比較的規模の大きな地下活断層の特徴とその調査手法の検討

A Feasiblity study on research techniques for subsurface active faults

木下 博久; 野原 壯   ; 中田 高*; 池田 安隆*; 伊藤 潔*; 大槻 憲四郎*; 鷺谷 威*; 高田 圭太*; 遠田 晋次*

Kinoshita, Hirohisa; Nohara, Tsuyoshi; Nakata, Takashi*; Ikeda, Yasutaka*; Ito, Kiyoshi*; Otsuki, Kenshiro*; Sagiya, Takeshi*; Takada, Keita*; Toda, Shinji*

活断層のうち比較的規模の大きい地震を発生させるが顕著な地表地震断層を伴わないものを新たに「地下活断層」と呼び、2000年鳥取県西部地震を事例にそれらの分布の特徴と調査手法を検討した。数値解析に基づく地形地質調査の結果、地下活断層の存在の可能性やその方向,活動履歴などの推定根拠となりうると考えられる地形地質学的特徴が抽出された。

Results of numerical analyses and field studies in the epicenter area of the 2000 Tottoriken-seibu earthquake suggest that several research techniques are effective for identifying subsurface active faults unaccompanied with remarkable surface earthquake faults. Field observation of the lineaments found in the area by detailed air-photo interpretation revealed that most of them are accompanied by faults and/or dikes, and some of them show minor slips at the time of the earthquake. Crustal deformation caused by the activity of the subsurface seismogenic faults is deduced by numerical analyses based on seismic and geodetic data. The vertical deformation assumed from the height changes of the terraces along the river across the epicenter area suggests that cumulative uplift and subsidence associated with left-lateral strike-slip has been continuing. Offsets rate of the streams and uplift rate of the river terraces surfaces show good agreement with the estimate from the parameter of the earthquake faults model. Identification of active faults without clear surface evidence before occurrence of earthquakes is a difficult issue, and the techniques adopted in this study will probably provide relevant information effective to locate subsurface active faults.