[S-CG45] Science of slow-to-fast earthquakes

[SCG45-P04] Frictional properties of an imbricate thrust in the late Miocene Miura accretionary complex: implication for the faulting behavior along the Sagami Trough

*Akito Tsutsumi¹, Yuki Kasai¹, keigo Tashiro¹, Gonghui Wang², Yuzuru Yamamoto³ (1.Graduate School of Science, Kyoto University, 2.Disaster Prevention Research Institute, Kyoto University, 3.Graduate School of Science, Kobe University)

Keywords:friction of fault, frictional velocity dependence, Sagami Trough

In the Sagami Trough area, two regions of shallow SSEs with high cumulative slip have been observed in the same depth range as the large inter-plate earthquake along the trough (Nishimura, 2021). Identifying the factors that produce such a spatial distribution of different types of earthquakes is important to understand the mechanism of the SSEs. In the upper Miocene Misaki formation of the Miura-Boso accretionary complex, Miura Peninsula, shallow deformation structure during subduction at the Sagami Trough are preserved owing to its burial depth of only ~1 km depth (Yamamoto et al., 2005). To explore the frictional properties of the material that could represent the incoming inter-plate fault-zone material at the shallow SSE regions, we conducted two types of deformation experiments on the samples collected from the Misaki formation: velocity-step friction experiments and torque-controlled large ring-shear experiments. Experimental samples used for the velocity-step tests are collected from the alternating hemipelagic silt and tuff layer and from the Sengen thrust, which develops in the Misaki formation. The Sengen thrust is considered to be a branch of décollement (Yamamoto et al., 2005). The velocity step experiments were conducted under a 5 MPa normal stress in wet conditions at the slip velocities ranging from ~0.01 μm/s to ~3 mm/s. The large ring-shear experiments were performed under either undrained or drained conditions, using the machine designed for studying landslides (Sassa, et al., 2004).
In the velocity-step tests, all the samples tested thus far show negative velocity dependence of friction for the range of velocities V from ~1 to ~10 μm/s (in terms of the up-step velocity of the velocity-step tests). The hemipelagic siltstone, which hosts the Sengen thrust, shows positive velocity-dependence at the lowest velocity of V = 1 μm/s initially, then shows a transition from positive velocity dependence to negative dependence as slip proceeds. The large ring-shear tests on the hemipelagic siltstone performed under the undrained tests showed that the increase in pore-fluid pressure due to fault deformation before the initiation of fault slip is of the same magnitude as the increase in pore-fluid pressure during slip. Considering the SSEs distribution along the Sagami Trough, the estimated negative velocity dependence of the material collected from the Misaki formation suggests inhomogeneous pore-pressure distribution along the plate interface.