臺灣博碩士論文加值系統

台灣造山帶座落於歐亞板塊與菲律賓海板塊聚合區域，探討台灣及其周圍地體構造除了需在台灣島上進行各類的地質和地球物理調查及分析外，也需要調查台灣周圍海域的地體構造，如此才能夠完整解釋台灣區域的複雜構造。因為台灣位於板塊交界處，地體構造活動相當活躍，地震頻繁，雖然台灣島上有較完整且密集的陸上地震觀測網，但陸上地震觀測網並無法覆蓋海域地震，因此必須藉由海底地震儀(Ocean Bottom Seismometer, OBS)的佈放，才能觀察到陸上不易偵測的較小規模地震或微震活動。本論文即以此觀點出發，期盼能結合海域的研究，透過臨時海底地震儀觀測網所收集之資料進行局部的台灣周遭地震之相關研究和分析。
本論文第一個研究主題為分析台灣西南部外海因海床天然噴氣而誘發的微震訊號，了解海底氣體噴發的可能機制。台灣西南部海域有常見的泥貫入體與海底泥火山地質構造，相關的海床噴氣現象在該區域普遍可以被觀測到。因此藉由海底地震儀所組成的臨時觀測網在泥火山MV1與周圍區域持續地記錄22天，我們從每個測站的波形紀錄中，可明顯辨識脈衝微震與長時微震兩個訊號，前者震動時間僅2~3秒短，後者則因與脈衝微震有相似的單頻震盪特徵而被視為是脈衝微震疊加而成。我們推論出海床噴氣活動與其誘發的微震訊號與潮汐的漲退活動相關，其中又以半日潮與全日潮最為明顯。劇烈的噴氣活動與伴隨的長時微震通常出現在潮汐上升至最高高潮與下降至最低低潮期間，而在接近平潮時期時，兩者活動度接明顯下降。整體而言，噴氣活動和微震振幅與每個潮汐日內潮差呈正比，相關性越高暗示在測站下方的淺部沉積層中的氣體含量越高。
本論文第二個研究主題則為探討台灣東北部和平海盆至南澳海盆地區的孕震構造。影響台灣地區的地震有將近百分之七十分布在台灣陸上地震站網以外的海域，然受限於東部外海地震測站包覆度不足，造成地震定位上有一定的誤差，增加解析東部外海複雜構造的難度。2015年4月20日，在南澳海盆發生了規模Mw 6.3的地震，我們隨即在震央附近佈放6顆海底地震儀並收集近15天的地震資料。配合8個陸上地震站組成臨時觀測網，期間共有501個地震事件被使用HypoDD重新定位。定位結果顯示震源深度集中在17至35公里間。研究區域之地震依據震源機制可分為兩種截然不同型態：一是屬實板塊介面地震帶之地震，屬逆斷層型態的地震活動，深度落於20-30公里左右，震央分布呈西北-東南向，深度往北漸增，此地震群與琉球隱沒系統有關，特色為低角度之逆衝斷層：二是屬於和平群震淺部地震，屬正斷層型態之地震活動，其分布沿著高角度南傾的斷層面，自南澳海盆下方之低速帶向上延伸，其主因可能是位於南澳海盆東南與西北兩側之右移橫移斷層造成此區域局部擠壓應力，使得隱沒中的菲律賓海板塊受到擠壓而突起，在在突起構造上方則誘使上覆的弧前地層陷落造成一系列的正斷層。

