台灣東部海域地震觀測研究暨儀器研發; A study of earthquake observation in the eastern offshore Taiwan and Developing of OBS instruments

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Title:	台灣東部海域地震觀測研究暨儀器研發;A study of earthquake observation in the eastern offshore Taiwan and Developing of OBS instruments
Authors:	林慶仁;Ching-ren Lin
Contributors:	地球物理研究所
Keywords:	地振動背景雜訊;海底地震儀;感震器;花東海盆;Ambient noise;ocean-bottom seismometers(OBSs);Huatung basin;geophone
Date:	2009-12-21
Issue Date:	2010-06-11 15:48:13 (UTC+8)
Publisher:	國立中央大學圖書館
Abstract:	本論文主要是使用首次在台灣東部海域收集到的長期寬頻海底地震資料，分析海床的地震動背景雜訊特性、探討BBOBS記錄地震資料的品質以及應用在探討地下構造和改善地震定位上的成果，並詳述自行研發之中週期OBS的設計、元件規格及初步成果。在海床的地震動背景雜訊部分，水平分量的PSD高峰值在頻段0.2~0.5Hz之間大約為-90dB，垂直分量則約-100dB，海底測站的地振動背景雜訊PSD比海邊測站高，陸上測站離海岸越遠PSD越低，推論在0.2 - 0.5Hz之間的雜訊高峰很可能是由於剪力波在海床沈積物間共振[Stephen et al. 2003a]及海風吹襲[Bromirski et al. 2005]、海面風浪[McCreery et al., 1993]等綜合效應產生的結果。在6 - 7Hz之間OBS測站都有一個比前後頻帶高約40dB的雜訊PSD高峰，這個雜訊高峰可能是由於海面航行的船隻或海浪碎波所造成的[Bromirski et at.,2005]。在低頻方面，自0.05Hz以下水平分量的雜訊與海潮水流有關[Bromirski et at.,2005]，在0.005-0.03Hz頻帶內，水深小於2000米的測站，垂直分量可以很明顯的看出有10- 30dB的雜訊凸起，推論是因為波浪由深海傳至淺海的過程中，由於淺化的影響導致波浪尖銳度增加而引發的長波振盪現象（稱為亞重力波，infra-gravity wave）所引起。隨著頻率的降低，OBS測站的水平分量地震動背景雜訊PSD成線性增加的趨勢。本論文也探討OBS記錄地震資料的品質，利用寬頻海底地震儀收錄之東部外海地震資料，評估對於地震定位精確度之改善程度，比對加入海底地震儀資料前後，板塊邊界地震活動度之統計。在波形應用實例上，利用單一OBS測站配合陸上寬頻地震網資料，利用雷利波的波傳效應可用來研究地下構造。透過部署於跨越花東海盆兩邊測站收到的雷利波波傳播路徑，使用有限頻率的分析證明這種雙站雷利波方法的有效性，逆推結果顯示花東海盆岩石圈的Vs比西非海盆減少約0.1 km/s，年齡估計是15-30Ma。在picking過程中採用微分手段，把速度記錄轉成加速度記錄比較準確地標出P波和S波到時。由於海底測站之數量與分布都還太少，因此不管是一維或是三維速度模型在海域部分並不是非常精確，想要有精確的速度模型必須有較多長期的海底測站收錄大量的地震資料以獲取理想的波線，建議未來能部署更多OBS，除了建立精確的速度模型之外，更有助於改善海底地震定位之誤差。目前結合中央研究院、國立中山大學和國家實驗研究院等單位組成的研發團隊，已經能成功研發建造中週期OBS，本論文也針對中週期OBS的設計與元件規格做探討。實驗證明自行研發的資料記錄裝置和感震器具有記錄長週期震波的能力。 Broadband records from ocean-bottom seismometers(OBSs) deployed in the Okinawa trough and the Huatung basin were analyzed to provide seafloor noise characteristics and the detection thresholds for teleseismic body and surface waves. Ambient noise levels on the horizontal components are 10-40 dB higher than on the vertical component, with the sensor seated on the surface of the sediment. On the vertical components, infragravity waves are 10-30 dB more energetic at the shallower Okinawa trough sites ( 2000 m depth) than at the deeper Huatung basin site (~4700 m). From 0.03 Hz to 0.2 Hz, the Huatung basin noise levels are comparable to that of the broadband stations in Taiwan on a quiet day. The microseism peaks (~0.2-0.5 Hz) of OBSs reach or exceed the high noise model of continental stations. At regional distances Mw 6.5 is required for recording prominent Rayleigh waves if the source radiation is unfavorable, but 6.2 is sufficient for a favorable focal mechanism. Several tens to over one hundred high-fidelity P, Pdiff and PKP waveforms have been recorded per year by OBSs at high corner frequency 0.1 Hz with a minimum Mw 5.3-6.0. The number of recording drops to less than 5 per year at 1 Hz with Mw 6.4 and distances less than 90o. The OBS deployed ~180 km east of Taiwan provides a first glimpse into the upper mantle beneath the westernmost section of the Philippine Sea or the Huatung basin. We measured inter-station phase velocities of Rayleigh waves between the OBS and stations on the eastern coast of Taiwan. The phase velocities show smooth variations from 3.8 to 3.9 km/s for periods of 25-40 s. In this short period range, phase velocities are comparable to those characterizing the 15-30 Ma Parece-Vela basin of the Philippine Sea. The seismological evidence, together with the correlation in seafloor depth, suggests that the Huatung basin and the Parece-Vela basin may have a similar age. This is at odds with the previous geochronological study suggesting an early-Cretaceous age for the Huatung basin. We have joint-forced with the National Sun Yat-Sen university and the Taiwan Ocean Research Institute in designing and building a wide-band OBS that is aimed for 3 month deployment in shallow waters around Taiwan. An array of 10 of this low-coast, low power budget instrument is scheduled for deployment in the northeast corner of Taiwan in summer of 2010. The experimental test is reported in this thesis.
Appears in Collections:	[Graduate Institute of Geophysics] Electronic Thesis & Dissertation

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