The record of physical processes that occur during seismic slip events is well preserved in fault rocks from the active Nojima fault in Japan. The fault rocks formed at about 3 km depth, and comprise thin alternating layers of very fine gouge and pseudotachylyte derived from granite. Each layer is thinner than a few millimeters, and corresponds to one seismic slip event. The very thin slip zone width suggests that some mechanisms of slip weakening operated, and our studies of the fault rocks suggest that fluidization and melting of gouge were particularly important. Fluidized and nonfluidized gouges were distinguished using the detection probability of fragmented counterparts method. It is known from granular material science that the phase transition from a grain friction regime to fluidization of granular materials can occur only by a very small decrease in volume fraction of solid grains. Once fluidization occurs, the frictional resistance decreases abruptly to nearly zero even before thermal pressurization reaches its extreme state. When fault gouge is melted, frictional resistance is governed by the viscosity of melt which depends mainly on temperature, chemical composition, H2O concentration and the volume fraction of unmelted grains. For each pseudotachylyte layer, we estimated the temperature of melt using various temperature indices, and measured volume fraction of unmelted grains. We synthesized these data to reconstruct the change in viscosity during seismic slip events. The melt viscosity is very high (107–9 Pas) during initial melting due to the combined effect of low temperature (750°–800°C) and large volume fraction of solid grains. Thus seismic slip is inevitably restrained instantaneously. Once this mechanical barrier is overcome, the viscosity reduces continuously and dramatically as slip increases. At 1000°C, the viscosity reduces to 104Pas and eventually to 10Pas at 1280°C. Thus stress drops almost completely and rupturing tends to run away.

We report the viscoelasticity of the thin film of aqueous NaCl solution confined between mica surfaces measured by shear resonance apparatus. The observed shear resonance curves at separations less than ca. 2 nm indicated that the solution exhibits the high lubrication effects under some loads. The effective viscosity (0.1-10 Pa s) obtained for the separations less than 1 nm from a mechanical model was 2-4 orders of magnitude larger than the bulk value. Our study employing a novel shear measurement provided a comprehensive picture for the dynamics of confined water thinner than a few nanometers.

The Taiwan Chelungpu‐fault Drilling Project penetrated three fault zones as the Chelungpu fault system, which slipped during the 1999 Chi‐Chi earthquake, discovering disk‐shaped black material (BM disk) within the middle and lower fault zones in Hole B. The microscopic features of the BM disks indicated that they were pseudotachylytes, and they showed high magnetic susceptibility, possibly the result of intense shearing or high temperature conditions. Inorganic carbon content of the BM disks was low, possibly because of thermal decomposition of carbonate minerals. The high temperatures might be related to frictional heating during the earthquake, implying that the BM disks were produced under intense shearing with frictional heating that reached melting temperature. Because the disks, which provide the only evidence of melting, pre‐date the 1999 earthquake, we concluded that frictional melting did not occur during the earthquake.

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The Izanagi plate subducted rapidly and obliquely under the accretionary terrane of Japan in the Cretaceous before 85 Ma. A chain of microcontinents collided with it at about 140 Ma. In southwest Japan the major part of it subducted thereafter, but in northeast Japan it accreted and the trench jumped oceanward, resulting in a curved volcanic front. The oblique subduction and the underplated microcontinent caused uplifting of high-pressure (high-PI metamorphic rocks and large scale crustal shortening in southwest Japan. The oblique subduction caused left-lateral faulting and ductile shearing in northeast Japan. The arc sliver crossed over the high-temperature (high-T) zone of arc magmatism, resulting in a wide high-T metamorphosed belt. At about 85 Ma, the subduction mode changed from oblique to normal and the tectonic mode changed drastically. Just after this the KulafPacific ridge subducted and the subduction rate of the Pacific plate decreased gradually, causing the intrusion of huge amounts of granite magma and the eruption of acidic volcanics from large cauldrons. The oblique subduction of the Pacific plate resumed at 53 Ma and the left-lateral faults were reactivated.