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

[SCG45-P02] Experimental study on shear-induced liquefaction along a fault in shallow portion of accretionary prism

*Keigo Tashiro¹, Akito Tsutsumi¹, Chengrui Chang¹, Yuzuru Yamamoto², Gonghui Wang³ (1.Graduate School of Science, Kyoto University , 2.Graduate School of Science, Kobe University , 3.Disaster Prevention Research Institute, Kyoto University)

Keywords:liquefaction, fault, shallow portion of accretionary prism

Within the Miura-Boso accretionary complex distributed in the southern Miura and Boso Peninsulas, initial deformation of the accretionary complex in the shallow part of the subduction zone is preserved due to its shallow burial depth (Yamamoto, 2005). In the imbricate thrust faults that develop in this accretionary complex and in the shear deformation concentration zone (thrust unit) immediately above these faults, fault gouge material has been observed to be injected into the hanging wall strata, which has attracted attention as a trace of high pore pressure generation during deformation (Yamamoto et a., 2005; , 2006).The process of pore pressure increase during deformation in fault zones results in a reduction in fault strength due to a decrease in effective stress. Therefore, elucidating the mechanism of such liquefaction is important for understanding the mechanism of strength weakening of faults during earthquakes. In this study, We attempted to clarify the process of pore pressure increase leading to liquefaction and the internal structural development of the fault by conducting shear experiments using accretionary (Misaki Formation) materials distributed in the southern part of the Miura Peninsula. The Misaki Formation consists of alternating layers of scoria sandstone and semi-pelagic siltstone. In this shear experiment, semi-pelagic siltstone was used as the sample. A large ring shear apparatus (Sassa, 2004) at the Disaster Prevention Research Institute, Kyoto University was used for the experiments. This apparatus can perform shear tests under undrained conditions and can spontaneously increase the pore water pressure of the fault by applying shear stress. This allows simulation of the entire rupture process from the increase in pore water pressure under shear stress, reaching the rupture line, to the decrease in shear strength, and finally to steady-state slip. Weathered portions of the samples were removed, dried in a dryer at 60°C for 24 hours, and crushed. The upper and lower sides of the sheared sections filled with semi-pelagic siltstone were filled with silica sand No. 8. The silica sand used was 805 g and the sample was 689 g. Prior to the start of shearing, the pore space (air) in the sample was replaced with carbon dioxide over a period of 6 hours, then filled with water and consolidated at 500 kPa for 1 day. After consolidation was completed, the shear stress was increased at 0.1 kPa/sec, and the failure-line condition was reached at 251.8 kPa. The effective stress decreased from 500 kPa to 320 kPa as the pore water pressure increased during the process of reaching the failure line. After reaching the fracture line, macroscopic rotation (slip) began and pore water pressure increased rapidly. Pore water pressure increased to about 350 kPa and became steady, while shear stress decreased to about 40 kPa. Therefore, the fault strength was reduced by about 84% due to the increase in pore pressure caused by shear slip, resulting in mechanical liquefaction. After the experiment, the collected specimens were impregnated with epoxy resin, and thin sections were made to observe the deformation structure of the shear zone. The particle array and other aspects are being analyzed. In this presentation, we will report the results of our investigation of the differences in structure between the samples sheared at drained and undrained conditions. In the future, we will conduct drained experiments on the same siltstone samples and compare the mechanical behavior and internal structure to further investigate the liquefaction process and the characteristics of the internal structure.