台灣北部短週期噪訊研究 1. 周遭噪訊層析成像 2. 噪訊來源研究

Abstract

由於操作簡單以及相對於傳統地震學在表面波層析成像法上佔有許多優勢，因此藉由計算測站之間連續紀錄的交互相關函數而得到測站之間格林函數的技術，現在已被廣泛應用在地震學的研究上。本研究應用此技術於分析2006年北台灣三個地震網三方向的連續紀錄，其中包含中央氣象局地震觀測網北部測站以及中央研究院在新竹和陽明山所架設的微震觀測網。針對每條波徑，我們分別計算垂直、徑向與橫向三個方向的交互相關函數，垂直與徑向的交互相關函數代表雷利波的格林函數，而橫向的則代表洛夫波的格林函數。接著我們測量交互相關函數1秒至5秒之間的群速度與相速度值。經過資料篩選之後，我們使用品質穩定的頻散曲線，並配合多重尺度參數法，反演北台灣雷利波與洛夫波的二維相速度與群速度速度構造，再利用二維速度構造的結果，建構北台灣淺層地殼三維速度構造。結果顯示，高解析度的速度模型與地質構造單元十分吻合。 我們也藉著以下方法來研究周遭噪訊的特性：（1）分析交互相關函數正負方向訊號的相對強度；（2）測量交互相關函數相對於一整年之平均值的振幅變化，建立背景能量起伏與時間和方位角之間的關係；（3）計算沿海測站連續紀錄的頻譜密度以及方均根值隨時間的變化。結果顯示：（1）近岸的海浪可能是主要的噪訊來源，而當能量從海洋傳到陸地時，海底地形可能扮演著重要的角色；（2）不同方向之交互相關函數在時空變化的特性上十分相似，意味著不同方向的噪訊在淺層地殼中藉由散射而被充分混和，達到近似散射場的環境；(3) 大氣的擾動可能是影響交互相關函數時空變化特性的原因。

Retrieving Green functions between stations by cross-correlating continuous seismic records has quickly become a popular technique in seismology for its operational simplicity and various advantages over traditional surface wave tomography. We apply this technique to three component continuous seismic data recorded from three networks in northern Taiwan, including Tatun Volcanic Area array, Hsinchu array and northern part of Central Weather Bureau Seismic Network, for the time period from Jan, 2006 to Dec, 2006. For each station pairs, we derive Love waves from T–T (transverse) component cross-correlation functions (CCF), and Rayleigh waves from Z-Z (vertical) and R-R (radial) component CCF respectively. We measure group and phase velocities for the period range from 1 to 5 seconds. With careful data selection, the qualified dispersion curves are used to derive two dimensional (2-D) phase and group velocity maps for both Rayleigh and Love waves with multi-scale inversion technique. The 2D maps are then used to develop a 3-D shallow velocity structure of the northern Taiwan. We also attempt to probe the sources of ambient noise by several approaches: (1) analyzing the relative strength between the causal and acausal CCF; (2) measuring the relative strength of CCF amplitudes with respect to their own annual average as a function of time and azimuth to determine the background energy flow; and (3) computing power spectra density and rms amplitudes as a function of time for representative costal stations. The results show that (1) offshore ocean waves are likely the major ambient noise source, and bathymetry might play a role in the process of energy transfer from ocean to continent; (2) there are clear and similar temporal variations for different component of CCF, implying different component of noises are well mixed during scattering in the shallow crust, and a quasi-diffuse field for ambient noise is achieve; and (3) the atmosphere perturbations might be responsible for the observed temporal variations of CCF.