midway along the surface rupture, close to where the largest coseismic displacement occurred. High-velocity friction experiments reveal exponential slip weakening from a peak friction toward a steady state value. Slip weakening is more pronounced for water-dampened gouge than dry gouge, suggesting thermal pressurization. The fault gouge has a very low permeability (< 10 À21 m 2 at 165 MPa effective pressure) and is surrounded by fault breccia with a permeability of 10 À19 to 10 À17 m 2 , grading into less permeable, fractured country rocks. The fault zone thus exhibits a "conduit/barrier" structure, allowing fluid flow only in the breccia zone. We numerically modeled coseismic slip weakening including thermal pressurization and mineral dehydration/decarbonation, basing our calculation on measured frictional and transport properties and on the slip history inferred for the Wenchuan earthquake. The results indicate that (1) thermochemical pressurization played an important role in causing dynamic slip weakening, (2) the slip-weakening distance is similar to the seismologically determined values, and (3) pore pressures might have exceeded the normal stress, thus maintaining temperatures below 600°C. Interestingly, enough heat was generated to fully remove and thermally pressurize the interlayer water from smectite, contributing an excess pore pressure of~6 MPa. In addition, we found that the incorporation of state-dependent fluid properties predicts much more efficient fluid pressurization than using constant properties. The dramatic weakening predicted probably offers a compelling explanation for the large coseismic displacement and slip acceleration observed near Beichuan city.

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Fluid infiltration within fault zones is an important process in earthquake rupture. Magnetic properties of fault rocks convey essential clues pertaining to physicochemical processes in fault zones. In 2011, two shallow holes (134 and 54 m depth, respectively) were drilled into the Yingxiu-Beichuan fault (Longmen Shan thrust belt, China), which accommodated most of the displacement of the 2008 Mw 7.9 Wenchuan earthquake. Fifty-eight drill core samples, including granitic host rock and various fault rocks, were analyzed rock-magnetically, mineralogically, and geochemically. The magnetic behavior of fault rocks appears to be dominated by paramagnetic clay minerals. Magnetite in trace amounts is identified as the predominant ferrimagnetic fraction in all samples, decreasing from the host rock, via fault breccia to (proto-)cataclasite. Significant mass-losses (10.7-45.6%) are determined for the latter two with the ''isocon'' method. Volatile contents and alteration products (i.e., chlorite) are enriched toward the fault core relative to the host rocks. These observations suggest that magnetite depletion occurred in these fault rocksexhumed from the shallow crust-plumbed by fluid-assisted processes. Chlorite, interpreted to result from hydrothermal activity, occurs throughout almost the entire fault core and shows high coefficients of determination (R 2 > 0.6) with both low and high-field magnetic susceptibility. Close relationships, with R 2 > 0.70, are also observed between both low and high-field magnetic susceptibility and the immobile elements (e.g., TiO 2 , P 2 O 5 , MnO), H 2 O 1 , and the calculated mass-losses of fault rocks. Hence, magnetic properties of fault rocks can serve as proxy indicators of fluid infiltration within shallow fault zones.

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