The Taiwan mountain belt belongs to a collision zone, located between the Eurasian plate (EP) and the Philippines Sea Plate (PSP). In such a complex tectonic regime, lots of seismic activities are expected. However, due to the limitation of the onland seismic stations, the seismotetonics setting of the related offshore area is usually not well understood. In order to better understand the tectonics of the offshore area, we have combined the Ocean bottom Seismometer (OBS) and the onland seismic stations. Different from the onland station, OBS data can record micro-earthquakes and tremors that are outside the range of the onland seismometers. The joint use of the OBS and the onland seismic stations can help understand detailed structures in the offshore area of Taiwan.
The first topic of this dissertation is to understand the mechanism of gas emission off SW Taiwan. The area off SW Taiwan belongs to an incipient stage of orogeny and consists in numerous active mud diapirs and mud volcanoes. Gas emissions out of the seabed off SW Taiwan are a common phenomenon. However, the mechanism of gas emissions is still poorly understood. For that, we collect the gas-related impulse tremor and long-duration tremor by using OBS around mud volcano MV1. We show that the gas emissions out of the seabed and the associated tremors off SW Taiwan are strongly correlated with the ocean tides, especially with the diurnal and semidiurnal constituents. For each tidal day the strong gas emissions and long-duration tremors occur mainly at the rising periods to the higher high water and the falling periods to the lower low water. Both the gas emissions and tremor activities are generally quiet near the tidal datum. On average, the daily intensity of gas emissions and the magnitude of tremors are positively proportional to the daily tidal range; we suggest that the larger slope site has a greater gas concentration in the shallow sediments.
In the second topic, we address the seismotetonics of Nanao-Hoping forearc basin off NE Taiwan. In the Taiwan area, over 70 percent of the earthquakes occurred in the offshore area. Based only on the onland seismic network, the observation and monitoring of those earthquakes are apparently insufficient. In order to better understand the nature of the seismic behavior in that region, 6 OBS were deployed around the Nanao basin immediately after the Mw 6.3 earthquake on April 20, 2015. We have combined the recorded OBS data with 8 CWB onland stations for a detailed earthquake analysis. In total, 501 earthquakes have been relocated by using HypoDD. The distribution of relocated hypocenters shows a NW-SE (NE-SW??) trending and the focal depths range from 15 to 35 km. The earthquake locations display two major features: (1) A group of events at depths between 20 and 30 km beneath Nanao Basin is aligned with the subduction interface. We consider that this group belongs to subduction earthquakes, because the earthquakes cluster in the vicinity of the expected subduction interface. (2) Another seismicity is beneath the NW Nanao Basin and Hoping Rise and shows a high-angle south-dipping structure. We suggest the feature is associated with normal faults. Two parallel right-lateral transform faults on Hoping Basins and Ryukyu accretionary prism (Yaeyama fault) could cause a transpressional stress and induced a local deformation on subducting PSP slab. The subducting PSP is therefore locally arching, which has generates a series of normal faults on the NW Nanao basin and Hoping Rise. 

摘 要 i
Abstract iii
目錄 v
圖目錄 viii
表目錄 x
第一章、緒論 1
1.1前言 1
1.2研究動機與目的 2
1.3本文內容 3
第二章、海底地震儀 8
2.1 前言 8
2.2 海底地震儀 8
第三章、台灣西南海域海床氣體噴發機制 16
3.1 前言 16
3.2 西南海域地質背景與調查概況 16
3.2.1研究區域地質背景 16
3.2.2 台灣西南海域之天然氣水合物賦存潛能 17
3.2.3 泥火山與海床噴氣現象 17
3.2.4 台灣西南外海潮汐特性 19
3.3研究資料收集與處理 30
3.3.1 海底地震儀資料收集 30
3.3.2科學魚探儀系統與資料收集 30
3.3.3 潮汐資料 31
3.4結果 34
3.4.1泥火山MV1噴氣機制 34
3.4.2微震活動 35
3.4.3 潮差與微震振幅 37
3.4.4 微震與潮汐分潮 37
3.5討論 50
3.5.1 甲烷氣體噴發機制 50
3.5.2 沉積物中氣體蘊藏量 51
3.5.3 微震震源位置 52
3.5.4 甲烷氣體逸出與影響 52
3.6結論 56
第四章、台灣東北海域琉球隱沒帶的地震地體構造 57
4.1 前言 57
4.2 台灣東北部海域構造環境與地震活動 57
4.2.1台灣東北部外海的地體構造 57
4.2.2 台灣東北部外海的地震特性 59
4.3資料處理方法與收集 68
4.3.1 海底地震儀重定位 68
4.3.2 地震波到時挑選與初步定位 68
4.3.3 地震重新定位 69
4.3.4 地震規模 71
4.3.5資料收集 72
4.4資料處理與結果 76
4.4.1 海底地震儀位置校正 76
4.4.2地震定位結果 76
4.4.3 地震規模 78
4.4.4震源機制解 78
4.5 討論 88
4.5.1 板塊介面地震群 88
4.5.2 和平地震群 89
4.5.3 海域觀測網優點 91
4.6 結論 99
第五章、總結 100
參考文獻 102

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