利用航空影像關聯和DSM時間序列測量 台灣西南部車瓜林斷層和旗山斷層之間的地表變形;Measuring ground deformation across the Chegualin and Chishan faults, Southwestern Taiwan, using aerial image correlation and DSM time series

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Title:	利用航空影像關聯和DSM時間序列測量台灣西南部車瓜林斷層和旗山斷層之間的地表變形;Measuring ground deformation across the Chegualin and Chishan faults, Southwestern Taiwan, using aerial image correlation and DSM time series
Authors:	陳愷風;Chen, Kai-Feng
Contributors:	應用地質研究所
Keywords:	影像關聯;攝影測量;地表位移;中寮隧道;活動斷層;潛移斷層;Image correlation;Photogrammetry;Surface displacement;Zhongliao tunnel;Active faults;Creeping faults
Date:	2024-08-23
Issue Date:	2024-10-09 17:23:37 (UTC+8)
Publisher:	國立中央大學
Abstract:	台灣位於菲律賓海板塊與歐亞板塊的聚合邊界，前者以每年82毫米的速度向東北方與後者碰撞。其中約一半的板塊擠壓變形集中在台灣西南部，主要通過褶皺逆衝和構造脫逸作用來吸收變形。大地測量結果揭示了台灣南部西麓山帶的顯著變形。本區有兩條相距僅500米且平行的向東南傾斜的逆衝斷層。車瓜林斷層截切上新世至更新世的古亭坑層泥岩，並以其低起伏的惡地地形著稱。東側的旗山斷層將中新世晚期的砂岩抬升至古亭坑層之上，並在斷層上盤形成了與植被茂密的顯著山脊，且具有較不穩定的邊坡。根據安裝於國道一帶的全球導航衛星系統（GNSS）、水準測量和導線測量等，跨越這兩個斷層測量結果顯示此二構造的總縮短速率僅為每年15毫米。但在車瓜林斷層的壓縮率達到每年50毫米，而在旗山斷層的伸張速率達到每年32毫米。在這兩個斷層之間的地區，相對於澎湖的抬升速率從每年33毫米往東南方增加到每年100毫米，但在旗山斷層以東，抬升速率下降到每年20毫米。引發這種顯著無震地表變形的機制仍須深入探究。本研究通過影像相關技術來監測地表變形和測量水平位移，並且透過數值地表模型（DSM）時間序列來評估垂直位移。該方法透過提供覆蓋面積更廣且高解析度的觀測來補充現有的大地測量和合成孔徑雷達干涉（InSAR）的成果。我們使用2008年至2015年的8組航拍影像，測量了一個以國道及大地測量網為中心，面積達3×3平方公里的區域。影像通過Micmac進行處理，這是一套免費開源的攝影測量軟體。由影像關聯所測得位於國道沿線的水平位移與大地測量觀測結果相當吻合。在車瓜林斷層一帶，我們在2008年至2015年間觀察到垂直斷層方向的累積水平壓縮約有17至44公分，且該水平壓縮的側向延伸至少有2000公尺，分布於車瓜林斷層及其主要分支斷層上；國道以南帶有13至42公分的右移分量，而國道以北則有略微的左移分量。不過由於植被覆蓋的關係，旗山斷層的定位及錯移量仍難以偵測。此外，我們的量測成果揭示了在兩條斷層之間此前從未偵測到的旋轉位移，國道以北的地塊為順時針旋轉，國道以南則為逆時針旋轉。另外，利用DSM時間序列來測量的垂直位移的初步結果也符合大地測量網先前所觀察到的抬升模式。地表地質調查揭示了數個具有滑移方向指標的斷層帶。我們在車瓜林斷層沿線設法追踪到至少延續2公里的主要剪切帶，其逆衝滑動特性與影像關聯測量的結果一致。在旗山斷層附近，我們發現了向東南傾斜且呈雁形排列的高角度正斷層，這些斷層均帶有右移分量，地表出露最遠可達國道西北方1.4公里。透過剪切面上的擦痕，我們還觀察到這些右移正斷層的滑移角有所變化。總結影像相關性和地質調查結果，我們總結出一張局部活動構造分布圖和一個2.5×2.5平方公里範圍的三維地表變形模型。我們提出了一個與泥貫入體相關的半穹頂系統來解釋局部的地表變形模式。穹頂的中心位於中寮隧道西北端附近，穹頂的西北側則是以車瓜林斷層為界。我們可能還需要透過地球物理方法或地質定年來進一步解釋此一現象的機制。 ;Taiwan is located at a convergent boundary where the Philippine Sea Plate is moving northeastward to the Eurasian Plate at the rate of 82 mm/year. About half of the convergence is accommodated in southwestern Taiwan through folding and thrusting, as well as tectonic extrusion. Ground-based geodetic observations revealed a remarkable deformation pattern within the Western Foothills of southern Taiwan. There, two sub-parallel southeast-dipping thrusts are separated by only 500 m. The Chegualin fault cuts through the Plio-Pleistocene Gutingkeng mudstone, characterized by a scenic low-relief badland landscape. To the east, the Chishan fault has brought Late Miocene sandstone on top of the Gutingkeng mudstone: the hanging wall consists of a significant ridge topography associated with vegetated and locally unstable slopes. According to GNSS, levelling and traverse measurements along a freeway and tunnel crossing both thrusts, there is limited net shortening across the two major structures at a rate of 15 mm/year, but 50 mm/year of compression across the Chegualin fault and 32 mm/year of extension across the Chishan fault. In-between the