運用河流地形參數探討日本東北奧羽山脈兩側構造系統的活動度

Abstract

日本在板塊交互作用之下，地震活動頻繁，境內分布許多活動斷層與山間盆地。以日本東北奧羽山脈兩側緊鄰的北上低地與橫手盆地為例，此區域具有明顯的構造地形，分別在東西兩側的山前緣，具有北上低地西緣斷層系統 (WFZKL) 與橫手盆地東緣斷層系統 (EFZYB)。由於河流能夠記錄人為因素、氣候條件、岩性差異或是構造作用所產生的地形變化，因此本研究希望運用河流地形參數的方法探討奧羽山脈兩側構造系統的活動特徵。 本研究使用的方法為河流地形參數的χ值與標準化河流陡峭度指標 (k_sn)，其中χ值圖形的斜率，能夠顯示標準化河流陡峭度指標。然而在奧羽山脈的部分河流剖面中具有遷急點，為了探究遷急點是否與構造作用有關，本研究藉由衛星影像、地質圖與實地的野外調查，發現這些遷急點的成因多為水壩建物或是局部岩性差異，可能與構造作用的關連性不大。 在排除遷急點、氣候與岩性對河流地形的影響後，本研究在奧羽山脈東西兩側所得的標準化河流陡峭度指標與流域平均陡峭度指標 (MLS) 具有不同的高低趨勢，於西北側流域比西南側與東側流域高，代表西北側的侵蝕速率與抬升速率具有較快的趨勢，而造成抬升速率較快的原因，可能與構造活動有關，因此推論奧羽山脈西北側的構造活動度比西南側與東側高。這個結果似乎與1896年發生於奧羽山脈西側山前緣的陸羽地震，地表抬升集中於北側的結果可以互相呼應。然而奧羽山脈東南側流域亦具有較東北側略高的標準化河流陡峭度指標與流域平均陡峭度指標，此結果可能與燒石岳 (Yakeishi Dake) 和栗駒山 (Kurikoma Yama) 這兩座第四紀火山噴發使得地形回春有關。綜合本研究結果推論日本東北奧羽山脈能夠適用河流地形參數的方法，若於日後應用此方法於日本其他地區時，應考慮第四紀火山對河流地形參數的影響。

Japan is located at a complex plate boundary between the Okhotsk, Pacific, Philippine Sea, and Eurasian plates. Many volcanic arcs and intra-arc fault systems are developed with the interactions of these plates. Due to the activities of the fault systems, many mountain ranges and intermontane basins developed on the Japan islands. A good example is the Ou Backbone Range in the Tohoku region, with the Kitakami Lowland on its eastern side and the Yokote Basin on its western side. Along each flank of the range there is a major active fault system. On the eastern side is the Western Fault Zone of Kitakami Lowland (WFZKL) and on the western side is the Eastern Fault Zone of Yokote Basin (EFZYB). Previous paleoseismic and geomorphic studies suggest that the long-term slip rate of the WFZKL is about 0.2-0.4 mm/yr, and that of the EFZYB is higher, about 1 mm/yr. Since these previous studies were mostly concentrated along the fault trace, we analyzed fluvial geomorphic characteristics of the range, in the hope that such analysis would provide us information of a wider spatial coverage of the hanging-walls of the two fault systems. We analyzed the normalized river steepness index (ksn) and the geomorphic index χ of river systems along both flanks of the Ou Backbone Range. Our results show that the eastern flank generally has rivers with lower ksn values, but rivers along the northwestern flank have distinctively higher ksn values. However, since knickpoints are present in many of the river systems, these rivers may not be under steady state conditions, and the different ksn values may not represent differences in tectonic uplift rates. To rule out this possibility, we conducted field investigations to prominent knickpoints, and found that they are mostly artificial check-dams or waterfalls produced by local lithologic difference. Therefore, we suggest that most of the river systems are under steady state conditions, and the higher ksn values may indeed indicate a higher uplift rates of the area. This would suggest that the northern segment of the EFZYB has a higher slip rate, consistent with its more prominent geomorphic expressions, and the fact that the 1896 Riku-u earthquake rupture was only limited along the northern segment of the EFZYB.