利用反射震測探討宜蘭平原南部之基盤深度及斷層分佈 ; Detecting the basement and the faults under the Ilan plain, northeastern Taiwan, using seismic reflection method

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Title:	利用反射震測探討宜蘭平原南部之基盤深度及斷層分佈;Detecting the basement and the faults under the Ilan plain, northeastern Taiwan, using seismic reflection method
Authors:	張峻瑋;Jun-wei Zhang
Contributors:	地球物理研究所
Keywords:	反射震測;濁水斷層;三星斷層;基盤面;宜蘭平原;seismic reflection method;Zhuoshui fault;Sansing fault;basement depth;Ilan plain
Date:	2010-07-12
Issue Date:	2010-12-09 10:51:34 (UTC+8)
Publisher:	國立中央大學
Abstract:	台灣東北部宜蘭平原底下的基盤面深度及地下構造分佈是探討該區構造演化的重要資料。宜蘭平原南部存在一條帶狀的地震分佈，推測底下可能有斷層存在，雖然地質圖標示兩側山區有多條斷層出露，不過進入平原區，由於沉積物覆蓋無法得知確切構造位置，因此本研究欲利用反射震測法測繪宜蘭平原南部地下構造，並推測其可能的演化。本研究以「震盪震源車」在宜蘭平原南部施測十餘條測線，測線長度約為1至3公里，為了與江新春(1976)的長測線結果相比對，在測線安排上分為三個部份：其一為靠山區測線，在蘭陽溪南北兩側各放上一組垂直測線，確定地層分佈；其二為靠海測線，位置均在蘭陽溪以南，且從蘭陽溪旁一路延伸至平原山邊；其三為確定三星斷層分佈所施測之測線。早年江新春(1976)的震測剖面為3重合，測站間距25公尺，本研究震測則為24重合，測站間距4公尺，解析度提高許多。研究結果分為基盤深度及斷層分佈兩部份。在基盤深度方面，靠山區剖面顯示，基盤深度大約為250至350公尺；而靠海剖面顯示，基盤面在蘭陽溪附近可深達760公尺，愈靠山區愈淺，直至南北測線南測尾端基盤面為200公尺。基盤以上的第四紀地層分為下段的礫石層及上段的砂泥層，在礫石層及砂泥層之間為江新春(1976)地質剖面的A反射層，礫石層反射相當絮亂複雜，致使基盤面呈現破碎散亂分佈，極不平整，代表早期三角洲的高能量沉積環境。震測剖面上出現多條斷層，但其中有兩條斷層特別明顯，與江新春(1976)震測剖面比較後發現為濁水斷層(該斷層可能與梨山斷層有關)及三星斷層。其中，震測剖面顯示三星斷層為橫移斷層，且斷層已切穿並進入基盤以上地層，滑移方向不明顯，參考地震及外海震測資料顯示，此條斷層可能與2005年3月5日所發生的宜蘭平原外海雙主震有關，再輔以地質圖，三星斷層可能為Ku et al.,(2008)所測繪的Shelf fault的上陸延伸且可能進入宜蘭山區。另外，震測剖面上濁水斷層兩側的基盤面深度驟變，在斷層南側深度為640公尺而在斷層北側深度驟降為760公尺，由於斷層兩側無明顯錯動，基盤面以上地層皆穩定沉積，所以判定為介於不同岩性地層間的界限斷層，可能為中央山脈板岩地質區分佈的北界。The Ilan plain is located at northeastern Taiwan, and is considered as a delta fan formed under the influence of Okinawa Trough. There exists a conspicuous linear distribution of earthquakes in the southern part of Ilan plain, a significant indication of buried faults. Although many faults were identified in the surrounding mountain areas, their features are not examined under the plain due to thick sediment deposits. In this study, we use the seismic reflection method to depict the subsurface structure under the Ilan plain and to infer its possible structural evolution. A ‘Minivibe’ source and a 192-channel system are used to do the survey. For comparing with Chiang’s (1976) seismic results, the surveys are divided into three parts to detect different structural distributions: basement shape, sediment layers, and faults. The seismic profiles in 1976 are 3 folds and the receiver interval 20 m. However, this study used 24 folds and 4 m interval. It is expected to reveal more detailed structure features. The results can be described in two parts: basement depths and fault distributions. In the part of the basement depth, the profiles near the mountain show that the basement depths are about 250 to 350 m. The profiles near the coast show that the basement depth near the Lanyang River reaches 760 m and gets shallower southward. Quaternary strata above the basement have two parts: the lower one is full of gravel deposits and the upper one is sand and silt. The strong reflector ‘A’ in Chiang’s profiles is identified as a separator of these two parts. The reflection in the gravel deposit is complex which even leads a broken distribution of the basement. This represents a high energy deposit environment in the early stage of delta fan deposit. Several faults appear in seismic profiles, among them, two faults are quite distinct. After comparing with the 1976 seismic profile, we recognize them as the Zhuoshui fault, which is probably related to the Lishan fault, and the Sansing fault. The Sansing fault is a strike-slip fault and is found to cut though the Quaternary strata to very shallow depth. This implies that it could be an active fault. This fault may be the main fault of the earthquake doublet (M=6.2) happened at 2005. The seismic profiles across the Zhuosui fault show shat the basement depths change suddenly. To the north of the Zhuosui fault the basement depth is 760 m, but decreases abruptly to the south of the fault as 640 m. The sediments above the basement are quite flat and stable. The fault should be the boundary to distinguish the different rock contact and could be the northern boundary of the Central Range geologic province.
Appears in Collections:	[Graduate Institute of Geophysics] Electronic Thesis & Dissertation

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