嘉義地區的相對構造活動速率及其與地表地形的相關性：利用河流水力侵蝕模型

Abstract

透過大地測量及地震觀測，位於台灣西南部的嘉義地區被認為是一個強烈受到構造活動影響的區域，頻繁的構造活動造成此區域明顯的地表變形，也因此被公認為一個適合用來研究構造活動與地表變形關係的主要地區。本研究使用地形計測指標（Geomorphic Indexes）中的河流水力侵蝕模型（stream power erosion model）來討論嘉義地區河流基岩河道（fluvial bedrock channel）的地形與構造活動的關係。河流水力侵蝕模型是一個有別於以往的地形計測指標，它同時也將許多影響侵蝕速率的水文參數、流域盆地特性、氣候等因素加入討論。 經由河流水力侵蝕模型的分析，本研究所選取的9條河流中，其河流凹曲度（concavity index；m/n ratio）介於0.42∼0.91之間，相較於理論值（0.5），本研究區中大部份河流的凹曲度均有偏高的趨勢（＞0.5），此一現象顯示了本研究區的河流下切（侵蝕）速率是較快的。倘若是在地體均衡（steady-state）的情況之下，便能同時代表抬升速率也相對較快，意即此研究區的構造活動可能是較為頻繁的。 在本研究中，我們也利用短期（1984年∼2001年，共18年）或較長期（40年∼100年）的年平均降雨量、輸砂量濃度與流量，試圖說明降雨侵蝕與河流能量對於地表形貌的影響。根據結果發現，降雨侵蝕可能為造成嘉義地區較南部區域河流凹曲度偏高的主要原因之ㄧ。本研究也透過不同統計方法發現，DEM解析度對於本研究的影響並不大，反而是取樣方式的不同會對結果造成不等程度上的影響。 此外，河流水力侵蝕模型中，有一可用來代表相對構造抬升速率的河流陡峭程度指標（normalized steepness index，ksn）指出，本研究區域的構造抬升速率可以以八掌為界，在八掌溪以南的區域，其構造抬升速率較八掌溪以北的區域高；同樣的趨勢也可從河流的蜿蜒程度看出。 除了構造活動會影響河流地表形貌，本研究也討論許多流域盆地特性，如：岩性、地表構造、支流分布情形與流域面積和河流凹曲度的關係。藉由這些綜合討論得知，影響河流形貌的因素除了構造活動，還有受到許多其他因素，其與構造活動的相關性並非一致性的，不同的時間、地點、範圍，不同的測量尺度、分析方法都會有不同的反應，因此，在解釋上必須更加小心謹慎。

Chiayi area revealed by geodetic measurements and background seismicities, in the Southwestern Taiwan has been undergoing active intensive crustal deformation. We used the stream power erosion model of 9 rivers, easy and less time consuming, to characterize the occurrence and morphology of fluvial bedrock channels and to establish their relationships with tectonic activities. Besides, the stream-power erosion model also takes some hydrological factors such as water discharge, precipitation, and sediment export into consideration to quantify the influence of erosion process in the stream power erosion. Most of the selected rivers in our study area showed high values of concavity (0. 42 ≤ m/n ≤ 0.91), comparing to the global mean river concavity (m/n ratio 0.5), implying the high tectonic activity and high erosion rate in Chiayi area. The mean annual precipitation, water discharge and suspended sediment export concentrations (Cs) for a period of eighteen years (1984~2001) were calculated to figure out the reason for such high m/n ratio. The result showed that precipitation might be one of the important factors to result in high river concavity in the south of the study area. DEM resolution plays much less important role than sampling methods. Because both normalized steepness index (ks) and concavity index (θ) indices are related to lithologic variations, sediment flux, orographic precipitation and uplift rate, thus the normalized steepness index (ksn) is used to quantify the “real” uplift only. The result has pointed out that high uplift rate is predicted at southern part of Pachang river while low uplift rate is predicted at the northern part even though the sinuosity of all rivers are displaying the same pattern. Finally, the characteristics of river basin such as the lithology, structure, hydraulic geometry and so on are also been discussed. We conclude that the river concavity and normalized steepness index deduced from stream power erosion model not only could reflect the influence of tectonic activity and the erosion process but also associate with the drainage basin characteristics.