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[SCG52-P07] Relationship between frictional properties of minor faults in the Sanin strain concentration zone and seismic activity
Keywords:2000 Western Tottori Earthquake, Friction experiments, Fault gouge
[Introduction]In the Chugoku region, there is a seismic zone along the coast of the Sea of Japan (Nakao et al., 1991, Annual Report of Disaster Prevention Research Institute, Kyoto University), and the orientation of the maximum principal compression axis there is rotated by about 30 relative to the E-W of the entire Chugoku region (Kawanishi et al., 2009, JGR). This is thought to be due to the right lateral displacement of the lower crust directly beneath the seismic zone, and in order to reproduce it quantitatively, it is necessary to reduce the differential stress in the upper crust. However, there is no evidence in this region to suggest high pore fluid pressure, which is generally assumed to be a factor in the reduction of differential stress (Iio et al., 2018, Tectonophysics). Mori (2020, thesis) conducted friction experiments using fault gouge from the 2000 Tottori-ken Seibu earthquake aftershock area, and showed that clay minerals may be a factor in the weakening of the upper crust. However, to determine the cause of the weakening, it is necessary to determine the friction coefficients in other regions. In addition, the materials in the fault may show velocity weakening. In addition, geodetic observations show that the rupture process of the 2000 Tottori-ken Seibu earthquake had regional characteristics (Iwata and Sekiguchi, 2002, Monthly Earth Bulletin).
In this study, we investigate the influence of clay minerals on the strength of the shallow upper crust (shallower than 4 km) and the relationship with seismic activity based on friction experiments using fault gouge in small faults in the Sanin strain concentration zone.
[Experimental Methods]The samples were divided into three sections in western Tottori Prefecture: northern, central and southern, and one sample was collected from the northern section, three from the central section and one from the southern section. One sample was collected from Kamata outcrop in central Tottori Prefecture, and one sample was collected from Satani outcrop along the main fault and lineaments of the Kano and Iwatsubo faults in eastern Tottori Prefecture. The samples were subjected to quantitative mineral analysis based on powder X-ray diffraction. The friction coefficient and velocity dependence were determined using a triaxial compression test machine owned by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC). The experimental conditions were 26 to 74 MPa of sealing pressure, 10 MPa of pore pressure, and 45 to 105 , respectively, for depths up to 4km.
[Results]Qualitative analysis of the minerals revealed the presence of clay minerals such as smectite. Quantitative analysis of the minerals showed that the sample contained 35.5-83.8 % clay minerals. The steady-state friction coefficient of the samples ranged from 0.12 to 0.32. The friction coefficients of the samples decreased as the ratio of clay minerals in the samples increased. The velocity dependence of friction decreased with depth for almost all the samples and became negative at 3 to 4 km. In the northern part, the velocity dependence of friction was positive for all depth conditions. The frictional strength obtained in this study is significantly lower than that of common rocks (0.6-0.85). It is also harmonious with the friction coefficient (less than or equal to 0.2) obtained from seismic data by Yukutake et al. (2020, Tectonophysics). The velocity dependence of the friction obtained in this study is considered to be a result reflecting the relationship between high-speed sliding at depth and shallow-reaction sliding. In the northern part of western Tottori Prefecture, the velocity dependence of friction is positive for all depth conditions due to the characteristic occurrence of halloysite. This frictional property may have played a role as a barrier to suppress the propagation of slip during earthquakes.
In this study, we investigate the influence of clay minerals on the strength of the shallow upper crust (shallower than 4 km) and the relationship with seismic activity based on friction experiments using fault gouge in small faults in the Sanin strain concentration zone.
[Experimental Methods]The samples were divided into three sections in western Tottori Prefecture: northern, central and southern, and one sample was collected from the northern section, three from the central section and one from the southern section. One sample was collected from Kamata outcrop in central Tottori Prefecture, and one sample was collected from Satani outcrop along the main fault and lineaments of the Kano and Iwatsubo faults in eastern Tottori Prefecture. The samples were subjected to quantitative mineral analysis based on powder X-ray diffraction. The friction coefficient and velocity dependence were determined using a triaxial compression test machine owned by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC). The experimental conditions were 26 to 74 MPa of sealing pressure, 10 MPa of pore pressure, and 45 to 105 , respectively, for depths up to 4km.
[Results]Qualitative analysis of the minerals revealed the presence of clay minerals such as smectite. Quantitative analysis of the minerals showed that the sample contained 35.5-83.8 % clay minerals. The steady-state friction coefficient of the samples ranged from 0.12 to 0.32. The friction coefficients of the samples decreased as the ratio of clay minerals in the samples increased. The velocity dependence of friction decreased with depth for almost all the samples and became negative at 3 to 4 km. In the northern part, the velocity dependence of friction was positive for all depth conditions. The frictional strength obtained in this study is significantly lower than that of common rocks (0.6-0.85). It is also harmonious with the friction coefficient (less than or equal to 0.2) obtained from seismic data by Yukutake et al. (2020, Tectonophysics). The velocity dependence of the friction obtained in this study is considered to be a result reflecting the relationship between high-speed sliding at depth and shallow-reaction sliding. In the northern part of western Tottori Prefecture, the velocity dependence of friction is positive for all depth conditions due to the characteristic occurrence of halloysite. This frictional property may have played a role as a barrier to suppress the propagation of slip during earthquakes.