利用持久散射體雷達干涉技術與ALOS影像探討台灣東部縱谷斷層北段的分段特性

Abstract

花東縱谷位於歐亞板塊與菲律賓海板塊的碰撞帶上，其中縱谷斷層(LVF)吸收兩板塊間約三分之一聚合縮短量，為縱谷中最活動的斷層系統，縱谷斷層也是孕震斷層，在1951年的花蓮台東地震序列中，曾造成四個斷層分段的破裂，前人研究預測此斷層系統可有規模高達7.2的地震潛勢，因此在地震災害評估上具有重要的議題。大地測量觀測與構造地形研究都顯示縱谷斷層的各分段有不同的特性，因此本研究利用太空遙測技術所得之震間變形分布來探討斷層分段特性。本研究的範圍重點放在縱谷斷層北段，應用永久散射體雷達干涉技術軟體StaMPS，以ALOS L-band影像分析縱谷的地表變形，研究成果和ERS C-band影像的研究相比，永久散射體的密度有大幅提升，可提供地表變形的空間連續性資料。在花蓮市地區，其東北部的雷達視距方向(LOS)速度比本市其他部分偏低，米崙斷層兩側並無觀測到明顯的速度變化。從花蓮溪口到南富，嶺頂斷層兩側的LOS速度並無明顯變化，暗示此段斷層在間震期活動性較低，但在北緯23.85° 到 23.73°間，縱谷兩側與海岸山脈的雷達視距方向速度有局部減少的現象(與此區南北相比，速度從7 mm/yr降為-7 mm/yr)，在此區以南和以北，LOS方向速度則又增加。從南富到春日，PSI的成果指出在瑞穗斷層兩側的LOS方向速度並無明顯變化，但在北緯約23.63° 到23.68°間，觀測到疑似中央山脈斷層造成的速度變化，且此變化只侷限在這個小範圍內；在奇美斷層附近的海岸山脈LOS速度相對縱谷而言，相對較高，可能和奇美斷層的活動有關。我們用PS-InSAR算出來的LOS速度和連續GPS與水準所觀測到的垂向變形大致相近，顯示PS-InSAR技術可用於大面積變形的監測。

Taiwan is located at the boundary between the Eurasian Plate and the Philippine Sea Plate. The Longitudinal Valley at the eastern Taiwan is considered as the oblique collision boundary. There are several faults in the valley and the most active one is the Longitudinal Valley Fault (LVF) which accommodates about one third of convergence rate between these two plates. LVF is also seismogenic fault zone which ruptured four segments of LVF in 1951 Hualien-Taitung earthquake sequences. Besides, precious studies have proposed that this region could occur a potential big earthquake with a magnitude 7.2, thus it is a crucial issue for the assessment of earthquake hazard in this area. Both geodetic survey and geomorphologic study highlight relationship of fault activity and segmentation of LVF. Therefore, a space-borne geodetic technique is used to detect the spatial distribution of interseismic deformation along the Longitudinal Valley to further access the segmentation of LVF. This study area is mainly focus on the northern LVF. To achieve our goal, we apply a Persistent Scatterer SAR interferometry approach (StaMPS) using L-band ALOS data in eastern Taiwan. The result shows a considerable improvement of density of measurement compared to previous studies using C-band ERS data. The high density of measurement allows us to get a continuous view of active deformation, which can indicate the fault segmentation of the LVF. In Hualien city, the LOS velocities in northeastern part are lower than the rest area. No velocity change is observed across the Milun fault. From Hualian river mouth to Nanfu, there is no clear LOS velocity offset across the LVF except for the northernmost part, which indicates activity of LVF might be low during the interseismic period. From our PSI (abbreviation of Persistent Scatterer SAR interferometry) result, the Linding fault can be further divided into three sub-segments by their distinct LOS velocity. In the central part of the Linding fault (between latitude of 23.85° to 23.73°), a local subsidence in LOS direction with the mean LOS velocities ranging from about 7 to -7 mm/yr is observed, while toward the north and south, the velocity is increased again. Our result shows that the Juisui fault is not active, too. However, a clear velocity gradient is observed near the eastern flank of the Central Range between latitude of ~23.63° to ~23.68°. It may be related to activity of the Central Range fault in this area. North or south of this region, the velocity change vanished. The velocity around the Chimei fault is generally higher than around the Juisui fault. The LOS deformation rate by using persistent scatterer approach is similar to the vertical deformation from continuous GPS and leveling data, which is suitable for monitoring of surface deformation in large area.