Using historical aerial images to accurately locate the urban "invisible" active faults: A case study of the Shuiyu fault of the Datong Basin in Shanxi province
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摘要: 活动断层是城市地震灾害的重要风险源，准确厘定活动断层的空间几何展布是有效降低城市地震灾害和开展活动断层避让的基础。而城市化建设对原始地貌的严重改造使得原先出露地表的活动断层成为"隐形"断层，难以准确确定断层的几何展布。大同盆地中部的水峪断裂北段位于马铺山东缘，断错地貌明显，但南段进入大同市御东新区被城市建筑物覆盖，成为"隐形"断层。研究基于大同地区1965年历史航片影像资料，结合1∶10000地形图，运用航片像对和航空立体摄影的方法重建了该地区的数字高程模型（DEM）和正射影像（DOM），重点对水峪断裂"隐形"段进行识别和定位，厘定了水峪断裂断层陡坎的几何展布特征。研究结果表明，水峪断裂北段山前陡坎线性特征明显，由影像可以准确解译出断层几何展布位置；水峪断裂南段可基于2000年之前的Keyhole历史遥感影像、DOM与DEM资料，根据断层两侧的颜色差异及陡坎高度差异，精确厘定该段的断层几何展布位置；DEM提取的地形剖面表明"隐形"段陡坎高度在19 m左右。同时，基于野外调查的断层剖面与浅层人工地震剖面证明该陡坎就是水峪断裂的位置，这也表明利用历史航卫片对城市"隐形"活动断层进行精确定位的方法是可行的。该研究不仅为大同地区的地震危险性评估提供了重要依据，而且为城市"隐形"活动断层探测提供了一个新的思路和途径。Abstract: Active fault is a major risk source of urban earthquake disaster. Accurately identifying the spatial geometry distribution features of active fault is the basis for urban seismic risk migration. However, due to the large-scale and deep urban renewal by human activities and urbanization, the traces of active faults are obscure and invisible on the surface, which makes it hard to identify the surface geometry of this kind of active faults. Though the northern Shuiyu fault in the central part of the Datong Basin is bounded by the eastern edge of the Mapu Mountain and characterized by linear displaced landforms, the southern section of the Shuiyu fault crossing the Yudong District of Datong City is covered by dense buildings and roads, becoming an "invisible" fault. In this study, based on the 1965 historical aerial photos of the Datong region in the 1960's and the 1∶10000 topographic maps, we reconstructed the original DEM and DOM of this region using the aerial photo stereopair and stereoscopic photography. These original data revealed the geometry distribution characteristics of the Shuiyu fault. Our results show that the piedmont fault scarps are marked by prominent linear features along the northern Shuiyu fault. We can accurately locate the fault by the images. Based on the previous Keyhole satellite images, DOM and DEM data, the geometric distribution of the fault in the southern Shuiyu fault can be accurately determined. Topographic profiles extracted from the original DEM show that the vertical offset on terrace T3 along the Shuiyu fault is about 19 meters. On this basis, the fault natural exposures and shallow seismic reflection data demonstrate that it is feasible to accurately locate the "invisible" active fault in the city by using historical aerial photos and stereoscopic photography. This study provides not only an important basis for the seismic hazard assessment in the Datong region but a useful technique for the detection of "invisible" active faults beneath a city.
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图 1 大同盆地水峪断裂构造位置图(底图为2020年Landsat影像)
F1—口泉断裂；F2—水峪断裂；F3—采凉山东麓断裂
Figure 1. Regional tectonics of the Shuiyu fault, Datong basin (The base map is the Landsat image of 2020)
F1—The Kouquan fault; F2—The Shuiyu fault; F3—The eastern Cailiangshan fault
图 3 基于大同历史航片的立体摄影技术流程图
Figure 3. Flow chart of the stereoscopic photography using historical aerial photos
图 4 大同水峪断裂历史影像
a—2020年Google earth影像(黄色虚实线为水峪断裂)；b—1985年美国锁眼(Keyhole)卫星影像；c—1965年历史航片正射影像(DOM)；d—1965年历史航片数字高程模型(DEM；红色箭头指示断层陡坎位置)
Figure 4. Historical images of the Shuiyu fault. (a)Google earth image of 2020. (b) Keyhole satellite image of 1985. (c) DOM data of 1965. (d) DEM data of 1965
The yellow pseudo-solid line is the Shuiyu fault, and the red arrows indicate the position of the fault scarps
图 6 御东新区水峪断裂在历史航片DEM上的地貌表现
a—水峪断裂历史航片DEM；b—水峪断裂地貌表现
Figure 6. Geomorphic features of the Shuiyu fault in the Yudong district based on historical DEM data.
(a) Historical DEM of the Shuiyu fault. (b) Geomorphic features of the Shuiyu fault.
图 7 跨水峪断裂的浅层人工地震探测剖面(位置见图 6b)
FPk8, FPk20—断层点；TN，Tg—地震波反射界面
a—KL2测线；b—KL1测线Figure 7. Shallow seismic reflection profiles across the Shuiyu fault (The sites are shown in Fig. 6.)
(a) Surveyline KL2. (b) Surveyline KL1
FPk8 and FPk20 are the fault points; TN, Tg are the reflection interfaces of seismic waves
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