利用震波波速變化 辨識斷層於近地表位置之可行性研究

Feasibility of Using the Near Surface Seismic Waves Velocities to Identify a Fault Location

陳嘉榮(Chia-Jung Chen)

指導教授 ： 石瑞銓

國立中正大學/理學院/地球與環境科學系地震研究所/碩士(2013年)

摘要

目前在台灣共發現有33條活動斷層，而大部分的斷層帶上因覆蓋沉積物使斷層在近地表的詳細位置並不清楚，通常需透過槽溝開挖或地質鑽探才能了解。淺層反射震測可以獲得地下的構造影像，但近地表的影像大部分會因抹除，而無法了解斷層活動於近地表的變化關係。本研究針對淺層震測影像遭抹除的位置，分析地震波於三種不同形式的斷層在近地表處的速度變化，以找出切確的地表斷層位置。這三條斷層分別為：新化斷層(走向滑移斷層)，從淺層反射剖面可看到花狀地下構造；山腳斷層(正斷層)，在下盤有厚層的沉積物覆蓋；恆春斷層(逆斷層)，有沉積物與混雜泥岩覆蓋。本研究所使用P波折射法與多波道表面波分析方法(MASW)，分析地下18公尺深以上的P波與S波速度變化。施測時測線均設法跨越估計的斷層帶位置擺設。獲得的P波與S波速度變化，亦經比對先前相關鑽井資料與淺層反射震測影像。新化斷層在P波與S波速度變化均發現於跨越斷層時於橫向上有速度變低之現象，配合鑽探資料顯示，岩性變化趨勢與速度變化趨勢一致，且地下速度變化帶從西往東有逐漸往東北方偏移的情形。金山斷層亦出現低速帶，經比較反射影像發現斷層位置與低速帶位置一致，而較東邊測線也發現低速帶，可能為斷層延伸處。恆春斷層的測線中P波與S波之低速變化出現在同一位置處，極可能與斷層位置相關。本研究結果顯示，三種不同形式斷層均出現低速帶現象，其位置與鑽井資料的橫向岩性變化，和反射影像的信號不連續處有良好的關聯，與斷層帶有極大的關聯，說明了利用近地表地震波速度變化去辨識斷層位置的可行性。

關鍵字

近地表速度變化

並列摘要

Currently, thirty-three active faults are found on the island; however, more detailed studies such as excavation and borehole are still needed. Since most of those fault zones are covered by sediments, actual locations of the faults are not clear on the surface. Although the shallow seismic reflection method is useful for obtaining the subsurface image of the faults; however, the image is commonly muted from the ground surface down to the depth around 20 m. Many studies have shown that the velocities variations of the seismic waves at the near surface could be used for locating the accurate fault location. However, limitation of using the seismic wave methods to locate a fault location is not clear. In this paper, we will show feasibility of the seismic wave methods for identifying the fault locations on the surface. The applied seismic methods consist of the P-wave refraction and the multi-channel surface wave analysis methods. The two methods are conducted on three active faults of different fault types. The Hsinhua fault is a strike-slip fault with a flower structure shown on the shallow reflection seismic profile; the Sanchiao fault is a normal fault covered with thick sediments on the hanging wall; and the Henchun fault is a thrust fault covered by sediments and mélange. Each fault was studied by using the shallow seismic reflection method and core borings. This study uses variations of the P-wave and S-wave velocities across the fault zones to compare with boreholes and shallow seismic reflection images. The integrated results will be used to illustrate the feasibility of using the seismic wave methods to identify the actual fault location. Data collected for this study were recorded by using 4.5-Hz geophones of 24-channel, down to the depth of 18 m were analyzed. We may see that low velocities were observed in fault zone, which is well correlated to the results from core borings or seismic reflection image.

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