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http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/9429
標題: | 美國西南部地區上部地幔震波層析成像 Multi-Scale Finite-Frequency Travel-time Tomography Applied to Imaging 3-D Velocity Structure of the Upper Mantle beneath the Southwest United States |
作者: | Yi-Fan Yin 尹一帆 |
指導教授: | 洪淑蕙 |
關鍵字: | 美國,層析成像,多尺度,有限頻寬,小波, tomography,United States,finite frequency,multi-scale,wavelet, |
出版年 : | 2009 |
學位: | 碩士 |
摘要: | 震波層析成像是瞭解地球深部之動力機制的重要工具,在研究美國西南部地質及構造的研究中為其深部的動力解釋提供重要證據。其中,從1999至2001年,長950公里、橫跨Colorado Plateau、Rio Grande Rift及Great Plains的La Ristra線性陣列在此設立,希望能觀測穿過此三個不同地質區的地幔剖面。利用前述資料所建構的二維模型由於在逆推速度構造時無法避免的正則化與平滑化問題,其所解出之速度異常幅度太小,無法解釋P波及S波沿著陣列觀測到的波形變化。本研究利用La Ristra陣列和附近所有能取得的資料,結合新近設立的USArray地震觀測網得到更完整的資料覆蓋。P波及S波走時資料經濾波成高低兩個頻帶(高頻:P波0.3-2 Hz,S波0.1-0.5;低頻:P波0.03-0.125 Hz,S波0.03-0.1 Hz),並利用波形間互相比對求得最佳化的走時異常。不同於傳統的震波層析成像採用波線理論與格點參數化,本研究使用有線頻寬理論以及以小波理論為基礎的多尺度參數化來建構其速度模型。有線頻寬理論能更真實的反應三圍速度構造與走時異常的對應,多尺度參數化則使模型能隨資料密度自行調整解析度。
反演結果顯示由50公里深度直到250公里處,速度異常分佈大致與地表地質構造相關。最明顯的為環繞Colorado Plateau南部,並隨晚新生代火山帶延伸向東的慢速構造。250公里以下,速度構造與地表地質構造的關聯漸消失。在Colorado Plateau下方,300公里以下有明顯慢速構造存在,此一慢速異常可能為高原的海拔高度提供浮力來源。 Seismic tomography has played a key component in unraveling the deep processes that caused the surface morphology and rift magmatism in the southwest United States. Earlier study used teleseismic body-wave arrivals recorded by the La Ristra experiment, a dense broadband array of 950-km in length deployed during 1999-2001 and run through the Great Plains, the Rio Grande Rift (RGR), and the Colorado Plateau, to construct a 2-D tomographic image of the upper mantle structure beneath this linear array. However, because of the inevitable smoothing and damping imposed in the inversion, the resulting velocity contrast is too weak to explain distinct P and S waveform changes across the array. In this study, all the available data from the La Ristra, nearby arrays, and newly deployed USArray are utilized for the determination of frequency-dependent travel-time shifts by inter-station cross correlation of waveforms at both high- (0.3-2 Hz for P and 0.1-0.5 Hz for S) and low-frequencies (0.03-0.125 Hz for P and 0.03-0.1 Hz for S). Different from the previous models that rely on classical ray theory and regular grid parameterization, the new tomographic images are built based on state-of-the-art 3-D, finite-frequency sensitivity kernels that relate observed travel-time data to velocity heterogeneity and a wavelet-based, multi-scale parameterization that enables to yield robust structures with spatially-varying resolutions subject to data sampling. The resulting P and S models reveal a prominent smile-shaped region of low wave speed anomalies encircling the Colorado Plateau and confined in the uppermost 250 km depth. The lowest velocity anomalies are highly correlated with the late Cenozoic volcanic fields but do not underlie the rift center. At depths greater than 250 km, the variations of P and S velocity structures appear less coherent with the surface geological features. A very slow-velocity core inside the Colorado Plateau extending from ~300 km to greater depths may provide a plausible explanation for its elevated topography. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/9429 |
全文授權: | 同意授權(全球公開) |
顯示於系所單位: | 地質科學系 |
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