以Sp轉換波相探討台灣嘉義地區之沉積層厚度變化

Abstract

台灣位於歐亞大陸板塊與菲律賓海板塊之交界，因此全島皆有高地震活動度。由於未膠結固化的地表沖積層與堅硬基盤，兩者之波傳阻抗 ( impedance ) 有著相當大的差異，這也使得沖積層對於地表振動的形式與大小有著十分大的影響，因此了解一區域之沖積層變化對於地震防災是一項重要的課題。本研究利用密布於台灣全島之強地動觀測網 ( Taiwan Strong Motion Instrumentation Program，TSMIP ) 所蒐錄的的地震資料，分析Sp轉換波相到達時間估算地表沉積層厚度，以描繪位於台灣西南部的嘉南平原之沉積層厚度變化，並藉以仔細分析這人口稠密區域的地下基盤幾何形貌。 研究區位於台灣西南，為一南北狹長之平原，西鄰台灣海峽，東靠西部麓山帶，為新生代晚期以來，由被動大陸邊緣轉成前陸盆地，再經過聚合抬升之沖積平原。地形高度變化為由東向西漸緩，地表為第四紀沖積層所覆蓋，下方為第三紀岩層。根據鑽井與震測剖面資料，此區沖積層厚度變化受到北港高區與西部麓山帶之影響，平原區之沉積層厚度變化大致為由北向南、由西向東逐漸增厚。 此方法為利用Sv波與Sv波傳至近地表低速沖積層時所轉換成P波的時間差，即S-Sp到時差，描繪研究區域沉積層厚度變化。而這項利用Sp轉換波相估算沖積層深度的方法，已成功地應用在宜蘭平原。為避免高估沉積層之厚度，在此選用之震源深度大於6公里，且震源出射角大於150°的地震，最後於本區域內1992至2010年五月共142個TSMIP測站中的18,892筆地震波型，在109個測站中挑選出1,200筆具有Sp轉換波相之地震紀錄。 S-Sp到達時間差最小為0.3秒，最大可達3.2秒。由北至南到時差由小變大，研究區除了麓山帶所得到時差在1秒以下外，研究區北部受到北港高區影響與南邊的台南背斜皆有相對低值。並且在研究區域之南端亦觀察到到時差急遽增加的情形。配合震測剖面與鑽井資料所得之平均P波速度，與Brocher (2005) 的經驗公式所得之Vp/Vs比值，計算此區之深度變化，與前人之震測、鑽井資料比較顯示，本研究Sp轉換波所反映之速度不連續面可能於中新世地層間。然而靠近麓山帶之地底構造複雜，且地層已非水平疊置，使得深度有所誤差。 本研究以密布於台灣的強地動觀測網，觀察Sp轉換波相估算嘉南平原沖積層之深度變化，所得結果與震測剖面及鑽井資料的地底形貌變化大致吻合，並可明顯觀察到北港高區所影響，沖積層深度有變淺的現象，以及明顯勾勒出台南背斜，與其南邊的增厚之地層。這項簡易、具經濟效益的方法，可得到一研究區域地底形貌之大致輪廓，並提供沒有良好野外露頭之低緩地區地底構造資料。

The Chia-Nan Plain is a long and narrow alluvial plain, located in the west of Central Mountain Range and the east of Taiwan Strait. Accelerograms recorded from 1992 to 2010 May by the Taiwan Strong Motion Instrumentation Program (TSMIP) stations are used to study the thickness variations of the Quaternary alluviums beneath the Chia-Nan Plain, the biggest alluvial plain in Taiwan, by examining Sp converted phase. The thicknesses are estimated by the travel-time difference of S and Sp waves. This technique of using P-S converted phases had a quite success in the previous study of Lan-Yang Plain. The records of seismic events that were located in the southwestern Taiwan region and recorded by 142 TSMIP stations of Chianan Plain are used. After choosing local earthquakes that have a takeoff angle greater than 150° and a focus depth deeper than 6 km to better conform vertical incidence of local propagating body waves when crossing the sediment–basement interface, there are 1,200 manually picked time differences of S-Sp arrivals from 109 TSMIP stations. With this time differences and the average P waves velocity from seismic reflection profiles and boreholes information, the thickness of the sedimentary in this area could be estimated. By the high seismic activity and dense of TSMIP stations in this study area, we used this technique to depict the variations of sediments thicknesses in Chia-Nan Plain. In general, the thicknesses become thicker toward south and east, and the boundary between sediments and basement which generates the Sp converted waves might be located in Miocene strata. This approach is simple and time-effective and can be used to preliminarily define the general characteristics of thickness structure for a region with available converted phases before conducting comprehensive onsite geophysical surveys.