利用噪訊成像反演宜蘭平原上部地殼 三維高解析度S波速度構造;High-resolution 3-D Shear Wave Upper-crust Structures in Ilan Plain using Ambient Noise Tomography

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题名:	利用噪訊成像反演宜蘭平原上部地殼三維高解析度S波速度構造;High-resolution 3-D Shear Wave Upper-crust Structures in Ilan Plain using Ambient Noise Tomography
作者:	陳凱勛;Chen,Kai-Xun
贡献者:	地球科學學系
关键词:	周遭噪訊;宜蘭平原;快速推進法;層析成像;ambient seismic noises;Ilan Plain;fast marching method;wavelet-based sparsity-constrained tomographic inversion
日期:	2015-08-14
上传时间:	2015-09-23 11:30:28 (UTC+8)
出版者:	國立中央大學
摘要:	宜蘭平原位在台灣的東北方，處於琉球隱沒系統中沖繩海槽的最西端，平原內的高地溫梯度與多處溫泉可能是弧後擴張伴隨的岩漿活動有關(Tong et al., 2008)。宜蘭平原大致上可被蘭陽溪分割成南北兩半邊，相較於北半邊，南半邊近地表的未固結第四紀沉積物較薄(江新春, 1976)，厚度不超過1 km，較多的島內地震集中在三星斷層，相較於澎湖呈現東南方向的位移(Hou et al., 2009)。本研究透過反演得到宜蘭平原地區三維上部地殼剪力波速度構造來達到兩個主要目的，第一是根據速度構造中所呈現的低速異常來評估地下地熱源的範圍；第二是配合震測調查結果，根據速度低速區繪製沉積物的立體形貌。我們從2014年8月至2015年1月在宜蘭平原及其周圍佈放了89個單分量短週期的Texan儀器，站間距約2 km，使用了周遭噪訊層析成像法來反演得到高解析度的三維上部地殼剪力波速度構造。在資料處理上，第一步從兩兩測站配對的地震儀連續紀錄中重建兩測站之間的基態雷利波經驗格林函數；第二步分別使用了多重濾波法以及影像轉換技術得到頻寬0.25-1.67 Hz的群速度以及相速度頻散曲線；最後先利用快速推進法計算在非均勻介質中測站配對的波線路徑與速度，再以小波為基底的模型參數化方式直接反演得到三維剪力波速度結構。從速度構造結果顯示剪力波最低速在地下0.5 km內可慢至0.4 km/s，與震測資料分析的垂直剖面相比(江新春, 1976)，慢速沉積物的低速區可對應到相同的速度構造剖面。平原西部的速度構造顯示在深度1 km內呈現局部的低速異常，而這位置與已存在的地熱井位置相同。 ;The Ilan Plain (IP) in NE Taiwan locates on the western end of the Okinawa trough and exhibits high geothermal gradients with abundant hot springs, likely resulting from magmatism associated with the back-arc spreading as manifested by the offshore volcanic island (Kueishantao)(Tong et al., 2008). North and south sides of IP are divided by Lan-Yang River with distinctive characteristics. Comparing to the northern part, the southern part exhibits, relatively, thin unconsolidated Quaternary alluvium layer with depths ranging from 0 to 1 km (Chiang, 1976), high on-land seismicity and significant SE movements relative to Penghu island. Purposes of this study are two folds. By obtaining a high-resolution 3-D shear wave upper-crust structures, we aim at (1) assessing the extent of underground geothermal sources as revealed by low velocity anomalies, (2) mapping 3-D sedimentary structures as revealed by the structures of very low velocity zones at surface. To fulfill this goal, we deployed 89 Texan instruments (~2 km station interval) between Aug. 2014 and Jan. 2015, covering most of the IP and its vicinity. We conduct methods of ambient noise tomography for inversion of high-resolution 3-D shear wave upper-crust velocity structures. Firstly, we estimate empirical Green’s functions (EGF) of Rayleigh wave between station pairs by ambient noise cross-correlation. Secondly, dispersion curves of group and phase velocities are measured at the frequency range between 0.25 and 1.67 Hz from each EGFs. Multiple filter Technique and Image transformation technique are used to measured group and phase velocities at each period, respectively. Finally, we apply a fast marching method for inhomogeneous-medium ray tracing and for calculations of velocities between station pairs. We also adopt a wavelet-based sparsity-constrained tomography method for the direct inversion of 3-D shear wave velocity structures. Results show that the lowest shear wave velocity can be as low as 0.4 km/s. mostly at depths shallower than 500 meters. Having examined the vertical cross-sections of each profiles, the spatial distributions of low velocity zones well match to those of sedimentary structures as shown by seismic reflection survey (Chiang, 1976). Results in west IP show that local low velocity anomalies with depth shallower than 1 km display in regions of known geothermal wells.
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