Understanding the physical processes of frictional slip of subduction zone earthquakes from the characteristics of fault rocks.

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Followings were known from the analysis of the core samples from the southern and northern boreholes penetrating the Shelungpu fault, the seismic fault of the 1999 Chi-Chi earthquake in Taiwan.1. At the depth of 176m of the southern borehole, we found the crushed pseudotachylyte layer about 2m thick. Recycled pseudotachylyte grains are common, indicating that seismic slips have repeated in this shear zone. Very small part of grains of crushed pseudotachylyte record high temperature (up to 1730℃) signatures, but major part of grains shows the texture of very low degree of melting.2. At the depth of 223-227m and 283-320m of the northern borehole, we found the several shear zones of the Shelungpu fault. The fault core of these shear zones is composed of soft clay layer of 1-3cm thickness, and it is commonly associated with a layer of about 1m thick fault breccia with mosaic texture. Clay veins from the fault core frequently intrude the fault breccia. This clay injection structure suggests … More the generation of instantaneous and abnormally high pore pressure.3. The southern and northern parts of the Shelungpu fault behaved very differently during the 1999 Chu-Chu earthquake; moderate slip associating the radiation of high frequency wave, while very large and smooth slip lacking the radiation of high frequency wave. These contrasting slip behaviors appear to be consistent with the fault rocks. Namely, the highly viscous melt of very low degree of melting in the south would have restrained the seismic slip, while the slip would have been promoted by the elastohydrodynamic lubrication (Brodsky and Kanamori, 2001) of fault clay layer.We studied the comminution and fluidization of fault gouges through analyzing fault gouges.1. Comminution starts at the fractal dimension 2.5 of grain size frequency, and the fractal dimension increases as further comminution. Fractal granular materials can be packed most densely when the fractal dimension is 2.5. Therefore, comminution itself is a slip-weakening mechanism.2. We developed a new method how to distinguish the gouges whether they have experienced fluidization or not.3. Normal interface vibration by Brune et al. (1993) is a most plausible cause of fluidization, and we presented two field evidences that support this mechanism.4. The knowledge of powder science teaches us that fault plane becomes nearly frictionless when gouge is fluidized. Less