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

[SCG45-P13] Internal structure and frictional properties of faults in shallow portion of subduction zones: the Sengen Fault, southern Miura Peninsula, central Japan

*Kasai Yuki¹, Akito Tsutsumi¹, Yuzuru Yamamoto² (1.Graduate school of science, Kyoto university, 2.Graduate school of science, Kobe University )

Keywords:very low frequency earthquakes

Fault material had been considered to have positive velocity dependence of friction under 3-4km depth (Scholz, 1998). However, recent seismological observation indicates that very low frequency earthquakes (VLF) occur at shallow accretionary prism. VLF is considered to occur at décollement and imbricate thrust in the accretionary prism (Ito, Obara, 2006). The occurrence of VLF implies the fault materials have negative velocity dependence of friction. In fact, some fault gouge collected from the Nankai accretionary prism indicates negative velocity dependence (Ikari and Saffer, 2011, Tsutsumi et al., 2011). Research of the internal structure and the frictional properties of faults in accretionary prism has been conducted mainly by using deep sea drilling samples, though we are not able to collect much amount of sample and observe fault structure directly. In this study, we focused on the Sengen thrust exposed in southern Miura peninsula, an imbricate thrust. The Sengen thrust cut accretionary prism named Misaki formation, deposited 10-6 Ma (Yamamoto et al., 2005). The Sengen thrust is the branch of the Jogashima thrust, which branches from decollement (Yamamoto et al., 2005). Therefore, Sengen thrust and the out-of-sequence thrust in Nankai have same feature that they are branches from decollement. In this study, we firstly observed the internal structure of the Sengen thrust and the examined the frictional properties of the Sengen thrust. The fault core of the Sengen thrust is about 20-mm-thick fault gouge. Hanging wall side of the gouge is fault breccia, which contains clasts. The size of clasts increases with the distance from the gouge. The footwall side of the Sengen thrust is damage zone. Damage zone is distinguished from the host rock by the existence of cracks. The observed fault structure is consistent with the observation reported in Yamamoto et al [2005]. We find the alignment of fine-grained shear zone, clay minerals and the long axis of < 1mm-sized clasts in the gouge. We conducted XRD analysis of the gouge. XRD pattern indicates the existence of smectite. The observed fault-core structure suggests that past fault displacement and intense deformation has localized into the gouge. We conducted velocity step frictional test using the gouge material. We conducted the test using 1.00 g of the gouge, which was gently disaggregated until it passes #150 mesh sieve. To prepare a wet condition, we added 1.00 ml distilled water to the gouge sepecimen. The step in the imposed slip rate was from few μm/s to 10 μm/s. The axial load was set constant at 5 MPa. Experimental result reveals that the gouge from the Sengen fault shows positive velocity dependence of friction. The velocity-strengthening property of the fault at this slip velocities would act to lower the maximum slip velocity of a slip event. Velocity step experiment at lower velocity is needed to reveal the whole behavior of the Sengen thrust.