利用多感測器合成孔徑雷達干涉技術計算縱谷縫合帶三維地表變形場

Abstract

地震週期中的間震形變（interseismic deformation），是地殼沿著板塊邊界逐漸累積應變（strain），最終以地震的形式突然釋放的過程。花東縱谷斷層是位於菲律賓海板塊與歐亞板塊聚合邊界上的著名潛移型斷層，前人透過多種測地觀測方法得知，其在池上鄉附近的潛移速率高達每年 20 毫米，然而，斷層沿線的間震期潛移行為仍不清楚。觀測間震形變的典型方法是使用高精度的全球導航衛星系統（GNSS），但 GNSS 觀測僅能提供特定站址的三維地表變形，缺乏足夠的空間解析度來揭示較小尺度的潛移或鎖定斷層。若無高空間解析度的地殼形變資料，將會無法準確推估間震期斷層鎖定深度。相比之下，合成孔徑雷達干涉測量（InSAR）可提供公分級精度與高空間解析度的地表變形觀測。過去研究多使用視線方向（LOS）的速度或結合 LOS 與 GNSS 資料來觀測斜向縱谷斷層的潛移行為，然而，這些觀測僅限於單一方向，或者是在南北方向的解析度不足。因此，我們利用 Sentinel-1 與 ALOS-2 衛星影像（2015–2020年），透過短基線差分干涉合成孔徑雷達（SBAS-InSAR）技術生成三維速度場，透過高解析度的速度場，我們得以辨識出更細微的地質構造及其變形行為。我們發現2022 年關山池上地震發生前，玉里斷層在東西向有每年 2–3 毫米的潛移現象，此為先前尚未被提出的觀測結果。此外，根據我們的南北向與垂直速度場及水準測量資料，在已知東傾的奇美斷層附近，推測存在一條西傾構造，此構造可能為中央山脈斷層的延伸，並可能與奇美斷層交會。我們的研究結果有助於發展更準確的地震災害風險模型。

Interseismic deformation is the gradual accumulation of strain in the Earth's crust along tectonic plate boundaries, culminating in the abrupt release manifested as earthquakes. The Longitudinal Valley fault is a well-known creeping fault located at the convergent boundary between the Philippine Sea Plate and the Eurasian Plate. Its creeping rate is as high as 20 mm/yr near Chihshang Township through varied geodetic observations. However, the comprehensive interseismic creeping behavior along the fault trace remains unknown. The typical method to observe interseismic deformation is to use the GNSS network with the highest precision. However, GNSS measurements in previous studies offer three-dimensional surface deformation at specific sites, lacking the spatial detail to reveal smaller-scale creeping/locked fault traces. Determining interseismic fault locking depth is challenging without high-resolution crustal deformation data. In contrast, InSAR provides centimeter-scale precision and high spatial resolution observations of surface deformation. Previous research utilized line-of-sight (LOS) velocity or LOS velocities combined with GNSS to observe the creeping behavior of the oblique Longitudinal Valley fault. However, these observations are either limited to a single direction or lack sufficient resolution in the north-south direction. Therefore, we used Sentinel-1 and ALOS-2 imagery (2015-2020) to generate 3-D velocity fields through Small Baseline Subset InSAR (SBAS-InSAR). With the high-resolution velocity fields, we identified more detailed geological structures and their behaviors. We found the Yuli fault exhibited 2-3 mm/yr of creep in the E-W direction before the 2022 Guanshan-Chishang earthquake, a phenomenon not previously reported. Additionally, an inferred structure near the previously mapped east-dipping Chimei fault appears to be west-dipping based on our N-S and U velocities and leveling data. This inferred structure may represent an extension of the Central Range fault, potentially intersecting the Chimei fault. Our findings could facilitate the development of accurate models for mitigating seismic hazards.