花東縱谷一般被認為是菲律賓海板塊與歐亞板塊的縫合帶，地震活動頻繁。1951年Mw7.3的花蓮地震序列與2018年Mw6.26的0206花蓮地震，皆觀測到縱谷北段的米崙斷層產生破裂。近年來在縱谷北段的研究中，前人提出許多縱谷北段的地下構造模型，但仍有許多問題尚未釐清。為了瞭解斷層位置與地層走向，本研究使用重力異常值推算密度構造來探討縱谷北段地下構造。
本研究中搜集了研究區域內陸域及海域的重力資料，經過重力修正流程後，繪製出縱谷北段的重力異常圖，並進行地下構造之模擬。使用Talwani et al.(1959) 所提出的二維理論重力值公式計算2D密度模型的重力效應。為了減少重力建模中的非唯一解的問題，本研究使用地震層析成像、反射震測、地表地質圖和此區域的先前結果來約束密度模型。在使用嘗試錯誤法修改模型後，本研究最終得到一個符合觀測重力值的密度構造，並且提出了四個垂直於縱谷走向的二維密度剖面。
本研究確認了縱谷北段的地層走向和斷層位置。由於菲律賓海板塊與歐亞板塊之間的碰撞，縱谷北段的沉積物變厚並向東傾斜，嶺頂斷層可能是因海岸山脈抬升造成的沉積層邊界。構造觀察還表明，呂宋島弧開始向縱谷北段俯衝，導致米崙台地抬升。本研究所提出的密度模型為縱谷北段的地下構造提供了更多了解。

The Longitudinal Valley in eastern Taiwan is characterized by frequent earthquakes and complex geologic structures. It is generally considered as the suture of the collision between the Philippine Sea plate and the Eurasian plate. When the M7.3 earthquake in October 1951 and the M6.4 earthquake in February 2018 happened, the rupture of the Milun fault was observed in the northernmost Longitudinal Valley (NLV). To understand the fault strike and geological formation in the NLV, we use gravity anomalies to model the density structures and then explored the tectonic structure mechanism.
In this study, we collected the gravity data including onshore and offshore in NLV. After the gravity corrections, we obtained the gravity anomaly map of the NLV and proceeded to the 2-D density profiles modeling. Then using the Talwani method to calculate the effect of the 2D density model. To reduce the non-unique problem in gravity modeling, the density model was constrained by using the tomography, the reflection seismic data, the surface geological mapping and the previous results in this area. After using trial and error method to modify the model, we finally get a reliable density structure that conform with observed gravity data. We present four 2D density profiles perpendicular to the strike of the NLV.
After the gravity modeling, we get the locations of strata and faults in the NLV. Because of the collision between the Philippine Sea plate and the Eurasian plate, the sediment in the NLV becomes thick and dipping to east. The Lingding fault might be a sediment boundary made by the Coastal Range uplifted. Structural observations also show that the Luzon arc has started to subduct in the NLV, caused the Milun tableland uplifted. Those density models provide more details to understand the tectonic structure beneath the NLV.

第一章　緒論　　1
第二章　研究區域概述　　23
第三章　重力修正流程與重力異常　　33
第四章　地下密度構造模擬　　49
第五章　結果與討論　　63
第六章　結論　　75

Brocher, T.M., (2005). Empirical relations between elastic wave speeds and density in the Earth's crust. Bull. Seismol. Soc. Am. 95, 2081–2092. http://dx.doi.org/10.1785/0120050077.
Chen, W.S. and Y. Wang, (1988). Development of deep-sea fan systems in coastal range basin, Eastern Taiwan. Science Reports of the National Taiwan University, Acta Geologica Taiwanica, No.26, PP. 37-56.
Cooper, G. R. J. (1998). Geomodel for Windows, 2.5D interactive magnetic and gravity data modeling and inversion.
Chang, C. P., Angelier, J., C. Y. Huang, & C. S. Liu, (2001). Structural evolution and significance of a mélange in a collision belt: the Lichi Mélange and the Taiwan arc–continent collision. Geological Magazine, 138(06). doi:10.1017/s0016756801005970
Chung, L.H., Y.G. Chen, Y.M. Wu, J.B.H. Shyu, Y.T. Kuo, and Y.N.N. Lin, (2008). Seismogenic faults along the major suture of the plate boundary deduced by dislocation modeling of coseismic displacements of the 1951 M7.3 Hualien-Taitung earthquake sequence in eastern Taiwan, Earth Planet. Sci. Lett., 269, 416-426.
