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

[SCG45-P04] 後期中新世三浦付加体中のインブリケート衝上断層の摩擦特性：相模トラフにおける断層すべり挙動理解に向けて

*堤 昭人¹、笠井 佑樹¹、田代 圭吾¹、王 功輝²、山本 由弦³ (1.京都大学大学院理学研究科、2.京都大学防災研究所、3.神戸大学大学院理学研究科)

キーワード：断層摩擦、速度依存性、相模トラフ

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.