The investigation of seismogenic structure of historical strong earthquakes and the research on the genetic link between earthquakes and active faults are a basic seismogeologic work. In particular, the investigation of seismic surface rupture zones and the study of seismogenic structures are extremely important for understanding the characteristics of their tectonic activities. The determination of the macro-epicenter provides important evidence for the site selection for post-disaster reconstruction and avoidance. Due to the diversity of the rupture process in the focal area, the macro-epicenter and the micro-epicenter may not be identical. As the magnitude increases, the larger the focal area of an earthquake is, the more significant the gap between the macro-epicenter and the micro-epicenter will be.

The northern margin of the Qaidam Basin is an area with frequent earthquakes, where many earthquakes with magnitude above 6.0 occurred in the history. In the early and late 1990s, small earthquake swarms with long duration and high frequency occurred in this area, which caused considerable losses to the local industry. Since the Delingha earthquake of magnitude 6.6 in 2003, two earthquakes with magnitude 6.3 and 6.4 occurred in the northern margin of the Qaidam Basin in 2008 and 2009, which aroused great attention of researchers. A new research focus has emerged on this area, and many scholars conducted in-depth research on the faults of the northern margin of the Qaidam Basin.

The author conducted a preliminary remote sensing interpretation of the Amunikeshan Mountain segment of the northern margin of the Qaidam Basin and found that there is a very straight linear feature in the image of the Amunikeshan mountain front. On the basis of remote sensing interpretation, a related study was carried out on the Amunikeshan segment of the northern margin fault of the Qaidam Basin, which was considered to be a Holocene active fault. Since the late Holocene, the horizontal movement rate of the fault is 2.50~2.75mm/a, and the vertical movement rate is(0.43±0.02)mm/a. A 30km-long earthquake surface rupture zone was found in front of Mount Amunikeshan. It is preliminarily believed that the rupture might be caused by a strong historical earthquake. According to the catalogue of historical strong earthquakes and local chronicles, there were earthquakes of magnitude 6.8 and 6.3 occurring in this area on May 21, 1962 and January 19, 1977, respectively. There has been no detailed research report on these two earthquakes.

Through on-the-spot geological investigation, it is found that there are fault scarps, fault grooves, seismic bulges and ridges, twisted water system and other landforms developed along the line, forming a surface rupture zone with a strike of N30°-40°W, a coseismic displacement of 2.3m, and a length of about 22km. Through trenching and excavation, the trench section reveals several faults, indicating the characteristic of multi-stage activity. In the section, the faults ruptured to the surface, and the late Quaternary activity is obvious. Combining surface relics, geological dating, and micro-geomorphic measurements, it is determined that the nature of the fault is mainly strike-slip with thrust. The investigation has found many seismic geological disasters, such as landslides, rockfalls and ground fissures along the fault, which are judged to be generated in recent decades or centuries.

Based on the empirical statistical relationship between magnitude and surface rupture, and the empirical relationship between strike-slip fault and rupture length, the average magnitude required for producing a 22km-long earthquake surface rupture is 6.79, and the average magnitude for producing a 2.3m coseismic displacement is 7.03. In combination with the surface rupture, trench profile, geological dating, seismic geological disasters, empirical formula calculation, historical earthquake catalogue, local chronicles and other documents, it is considered that the rupture zone is most likely produced by the North Huobuxun Lake M6.8 earthquake on May 21, 1962, and its seismogenic fault is the Amunikeshan Mountain segment of the northern margin fault of the Qaidam Basin.

Since the study area has no permanent residents or buildings(structures), which are taken as the basis for inquiring and investigating the earthquake intensity, we are unable to draw the earthquake intensity map.

