[S-CG40] Science of slow-to-fast earthquakes

[SCG40-P29] The frictional properties of an imbricate thrust of the Miura accretionary complex for a wide range of slip velocities.

*Yuki Kasai¹, Akito Tsutsumi¹, Yuzuru Yamamoto², Horst Zwingmann¹, Andrew Todd³ (1.Graduate School of Science, Kyoto University, 2.Department of Planetology, Graduate School of Science, Kobe University, 3.CSIRO Energy, Kensington, WA, Australia)

Keywords:Accretionary prism, velocity dependence of friction, friction test, fault dating

Recent seismological observations have revealed that various types of slow earthquakes occur in subduction zones (e.g., Hirose et al., 1999). Some numerical and observational research in the framework of the RSF law claim that the occurrence of SSEs is related to small value of critical stiffness, k_cr induced by high pore fluid pressure along a fault with negative velocity dependence (Shibazaki, 2008; Kodaira et al., 2004). In order to examine if the friction velocity dependence of a fault within shallow accretionary complex can be negative, we conducted velocity step friction test using fault and the host rock samples collected from the Sengen thrust, which develops in the late Miocene Miura accretionary complex, Misaki formation. The Sengen thrust is considered to be a branch of décollement (Yamamoto et al., 2005). The depositional age of the Misaki formation is considered to be 10-6 Ma (Kanie et al., 1991; Kawakami, 2001; Yamamoto et al., 2005), and the maximum experienced temperature is estimated to be 52±15 (Yamamoto et al., 2005). The Sengen thrust consists of three parts; fault gouge, fault breccia in hanging wall and shear-band-zone in footwall (Yamamoto et al., 2005). Fault gouge and breccia contains quartz, plagioclase, smectite, illite, kaolinite. XRD analysis revealed that qualitative difference of mineral composition between gouge and breccia was not detected. The velocity step experiments were conducted under a 5 MPa normal stress in wet condition (i.e., 0 MPa pore pressure) at the velocities ranging from ~10^-2 µm/s to few mm/s. Gouge showed both negative and positive velocity dependence of friction at the velocities of ~1 to ~10 μm/s, however, only negative velocity dependence of friction at the velocities of ~10 μm/s to ~1 mm/s. Host rock showed transition of velocity dependence of friction positive to negative as slip proceeds at the velocities of ~10^-2 to ~10^-1 µm/s. We recorded only negative velocity dependence of friction for breccia and shear-band-zone samples at the velocities of ~1 μm/s to ~1 mm/s. The estimated negative velocity dependence of the Sengen fault material indicate that the proposed mechanism of slow earthquakes within the context of conditionally stable fault at high pore pressure is applicable at shallow depth of subduction zones.
We also conducted K-Ar dating of the Sengen thrust. We will report ongoing result of the dating in this presentation.