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[1] Frictional properties of natural kaolinite-bearing gouge samples from the Median Tectonic Line (SW Japan) have been studied using a high-velocity rotary shear apparatus, and deformed samples have been observed with optical and electron (scanning and transmission) microscopy. For a slip velocity of 1 m s À1 and normal stresses from 0.3 to 1.3 MPa, a dramatic slip-weakening behavior was observed. X-ray diffraction analysis of deformed samples and additional high-velocity friction experiments on pure kaolinite indicate kaolinite dehydration during slip. The critical slip-weakening distance D c is of the order of 1 to 10 m. These values are extrapolated to higher normal stresses, assuming that D c is rather a thermal parameter than a parameter related to a true characteristic length. The calculation shows that dimensionally, D c / 1/s n 2 , where s n is the normal stress applied on the fault. The inferred D c values range from a few centimeters at 10 MPa normal stress to a few hundreds of microns at 100 MPa normal stress. Microscopic observations show partial amorphization and dramatic grain size reduction (down to the nanometer scale) localized in a narrow zone of about 1 to 10 mm thickness. Fracture energy G c is calculated from the mechanical curves and compared to surface energy due to grain size reduction, and energies of mineralogic transformations. We show that most of the fracture energy is either converted into heat or radiated energy. The geophysical consequences of thermal dehydration of bonded water during seismic slip are then commented in the light of mineralogical and poromechanical data of several fault zones, which tend to show that this phenomenon has to be taken into account in most of subsurface faults and in hydrous rocks of subducted oceanic crust.Citation: Brantut, N., A. Schubnel, J.-N. Rouzaud, F. Brunet, and T. Shimamoto (2008), High-velocity frictional properties of a claybearing fault gouge and implications for earthquake mechanics,

[1] Compressional, shear wave velocities and their ratio, V p /V s , were measured along with porosity variations during wet and dry hydrostatic compaction of Bleurswiller sandstone, a 25% porosity Vosgian sandstone. At first, increase in hydrostatic pressure was accompanied by a simultaneous increase of both V p and V s as expected. At a critical effective confining pressure P*, a large mechanical decrease of porosity was observed that was due to pore collapse and grain crushing. Theoretically, two different processes are affecting the elastic wave velocities in counteracting ways during cataclastic compaction: cracking and porosity decrease. Our experimental results show that cracking is the dominant effect, so that grain crushing and porosity reduction were accompanied by a large decrease in velocities. The ratio V p /V s was also observed to change during our experiments: In the wet specimen, V p /V s value increased from 1.72 to 1.84, while in the dry specimen, it increased from 1.59 below P* to 1.67 beyond P*, respectively. To quantitatively interpret these results, an isotropic effective medium model (EM) was used that considered the sandstone as a mixture of spheroidal pores and penny-shaped cracks. In particular, the increase in V p /V s , in the wet case, is well reproduced and shows the important role played by the mechanical coupling of fluid with low aspect ratio cracks (<10 À2 ). In the dry case, however, our experimental results highlight an increase of V p /V s ratio during cataclastic compaction, in apparent contradiction with the predictions of the EM model. Indeed, increases in V p /V s ratio, and hence in Poisson's ratio, are, in general, attributed to fluid saturation. A closer look to the microstructure may provide a possible interpretation: Beyond P*, grains are no longer cemented. Using Digby's granular model as an alternative model, we were able to reach a quantitative agreement with the experimental results. The possible implication is that in both dry and wet conditions, cataclastic compaction due to grain crushing induces an increase in V p /V s ratio.Citation: Fortin, J., Y. Guéguen, and A. Schubnel (2007), Effects of pore collapse and grain crushing on ultrasonic velocities and V p /V s ,

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[1] We observe the nucleation phase of in-plane ruptures in the laboratory. We show that the nucleation is composed of two distinct phases, a quasi-static and an acceleration stage, followed by dynamic propagation. We propose an empirical model which describes the rupture length evolution: The quasi-static phase is described by an exponential growth while the acceleration phase is described by an inverse power law of time. The transition from quasi-static to accelerating rupture is related to the critical nucleation length, which scales inversely with normal stress in accordance with theoretical predictions, and to a critical surfacic power, which may be an intrinsic property of the interface. Finally, we discuss these results in the frame of previous studies and propose a scaling up to natural earthquake dimensions.