利用海底地震儀記錄研究台灣東部外海淺層地殼速度構造和噪訊特性

Abstract

周遭噪訊層析成像利用台站間的互相關函數，近年來被大量應用於地殼剪力波速度構造層析成像的研究。對速度構造的解析度，透過架設密集的測站網長時間觀測連續記錄得到很大提升。自2006年以來，中研院地球所，以及臺美合作的TAIGER計畫，在臺灣東部外海進行一系列的海底地震儀實驗，其主要目的在於擴增台灣地震網的覆蓋範圍，同時改進對台灣東部外海速度構造的解析度。 本次研究使用過去10年間在臺灣東部外海海域所佈放的67个海底地震儀記錄的垂向分量和差壓力儀的記錄，提取出短週期的雷利波和長週期的亞重力波訊號，來研究臺灣東部外海地下構造以及臺灣短週期的secondary microseism和長週期的亞重力波的來源。本研究應用基於連續小波變換的方法來描述訊號的頻譜特徵隨時間的變化，並量取群速度和相速度。結果顯示，基階瑞雷面波可從位移和壓力分量恢復的噪訊互相關函數中被觀測到，主要頻段位於3-8 秒，速度位於0.3-1.5 km/s。長週期的亞重力波，於壓力分量和部分位移分量被觀測到，主要頻段為50-180 秒，速度為0.05-0.1 km/s,符合線性理論的預測。 根据每一區域量测得到的平均频散曲线，利用較準確的本徵震盪的線性疊加理論來計算相速度對P波和S波速度隨深度變化的敏感度算核，然後採用阻尼最小平方線性反演方法求得隨深度變化的一維P和S波速度模型。結果顯示東北外海的沖繩海槽淺層速度較慢，且Vp/Vs比值較高；琉球前弧速度相對較快，在淺層的Vp/Vs比值較高；花東盆地具較快的速度結構，並且Vp/Vs比值較低。另外，由於台灣東部外海淺層剪力波速度因缺少良好的參考模型，線性反演的結果會非常受限於假設的起始模型，未來必須採用非線性的貝葉斯反演以求得統計上可能速度模型的機率分佈。

In recent years, noise cross-correlation functions (NCFs) from ambient noise between paired stations provide the unprecedented interstation path coverage within highly instrumented regions for high resolution shear wave speed tomographic imaging. Since 2006, a number of OBS experiments from the Institute of Earth Sciences (IES), Academia Sinica of Taiwan and TAIGER, a US-Taiwan cooperative research project, have been conducted in deep sea east of Taiwan to expand the path coverage of seismic network and improve the resolution of the subseafloor velocity structure. In this study, we use continuous recordings of vertical records and differential pressure gauges in 67 OBSs deployed offshore eastern Taiwan during the past 10 years to investigate the subseafloor structures from extracted Rayleigh waves and the source origin contributing to the short period secondary microseism and generation of very long-period infragravity waves. We apply a wavelet-based method to characterize the time-varying spectral properties and measure the frequency-dependent group and phase velocities of these waves. The results show fundamental mode Rayleigh waves in both the vertical records and DPG derived NCFs with a dominant period of 3-8 s and relatively slow speed of 0.3-1.5 km/s. The infragravity waves at periods ranging from 50 to 180 s found in DPGs records and some of the vertical records show the speed of 0.05-0.1 km/s. With these robust dispersion measurements, we employ an accurate normal mode summation method to calculate the sensitivity kernels of phase velocity anomalies with respect to fractional perturbations in P- and S-wave velocity Vp and Vs as a function of period and depth. A damped least-squares linear inversion is then conducted to constrain the radial velocity structures beneath the seafloor offshore eastern Taiwan. Our results show that the subseafloor S-wave velocity structure in Okinawa Trough (OT) is slowest among all the investigation areas offshore eastern Taiwan, with a very high Vp/Vs ratio in the topmost 2 km of the crust. The S wave velocity in the Ryukyu forearc is relatively higher but also with a high Vp/Vs ratio in the shallow depths, while that in the Huatung Basin is high with a low Vp/Vs ratio. Because of the lack of constraints on the shear velocity structure offshore eastern Taiwan, the linear inversion results are highly dependent on the assumed initial models. A nonlinear Bayesian inversion is needed to obtain the probability distribution of the possible 1-D structures in the future study.