日本地球惑星科学連合2022年大会

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[J] 口頭発表

セッション記号 S (固体地球科学) » S-CG 固体地球科学複合領域・一般

[S-CG52] 変動帯ダイナミクス

2022年5月27日(金) 10:45 〜 12:15 301A (幕張メッセ国際会議場)

コンビーナ:深畑 幸俊(京都大学防災研究所)、コンビーナ:岩森 光(東京大学・地震研究所)、大橋 聖和(山口大学大学院創成科学研究科)、座長:大橋 聖和(山口大学大学院創成科学研究科)、岡田 知己(東北大学大学院理学研究科附属地震・噴火予知研究観測センター)

11:30 〜 11:45

[SCG52-10] プレート境界断層の高密度ランダム組織:力学実験からの検証

*山本 由弦1、鈴木 雄大2、栗本 悠平3、張 鋒2、阪口 秀4 (1.神戸大学、2.名古屋工業大学、3.清水建設、4.笹川平和財団海洋政策研究所)

キーワード:プレート境界断層、圧密試験、海洋掘削

The initiation of décollement at plate boundaries is a fundamental geological process that makes mountain belts, and controls subduction zone geometries. ODP (Ocean Drilling Program) and IODP (Integrated Ocean Drilling Program 2003-2013; International Ocean Discovery Program after 2013) expeditions have penetrated the shallow portions of décollements at plate boundaries and identified unique fabrics within. That is, the shallow parts of décollement zones at Nankai (off Muroto Peninsula, Japan) and Costa Rica (off Nicoya Peninsula) are composed of fragmented mudstone which possess a higher bulk density. Although the surfaces of fragments are represented by slickenlines and narrow zones of clay-minerals with preferred orientations, the core of the fragments is characterized by random fabrics. The identification of these characteristics is quite notable, if contrary, because it requires densification to occur without the formation of shear-induced fabrics within highly sheared fault zones. Similar fabrics are also reported from ancient shear zones at plate boundaries such as the Northern Apennines, Italy and Boso Peninsula, Japan. Our hypothesis is that weakly cemented intervals developed at our reference sites, and these intervals were sensitive to the fluctuations of effective stress that occurred due to earthquakes or fluid flow along plate boundaries. In fact, at some of the aforementioned reference sites décollement-zones have equivalent horizons that are characterized by weak inter-granular cementation.
To verify the hypothesis, we established a new-type of experimental apparatus which achieves fluctuations in consolidation stress of up to 5 Hz and overburden pressures up to 80 MPa. Then we conducted classical consolidation and dynamic consolidation tests and examined the fabric changes before and after the experiments using Anisotropy of Magnetic Susceptibility (AMS). The former test applied up to 80 MPa (0.01%/min strain rate) and obtained consolidation yielding stress (Pc). The latter applied 0.94 Pc ± 0.68 Pc (0.5Hz), with the number of fluctuations applied being 1000, 5000, and 10000. Specimens were taken from just above the décollement zone (IODP Hole C0023A) and its equivalent interval at a the the reference site (ODP Hole U1173A).
After the classical consolidation tests, specimens exhibited obvious flattening fabrics due to consolidation under uniaxial conditions. By comparison, after the dynamic tests, specimens retained their original fabric despite the accumulated strain being greater than 10%. More interestingly, overconsolidation states are apparent under the original confining pressure and are observed after the experiments. Therefore, we suggest that densification with a random fabric is a characteristic of the shallow part of décollement zones, and is produced by fluctuation of effective stress during the primary accretion processes.