集集大地震誘發九份二山崩塌的機制與過程

A study of the mechanism and processes of Chiufengershan landslide triggered by Chi-Chi earthquake

巫宗南(Chung-Nan Wu)

指導教授 ： 徐美玲

國立臺灣大學/理學院/地理環境資源學研究所/博士(2009年)

https://doi.org/10.6342/NTU.2009.02020

摘要

集集大地震所造成的九份二山崩塌，其滑動面初期大多被崩積物所覆蓋，也因而掩埋了許多有關崩塌的極重要線索。後經數年的流水侵蝕，這些被掩埋的地質證據逐漸浮現。經過仔細的調查，輔以新的鑽探資料，本研究對九份二山崩塌岩體的其崩塌次序與發生的機制提出新的詮釋。
九份二山於崩塌前呈現上尖下寬的順向坡，其由陡坡的坡型研判本區的地形演育受到地質構造與節理的控制。表面岩層為厚層砂岩，底部為泥質砂岩夾薄層頁岩。由崩塌後滑動面的位置研判，崩塌前滑動層並未在坡腳出現自由端。崩塌地東翼有順著斜交走向節理所形成的大型蝕溝，其與多處岩體的節理面均發現有銹染的情形，顯示地表水經由蝕溝與節理滲入至崩塌地的滑動層，為此次地滑埋下契機。
調查發現滑動面上有數處平行走向的左移斷層、伴隨衍生性左移斷層、垂直走向的錯動帶及坡度陡降帶，其中部分錯動處出現斷層泥與擦痕。由野外觀察及當地居民提供地震當時的現象推斷，本崩塌地乃地震造成岩體水平方向的錯動後，再產生岩體的上衝下擠，形成背衝斷層而崩落。
研究區因水文與地質特性，於地震時產生七層滑動面。龍南產業道路下方所發現的滑動面有三層，但滑動面的深度愈到坡腳愈淺。岩層沿著產業道路附近出現左移斷層，此等不連續面成為連接不同深度滑動岩層的滑動面。龍南產業道路中段上方有岩體向外突出的現象，乃因岩體下滑時無自由端，而發生地層被擠壓向上彎曲的現象。另由鑽探資料發現位於目前出露的最下層滑動面之下約22.5公尺處，還有一層滑動面，受到研究區稜線附近水池滲水的影響而降低強度，成為地震時造成多處逆衝斷層的關鍵。
此次地震所引發的崩塌次序為：大平角近粗坑溪處先因龍南產業道路附近左移斷層的推擠而產生落石；緊接著主崩塌地下部岩體下滑；跟著下滑的主崩塌地上部岩體因底部受阻，而局部拱起，待蓄積能量超過摩擦力，岩體再行下滑；並牽引崩塌地東翼的岩體，使呈逆時針旋轉，其中間岩體因下坡無自由端而被拱起形成褶曲，兩側則分向滑入粗坑溪與韭菜湖溪谷；最後，主崩塌地上部及西翼岩體因主崩塌地下滑後相繼下滑，並產生數種因岩石碰撞擠壓的特殊地形現象。

關鍵字

崩山 ； 平面型地滑 ； 地震型崩山 ； 集集大地震 ； 九份二山

並列摘要

Immediately after the Chi-Chi earthquake, the sliding surface of the Chiu-fen-erh-shan landslide was covered by erratic materials, so were the potential clues for the landslide processes. Years later, bedrock began to expose along with the crucial geological evidence. Based on careful investigation and new bore hole data, a new theory concerning the landslide process and mechanism is proposed.
The sliding slope is a triangular dip slope. Its formation is controlled by the geological structure and discontinuities, with surface massive sandstone underlain by clayey sandstone alternated with thin shale. Judged from the thickness of the sliding strata and the position of the sliding surface, there should be no free fall end before the sliding. Large rills developed along joints oblique to the strike of the rock strata. Stains of ferric oxide were observed on the joint surface in many places, signifies the infiltration of surface runoff into the sliding surface, which eventually induced the landslide.
Several left-slip faults and splay faults , as well as some ruptures perpendicular to strike were observed during investigation. According to the phenomena and the information provided by local residents, it is postulated that the earthquake induced horizontal rupture first, and then the up-and-down thrusts which caused collision and compression of the rock mass, and eventually resulted in the back thrust-like fault landform. Based on the core data, the depth of rock affected by the earthquake could have reached 25 m under the main sliding surface.
Three sliding surfaces are found on the slope below the Long-nan industrial road. They exhibited a rare situation with the depth of the sliding surfaces decreased down-slope. Another unusual phenomenon is the middle part slope above the Long-nan industrial road is convex outward. It is postulated that the massif slipped up by the earthquake and then slipped down with a momentum exceeding the frictional force of the rock and caused the upper thicker rock strata slip over the surface of the left-slip fault and push the lower thinner rock strata down-slope. The remaining sliding mass at the down-slope side of the back thrust fault testifies its occurrence.
The process of the earthquake triggered landslide is postulated as below:
1. Earthquake induced rock glide which in turn triggered rock fall at Dar-ping-chou with thunder-like noise.
2. The sliding of the lower massif of the main landslide drew the left-flank rock mass to turn counterclockwise to the edges of the slope and induced rockfalls into Sezi-ken and Jiu-tsai-hu streams. The middle part formed into two folds and tilted the “earthquake house”.
3. The upper massif of the main landslide and the right-wing massif, losing their end support, eventually slide down, and cause rock collisions and compressions.

並列關鍵字

landslide hazard ； block glide ； earthquake-triggered landslide ； rock collision ； Chiufengershan

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