At 01:45 on January 8, 2022, Beijing Time, an M_S6.9 earthquake occurred in Menyuan County, Haibei Prefecture, Qinghai Province, with a focal depth of 10km. The microscopic(instrument)epicenter is located at 37.77°N latitude and 101.26°E longitude in the intersection between the Toleshan fault zone and the Lenglongling fault zone in the northern Qilian-Qaidam block. The epicenter is 54km away from Menyuan County in Qinghai, 99km away from Qilian County, 100km away from Haiyan County, 83km away from Minle County in Gansu Province, 83km away from Yongchang County, and 141km away from Xining City. When the earthquake occurred, Menyuan County and Xining City, the capital of Qinghai Province, were strongly felt, and Yinchuan, Lanzhou, Xi'an and many other places were felt. At the same time, affected by the earthquake, the Lanxin high-speed rail line, an important railway transportation hub of the Belt and Road, was suspended. This earthquake is the largest earthquake in the world since 2022. It is also another earthquake of magnitude 6.0 or above in Qinghai Province following the Maduo M_S7.4 earthquake on May 22, 2021. Besides, this earthquake is the event with the highest magnitude and the longest surface rupture in the region after the two M6.4 Menyuan earthquakes of August 26, 1986 and January 21, 2016. Therefore, this earthquake has attracted much attention from the society. The coseismic surface rupture distribution, combination characteristics, development properties and coseismic displacement of this earthquake were identified in time to help to have a correct understanding of the earthquake seismogenic structure, rupture process, and assessment of short-term earthquake hazards. It is also of great significance for major project route selection, earthquake fortification and rescue and disaster relief. On the basis of the on-site seismic geological investigation, based on the interpretation and analysis of high-resolution satellite remote sensing images, and combined with the low-altitude photogrammetry of unmanned aerial vehicles(DJI PHANTOM 4RTK), the author obtained the coseismic rupture data of five typical sites along the surface rupture zone generated by the earthquake. Using Agisoft Metashape Professional software to process the aerial photos of each section indoors, a high-resolution orthophoto map(DOM)was generated. At the same time, the five typical earthquake surface rupture sections were described in detail in ArcGIS Pro software based on the orthophoto map. Preliminary research shows that the surface rupture zone of the Menyuan M_S6.9 earthquake is more than 22km long and consists of the main rupture of the northern branch and the secondary rupture of the southern branch. The north branch main rupture zone is distributed in the middle-western segment of the Lenglongling Fault of central Haiyuan fault zone, with a length of more than 18km and an overall strike of 295°. The maximum co-seismic horizontal displacement is located in the middle of the rupture zone at Liuhuangou(37.799°N, 101.2607°E), which is about 3.1m and gradually decays towards both ends. The secondary rupture of the southern branch is distributed on the local segments of the eastern Toleshan Fault in the central-western Haiyuan fault zone, with a length of about 4km and a strike of 275°, constituting a secondary branch rupture zone arranged in a left-stepped en-echelon pattern to the western segment of the main rupture zone. There are en-echelon extensional stepovers between the two rupture zones of the north and south branches. The whole surface rupture zone is mainly composed of linear shear cracks, oblique tension cracks, tension-shear cracks, compressional bulges and other structural types. The coseismic surface rupture has the characteristic of typical left-lateral strike-slip motion with a thrust component, and the maximum vertical dislocation is 0.8m.

