利用 InSAR 時間序列探討縱谷斷層南段的地表潛移

Abstract

地表潛移為重要的斷層滑移行為，其透過無震滑移釋放能量，然而有時斷層面有部分未潛移也能夠產生地震，因此攸關地震危害的評估。臺灣位於菲律賓海板塊和歐亞板塊的活動碰撞帶，跨過台灣每年約有82毫米的縮短量，其中43%集中於東部的花東縱谷聚合帶，該區域最主要的斷層為縱谷斷層，此斷層的南段也是世界上著名的潛移斷層之一。縱谷斷層為一條逆衝兼左移的斷層，根據前人的研究成果，其南段玉里至利吉具有地表潛移，速率約為10－30 mm/yr。前人之Envisat和ALOS-1雷達影像的InSAR時間序列成果也觀測到，斷層南段在視衛星方向的地表速度場有明顯的地表潛移。然而，現地觀測與影像的測地觀測結果不一致，測地資料顯示了沿斷層的地表潛移廣泛分布於縱谷斷層南段，而現地觀測可辨識的斷層地表潛移僅局限在少數特定地點。此外， InSAR時間序列推導的速度場顯示，縱谷斷層南段的地表潛移速率會隨時間和空間變化，Peyret et al.（2011）觀測最大的地表潛移速率在富里，往北往南遞減，於玉里甚至趨近於0 mm/yr；Champenois et al.（2012）所觀測到的則是富里至玉里均有較高的地表潛移速率，於玉里有約為26 mm/yr，與Peyret et al.（2011）的觀測成果有很大的差異。 因此，本研究使用2015至2020年27幅（9個時間 x 3幅）ALOS-2雷達影像，應用於InSAR時間序列小基線法，分析縱谷斷層南段地表潛移的時空分布情形。並使用ISCE軟體進行雷達影像的前處理以及產製干涉圖，其中包含影像對位、圖幅拼接、電離層校正等。根據視衛星方向速度場成果，縱谷南段存在明顯的速度不連續邊界，跨過此邊界的速度差約5－30 mm/yr，其位置也與前人繪製的縱谷斷層大致重疊，且在玉里至池上跨斷層的速度變化寬度約為0.5至1公里，代表此處有非常近地表的潛移。另外，在鹿野斷層也觀測到明顯的速度變化邊界，約為5－16 mm/yr，且鹿野斷層的長度較前人調查的還要更長，在檳榔向南延伸約5公里有皆有地表潛移，而在東側卑南山西緣則沒有顯著的速度差異。

Surface fault creep, a significant behavior related to seismic hazard assessments, releases energy through aseismic slip and can potentially lead to earthquakes. The Longitudinal Valley fault (LVF) in eastern Taiwan, situated within the active collision boundary of the Philippine Sea and Eurasian plates, is globally renowned for its observed surface creep with oblique motion in southern portion from Yuli to Lichi. Geodetic data indicate a reported surface creep rate of approximately 10-30 mm/yr at specific sites along the fault. InSAR time series using Envisat and ALOS-1 images also reveal substantial surface creep along the southern LVF. Despite widespread surface creep in geodetic data, in-situ observations do not consistently align with imaging geodetic observations, showing localized surface creep at specific locations. Moreover, fault creep patterns derived from InSAR time series exhibit variations over time. Peyret et al. (2011) observed the highest surface creep rate in Fuli, which decreases towards both the north and south, approaching nearly 0 mm/yr in Yuli. In contrast, Champenois et al. (2012) observed higher surface creep rates from Fuli to Yuli, with approximately 26 mm/yr in Yuli, showing significant differences from the observations by Peyret et al. (2011). Hence, in this study we conduct a spatial-temporal analysis of southern LVF surface creep by using the Small Baseline Subset (SBAS) InSAR technique. Employing ISCE software, we preprocess, co-register, and apply DInSAR to 3 frames and 9 ALOS-2 SAR images from 2015 to 2020 in the ascending orbit in the stripmap (SM3) mode. The split-spectrum method is employed for ionospheric correction. The results reveal a velocity boundary ranging from 5 to 30 mm/yr in the line-of-sight (LOS) direction along the southern Longitudunal valley. The major velocity change boundary migrates westward in the southernmost valley, and a noticeable change in LOS velocity, approximately 5 to 16 mm/yr, is observed along the northern section of the Luyeh fault. Conversely, the southern part of the Luyeh fault along the western edge of Beinan Mountain does not display a significant difference in velocity. Instead, extending 5 kilometers south from Pinglang on the western side exhibits a gradual velocity change due to surface creep.