Chen, C.Y., J.C. Lee, Y.G. Chen, and R.F. Chen, (2014). Campaigned GPS on Present-day crustal deformation in northernmost Longitudinal Valley preliminary results, Hualien Taiwan. Terr. Atmos. Ocean. Sci., 25, 337-357, doi: 10.3319/TAO.2013.12.25.01(TT)
Doo, W.B., C.L. Lo, S.K. Hsu, C.H. Tsai, Y.S. Huang, H.F. Wang, S.D. Chiu, Y.F. Ma, C.W. Liang, (2018). New gravity anomaly map of Taiwan and its surrounding regions with some tectonic interpretations. Journal of Asian Earth Sciences,154, 93-100. doi: 10.1016/j.jseaes.2017.12.010.
Doo, W.B., Kuo-Chen, H., Brown, D., Lo, C.L., Hsu, S.K., Huang, Y.S., (2016). Serpentinization of the fore-arc mantle along the Taiwan arc-continent collision of the northern Manila subduction zone inferred from gravity modeling. Tectonophysics 691, 282–289. http://dx.doi.org/10.1016/j.tecto.2016.10.019.
Dehlinger, P., (1978): Marine gravity, Elsevier Scientific Pub. Co., Netherlands, 137.
Hu, C.C., Chen, W.S., (1986). Gravity and magnetic anomalies of eastern Taiwan. Mem. Geol. Soc. China 7, 341–352.
Hsu, S.K., Liu, C.-S., Shyu, C.T., Liu, S.Y., Sibuet, J.-C., Lallemand, S., Wang, C.-S., Reed, D., (1998). New gravity and magnetic anomaly maps un the Taiwan-Luzon region and their preliminary interpretation. Terr. Atmos. Ocean. Sci. 9, 509–532.
Huang, S. Y., J. Y. Yen, B. L. Wu, I. C. Yen, and R. Y. Chuang, (2019). Investigating the Milun Fault: The coseismic surface rupture zone of the 2018/02/06 ML 6.2 Hualien earthquake, Taiwan. Terr. Atmos. Ocean. Sci., 30, 1-25, doi: 10.3319/ TAO.2018.12.09.03
Kuo-Chen, H., Wu, Y.-M., Chang, C.-H., Hu, J.-C., Chen, W.-S., (2004). Relocation of the eastern Taiwan earthquakes and its tectonic implications. Terr. Atmos. Ocean. Sci., 15, 647–666.
Kuo-Chen, H., F. T. Wu, and S. W. Roecker (2012). Three-dimensional P velocity structures of the lithosphere beneath Taiwan from the analysis of TAIGER and related seismic data sets, J. Geophys. Res., 117, B06306, doi:10.1029/2011JB009108.
Ludwig, W.J., J.E. Nafe, and C.L. Drake, (1970). Seismic refraction In the Sea, Vol. 4, Maxwell, A. E., ed., Wiley-Interscience, New York.
Morelli, C., (1976), Modern standards for gravity surveys, Geophysics, 41, no. 5, 1051.
Rau, R.J., K. H. Chen, and K.-E. Ching (2007). Repeating earthquakes and seismic potential along the northern Longitudinal Valley fault of eastern Taiwan, Geophys. Res. Lett., 34, L24301, doi:10.1029/2007GL031622.
Shyu, J. B. H., Y. R. Chuang, Y. L. Chen, Y. R. Lee, and C. T. Cheng, (2016a). A new on-land seismogenic structure source database from the Taiwan Earthquake Model (TEM) project for seismic hazard analysis of Taiwan. Terr. Atmos. Ocean. Sci., 27, 311-323, doi: 10.3319/TAO.2015.11.27.02(TEM)
Shyu, J. B. H., C. F. Chen, and Y. M. Wu (2016b). Seismotectonic characteristics of the northernmost Longitudinal Valley, eastern Taiwan: Structural development of a vanishing suture, Tectonophysics 692, 295–308.
Sandwell, D. T., Muller, R. D., Smith, W. H. F., Garcia, E., & Francis, R. (2014). New global marine gravity model from CryoSat-2 and Jason-1 reveals buried tectonic structure. Science, 346(6205), 65–67. doi:10.1126/science.1258213
Talwani, M., J. L. Worzel, and M. Landisman, (1959): Rapid gravity computations for two-dimensional bodies with application to the Mendocino submarine fracture zone. J. Geophys. Res., 64, 49-59, doi: 10.1029/JZ064i001p00049.
Turcotte, D. L., and G. Schubert, (1983), Geodynamics, Cambridge University Press.
Teng, L.S., W.-S. Chen, Y. Wang, S.R. Song and H.J. Lo, (1988): Toward a comprehensive stratigraphic system of the Costal Range, Eastern Taiwan. Science Reports of the National Taiwan University, Acta Geologica Taiwanica, No.26, PP. 19-25.
