Abstract:

Strong earthquakes(magnitude>6.5)typically cause coseismic surface ruptures of several kilometers or even hundreds of kilometers long on the surface. Coseismic surface rupture is the most intuitive geomorphic representation of an earthquake on the surface, and its geometry and distribution characteristics provide important information about the fault activity. Field investigation is the most basic means for research on coseismic surface fractures, but for areas that are hard to access or have harsh climatic environments, field investigation is often greatly limited. In recent years, the increasing abundance of high-resolution remote sensing images and the rapid development of photogrammetry methods can help us quickly obtain high-resolution topographic and geomorphic data of the study area, to better identify the fine geometry of the earthquake surface rupture zone and measure the offsets of geomorphic markers along the fault. The Litang Fault is a sinistral strike-slip fault located within the Sichuan-Yunnan rhombic block on the eastern edge of the Qinghai-Tibetan plateau. Several historical earthquake events have occurred on this fault, such as the 1890 and 1948 earthquakes, and clear seismic surface ruptures still exist along the fault so far. Previous studies have conducted a series of works on the coseismic surface rupture of this fault, but most of these works were based on field investigations or relatively low-resolution remote sensing images, and there is still a lack of fine research on the coseismic surface rupture of the fault. In this paper, the coseismic surface rupture of the 1890 earthquake which occurred on the Litang Fault was selected as the study object. To obtain high-resolution topographic data of this fault, the WorldView satellite stereo images were used to generate a 0.5-m-resolution orthophoto and a 1-m-resolution Digital Elevation Model(DEM)of the Litang fault based on the photogrammetry method. With the high-resolution topographic data, the fine geometry of the 1890 earthquake surface rupture zone was mapped in detail. The mapping results show that the total length of the surface rupture is about 27km, with an overall strike of N40°W. The rupture is mainly characterized by sinistral strike-slip motion, with a certain degree of dip-slip component in local areas. Except for the interval of approximately 6km with no surface rupture at the Wuliang River floodplain in the Litang Basin, the surface ruptures are relatively continuous at other locations. In addition, various rupture styles have been identified along the fault, including en echelon tension cracks, mole tracks, sag ponds, fault scarps, and displaced gullies. Furthermore, the sinistral offsets of 90 groups of linear geomorphic markers such as gullies and ridges were measured along the fault, which range from 1m to 82.4m. We further estimated the Cumulative Offset Probability Distribution(COPD)of the offsets located on the terrace I of the Wuliang River, which are all in the range of 0-9m. The COPD plot displays four distinct peaks at 1.3m, 2.4m, 4.3m, and6.1m, respectively. Previous studies have reported that the terrace I of Wuliang River formed at about(4 620±40)a BP. Thus, it can be indicated that the Litang fault may have experienced at least four strong earthquake events since(4 620±40)a BP, and the smallest peak of 1.3m may represent the coseismic displacement of the most recent 1890 earthquake. The rupture length of the latest 1890 earthquake was about 27km, and the coseismic sinistral offset was about 1.3m, yielding an estimated moment magnitude of M_W6.8-7.1. The coseismic offset of the other three earthquakes was about 1.8m, 1.9m, and 1.1m from old to new, respectively, yielding a magnitude estimate of M_W7.3, M_W7.3, and M_W7.0, with a size comparable to the 1890 earthquake. The research results fully demonstrate the potential of high-resolution remote sensing images in the study of fine characteristics of earthquake surface rupture.

Key words: high-resolution remote sensing image, photogrammetry method, coseismic surface rupture, offset distribution, Litang Fault

摘要：

同震地表破裂是地震在地表最直观的地貌表现, 其几何形态和展布特征记录了丰富的断层活动信息。近年来, 高分辨率遥感影像的日益丰富和摄影测量方法等的快速发展, 能帮助我们快速获取研究区高分辨率地形地貌数据, 以便更好地识别地震地表破裂带的精细几何结构, 并测量沿线断错位移分布等信息。文中选取川西理塘断裂1890年地震的同震地表破裂为研究对象, 利用WorldView遥感卫星影像立体像对和摄影测量方法生成了研究区0.5m分辨率的正射影像和1m分辨率的数字高程模型(DEM)。基于此数据解译获取了1890年地震地表破裂带的空间展布范围和精细几何结构, 沿破裂带测量了90组冲沟、 田埂等线性地貌标志的左旋位移, 并统计了其累积概率密度分布(COPD)。结果表明, 理塘断裂全新世中期以来至少经历过4次规模相当的强震事件, 其中最近一次的1890年地震事件的破裂长度约为27km, 同震左旋位移约为1.3m, 估算震级为 M_W6.8 ~7.1, 其他3次由老到新的地震事件的同震位移约为1.8m、 1.9m、 1.1m。研究结果充分展示出高分辨率遥感影像数据在同震地表破裂研究中的应用潜力。

关键词: 高分辨率遥感影像, 摄影测量方法, 同震地表破裂, 位移分布, 理塘断裂