HR: 0800h
AN: S41B-1001    [Abstracts]
TI: Depth Dependence of the Fault Strength in the Creeping Section of the Atotsugawa Fault, Japan
AU: * Mizoguchi, K
EM: mizo@bosai.go.jp
AF: National Research Institute for Earth Science and Disaster Prevention, 3-1, Tennodai, Tsukuba, 305-0006 Japan
AU: Fukuyama, E
EM: fuku@bosai.go.jp
AF: National Research Institute for Earth Science and Disaster Prevention, 3-1, Tennodai, Tsukuba, 305-0006 Japan
AU: Kitamura, K
EM: keigo-kitamura@aist.go.jp
AF: National Institute of Advanced Industrial Science and Technology, Central 7, 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AU: Takahashi, M
EM: miki.takahashi@aist.go.jp
AF: National Institute of Advanced Industrial Science and Technology, Central 7, 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AU: Masuda, K
EM: koji.masuda@aist.go.jp
AF: National Institute of Advanced Industrial Science and Technology, Central 7, 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AB: The Atotsugawa fault is located along a highly deformed region in central Japan with 60km long, striking to N60°E and dipping to 90° ± 10°. From the laser distance measurement survey, a creeping section (1.5mm/y) was found in the northeastern part [Geogr. Surv. Inst., 1997]. In this section, a low seismicity area down to a depth of 7km was found above the seismically active region down to 15 km [Ito and Wada, 1999]. In order to investigate the depth dependent feature of the fault strength, we conducted tri-axial friction tests of the Atotsugawa fault gouge under the conditions of 1, 3, 5 and 7km depth. The NIED drilled a borehole in the fault zone down to a depth of 350m in this creeping section [Omura et al., 2004] and obtained core samples consisting of fault gouge, fault breccia and fractured host rocks (granitic rocks and hornblende gneiss). The samples are taken in the gouge zone (8.5mm in thickness) located at a depth of 342 m. The samples were disaggregated in distilled water and passed through a 100μm diameter sieve for the friction tests. From the XRD analysis, the gouge sample consists of quartz, feldspar, smectite, kaolinite and micas. The average grain size in the sample was approximately 16.9μm measured by a laser diffraction particle size analyzer. The friction tests were run using a gas-medium tri-axial apparatus at the AIST, Japan [Masuda et al., 2002]. For each run, 0.5g gouge powder was put between 30° sawcut of an alumina ceramic cylinder (20mm in diameter) and sheared at a constant axial slip rate of 0.1μm/s. Each test was done with pore fluid of distilled water at the temperature-pressure conditions of 1-7 km depths assuming a hydrostatic pore-pressure gradient of 10MPa/km, a lithostatic confining pressure gradient of 26MPa/km and a geothermal gradient of 30°C/km. In all experiments, the friction increases rapidly to an axial displacement of about 0.1mm, and then it gradually increases or becomes steady state. We found a strong depth dependence of friction; it increases from 0.25 - 0.3 at 1km to 0.5 at 7km. We need additional experiments to obtain a physical explanation on this depth dependence. However this result gives us useful information for the creeping motion observed at the Atotsugawa fault. If the creeping motion terminates at a depth around 7km corresponding to the lower boundary of the seismic gap, the frictional strength deeper than 7km should be equal to or more than the shear stress applied to the fault from the tectonic stress field. As far as a linear dependence of stress along the depth is assumed, the ratio of shear stress to effective normal stress (normal stress - hydrostatic pore pressure) on the fault should be constant. At shallow part where the friction is smaller than 0.5, the fault cannot sustain the applied shear stress and it is forced to slip. Slip along the fault results in a decrease in the applied shear stress. We modeled the creeping motion along the fault to balance the applied stress and the depth dependent fault strength obtained by the experiments. The obtained model is that at a depth of 0-2km the fault creeps at a rate of 1.5mm/y and it decreases down to 0.75mm/y at a depth of 6-8km.
DE: 8034 Rheology and friction of fault zones (8163)
SC: Seismology [S]
MN: Fall Meeting 2005