HR: 12:05h
AN: S32B-08    [Abstracts]
TI: The Change in Force Chains Inside a Gouge Layer Under Stresses Inferred From Transmission Waves and a Numerical Simulation
AU: * Yoshioka, N
EM: yoshi@yokohama-cu.ac.jp
AF: Yokohama City University, Seto 22-2, Kanazawaku, Yokohama, 236-0027 Japan
AU: Sakaguchi, H
EM: sakaguchih@jamstec.go.jp
AF: Japan Agency for Marine-Earth Science and Technology, 3173-25, Showa-machi, Kanazawaku , Yokohama, 236-0001 Japan
AB: A laboratory experiment was performed to detect nucleation processes by transmission elastic waves across a fault with a gouge layer. The apparatus consists of an upper block, a lower block and a gouge layer between the two blocks. The gouge layer is formed with quartz sand with a given particle size distribution and a given thickness. Normal force is the constant self-weight of the upper block and shear force is applied to the upper block by a linear motor through a leaf spring. Elastic waves are continuously transmitted across the fault throughout the shear stress application up to a final stick-slip event. It is observed that a precursory slip and a dilatancy always precede a stick-slip event. The waveform of the transmitted waves significantly changes as the shear stress is increased even when the horizontal displacement is negligibly small. In particular, the amplitude becomes dramatically small with the onset of precursory slip. These facts suggest that the stress chain inside the gouge layer is changed with the application of shear stress. In order to elucidate the mechanism of the observation, a numerical simulation was performed for the gouge layer using the discrete element method (DEM). The loading procedure in the numerical simulation exactly follows the way of the laboratory experiment. The results show that the precursory slip and vertical movements of the upper block are well reproduced by the simulation. The initial stress chains which are homogeneously distributed in the gouge later are drastically changed by shear stress application. With the increment of shear force, some selected force chains become thicker forming columnar structures in the major principal stress direction (direction of the superimposed force of the shear force and the self-weight of the upper block). Dilation due to rotation of the thick column induces volume expansion and it reduces the number of passes to transmit wave from the lower block to the upper block as a result. The mechanism of the experimental observations is well understood in the light of the numerical simulation.
DE: 5104 Fracture and flow
DE: 5112 Microstructure
DE: 5144 Wave attenuation
DE: 5194 Instruments and techniques
DE: 7223 Earthquake interaction, forecasting, and prediction (1217, 1242)
SC: Seismology [S]
MN: Fall Meeting 2005