利用噪訊成像法以及Ps接收函數探究日本東北之地殼構造

Abstract

日本東北是日本各地區當中構造單純的區域，在此地區的研究結果是往後研究其他受板塊隱沒與岩漿活動區域的基石。在這裡，太平洋板塊向西隱沒到歐亞板塊下，島的東側由兩個相當古老的岩塊組成，中間有多座活火山，西側由零星的活火山以及第三紀、第四紀沈積物組成。相較於過去三四十年的研究多著重在地幔的構造，近期受惠於資料品質的提升以及技術方法的演進，地殼尺度的相關研究也逐漸增加中。 我們使用佈設於此區一百個左右Hi-net測站之連續噪訊紀錄，計算兩測站間的交相關函數，這些廣佈的高品質測站交織出相當密集的波線路徑，進而將擷取出的基態表面波訊號用於建立高解析的地殼剪力波速度與方位非均向的模型。在三維的剪力波速度模型中，幾乎所有的火山下方都在5-18公里的深度間表現出顯著的低速異常，而在9-13公里深的速度異常更是可以高達將近-10%。而三維的非均向性模型顯示日本東北的地殼是由兩個不同的變形機制所構成，其中淺部5公里的快方向主要為南北向，平行島嶼走向，推斷是岩石受應力後而產生的排列方式; 20公里以下的快軸主要是東西向的板塊聚合方向，成因可能是受到在地幔楔因隱沒的二維地幔楔流場所帶動的剪力所導致;另一種成因可能是這地區在上一次的日本海張裂時期就已經形成並冷卻殘餘留下的非均向性; 在這兩層之間的非均向性則呈現強度減弱、快方向排列不一致的結果，顯示火山下方的震波速度明顯受到火成活動影響。 另外，遠震的接收函數也被用來探討地殼內不連續面的分佈與可能的成因，在分析將近15年總計約1800個的遠震資料後，我們發現接收函數中負相位的訊號多集中在火山附近約8-10公里深，先前的研究多將這類訊號歸因於地殼內的非均向性，不過我們在反覆確認計算結果、並利用合成波型來測試後，發現單純用地殼內非均向性來解釋所觀察到的負相位強度是遠遠不夠的，同時，這些負相位訊號在我們的剪力波速度模型上正巧是處於低速異常的上邊界，意味著該處可能是因為火山熔體堆積在整個儲集區的頂部而產生的速度下降，經計算後得知約有5~10%的熔體含量，而零星分佈在下部地殼的負異常訊號可能是反應另一個較深處熔體儲集區的上邊界。 以上兩個研究使用了地震學中不同的分析方法，不但能夠相互驗證與解釋，與火山構造的吻合也將單純的速度分佈賦予了火山構造的含義，使我們對這地區的地殼構造能夠有進一步的認識。

Northeast Japan, or the Tohoku region, is the “simplest” area in Japan archipelago for fundamental study of subduction-magmatism system that will be the basis for the other similar regions. Here, the Pacific plate westward subducts beneath the Eurasian plate, the east two mountains are composed of Cretaceous rocks, and Holocene arc volcanoes dominate in the central belt and lie more sporadically in the west. The seismic structure of Tohoku, particularly in the mantle was broadly investigated in the past few decades. In contrast, crustal-scale study is increasing in recent years, contributing to the improvements in data quality and analysis method. In the first part of this thesis, continuous records from 123 Hi-net stations were used to construct cross-correlation functions between each potential station-pair. These high-quality, widely-deployed borehole stations yield a data set of fundamental-mode surface wave with a dense path coverage, allowing us to simultaneously inverts for high-resolution shear velocity and azimuthal anisotropy crustal models. In the Vs structure, under most of the active volcanoes, low velocity zones are more common in the upper crust and are strong at a depth of 9-13 km with anomaly up to -10%. In azimuthal anisotropy, a crustal two-layer pattern is resolved, which a near N-S, island-parallel fast direction in shallow 5 km depth that may map structural fabrics of crustal deformation, and a near E-W, convergence-parallel fast direction in lower crust that may result from shearing either imposed by the return flow in the mantle wedge or frozen-in from the last stage of extension of the continental margin. Between these two layers, coherent crustal fabrics collapse and anisotropy turns to a chaotic state, suggesting that continuous volcanic activities disrupt the otherwise consistent fabrics. In the second part of this thesis, Ps receiver functions were used to investigate crustal discontinuities and their origins. From ~1800 teleseismic events occurred in 15 years, the calculated receiver functions show prevalent negative phases at a depth range of 8-10 km near the active volcanoes. In a previous study, the negative phases were attributed to crustal azimuthal anisotropy. However, after a closer look at the results and a series of tests with synthetic seismograms, we inferred that azimuthal anisotropy is insufficient to explain the strong negative phases. These pronounced phases correlate well with the upper margins of low velocity zones beneath volcanoes, which this feature is consistent with model for crustal melt storage where the highest concentrations of melt are localized at the top the mush column, implying a 5-10% melt fractions across the Tohoku. Some groups of deeper negative Ps phases may reflect the upper boundary of melt storage at greater depths. In this thesis, observations from ambient noise tomography and Ps receiver function could explain well with each other and correlate well with surface geology that provides seismic velocity more implication in volcanic structure, letting us move forward in crustal structure of Tohoku.