利用土壤逸氣成分調查台北地區山腳斷層在地表投影可能之分佈

Abstract

一般而言，地殼的破碎行為是造成土壤氣量異常變化的原因之一，而常被用來做為追蹤地表斷層分佈和地震預測的有效工具。氦氣、氡氣、二氧化碳與甲烷氣等氣體是空氣中含量極低的氣體成分，而可以較敏感地偵測出土壤逸氣之異常值，所以廣為大家所採用。其中氦氣是一種具有高度遷移性、在水中低溶解度且無色無味的惰性氣體，不會與其他成分產生化學反應，最能保持原本逸氣性質；其在乾燥空氣中約5.24 ppm，因為地殼中放射性元素的自然蛻變，而富集了大量的氦氣成分，容易因為地震與斷層活動而沿著地殼裂隙逸散至地表，而被偵測出異常反應。 本研究分別在關渡地區，五股鄉二重疏洪道、泰山、新莊丹鳳、樹林山子腳等地區，沿著橫跨山腳斷層通過地表可能位置的剖面，有系統地採集在地表土壤下方約0.7-1.0 公尺深度的土壤氣樣品，沿著19條剖面共253個採樣點，進行土壤氣體成分分析（氦氣、氡氣、二氧化碳、甲烷、氬氣、氧氣），嘗試利用土壤氣中異常的氣體濃度分佈，追蹤山腳斷層在地表的可能位置。 由於氦氣、氡氣和二氧化碳濃度有較為顯著的異常變化，可以做為判斷有否斷層通過該區域的主要依據，依此將山腳斷層可能在地表通過的區域，描述如下：（1）在關渡地區：斷層可能通過關渡宮與關渡自然公園之間；（2）二重疏洪道地區：斷層在溫子川以東至蘆洲堤防以西約700 公尺之間可能為山腳斷層通過地表的位置；（3）泰山鄉地區：氦氣濃度異常集中在中港大排至明志路之間約120-700 公尺的區域，斷層可能經過泰山鄉之義學國中，及明志國小前緣，向南延伸至中山路旁山海關餐廳後面；（4）新莊市丹鳳地區：氦氣濃度異常集中在中正路以東塔坑寮溪以西約100-450 公尺寬的區域，斷層可能經過龍安路的雙鳳公園，且往南延伸切過丹鳳國中操場；（5）樹林山子腳地區：斷層可能通過樹林監理站後方與三俊街約300公尺之間。 由代表性樣品之氦氣、碳同位素分析結果顯示，本研究區異常氣體來源主要為地殼源的氣體端成份，僅有少量的岩漿源成份；本研究以山子腳地塊西緣且為採樣最南端的S剖面每星期的連續觀測數據顯示，斷層帶上的土壤逸氣成份變化與雨量、及鄰近之地殼活動可能有關，需要進一步觀測以釐清其關連。 由本研究結果顯示，土壤逸氣異常的範圍和地調所所公布的山腳斷層可能通過位置相當吻合，顯示利用土壤逸氣異常值來調查斷層之分佈，為一有效且快速的方法，若能進一步配合其他方法之調查結果，當能更精確地研判出斷層之分佈位置。

Anomalous soil gas compositions are generally revealed on surface of fractures and fault zones. Therefore, the distribution and variation of soil-gas components are often used to trace the locations of faults and even as to be precursors of earthquakes. Due to low concentrations in air, He, Rn, CO2 and CH4 are sensitive to reflect soil-gas anomalies. Helium, an inert gas with high mobility and low solubility, has higher concentration in the crust due to radioactive decay. Therefore, helium can easily migrate through fractures or faults without fractionation. We attempted to trace the surface distribution of the Shan-Chiao fault by utilizing soil-gas survey. Soil gas was systematically collected at depth around 0.7-1.0 meter with metallic probes. Several sampling profiles across possible location of the Shan-Chiao fault were proceeded in Er-Chong Flood-diversion Canal (Wu-Gu area), Tai-Shan area and Xin-Zhuang area of the Taipei Basin. In Er-Chong Flood-diversion Canal, anomalously high soil He, CO2 and CH4 was distributed over the possible location of the Shan-Chiao fault. In other areas, the soil-gas anomalies (mainly He) are listed bellow: (1) Wen-Zi Canal to Lu-Zhou Riverbank (Wu-Gu area),spreading about 700 meters wide; (2) area between Zhong-Gang Escape Canal and Ming-Zhi Road (Tai-Shan area), spreading about 120-170 meters wide the. Shan-Chiao fault may pass through Yi-Xue Junior High School, the front part of Ming-Zhi Elementary School, and extend southward to Zhong-Shan Road; (3) from Zhong-Zheng Road to Ta-Liao-Keng Stream (Xin-Zhuang area),spreading about 100-450 meters wide. The Shan-Chiao fault may pass through Shuang-Feng Park in Long-An Road and extend southward to the play ground of Dan-Feng Junior High School. Further investigations are required to check if the fault stretches to identify whether the Shu-Lin area. The preliminary results show that the distribution of soil-gas anomalies, especially He abundance, correlates well with the possible surface distribution of the Shan-Chiao fault. The results also show that soil gas survey is effective to identify the locations of faults. By combining this technique with isotopic analysis, we can have more detail and precise information of the Shan-Chiao fault.