東台灣花東縱谷位於歐亞板塊與菲律賓海板塊的縫合帶，在其東北方有琉球隱沒系統；而南方有呂宋島弧隱沒系統，介於海岸山脈地質區與中央山脈地質區之間。縱谷表層覆蓋了第四紀沖積層，其中，長達150公里的花東縱谷活動斷層系統貢獻了每年約三公分的壓縮量，佔台灣總板塊聚合量八公分的三分之一，而近一百年來在花東縱谷密集的地震分布也暗示花東縱谷活躍的構造特性。為探討縱谷深度兩公里內的地下構造，本研究利用高解析反射震測法，於花東縱谷南段的東里、池上、關山與鹿野等地區，以雙震盪震源車為震源施測四條長二至四公里，近垂直縱谷走向的測線，再結合前人於花東縱谷北段的反射震測剖面，討論縱谷內地層的變化。 震測剖面顯示縱谷下方的基盤向東傾斜，並且縱谷下呈現楔狀的沉積盆地型態，其上覆蓋的沖積層與沉積層普遍朝東增厚。在東里與池上地區，基盤深度約為0.2至0.4秒雙程走時 (約200至400公尺)，以上覆蓋近水平堆積的沖積層，並且在靠近中央山脈地質區一側，基盤深度變淺，而池上斷層在東里的G剖面造成淺部沖積層的不連續。在關山地區，向東傾斜的基盤深度可及0.7秒 (約900公尺) ，研判往縱谷東側可及更深。而鹿野地區下的基盤深度可及1秒 (約1.7公里)，鹿野斷層在剖面J的西側使淺部沖積層凸起。結合前人於花東縱谷壽豐至玉里的震測剖面，整體顯示縱谷中的沖積層有往南遞減的趨勢。 而在瑞穗以南的剖面中，和光復以北顯示的連續沉積層相比，可以觀察到朝東北傾沒的背斜構造、東傾的卑南山礫岩、受斷層變形的淺部沖積層與往南逐漸增厚的利吉層，透漏花東縱谷中南段地層的傾斜與變形可能與縱谷中活動斷層的性質與中南段海岸山脈朝西北向的擠壓有關。;Four seismic reflection profiles were used to construct the subsurface structures below the southern Longitudinal Valley (LV) in eastern Taiwan, a suture zone between Eurasian and Philippine Sea plates. In the Longitudinal Valley, considerable seismicity as well as significant crustal deformation are accommodated by the Longitudinal Valley fault system (LVF). Therefore, a thorough subsurface structure geometry is required to constrain this seismic active zone. In this study, we aim to better understand the subsurface structures in the upper 2 km of the southern LV. We conducted high-resolution seismic reflection survey across the LVF from Donli to Luyeh in 2017. We used two mini Vibroseis trucks with sweeping frequencies from 30-200 Hz as the source and the CDP spacing is 2.5 m with the nominal 30 fold. The stacked seismic profiles reveal an east-dipping basement beneath the LV, which is characterized by a wedge-shaped basin bearing the sedimentary strata and the alluvium that thicken eastward. In Chihshang, the basement reaches 0.4 s (two-way travel time, TWT) and becomes shallower in the Central Range area. In Guanshan, the basement reaches 0.7 s and reaches near 1s (around 1.7 km in depth) in Luyeh. It is interesting to note that the thickness of the alluvium in the LV tend to decrease to the south if comparing the results with previous seismic profiles in the northern LV. Furthermore, the Chihshang fault is observed in the profile G in Donli which causes the undulations in the shallow alluvium, and the Luyeh fault is traced in the west of the profile J north of Peinanshan. Integrating the seismic profiles from previous to this studies in the southern part of the Longitudinal Valley, it is worthwhile to note the northeast-plunging anticline, east-dipping Peinanshan Conglomerate, the deformation of the alluvium due to the faults and the thickening Lichi mélange south of Guanfu. The deformation and tilting of the strata seem to relate to the faulting characteristics in the LV and the compression toward the northwest from the southern Coastal Range.
