| 摘要: | 沖繩海槽是琉球弧溝系統作用所造成的弧後擴張盆地，在這樣張裂性的地質作用區域內常有活躍的火成和熱液活動。在過去的研究中，利用地球物理(震測、重力和磁力)、地質(岩石採樣)和實際水下觀測方法，在南沖繩海槽內發現許多有潛力的熱液活動場址，包含火龍火山1、火龍火山2、石林隆堆、蓬萊斷層帶及第四與那國海丘場址等，但這些方法對於熱液相關活動分析所能涵蓋區域通常較小或是無法長期持續觀測。而海底地震儀具備一個水聽器及三分量的地動儀，可以用來持續監測天然地震及各種地動訊號，本研究即希望利用海底地震儀資料分析，來對於這些熱液活動資訊有更進一步了解。
本研究分析了在2017年4月7日至4月18間OR2-2231航次於南沖繩海槽佈放的6個海底地震儀資料，利用時頻分析、模型模擬、訊號來源分析及衰減因子分析等理論方法進行資料處理。地震儀震波時頻分析結果顯示，其中3個海底地震儀有記錄到諧波震顫(harmonic tremor)，一種常被認為與流體在火山內部的共振或火山氣體的排放有關的訊號。在測站距離小於7公里的情況下，諧波震顫只被單一個海底地震儀記錄，推測諧波震顫應該是由局部的活動造成。從時頻圖上觀察到不同測站的資料時頻特性相近，訊號能量基頻約都在4Hz左右。為了瞭解諧波震顫的震源特性，本研究透過火山震顫模擬測試不同物理參數下產生的不同火山震顫，結果顯示諧波震顫在氣體週期性供應時才會產生，且諧波震顫能量峰值的變化可能代表熱液活動情形的改變。將本研究訊號來源定位分析結果搭配前人研究南沖繩海槽內部的噴氣構造位置後，發現諧波震顫訊號來源與透過地球物理方法所獲得之噴氣較密集的區域及海底火山的位置對應良好。此外，本研究還觀察到訊號來源方向隨著時間的變化，推測可能是來自不同方向訊號活動程度差異或是流體在熱液系統內遷移導致的結果。依照能量頻率分布所計算出的衰減因子Q值基本上落在20以下，代表在研究時間內此區域熱液流體的組成非常穩定。
;Okinawa Trough is the back-arc basin formed behind the Ryukyu arc-trench system. In such extensional environments, magmatism and hydrothermal activities are very common. Many gas plumes and submarine volcanoes have been detected in the South Okinawa Trough (SOT), and several potential hydrothermal sites are mapped based on the various geophysical and geochemical survey data, which include Fire Dragon Volcano 1 (FDV-1), Fire Dragon Volcano 2 (FDV-2), Geolin Mounds (GLM), Penglai Fault Zone (PFZ), and Yonaguni Knoll IV (YK4-1). In order to monitor hydrothermal activity in the southwestern part of the SOT, a network of 6 OBSs was deployed during the OR2-2231 cruise in 2017, and the recording period was from April 7, 2017 to April 18, 2017. Harmonic tremor, a seismic signal that is generally associated with volcanic activity, was recorded at three of the OBSs. In this study, the methods used to study harmonic tremor include time-frequency analysis, model simulation, source direction analysis and quality factor estimation.
The result of time-frequency analysis of different OBSs demonstrates similar pattern, which shows a fundamental frequency of about 4 Hz. Each harmonic tremor event was not recorded in more than one station despite the fact that the distance between the stations is less than 7 km, suggesting that these signals should have originated very close to each receiver. In order to understand the source properties, the model simulation method proposed by Girona et al. (2019) was applied in this study. The result suggests that gas supply must contain certain degree of periodicity for the generation of harmonic tremor, and the variation of the dominant frequency may infer the condition of the hydrothermal sites. The source direction determined from the largest energy distribution shows a clear correlation with the gas plumes and submarine volcanoes position, inferring their causal relationship. However, the time-dependent variation for the source direction and the fundamental frequency may be associated to the different activity level of different sources or the fluid migration in the hydrothermal systems. Finally, the Q values are mainly lower than 20, which suggests that the composition of the hydrothermal fluids is stable during the studied period. |
