臺灣車籠埔斷層流體之地球化學特性

Abstract

1999 年集集大地震的地表破裂以及滑移速率皆相當高，尤其於地表破裂帶北段的錯動具較為平順、速率快且滑移量大的特徵，隱含了流體參與斷層面滑移而產生潤滑作用的可能性。斷層帶鄰近相關流體的研究對探討流體與車籠埔斷層系統的關係，以及在集集地震扮演的角色來說相當重要。 本研究利用車籠埔斷層鑽探計畫(TCDP)中，相距約40公尺的A井(深2003公尺)與B井(深1282公尺)井內的流體，分析其流體的地球化學特性，如離子濃度以及氫氧同位素等隨深度變化，以探討流體的來源，並配合區域構造狀況與岩心資料，來推估車籠埔斷層系統與流體的關係。結果顯示，B井流體氫氧同位素組成落在天水線上且接近自來水的氫氧同位素組成，表示B井之水體主要來自於光正國小的自來水。A井水體的離子濃度於200至300公尺突然變化，顯示A井中的流體主要來自兩個地球化學特性迥異的流體系統。淺部的流體離子濃度低，氫氧同位素組成落於天水線上且接近鄰近河水的值；深部流體離子濃度相對來得高，尤其是碳酸氫根濃度最高可達3000ppm，氧同位素值較重且偏離天水線，表示深部的流體於溫度較高的環境下與圍岩進行相當程度的同位素交換。由A井岩心方解石脈的氧同位素以及與各岩層的截切關係推測，深部流體應是來自桂竹林層的地熱水體。淺層與深層的流體具有不同的地化特徵，也隱示斷層上磐與下磐的流體系統迥異。 由上磐與下磐流體擁有迥異的地球化學特性，表示斷層帶可能為一個不透水的障礙面，將兩邊的流體阻隔開來。而A井岩心之上磐岩層普遍出現的方解石脈，提供了下磐含高碳酸氫跟濃度的流體上湧的證據，顯示在地震當時形成的岩石破裂與斷層面的錯動，提供下磐受壓流體湧升到上磐的管道。 根據流體的地化特性，區域地質與構造，以及岩心的資料等證據，我們可以初步推測流體在各個不同的時期與斷層系統的作用機制。A井內部地球化學特性不同的淺部與深部流體，分別來自於上磐與下磐的流體系統，顯示斷層帶能阻隔流體的流動， 將上磐與下磐的流體系統隔絕造成各自系統中流體之特性不同；A井岩心中普遍出現的方解石脈與岩石破裂呈現高度相關及其分布趨勢，提供了下磐桂竹林層裡的水湧升入上磐岩層的證據，隱示於同震時期，斷層滑移面可提供流體流通的管道。因此，由以上證據推測，斷層區域流體與斷層系統各個不同階段的關係，於間震時期，不透水的斷層泥將上下磐具有不同地化特徵的流體系統隔開，而地震發生時，斷層面錯動提供流體流通的管道，使下磐含有高濃度的碳酸氫根的地熱水向上湧升；地震過後，來自於下磐的碳酸氫根漸漸析出為方解石，充填於上磐岩石裂隙中，離桂竹林層越近，則方解石脈密度越高。地震過後，隨著斷層逐漸癒合，使得上下磐的流體系統再次被斷層帶所阻絕。

The 1999 Chi-Chi earthquake(Mw 7.6) is characterized by high rupturing and slip velocity in the North and the ground motion is dominated by large low-frequency displacements, suggesting the possibility of fault lubrication during co-seismic period. Thus, the characteristics of fluid involved in Taiwan Chelungpu fault system is important for us to realize the mechanism of the fault-fluid interaction. We analyzed geochemical characteristics, such as hydrogen and oxygen isotopes, and ionic concentrations, of fluid samples retrieved from various depth along boreholes of the Hole A and Hole B of Taiwan Chelungpu fault Drilling Project(TCDP) to trace the fluid sources. The results show that the source of fluid in the Hole B is mainly the tap water, while there are two probable sources in the Hole A owing to the abrupt shift of ionic concentrations at the depth of 200-300 m. The shallower fluid might be from the leakage above the depth of 300 m and is characteristic of lower ionic concentrations and the isotopic ratios are close to those of adjacent river water. However, the deeper fluid should be the thermal water from Kueichulin formation because of high ionic concentrations, especially HCO3-, and higher oxygen isotope, which suggests higher temperature and more isotope exchange. Two sources of fluid of the Hole A are representative of the fluid systems in the hanging wall and foot wall respectively. The geochemical characteristics of fluids in the Hole A imply that the fault zone serves as a barrier in the inter-seismic period, resulting in distinctly different fluid between the Hanging wall and the foot wall. The frequent occurrence and the distribution of calcite veins provide the evidence of the upwelling of HCO3--rich fluid of Kueichulin formation and indicate that the fault served as fluid conduit during faulting and allowed the fluid flow across the fault zone to precipitate calcite veins in fractures of the hanging wall. Thus, we can deduce the mechanism of local groundwater flow during different stages of fault development by evidences such as calcite veins distribution, regional groundwater geology, and fluids characteristics in boreholes of the Hole-A and Hole B. During inter-seismic period, groundwater flows below and above the fault zone are separated by the impermeable fault gouge layer. In co-seismic time, faulting breaks the gouge layer, providing openings that let the over-pressured thermal water which contained high concentration of bicarbonate ion to surge up. After co-seismic period, the gouge layer is sealed again, residual thermal water which contained high concentration of bicarbonate ion in the hanging wall gradually precipitated calcite in fractures and the closer precipitation took place, the more calcite veins.