At 02:04, May 22, 2021, an earthquake with M_S7.4 occurred in Maduo County, Guoluo Tibetan Autonomous Prefecture, Qinghai Province. The epicenter of the earthquake is about 70km(34.59°N, 98.34°E)south of the east Kunlun fault zone on the northern boundary of the Bayan Har block, with a focal depth of 17km. The Maduo M_S7.4 earthquake is the largest in China after the 2008 Wenchuan M_S8.0 earthquake. As of 07:00 on June 12, 2021, 58 aftershocks of M≥3.0 had been recorded, including 0 earthquakes of M7.0~7.9, 0 earthquakes of M6.0~6.9, 1 earthquake of M5.0~5.9, 17 earthquakes of M4.0~4.9 and 40 earthquakes of M3.0~3.9.
Field geological surveys after the earthquake showed that the earthquake occurred in the Yematan area, which is more than 30 kilometers south of the county seat of Machali Town. The seismic surface rupture shows obvious segmentation, which can be initially divided into 3~4 segments. The rupture spreads from east to west in a left step, gradually approaching the middle of the Yematan Basin. The nature of the fault is mainly left-lateral strike-slip.
The earthquake produced a large-scale continuous surface rupture in the area from the west of National Highway 214 to the south of Eling Lake, with a length of about 45km and a strike of N95°~105°E. The surface rupture zone is composed of a series of compressional bulges and right-hand echelon fractures, forming large-scale seismic bulges(ridges), seismic fissures, left-lateral displacement and other geomorphic features, and producing the seismic geological disasters such as sand and water gushing, soft soil seismic subsidence and so on. From the east of National Highway 214 to the east of Xueluodong, the fracture zone strikes N100°E, which is composed of discontinuous, small-scale tension shear cracks and small-scale bulge(ridge). In the vicinity of Xuema village, Changmahe Township, a section of about 10km long, N75°E striking, large-scale tension shear fracture and seismic bulge(ridge) with good continuity is developed.
The earthquake caused left-handed displacement of geological bodies, water system gullies, roads, etc. and formed strike-slip scratches in the strata. Through measurement, the horizontal displacement of this rupture is 1.5m in the Langmajiaheri area, 1.3m in the area of Yematanshangtou, and 1.1m west of Xuema Village. There is an obvious vertical displacement of 1.4~0.8m near Yematanshangtou, and the vertical displacement of other sections is not obvious. Generally speaking, the horizontal displacement is greater than the vertical displacement, and the rupture is dominated by strike-slip.Based on the field geological survey results, it is considered that the seismic rupture of this earthquake is large in scale and has a good continuity at its both ends, while the rupture scale is small and the continuity is poor in the middle. The preliminary inversion results of seismic rupture process, InSAR processing results and small earthquake precise positioning results show that the Maduo earthquake is a bilateral rupture with a rupture length of about 170km. The field geological investigation results are basically consistent with the geophysical inversion results.
The Maduo M_S7.4 earthquake(the instrument epicenter is located at 34.59°N, 98.34°E)occurred inside the Bayan Har block on the south side of the main Arak Lake-Tosuo Lake section of the east Kunlun fault zone. Existing data show that a number of nearly parallel NW-trending strike-slip faults are developed around the earthquake sequence. According to previous studies and this geological survey, the seismogenic structure of this earthquake is determined to be the Jiangcuo Fault. According to a comprehensive survey of the scale and length of the earthquake surface rupture and the damage to the buildings, it is believed that surface rupture zone in the Langmajiaheri area is large in scale with good continuity and multi types of surface ruptures. The area can be preliminarily determined as the macro-epicenter. The geographic coordinates of the macro-epicenter are 34.736°N, 97.794°E, which is nearly 50km away from the micro-epicenter. The difference is mainly due to the sparse seismic stations and weak monitoring capability in the area.
The fact that the Maduo earthquake occurred inside the Bayan Har block on the south side of the east Kunlun main fault demonstrates the possibility of generating earthquakes with magnitude 7 or greater in the interior of this block. Therefore, the seismogenic conditions and mechanism of strong earthquake activity inside the Bayan Har block should be a scientific issue that needs more attention in the future.

At 02:04 a.m. on May 22, 2021, a M_S7.4 earthquake occurred in the Maduo County, Qinghai Province, China. Its epicenter is located within the Bayan Har block in the north-central Tibetan plateau, approximately 70km south of the eastern Kunlun fault system that defines the northern boundary of the block. In order to constrain the seismogenic fault and characterize the co-seismic surface ruptures of this earthquake, field investigations were conducted immediately after the earthquake, combined with analyses of the focal parameters, aftershock distribution, and InSAR inversion of this earthquake.
This preliminary study finds that the seismogenic fault of the Maduo M_S7.4 earthquake is the Jiangcuo segment of the Kunlunshankou-Jiangcuo Fault, which is an active NW-striking and left-lateral strike-slip fault. The total length of the co-seismic surface ruptures is approximately 160km. Multiple rupture patterns exist, mainly including linear shear fractures, obliquely distributed tensional and tensional-shear fractures, pressure ridges, and pull-apart basins. The earthquake also induced a large number of liquefaction structures and landslides in valleys and marshlands.
Based on strike variation and along-strike discontinuity due to the development of step-overs, the coseismic surface rupture zone can be subdivided into four segments, namely the Elinghu South, Huanghexiang, Dongcaoarlong, and Changmahexiang segments. The surface ruptures are quite continuous and prominent along the Elinghu south segment, western portion of the Huanghexiang segment, central portion of the Dongcaoarlong segment, and the Huanghexiang segment. Comparatively, coseismic surface ruptures of other portions are discontinuous. The coseismic strike-slip displacement is roughly determined to be 1~2m based on the displaced gullies, trails, and the width of cracks at releasing step-overs.

