日本地球惑星科学連合2018年大会

講演情報

[EJ] ポスター発表

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-CG57] 変動帯ダイナミクス

2018年5月23日(水) 15:30 〜 17:00 ポスター会場 (幕張メッセ国際展示場 7ホール)

コンビーナ:深畑 幸俊(京都大学防災研究所)、竹下 徹(北海道大学大学院理学院自然史科学専攻)、岩森 光(海洋研究開発機構・地球内部物質循環研究分野)

[SCG57-P22] 2000年鳥取県西部地震余震域における断層系の発達過程と古応力場の解明

*内田 嗣人1,2向吉 秀樹1藤内 智士3小林 健太4金木 俊也5廣野 哲朗5 (1.島根大学大学院総合理工学研究科地球資源環境学領域、2.株式会社四国総合研究所土木技術部、3.高知大学理学部応用理学科、4.新潟大学理学部地質学教室、5.大阪大学大学院理学研究科宇宙地球科学専攻)

キーワード:断層発達史、古応力場、断層K-Ar年代

Fault topography in aftershock area of the 2000 Western Tottori Earthquake was poorly recognized. There are little information of earthquake in the poor topographic region and paleo fault activity may help to the sesmic evalustion. It is important to understand deformation process of fault systems in the poor fault topography for mitigation of geological hazard.

In this study, we discuss the paleostress fields and fault ages around aftershock area of the 2000 Western Tottori Earthquake. We adopted Hough transform inverse method to estimate the paleostress fields because of using incomplete fault-slip data (Sato, 2006) and we determined K-Ar dating of fault gouges in the aftershock area. The fault gouge contains auhigenic illite related to fault activity. It is difficult to separate the only authigenic illite from fault gouge with muscovite and detrital mica. Therefore, we conducted illite polytype analysis by the XRD patterns and evaluated mixure rate of muscovite and detrital mica. The 100% authigenic illite is considered as timing of fault activity in this study.

The Kawai and the Kuri formations (19-15Ma) and the Omori Formation (16-13Ma) is distributed in the northern area. The granitic rocks (65Ma) called ‘Neu Granitc Pulton’ is widely exposed around the central and southern area.

As a result of the paleostress analysis, we detected reverse faulting stress regime with NNW-SSE σ1 and high ratio in the northern area. This stress state is dominant after 13Ma and consistent with the stress field of formation of the Shinji Folded Zone. Two stress states, one is stress states Strike-slip stress regime with E-W σ1, N-S σ3 and intermediate stress ratio and the other is Strike-slip stress regime with N-S σ1, E-W σ3 and intermediate and high stress ratio in the Neu granite was also detected. The former is concordant with the contemporary stress in a whole Chugoku region (Kawanishi et al., 2009). And the latter is consistent with the stress regime which formed NE-SW trending geological faults distributed in Chugoku region (Kanaori, 1990). The stress change from N-S σ1, E-W σ3 to E-W σ1, N-S σ3 which was revealed from cutting-relationships between the dikes of the Yokota monogenetic volcanoes origin and faults considered to be caused when volcanic activity of the Yokota monogenetic volcano was occurred.
We constrained the timing of the authigenic illite in the fault gouge is approximately 22.8Ma from K-Ar age dating and XRD patterns. In the presentation, we discuss relationship between the timing of fault activities and palaestress fields.