faults, uplift increases southeastward from 33 to 100 mm/year relative to Penghu, but drops to 20 mm/year east of the Chishan fault. Sharp deformation gradients indicate aseismic slip on both structures. Such significant aseismic ground deformation has raised the question of the driving mechanism. To investigate this phenomenon, this study monitors ground-surface deformation using the image correlation technique to quantify horizontal displacements and using DSM (Digital Surface Model) time series to access vertical displacements. This approach aptly complements the existing ground-based transect and regional observations from GNSS and InSAR: it would provide high resolution observations covering a larger area along the strike of the geological structures. We used 8 sets of 8 aerial images acquired from 2008 to 2015 covering a 3 x 3 km area, centered on the freeway and ground-based geodetic network. Images were processed with Micmac, a free open-source photogrammetric software suite. The estimated horizontal displacements along the freeway closely match ground-based observations. Across the Chegualin fault, we observe during 2008-2015 cumulated horizontal compression along the fault-perpendicular direction ranging from 17 to 44 cm, 13 to 42 cm of right-lateral offset south of the freeway, and slightly left-lateral offset north of the freeway. The compression extends at least 2000 meters along the Chegualin fault and a major branch fault. However, it remains challenging to precisely quantify and locate the displacement discontinuity across the Chishan fault due to poor correlation caused by vegetation. Our results also reveal a previously unknown rotation pattern in-between the two faults, with a clockwise rotation north vi of the freeway and a counter-clockwise rotation south of it. Preliminary results on DSM time series also support the previously observed uplift pattern. Field geological surveys reveal several fault zones with kinematic indicators on the outcrop scale. Along the Chegualin fault, we manage to trace the main shear zones with reverse slip sense for at least 2 km, and geological evidence is consistent with the aerial image correlation results in terms of fault kinematics. Along the Chishan fault, we found steep SE-dipping en echelon pattern normal faults with a right-lateral component up to 1.4 km northwest of the freeway, and these dextral normal faults also present changes in rake, according to the slickenlines on the shear planes. To conclude on the aerial image correlation and geological survey results, we provide a map of active structures and a 3D ground deformation model with a range of 2.5 x 2.5 km. We propose a mud-piercement-related half-doming system to explain the local ground deformation pattern. The center of the dome is located near the northwestern end of the Zhongliao tunnel and the northwest side of the dome is bounded by the pre-existing Chegualin fault. Further explaining the mechanism may require more geophysical or geochronological research.
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