沖繩海槽與北台灣的地幔楔衰減特性及其與歐亞岩石圈之關係

Variation of Attenuation along the SW Okinawa Trough: Effect of the Eurasian Lithosphere

柯彥廷(Yen-Ting Ko)

指導教授 ： 郭本垣

共同指導教授 ： 洪淑蕙

國立臺灣大學/理學院/地質科學研究所/碩士(2010年)

https://doi.org/10.6342/NTU.2010.00717

摘要

地震波對於探究地幔動力學與地球內部構造無非是最佳的分析利器。無論分析波速之快慢或是能量之衰減，皆可有效地勾勒出隱藏在地底深處的構造形貌。相較於震波速度，震波之能量衰減分析，更能夠清楚的反應溫度及含水量的變化。地震波之振幅頻譜主要由震源與波傳介質所共同影響，因此如何區分兩者對於震波能量之影響是一極具挑戰的任務。在本研究中，我們首先發展一套全新的分析方法來有效分離震源與路徑的效應，其核心概念是假設相鄰地震傳至同一測站可分享相同的Q值，且一個地震只能擁有一個拐角頻率（fc），如此建構之多重震源-測站組合能”堅固的”限制共同路徑之Q值與不同地震各自擁有的fc。我們運用模擬退火進行非線性逆推，由觀測資料同時決定路徑與震源效應。結果顯示，fc與Mw呈現清晰的逆相關趨勢，更鞏固逆推架構與結果之可信度。

我們發現處於弧後擴張的沖繩海槽之下的地幔楔中，Q值僅有60左右。在東經122°以東附近Q值大約200-300，以西卻驟增至800以上，我們追朔兩種主要因素，溫度和水含量。若單就溫度的差異，弧後張裂向西減弱能貢獻約200-250℃的溫度差，與歐亞岩石圈接觸的熱傳效應估計應該很小，但菲律賓海板塊隱沒速率受到歐亞板塊阻擋而減緩，造成地幔迴流也隨之減慢所產生的溫度差大約有50℃，250-300℃左右的冷卻效應依然無法解釋Q值變化。本論文提議在北台灣下方，在PSP隱沒速率減慢的同時，可能伴隨板塊脫水速率下降，使地幔楔水含量變少。減少10倍水含量，加上250-300℃的冷卻足以解釋觀測的衰減變化。綜觀而言，台灣北部急遽之Q值變化可歸因於隱沒板塊接近台灣陸地時，被較厚且冷的歐亞板塊阻擋，造成隱沒速率減緩，弧後擴張停滯，導致溫度變冷及隱沒板塊脫水速率降低，進而使水含量減少所共同造成的結果。本論文提出的溫度－水的模型，綜合了區域構造、地幔動力與地震上的現有知識。此模型也未沖繩海槽的發展提供一個基礎架構。

關鍵字

震波能量衰減 ； 地幔楔 ； 沖繩海槽 ； 水含量 ； 板塊邊緣

並列摘要

Seismic imaging of wave speed and attenuation is one of the most effective probes to reveal the structures of and the geodynamic processes in the earth’s interior. Physical parameters like velocity and attenuation complement each other in interpreting mantle properties, with the latter sensitive to temperature and water content most and insensitive to compositions and errors in event times. However, both the source spectrum and the attenuation of the medium dictate waveforms. The challenge facing us is to isolate the influence of corner frequency fc from the determination of t* or Q. In this study, we present a new method, the cluster event method (CEM), to determine both fc and Q in a robust way and apply it to depict the Q variations in the mantle wedge along the axis of the westernmost Okinawa trough (OT). The CEM features sharing of Q among neighboring paths and allowing single fc for each event, yielding a formulation solved best with the optimization algorithm of simulated annealing.

The two ocean-bottom seismometers deployed in the OT joined the BATS in northern Taiwan to form 3 profile groups that facilitates an observation of how the mantle wedge changes as approaching the edge of the subduction zone. Our results reveal a dramatic increase in Q value, from 60 beneath the moderately rifting OT to about 1000 beneath northeast Taiwan where the backarc rifting is incipient. A quantitative estimation indicates that ~400 C cooling is required to explain the increase. Several mutually-interrelated causes are pursued, and the most significant contributors is the 200-250 C cooling due to the weakening of the OT toward Taiwan. Other thermal factors are derived from the contact between the Eurasian lithosphere and the slab and may contribute only 50 C. The total thermal effect amounts to 250-300 C.

In this thesis we propose that water plays an important role to raise the Q value. The retarded subduction by the resistance along the Eurasia-slab boundary cuts short the water supply from the slab, decreasing the amount of water in the mantle wedge. A factor of 10 depletion of water in the mantle wedge can fill the 100-150 C deficit left from the inferred thermal budget alone. The temperature-water scenario integrates regional tectonics, mantle dynamics, and seismology into a self-consistent working model that not only sheds light on the evolution of the OT but also helps direct new research effort to a global characterization of the plate edge dynamics.

並列關鍵字

seismic attenuation ； mantle wedge ； Okinawa trough ； water content ； plate edge

參考文獻

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