[S-SS30_29PO1] New perspective of great earthquakes along subduction zones

[SSS30-P24] Friction constitutive properties of shallow subduction zone material as estimated from rotary shear friction experiments

*Ryuji NAKANO¹, Yuka NAMIKI¹, Akito TSUTSUMI¹ (1.Graduate School of Science, Kyoto University)

Keywords:friction, subduction zone, rate- and state- friction constitutive law, Nankai Trough, Costa Rica

In order to understand the dependence of constitutive parameters, a, b, and Dc, on slip velocity, V, we conducted experiments by using a rotary shear high velocity friction apparatus. Samples used in this work were collected from the Nankai accretionary prism, offshore from Kii Peninsula, Japan, at Site C0004 during Integrated Ocean Drilling Program (IODP) Expedition 316 [Expedition 316 Scientists, 2009; Tsutsumi et al., 2011], and from the Costa Rica subduction zone, Cocos Ridge, at Site U1381 during IODP Expedition 334 [Expedition 334 Scientists, 2012 ]. All of the samples from the Nankai accretionary prism are clayey silt, whereas those from Costa Rica can be divided into 2 groups with respect to their composition: one is clayey silt (hereinafter referred to as "Costa Rica Unit I"), the other is silicic to calcareous ooze ("Costa Rica Unit II"). All experiments were carried out at 5 MPa normal stress and 0.0028-2.8 mm/sec slip velocity under wet condition (0.5 g samples with 0.5-0.9 ml distilled water). Moreover, we created a simulation program, which can estimate the values of constitutive parameters and system stiffness, k, with Levenberg-Marquardt method, supposing the spring-block model.The results are summarized as the following: (1) a, b and/or Dc increase with slip velocity; (2) the values are the highest at V = 0.028-0.28 mm/sec; (3) the values are the lowest at V = 0.028-0.28 mm/sec. The reason is not clarified yet, but it is remarkable that, despite the composition, the result of the clayey megasplay fault material from the Nankai accretionary prism resembles the result of Costa Rica Unit II. This implies that, as expected, constitutive parameters depend on not only material but also other conditions. Another remarkable point to be noted is that the values of system stiffness of Costa Rica Unit I decrease by a factor of 10 when compared with the measured apparatus stiffness value. This implies that the mechanical property of the material of Costa Rica Unit I may be more flow-dominated than others. This implies that the mechanical property of the material of Costa Rica Unit I may be more flow-dominated than others. Considering that the samples of the Nankai accretionary prism and Costa Rica contain 20-30 wt%, 60-70 wt% clay, respectively, it is possible that total clay content reflects the gouge behaviour.