- 學位論文
1999年集集大地震之車籠埔斷層摩擦溫度與蝕變作用
Frictional Temperature and Alteration of Chelungpu Fault Zone in 1999 Chi-Chi Earthquake
摘要
地震所釋放出的能量包括破裂能、輻射能以及摩擦熱，摩擦熱通常都被視為佔據地震釋放能量最大的一項。1999年集集台灣大地震中發生錯動的車籠埔斷層提供了一個很好的研究地震與斷層錯動機制的機會。臺灣車籠埔鑽探計畫在2004年於台中大坑鑽取車籠埔斷層的岩心，獲得新鮮未受風化的圍岩以及斷層泥。本研究著重在地震所釋放的摩擦熱，而計算摩擦熱所需要的參數中最為關鍵的是摩擦溫度；除了摩擦溫度外，本研究也嘗試模擬斷層帶熱液系統。由於黏土礦物本身對於溫度的反應較為靈敏，而且車籠埔斷層就位在錦水頁岩中，因此採用黏土礦物含量較多的頁岩當做研究材料，以研究斷層帶熱液系統的反應。 此研究中將樣本分為全岩以及黏土兩部分固體材料。其中，黏土部分作為對照組。將樣本置放入高溫爐中，分別在攝氏600度、700度、800度、900度、1000度和1100度的溫度下加溫五分鐘之後，以X光繞射分析儀鑑定礦物受熱後的反應，並以掃描式電子顯微鏡觀察是否有熔融現象。經由X光繞射分析後，可以發現以攝氏900度加溫5分鐘後，其黏土礦物就幾乎遭受熱分解，且在掃描式電子顯微鏡下可看出開始有熔融現象。並且，與斷層帶中樣本的掃描式電子顯微鏡結果比較，可以得知車籠埔斷層在1999年滑動所達到的溫度可能達攝氏900度以上。 根據前人研究，在TCDP- A井岩心1111.29m被視為是車籠埔主要滑動帶的深度中，有含量較高的膨潤土，而在圍岩中的含量卻很少；且在此滑動帶中，經由掃描式電子顯微鏡與穿透式電子顯微鏡發現有假玄武玻璃的存在，加上前人文獻中提到，在自然界中玻璃質可以經由蝕變形成膨潤土，因此推論車籠埔滑動帶經由錯動所產生的高溫摩擦熱，加熱斷層岩石形成假玄武玻璃後，再因斷層剪切作用帶進液體產生熱液蝕變作用形成膨潤土沉澱在斷層帶中。另外，Chen (2008)分析TCDP- A井來自深處的井管水，發現其碳酸氫根離子與鈉離子含量相當高，且推測其來源為桂竹林層。另外，斷層帶附近有方解石脈的生成，加上斷層泥經由三軸實驗壓出的液體其離子含量以氯離子和硫酸根離子為最多，因此設計了一套熱液實驗以確定是否斷層剪切之後，來自於桂竹林層的液體侵入，加上斷層摩擦的熱源使得過去海相沉積所留下鹽類之海水離子釋放出來，因此含有鈉離子、碳酸氫根離子、氯離子與硫酸根離子的液體與斷層帶反應產生膨潤土沉澱。實驗結果顯示膨潤土可以由頁岩經高溫形成之玻璃質與碳酸氫鈉或是氯化鈉反應而產生，且會釋放出鈉離子進入液體中。另外，陰離子分析也顯示熱液實驗過程會使得液體中的氯離子以及硫酸根離子增加，因此斷層帶中所含有相當高的氯離子以及硫酸根離子與熱液反應過程是相關的。 實驗結果可以讓我們建立初步的車籠埔斷層中熱液系統，車籠埔周邊地層含有較多的碳酸氫根離子以及鈉離子，在每一次錯動後會造成斷層帶中膨潤土以及方解石脈的生成，新礦物的生成會造成斷層帶的密封，因此能夠繼續累積應力到下一次錯動造成地震釋放能量；而膨潤土容易吸水膨脹造成地層不穩定，也跟車籠埔是個多次滑動的斷層有關。
並列摘要
Understanding energy changes during an earthquake is one of the most challenging problems confronting the earth science community. An earthquake releases accumulated elastic strain energy, which in turn is transformed into radiated energy, fractured energy and frictional heat. Among them frictional heat not only represents the largest share of energy but also the least known due to subsurface and surface weathering. The Chelungpu fault, which moved in the 1999 Chi-Chi earthquake, has provided us a good opportunity to shed light on the aforementioned questions via checking the reaction of clay minerals after frictional heating. In this study, starting material samples from the ChinShui Shale of TCDP, were each heated for 5 minutes in a furnace under different temperature conditions: 600°C, 700°C, 800°C, 900°C, 1000°C, and 1100°C, respectively. They were then analyzed by X-ray Powder Diffraction (XRD) and Scanning Electron Microscopy (SEM) to investigate mineral reactions and phase changes. Results of SEM observation indicated that grains of minerals did not change before 900°C. As soon as the heating temperature reached over 900°C, some melting phenomena began to be detected, with many vesicles were observed over 1000°C. The results of XRD show that the clay minerals began decomposing and melting at temperature over 900°C. Previous studies have shown that, in the abnormal content of clay minerals, smectite was relatively richer in fault-zones than in host rocks. Melting has also been observed in fault-zones. Based on the results from Chen (2008), the concentrations of sodium and bicarbonate were higher in borehole water at depth 1,110m in TCDP- A hole and they might come from the Kueichulin Formation. Also, calcite veins were found around the Chelungpu fault zone (Yeh, 2007). We interpreted that the smectite could originate from glass alteration, which the glass was generated by frictional melting in seismic events. We designed a series of experiments to simulate the hydrothermal reactions in order to confirm that the rock in the Chelungpu fault zone was first heated to glass, and then reacted with liquids to precipitate smectite. The results show that smectite could be formed in both of the hydrothermal systems of sodium bicarbonate and sodium chloride. Results of ion concentration analysis of fluid product associated with sodium bicarbonate reaction experiment reveal that the sodium and potassium ions would reduce to precipitate zeolite and the sulfate ion would increase after hydrothermal reactions, which could be caused by the relatively high concentrations of those ions in pore fluid of fault gauge. The result of sodium chloride reaction experiment indicates that sodium, sulfate and chloride ion would increase, and it may be related to precipitating smectite. These results from heating and simulating experiments of TCDP materials may provide us information for estimates of frictional heat and processes in the 1999 Chi-Chi Taiwan earthquake.
參考文獻
延伸閱讀
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