報告書番号

N06030

タイトル（和文）

2004年新潟県中越地震と地質構造の関係 －活褶曲地域における震源断層評価－

タイトル（英文）

Relationship of the 2004 Mid-Niigata Prefecture Earthquake with Geological Structure - Evaluation of Earthquake Source Fault in Active Folding Zone -

概要 （図表や脚注は「報告書全文」に掲載しております）

新潟県中越地震における臨時余震観測を通じて，活褶曲地域における震源断層を評価するための留意点を整理した．新潟県中越地震の余震分布は，北北東-南南西方向に伸び，両端部が狭くなる紡錘状の形態を呈する．この傾向は，震源域中部で複背斜が発達し，両端部で単背斜となる褶曲軸の分布形態とよく一致する．本震を含む西傾斜の震源分布面は東山背斜の軸直下に，最大余震を含む断層面は田麦山背斜と小松倉背斜の直下に位置し，この一群の余震分布の東縁が諏訪峠撓曲の位置とよく一致する．したがって，現在の褶曲構造は一連の断層運動の累積変位によって形成されたと考えられる．すなわち，震源断層の長さを評価する上では，撓曲構造を伴う褶曲軸の分布形態がポイントになる．また，断層関連褶曲を模擬した有限要素解析を実施し，堆積層厚や断層傾斜角の変化が褶曲構造の形態に及ぼす影響を評価した．逆断層運動はその上の堆積層に，断層の上端側が急勾配となる非対称な背斜構造を形成する．このとき，堆積層厚が増加するほど，背斜軸は上盤側に移動する．また，断層傾斜角が低角になるほど，褶曲構造の波長・非対称性が増大する．このように，褶曲構造は地下の断層運動を反映しているため，その形状が地下の伏在断層を推定するための指標になる．

概要 （英文）

We compile the important points to evaluate earthquake source fault in active folding zone through a temporary aftershock observation of the 2004 Mid-Niigata earthquake. The aftershock distribution shows spindle shape whose middle part is wide and both ends are narrow in NNE-SSW trending. The range of seismic activity corresponds well to the distribution of fold axes in this area, whose middle part is anticlinorium (some anticlines) and both ends are single anticline. In the middle part, the west dipping aftershock plane including the mainshock (M6.8) is located under the Higashiyama anticline. Another west dipping aftershock plane including the largest aftershock (M6.5) is located under the Tamugiyama and Komatugura anticlines, and the east margin of the aftershock distribution corresponds well with Suwa-toge flexure. Therefore the present fold structure should have been formed by an accumulation of the same faults movement. In other words, it is important to refer the fold axes distribution pattern, especially with flexure, for the evaluation of earthquake source fault. In addition, we performed FEM analyses to investigate the relation of fold structure to the thickness of the sedimentary layer and the dip angle of the fault. Reverse fault movement forms asymmetric fold above the fault, which steeper slope is formed just above the upper end of the fault. As the sedimentary layer became thicker, anticline axis moved to hanging wall side in the fold structure. As the dip angle became smaller, the wavelength of the fold became longer and the fold structure grew highly asymmetric. Thus the shape of the fold structure is useful as an index to estimate the blind thrust below it.

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