台東鹿野溪流域之地層滑動與河川化性在卑南地震後之相關性

Abstract

本研究嘗試探討台東鹿野溪集水區流域在2006年4月1日卑南地震(M L = 6.2)過後，其地層滑動與河川化性間之相關性。研究結果顯示，該集水區流域在卑南地震前之地層滑動的崩塌率為1.3%，地震後之崩塌率上升至1.7 %。同年5月17日珍珠颱風後之崩塌率為2.3%。隔年韋帕颱風後則下降至2.1 %。2006年之年輸砂量27百萬噸，也遠高於過去十年來之年平均輸砂量12百萬噸。此意義顯示，研究區域內之地層可能受到卑南地震擾動的影響，使得地質材料膠結較為鬆散，加上後續珍珠颱風帶來24小時276mm累積雨量的侵蝕，使得地表下原本已膠結鬆散之地質材料，在水壓力作用下，產生崩解破壞，並隨水流的動力流入河道內。 研究區域內屬於畢祿山層之崩塌率為3.6%，遠高於大南澳片岩層之1.4%。主要是由於畢祿山層的岩石強度(介於29.4 MPa至36.7 MPa)，比大南澳片岩層介於53.6 MPa至61.6 MPa強度弱，同時畢祿山層的岩層也較為破碎，單位體積節理數23.4 N/m3，高於大南澳片岩層的10.6 N/m3。 另外，本研究區主要之溶解態離子濃度在卑南地震前(2005年)，與地震後(2006年)之枯水期（11月至4月）的量測值比較發現，鈉離子（Na+）於地震前之月平均值為13.7 mg/l，地震後上升至27.2 mg/l ；鉀離子（K+）由2.8 mg/l，上升至地震後的3.3 mg/l，鎂離子(Mg2+)由15.7 mg/l上升至地震後的15.8 mg/l。卑南地震後接續而來之珍珠颱風，不僅提高了地層滑動及輸砂量，也同時增加了河川中主要溶解態離子的濃度。這些溶解態離子主要是源自於集水區內畢祿山層級大南澳片岩崩解岩塊內之斜長石、雲母，以及黏土礦物等主要礦物。 此外，顆粒性總碳含量與輸砂量呈現相反的變化趨勢，而豐水期有機碳含量由0.44 %上升至0.53 %，主要是來自於季節性變動的植被供應源。

This study focus on the relationships between landslides and stream chemistry in Lu-Ye river catchment after Bei-Nan earthquake (Mt = 6.2) and subsequent typhoon events. The result shows that the landslide ratio was 1.3% before Bei-Nan earthquake. It rose to 1.7% after the earthquake, 2.3% after typhoon Chan-Chu, 1.9% after typhoon Wipha. The annual sediment discharge in 2006 is 27 Mt/y which is 2.3 times of the mean value (12Mt/y) of past decade. These results indicate that the consolidation of bedrocks in Lu-Ye catchment was getting weaker when they underwent strong shaking movement after Bei-Nan earthquake. The following Chan-Chu typhoon resulted in heavy rainfall (276mm/day) and triggered the slope failure, and the landslides material was flushed into river channel. The main factors controlled landslide distribution are rock strength and joint number. The rock strength in Bilushan formation (29.4MPa-26.7MPa) is weaker than Dananao formation (53.6MPa-61.6 MPa), and the joint number in Bilushan formation is higher than Dananao formation. These factors caused the higher landslide ratio in Bilushan formation (3.6% in Bilushan formation and 1.4% in Dananao formation). The major dissolved ions in river water were getting higher after the earthquake and typhoon Chan-Chu but showed no remarkable trend after typhoon Wipha. This result indicate that the earthquake and Chan-Chu typhoon not only resulted in huge amount landslide and sediment delivered to the river , but also affect the river water chemistry. The rising ion concentration may be attributed to the minerals (feldspars, micas and clay minerals) in rock. The particulate carbon experiment result showed that the percentage were lower in wet seasons and increased 4 times in dry seasons. The rising of particulate organic carbon was due to plants decomposing on catchment slope.