沖繩海槽西南端之宜蘭平原震測研究

Seismic Images of the Southwestern Tip of the Okinawa Trough - the Ilan Plain, Taiwan

陳宗祺(Tsung-Chi 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

呈等腰三角形的宜蘭平原位於沖繩海槽西南端，其西北面與南面被雪山山脈和中央山脈圍繞。受到沖繩海槽擴張與大量沖積物沉積影響，宜蘭平原幾乎全被沖積物所覆蓋，平原底下沖積物構造環境複雜且斷層活躍。儘管目前已經有相當多研究進行於此但資料依然不足，因此我們對於宜蘭平原地下構造仍然有許多未知。
為了進一步瞭解宜蘭平原底下沉積物及活動構造的確切形貌，本研究利用淺層反射震測在宜蘭平原佈設十二條二維反射測線，並配合黃亭為(2011)發表「蘭陽平原東側沉積構造震測研究」的七條二維反射測線以及地調所的25口鑽井調查(蘇清全, 2011; 蘇品如等, 2009,2010)，整合進行討論，嘗試建立初步的宜蘭平原地下三維構造影像。
根據震測影像、地層震波速度與井下資料結果顯示，宜蘭平原位處地塹之上。推測有兩條主要的正斷層存在；一條向北傾斜的正斷層位於宜蘭平原南端，另一條向南傾斜的正斷層位於宜蘭平原北端。除此之外，兩條主要斷層之間也發現許多小規模的高傾角正斷層。
第三紀基盤面以上之沉積物，藉由地層震波速度、震測影像與鑽井資料結果，可以找到基盤面分隔第四紀沉積層與第三紀基盤，而第四紀沉積層可以找到末世冰期海侵面(18,000年)分隔海相與陸相沉積物。其中第三紀基盤面最深達880公尺，位置在宜蘭平原中間蘭陽溪北邊靠海處。從第三紀基盤面存在的深度位置變化，我們發現其深度位置向宜蘭平原南端、北端快速遞減至463公尺，且向西端遞減至320公尺，使基盤面呈現一半透鏡狀。由於末世冰期海侵面存在深度位置變化與基盤面相仿，故推斷第四紀以來下陷依然持續，然而第四紀沉積層的厚度變化相對較小，所以判斷宜蘭平原在第四紀時期沿著斷層的沉降速率是減緩的。

關鍵字

沖繩海槽 ； 反射震測 ； 宜蘭平原

並列摘要

The Ilan Plain is situated at the southwestern tip of the Okinawa trough. The triangle-shaped Ilan Plain is bounded by the Shiehshan range in the northwest and the Central range in the south. Although there were many studies conducted, subsurface structure of the Ilan Plain was still not clear. In this paper, we use 19 shallow seismic reflection profiles and 25 core borings to reveal the subsurface structure beneath the Ilan Plain. Results of the study should be able to provide a preliminary three dimensional image for understanding the sediments deposited and the active structures beneath the plain. Integrating seismic images with the interval velocities and results from core borings, we may find that the Ilan Plain is situated on top of a graben. The structure was mainly formed by subsidence of two normal faults; one is located in the south and dipping to the north, and the other is in the north and dipping to the south. In addition to the two main faults, many minor high angle normal faults can be traced in the seismic images as well. On top of the Tertiary basement, we may divide the Quaternary sediment into two zones by a significant interface. The depth of the deepest part of the basement was around 880 m, which is located in the center of the Ilan Plain at north of the Lanyang River. From the deepest part of the basement, we may see the depths of the basement were fast reduced to around 463 m toward the south and the north first, and then the variations were become smaller. It were also reduced to around 320 m toward the west slowly. Subsidence of the graben was gradually reduced along the two normal faults after the early Quaternary; therefore, thicknesses of the Quaternary sediment from south to northward have only minor variation.

並列關鍵字

Okinawa Trough ； Seismic Reflection Method ； Ilan Plane

參考文獻

蘇清全 (2011) 蘭陽平原末次冰期以來沉積環境變遷與構造作用特性。國立台灣大學地質科學研究所碩士論文，共129頁。

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