透過持久散射體合成孔徑雷達干涉分析新竹苗栗地區之地殼變形

Abstract

臺灣西北部屬西部麓山帶，是弧陸碰撞運動形成的褶皺逆衝斷層帶，有一系列的斷層與褶皺分布於此，例如新竹斷層、新城斷層、斗煥坪斷層、青草湖背斜等，其中新竹斷層與新城斷層為活動斷層，存在致災性地震的風險。近年儘管在新竹苗栗地區多有應用GPS和精密水準監測地表變形，但大抵聚焦在包含新城斷層與新竹斷層在內的一連串褶皺逆衝斷層帶，新竹苗栗的沿海地帶和斷層掩覆段則相對缺乏觀測點；另一方面，雖有少數研究應用合成孔徑雷達干涉技術(InSAR)探討地表變形與新竹苗栗地區構造之間的關係，但研究區域範圍與討論範疇甚少提及新竹苗栗的沿海地帶和斷層掩覆段，本研究旨在透過PSInSAR得出地表變形，分析其在空間及時間上的變化，結合背景構造探討區域內變形機制。 持久散射體差分干涉技術（PSInSAR）相較傳統大地測量，可獲取廣域且密集的高精度地表變形資料，以克服GPS或精密水準在研究區域內觀測點不足或分布不均的情況，本研究使用Sentinel-1衛星C波段雷達的升軌與降軌影像，時間跨度為2015年至2019年，透過ISCE與StaMPS軟體對影像以PSInSAR處理，獲得沿視衛星方向之平均位移量與時間序列，並拆分出東西向及垂直向的平均位移量，將前述成果結合前人研究的剖面、地震資料作通盤討論。 成果顯示研究區域中跨斷層的速度梯度變化並不明顯，顯示主要構造活動性不顯著，其中青草湖背斜南翼西端，以及新城斷層與斗煥坪斷層的交會處，可觀察到相比周圍較高的速度場，且升軌與降軌位移皆呈靠近視衛星方向運動，可知兩地皆以抬升變形為主導。青草湖背斜南翼西端的高變形區，與盲斷層的位置重合，根據右移與抬升位移以及地下形貌，可知此高變形區的地下形貌為帶右移的逆斷層所形成的正向花形構造；新城斷層西南段被斗煥坪斷層截斷處亦與高變形區重合，由震測剖面可知斗煥坪斷層具逆移性質，據此推論高變形區的局部抬升變形，由斗煥坪斷層的垂直層位滑移、新城斷層的西北向逆移、斗煥坪斷層下盤的右移共同作用，應力交會使此處地層累積較明顯的應變，導致地層向上拱起，並沿長軸向兩側形成張力，因而呈相反方向的位移。

The northwestern part of Taiwan belongs to the Western Foothills Belt and is a fold-thrust belt formed by arc-continent collision. A series of faults and folds are distributed in this region, including the Hsinchu Fault, Hsincheng Fault, Touhuanping Fault, and Chingtsaohu Anticline. Among these, the Hsinchu Fault and Hsincheng Fault are active faults with the potential for significant earthquake hazards. In recent years, although there have been applications of GPS and precise leveling to monitor surface deformation in the Hsinchu-Miaoli area, the focus has generally been on the continuous fold-thrust belt including the Hsincheng Fault and Hsinchu Fault, while the coastal areas and fault-covered segments of Hsinchu-Miaoli lack sufficient observation points. Additionally, while a few studies have utilized Synthetic Aperture Radar Interferometry (InSAR) to investigate the relationship between surface deformation and the tectonics of the Hsinchu-Miaoli region, these studies have rarely addressed the coastal areas and fault-covered segments. This study aims to use Persistent Scatterer Interferometry (PSInSAR) to analyze surface deformation and its spatial and temporal variations in conjunction with background structural exploration to understand the deformation mechanisms in the region. Compared to traditional geodetic measurements, Persistent Scatterer Interferometry (PSInSAR) can provide wide and dense high-precision surface deformation data. This method overcomes the limitations of inadequate or unevenly distributed observation points in the study area when using GPS or precise leveling. In this research, Sentinel-1 C-band radar images from both ascending and descending orbits between 2015 and 2019 were processed using ISCE and StaMPS software to obtain average displacement and time series along the satellite line of sight. The results were further separated into east-west and vertical average displacement components and combined with previous research profiles and seismic data for comprehensive discussion. The results indicate that there is no significant change in the velocity gradient across fault boundaries within the study area, suggesting that the main structural activity is not pronounced. In particular, at the western end of the southern limb of the Chingtsaohu Anticline and at the intersection of the Hsincheng Fault and Touhuanping Fault, higher velocity fields are observed compared to the surrounding areas. Both ascending and descending displacements show motion towards the satellite, indicating uplift deformation dominance in these locations. The highly deformed area at the western end of the southern limb of the Chingtsaohu Anticline coincides with the location of a blind fault. Based on the right-lateral and uplift displacements and subsurface morphology, it can be inferred that the subsurface features of this highly deformed area are positive flower structures formed by a right-lateral reverse fault. Similarly, the highly deformed area where the Hsincheng Fault is intercepted by the Touhuanping Fault coincides with the seismic reflection profile of the Touhuanping Fault, suggesting its reverse displacement nature. Based on this, it is inferred that the local uplift deformation in the highly deformed area is a result of the combined effects of vertical slip along the Touhuanping Fault, northwestward reverse slip along the Hsincheng Fault, and right-lateral movement along the down-dip side of the Touhuanping Fault. The stress convergence in this region leads to more significant strain accumulation in the subsurface layers, causing upward arching of the strata and tensional forces on both sides along the major axis, resulting in opposite displacements.