Ustaszewski, K., Y. M. Wu, J. Suppe, H. H. Huang, C. H. Chang, and S. Carena, Crust-mantle boundaries in the Taiwan - Luzon arc-continent collision system determined from local earthquake tomography and 1D models: Implications for the mode of subduction polarity reversal, Tectonophysics, special issue: Geodynamics and Environment in East Asia, 578, 31-49, 2012.
Yen, H.Y., W.T. Liang, B.Y. Kuo, Y.H. Yeh, C.S. Liu, D. Reed, N. Lundberg, F. C. Su, and H.S. Chung, (1995). A regional gravity map for the subduction-collision zone near Taiwan, TAO, 6, no. 2, 233-250.
Yu, S.B., Chen, H.Y., Kuo, L.C., (1997). Velocity field of GPS stations in the Taiwan area. Tectonophysics 274, 41–59.
Yen, J. Y., C. H. Lu, C. P. Chang, A. J. Hooper, Y. H. Chang, W. T. Liang, T. Y. Chang, M. S. Lin, and K. S. Chen, (2011). Investigating active deformation in the northern Longitudinal Valley and City of Hualien in eastern Taiwan using persistent scatterer and small-baseline SAR interferometry. Terr. Atmos. Ocean. Sci., 22, 291-304, doi: 10.3319/TAO.2010.10.25.01(TT)
Yen, J. Y., C.H. Lu, R. J. Dorsey, H. Kuo-Chen, C.P. Chang, C.C. Wang, R. Y. Chuang, Y.T. Kuo, C.Y. Chiu, Y.-H. Chang, F. Bovenga, and W.Y. Chang, (2018). Insights into seismogenic deformation during the 2018 Hualien (Taiwan) earthquake sequence from advanced modeling of InSAR and GPS data, Seismol. Res. Lett.doi:10.1785/0220180228.
王源，楊昭男，陳文山，1991，花蓮[地質圖幅及說明書1/50,000]，新北市中和區：經濟部中央地質調查所。
何春蓀，1986，台灣地質概論—台灣地質圖說明書，第二版，經濟部中央地質調查所。
林啟文、陳文山、劉彥求、陳柏村，2009，臺灣東部與南部的活動斷層二萬五千分之一活動斷層條帶地質圖說明書。經濟部中央地質調查所特刊，第23號，共178頁。
徐鐵良，1956，臺灣東部海岸山脈地質，臺灣省地質調查所彙刊，第8號,第15-41頁。
徐子媛，2013，台灣東部二維密度構造之模擬，國立中央大學地球物理研究所碩士論文。
陳文山，王源，1996，臺灣東部海岸山脈地質，臺灣地質，7，臺北縣中和市: 經濟部中央地質調查所。
陳文山，林益正，顏一勤，楊志成，紀權窅，黃能偉，林啟文，林偉雄，侯進雄，劉彥求，林燕慧，石同生，盧詩丁，2008，從古地震研究與GPS資料探討縱谷斷層的分段意義，經濟部中央地質調查所特刊，第20號，第165-191頁。
高宇軒，2016，利用大地測量和地球物理探勘探討花蓮市區斷層活動與地表變形，國立東華大學自然資源與環境學系碩士論文。
廖宏祥，2006，米崙斷層淺層研究，國立中正大學地震研究所碩士論文。
蔡坤哲，2001，重力地形修正之探討，國立中央大學地球物理研究所碩士論文。
鄧屬予，2007，台灣第四紀大地構造。經濟部中央地質調查所特刊，第18號，1-24頁。
謝文祺，2004，台灣中部二維密度構造之探討，國立中央大學地球物理研究所碩士論文。
謝獻祥，2009，利用重力資料探討台灣及其鄰近地區之三維地下構造，國立中央大學地球物理研究所博士論文。
謝一銘，2017，台灣花東縱谷北段反射震測調查，國立中央大學地球物理研究所碩士論文。
簡立凱，2015，花東縱谷斷層之地電研究，國立中央大學地球物理研究所博士論文。
顏滄波，1963，台灣大南澳片岩區中之變質帶，中國地質學會會刊，第六號，第72-74頁。
顏宏元，1991，台灣地區重力異常分布及其在地體構造上之含義，國立中央大學地球物理研究所博士論文。
經濟部中央地質調查所，1993，十萬分之一海岸山脈地質圖。
經濟部中央地質調查所，2006，都會區及周緣坡地整合性環境地質資料庫建置：坡地岩體工程特性調查研究(5/5)—東部地區期末報告，中央地質調查所報告第95-21號，共178頁。
經濟部中央地質調查所，2016，山崩與地滑地質敏感區劃定計畫書，L0017花蓮縣，取自https://www.moeacgs.gov.tw/newlaw/newlaw.htm。