清水地熱地區之微震與震波走時層析成像

A Study of Microseismicity and Traveltime Tomography in the Chingshui Geothermal Area

劉瀚方(Han-Fang Liu)

指導教授 ： 吳逸民

國立臺灣大學/理學院/地質科學研究所/碩士(2013年)

https://doi.org/10.6342/NTU.2013.01769

摘要

宜蘭清水地熱區曾經是1980年代著名的地熱能源產區，政府於此建置第一座地熱發電廠為台灣地熱發電史之先，但後來因為管線結垢的問題導致產能下降而停止營運至今。鑒於全球暖化日益嚴重，政府結合各種調查研究與探測技術，重啟清水地熱的計畫。為了探究清水地熱區之淺層速度構造與地震行為，於2010年至2011年間執行國科會清水地熱探勘計畫，於清水地區建置臨時觀測網。由人工標記2011年2月-4月之間的地震波到時資料以做地震定位，並結合氣象局的地震資料以增加地震定位的品質。本研究進一步將清水地熱區鄰近範圍內(121.56°-121.76°E, 24.53°-24.73°N)的地震資料，條件為(1)同時有4個以上測站收到清晰訊號、(2)震源深度小於30公里，總共12個測站與368個地震事件含2192個P波波相、1947個S波波相，採用LOTOS (Local Earthquake Tomography Software)反演地下速度構造。研究結果顯示，微震群集中於清水地熱下方3-5公里深處，與大地電磁結果之低電阻區相符。此區恰為高VP/VS比值，可能代表較含孔隙水較高之熱液儲集區。層析成像於水平面結果則顯示，高VP/VS比值構造的分布與清水溪斷層、小南澳斷層的分布趨勢相近，代表此地區的地體的速度構造與斷層密切相關。此外，地球化學證據顯示清水地熱水為來自高山之天水滲透、經地溫加熱的深循環。綜合上述結果推論，清水地熱3-5公里深的熱液儲集區為地熱深循環之其中構成角色，此儲集區恰好位於兩個斷層最破碎之交會處，其破碎的性質使熱液得以通往地表，進而形成地熱徵兆。

關鍵字

清水地熱 ； 微震 ； 震波層析成像

並列摘要

The Chingshui geothermal field was once the largest existing productive geothermal area in Taiwan. Following the restarting of the Chingshui geothermal exploration in recent years, the National Taiwan University Temporary Seismic Network (NTUTSN) was set around the Chingshui geothermal area from 2010 to 2011. After manual picking and earthquake locating, we merged and relocated the catalogs from NTUTSN catalog with the Central Weather Bureau Seismic Network (CWBSN). The microearthquake group beneath the Chingshui geothermal area distributed on the direction of NE with depth of 3 to 7 km and plunged to the NE. Most of the microearthquakes are bounded in between the C-fault and the Xiaonanao fault. In addition, they are in a low resistivity zone, which was inferred as a geothermal reservoir from the magnetotelluric studies. Through the improvement of the catalog quality, the distribution of the microearthquake group can be associated with previous geophysical observations.
We also obtained seismic traveltime tomography results through the Local Earthquake Tomography Software (LOTOS) in the Chingshui geothermal area (121.56°-121.76°E, 24.53°-24.73°N), using a total of 4139 arrivals from 368 events. Both P- and S-arrivals are included for velocity inversion. The results show that the microearthquake group was in a region with high Vp/Vs ratio at the depth of 3 to 5 km. It may indicate existence of a fluid-filled fractured matrix forming a deeper geothermal reservoir. Besides, the orientations of the high Vp/Vs ratio structures are ENE-WSW and NW-SE, which correspond to the trends of the Chingshuihsi fault and Xiaonanao fault, respectively. At the cross of the two faults, a high Vp/Vs ratio is obtained. It might be associated with the most fractured region. These fracture zones might serve as conduits for the geothermal activity, and geothermal fluid could circulate within the two fault zones.

並列關鍵字

Chingshui geothermal area ； microseismicity ； seismic tomography

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