Abstract:

Earthquake surface ruptures are the key to understand deformation pattern of continental crust and rupture behavior of tectonic earthquake, and the criteria to directly define the active fault avoidance zone. Traditionally, surface fissures away from the main rupture fault are usually regarded as the result triggered by strong ground motion. In recent years, the earth observation technology of remote sensing with centimeter accuracy provides rich necessary data for fine features of co-seismic surface fractures and fissures. More and more earthquake researches, such as the 2019 M_W7.3 Ridgecrest earthquake, the 2016 M_W7 Kumamoto earthquake, the 2020 M_W6.5 Monte Cristo Range earthquake, suggest that we might miss off-fault fissures associated with tectonic interactions during the seismic rupture process, if they are simply attributed to effect of strong ground motion. Such distribution pattern of co-seismic surface displacement may not be isolated, it encourages us to examine the possible contribution of other similar events. The 22 May 2021 M_W7.4 Madoi earthquake in Qinghai Province, China ruptured the Jiangcuo Fault which is the extension line of the southeastern branch of the Kunlun Fault, and caused the collapse of the Yematan bridge and the Cangmahe bridge in Madoi County. The surface rupture in the 2021Madoi earthquake includes dominantly ~158km of left-lateral rupture, which provides an important chance for understanding the complex rupture system.
The high-resolution UAV images and field mapping provide valuable support to identify more detailed and tiny co-seismic surface deformation. New 3 to 7cm per pixel resolution images covering the major surface rupture zone were collected by two unmanned aerial vehicles (UAV) in the first months after the earthquake. We produced digital orthophoto maps (DOM), and digital elevation models (DEM) with the highest accuracy based on the Agisoft PhotoScan^TM and ArcGIS software. Thus, the appearance of post-earthquake surface displacement was hardly damaged by rain or animals, and well preserved in our UAV images, such as fractures with small displacement or faint fissures. These DOM and DEM data with centimeter resolution fastidiously detailed rich details of surface ruptures, which have been often easily overlooked or difficult to detect in the past or on low-resolution images. In addition, two large-scale dense field investigation data were gathered respectively the first and fifth months after the earthquake. Based on a lot of firsthand materials, a comprehensive dataset of surface features associated with co-seismic displacement was built, which includes four levels: main and secondary tectonic ruptures, delphic fissures, and beaded liquefaction belts or swath subsidence due to strong ground motion. Using our novel dataset, a complex distributed pattern presents along the fault guiding the 158km co-seismic surface ruptures along its strike-direction. The cumulative length of all surface ruptures reaches 310km. Surface ruptures of the M_W7.4 Madoi earthquake fully show the diversity of geometric discontinuities and geometric complexity of the Jiangcuo Fault. This is reflected in the four most conspicuous aspects: direction rotation, tail divarication, fault step, and sharp change of rupture widths.
We noticed that the rupture zone width changed sharply along with its strike or geometric complexity. Near the east of Yematan, on-fault ruptures are arranged in ten to several hundred meters. Besides clearly defined surface ruptures on the main fault, many fractures near the Dongo section and two rupture endpoints are mainly along secondary faulting crossing the main fault or its subparallel branches. Lengths of fracture zones along two Y-shaped branches at two endpoints are about 20km. At the rupture endpoints, the fractures away from the main rupture zone are about 5km. Some authors suggested the segment between the Dongcao along lake and Zadegongma was a “rupture gap”. In our field investigation, some faint fractures and fissures were locally observed in this segment, and these co-seismic displacement traces were also faintly visible on the UAV images.
It is also worth noting that near the epicenter, Dongo, and Huanghexiang, a certain amount of off-fault surface fissures appear locally with steady strike, good stretch, and en echelon pattern. Some fissures near meanders of the Yellow River, often appear with beaded liquefaction belts or swath subsidences. In cases like that, fissure strikes are, in the main, orthogonal to the river. Distribution pattern of these fissures is different from usual gravity fissures or collapses. But they can’t be identified as tectonic ruptures because clear displacement marks are always absent with off-fault fissures. Therefore, it is difficult to determine the mechanism of off-fault co-seismic surface fissures. Some research results suggested, that during the process of a strong earthquake, a sudden slip of the rupturing fault can trigger strain response of surrounding rocks or previous compliant faults, and result in triggering surface fractures or fissures.
Because of regional tectonic backgrounds, deep-seated physical environments, and site conditions(such as lithology and overburden thickness), the pattern and physicalcause of co-seismic surface ruptures vary based on different events. Focal mechanisms of the mainshock and most aftershocks indicate a near east-west striking fault with a slight dip-slip, but focal mechanisms of two M_S≥4.0 aftershocks show a thrust slip occurring near the east of the rupture zone. On the 1︰250000 regional geological map, the Jiangcuo Fault is oblique with the Madoi-Gande Fault and the Xizangdagou-Cangmahe Fault at wide angles, and with several branches near the epicenter and the west endpoint at small angles. Put together the surface fissure distribution pattern, source parameters of aftershocks and the regional geological map, we would like to suggest that besides triggered slip of several subparallel or oblique branches with the Jiangcuo Fault, inheritance faulting of pre-existing faults may promote the development of off-fault surface fissures of the 2021Madoi earthquake. Why there are many off-fault distributed surface fissures with patterns different from the gravity fissures still needs further investigation. The fine expression of the distributed surface fractures can contribute to fully understanding the mechanism of the seismic rupture process, and effectively address seismic resistance requirements of major construction projects in similar tectonic contexts in the world.

Key words: Madoi earthquake, coseismic surface rupture, Jiangcuo Fault, distributed surface fractures, avoidance zone

摘要：

地震地表破裂是理解大陆地壳变形模式和地震破裂行为的关键, 也是活动断层避让带设置的直接依据。2021年5月22日青海玛多 M_W7.4 地震沿昆仑山口-江错断裂江错段形成了长达158km的同震地表破裂, 造成沿线野马滩大桥、 昌马河大桥坍塌。文中基于震后2次大范围现场调查资料和震区3~7cm分辨率的无人机航片, 获得了本次地震详细的地表破裂, 在精细填图的基础上, 阐述了玛多地震地表破裂、 地表裂缝、 砂土液化带和带状塌陷等多种类型裂缝的分布特征及其意义。除在断裂相交处存在多条次级破裂外, 局部存在大量延伸长、 走向稳定、 具有雁列特征的裂缝带, 最远处与主破裂带的距离>5km; 在震中附近及震中以西、 以东多个段落跨断层数千米范围内存在分支破裂、 斜列式地表裂缝、 砂土液化带、 带状塌陷和地裂缝等与同震变形相关的地表特征。玛多地震分布式同震地表裂缝的揭示, 主要得益于大范围、 密集的现场调查和厘米级高分辨率航片的精细解译, 使小位移量破裂或微弱裂缝得到充分识别。由于缺乏明确的位错标志, 难以甄别未在主破裂断层上的同震地表裂缝是构造成因还是震动成因, 分析其空间分布形态、 余震分布及其震源机制、 区域构造背景等, 经初步推测, 不排除有些裂缝代表区域先存断层的继承性活动与次级断层触发活动的可能性。对分布式同震地表裂缝的精细刻画有助于全面理解地震破裂过程的机理, 对于重要工程抗震减灾的有效设防具有现实意义。

关键词: 玛多地震, 同震地表破裂, 江错断裂, 分布式地表裂缝, 避让带