Abstract:

Detailed mapping of coseismic surface rupture can provide valuable information for understanding the geometrical complexities, dynamic rupture processes and fault mechanisms. Fault geometrical complexities, such as bends, branches, and stepovers are common in strike-slip fault systems and can control the coseismic surface rupture characteristics to a certain extent. Observational studies of surface ruptures in past earthquakes suggested that special rupture characteristics would form distributed ruptures and rupture gaps. The detailed mapping has become an important way to study the surface rupture. According to the China Earthquake Networks Center(CENC), the M_W7.4 earthquake occurred at 2:04 PM on May 22, 2021, in Madoi County, Qinghai Province. The epicenter is about 70km south of the eastern Kunlun Fault on the northern boundary of the Bayan Kera block. It is the largest earthquake that hit the Chinese mainland since the Wenchuan M_S8.0 earthquake in 2008. After field investigation and rupture mapping on the computer, Yao et al.(2022)estimated that the length of surface rupture of this earthquake is 158km. Surface ruptures of the M_W7.4 Madoi earthquake broke through the geometric discontinuities such as step-overs and bends, and formed various coseismic surface fractures, especially in the middle segment. In the survey of the Madoi earthquake, we rapidly acquired aerial image data using UAV aerial photogrammetry and obtained high-resolution digital orthograph models(DOMs)and digital elevation models(DEMs)using PhotoScan software based on the SfM algorithm processing. Those data provide an opportunity for detailed mapping of seismic rupture and also provide an important reference for fieldwork. Based on high-resolution topographic data, we carried out detailed surface rupture mapping, classification, geometric structure and strike analysis for the ~30km section of the epicenter segment. At the same time, we conducted field work to supplement and proofread the maps.
According to the characteristics of surface ruptures in the epicenter area, we divided the ruptures into six segments. The surface ruptures along segment S₁ and segment S₆ are concentrated near the main fault, while the surface ruptures in the stepover(segment S₃, S₄, and S₅)are distributed dispersively, and the secondary ruptures along the segment S₂ are also distributed scatteredly, with the main rupture missing. To reveal the distribution characteristics of surface fractures more clearly, the surface ruptures are divided into the main rupture, secondary rupture, surface rupture and collapse rupture, among which the genesis of the surface rupture is uncertain. There are a lot of typical tensile ruptures with left-lateral component in segment S₁, the strike of the ruptures is consistent with the strike of the main fault or intersects the main fault with a small angle. The maximum width of the main rupture in segment S₁ is ~50m. The main ruptures in segment S₆ are developed along with the preexisting tectonic topography and the offset of the left-lateral displaced gully is up to tens of meters in magnitude. The surface ruptures are distributed in an echelon pattern, and all intersected with the strike of the main fault at a large angle. The location and size of the step-over are determined according to the topography and rupture morphology of faults in segment S₁ and segment S₆. The surface ruptures on the floodplain and banks of the Yellow River are in various forms and difficult to classify accurately. Therefore, only the typical seismic ruptures developed along the accumulated tectonic topography are labeled as main ruptures, and other typical seismic ruptures inconsistent with the location of the main fault are labeled as secondary ruptures. The typically collapse ruptures distributed along the river bank or lake bank are labeled as collapse ruptures, while the rest are labeled as surface ruptures. Surface ruptures in segment S₃ are distributed on the planar graph, but they have a dominant strike in the NE direction that can be seen from the diagram map. In the floodplain of the Yellow River, there are typical “grid” cracks, “explosive” cracks, and tensile cracks, and many cracks are accompanied by sand liquefaction which is beadlike, single, and distributed along the cracks. After the earthquake, the geodesic and geophysical data obtained quickly from the InSAR co-seismic deformation map and precise positioning of aftershocks revealed the basic morphological characteristics of earthquake rupture and provided valuable information such as earthquake rupture length, which provided an important reference for the design of UAV aerial photography and fieldwork. Compared with the rupture trace in field investigation by Pan et al.(2021), the surface rupture coverage obtained by mapping based on UAV aerial photogrammetry technology in this study is more extensive and accurate.
In general, surface ruptures of the Madoi earthquake are widely distributed, and we have classified those ruptures into the main seismic ruptures, secondary seismic ruptures, collapse cracks, and other surface ruptures. In addition to the seismic rupture with the same strike, there are also a variety of distributed surface ruptures with different strikes from the main fault. In these distributed surface ruptures, there are also many surface ruptures whose cause is not clear and they may be affected by tectonics or strong quake. For example, the “grid” and “explosive” surface ruptures on the Yellow River floodplain may be related to the strong quake near the epicenter or may also be related to the three-dimensional dynamic ruptures process in the initial stage. In this study, the characteristics of earthquake surface rupture in the step-over and adjacent sections near the epicenter has been described in detail, which provides a deeper understanding of the distributed coseismic surface rupture in the strike-slip fault.

Key words: Madoi earthquake, high-resolution topographical data, epicenter section, distributed earthquake surface rupture, initial rupture stage

摘要：

同震地表破裂形态的精细刻画可为理解断裂带复杂几何结构、 动态破裂过程与破裂机理提供重要信息。2021年5月22日, 青藏高原内部青海省果洛藏族自治州玛多县发生了 M_W7.4 地震, 这是自2008年汶川 M_S8.0 地震后中国大陆地区发生的震级最大的一次地震。此次地震的同震地表破裂突破了沿线多个阶区、 弯折等几何不连续结构, 形成了长约158km的同震地表破裂带和多样化的断裂几何形态, 其中以震中区段落的地震地表破裂形态最为特殊和复杂。有助于全面认识震中区段落的地震地表破裂形态并深入理解其形成机理, 文中基于分辨率约为3cm的航空影像数据, 结合野外实地调查资料, 完成了本区域地表破裂的精细填图。对地表破裂的类型、 分布、 几何结构和走向等进行的综合分析表明, 震中区的地震地表破裂受阶区几何结构的影响而呈现分布式破裂的特点。并且, 震中附近的强震动效应和地震断裂初始发育阶段的影响, 进一步造成了该段落分布式地震地表破裂的形态。文中高清再现了震中区的阶区及其附近段落的地震地表破裂特点, 对走滑断裂带的分布式同震地表破裂有了更进一步的了解。

关键词: 玛多地震, 高分辨率地形数据, 震中区, 分布式地震地表破裂, 破裂初始阶段