The northern margin fault of Qaidam Basin(NMFQB)dominates the deformation of the northeastern part of the Qaidam Basin. The study on the Quaternary slip characteristics of NMFQB is of great significance to understand the regional strain-partitioning pattern for the south Qilian orogenic belt, and the extrusion process in the Qaidam Basin. In this paper, Quaternary activities of the fault are discussed based on the remote sensing interpretation, geological survey, trench excavation, GPS topographic profile measurement and OSL dating. The results show that the NMFQB has obvious linear characteristics from the remote sensing image of Xitieshan section. A series of geomorphic traces, such as fault scarps, fault facets, water system displacement, show that the Xitieshan section of the NMFQB is a Holocene active strike-slip fault with minor thrust. Four-stage alluvial fans were identified in the Xitieshan area. The DEM map shows a maximum horizontal displacement of 150m, 38m and 6.5m in the alluvial fan Fan3, Fan2 and Fan1, respectively. The geological age of Fan3 landform in the area obtained by OSL dating is(34.3±3.3)ka, the geological age of Fan2 landform is(11.6±1.0)ka, and that of Fan1 landform is(3.2±0.3)ka. Comparing with the analysis and collation results on the alluvial fans in the northern Qaidam Basin obtained by other researchers, the geological age of Fan3 alluvial fan in the northern Qaidam Basin is about 40ka, that of Fan2 is about 12ka, and the geological age of Fan1 is about 3.5ka. The age of the alluvial fan in the Xitieshan area is basically consistent with the development time of the alluvial fan in the region, indicating that the northern region of the Qaidam Basin was under a large-scale regional uplift during the same period, and the uplift activity was synchronous and recurrent.
Through GPS measurement of fault scarps across faults, the average height of the scarps formed in Fan1 is 1.2m. According to the geological dating of Fan1, the vertical movement rate is calculated to be 0.33~0.38mm/a. The average height of the scarps formed by the alluvial Fan2 is 2.35m. According to the geological dating of Fan2, the vertical movement rate is calculated to be 0.17~0.23mm/a.
We analyze the vertical displacement and related geomorphological ages of the two periods of alluvial fans at the two sites with one west to Xitieshan Town and one east to Quanjihe after measuring the horizontal and vertical displacement data of the geomorphic surface in this area. The Late Pleistocene strike-slip rate of this section is 3.55~4.72mm/a since 40ka and 2.68~3.65mm/a since 12ka, the Holocene strike-slip rate is 1.81~2.1mm/a since 3.2ka, and the Holocene vertical slip rate is 0.33~0.38mm/a. This amount of geological slip rate is consistent with the slip rate of 2~4mm/a from GPS observation.
According to the reverse “S” type structural system, natural profiles and trenching profiles of the northern margin fault of Qaidam Basin, it is believed that the fault was squeezed and uplifted by the Qilian Mountains block in the early stage, and the fault activity was mainly thrust, and in the latter stage, due to the impact of the Altun Tagh Fault, the fault activity takes the form of strike-slip. Controlled by the Altun Tagh Fault, North Qaidam Fault, the Elashan Mountain Fault and East Kunlun Fault, the Qaidam Basin behaves as a block rotating clockwise.

Datong Fault belt is a northwest trending fault in the north of Qinghai-Tibet plateau which controls the boundary of Xining Basin and Datong Basin.It consists of the Maziying-Miaogou(F₁)Fault and the Laoye mountain-Nanmenxia Fault(F₂).There is obvious displacement in vertical direction along the fault belt.The field investigation results show that this belt has long-term activity.There are several meters-long crushed zone and veins along the fault side in the basement rock.In the visible profile of fault,the Cambria system thrusts to the red brick Quaternary gravel,and there are several centimeters-thick fault gouges along the fault side.ESR dating of the fault gouge in the fault profile shows an age of(610?61)ka.The covering deluvial loess is not offset,and the OSL result is(14.6?1.5)ka.So it can be concluded that the fault belt was active in middle Pleistocene but not in later Pleistocene according to the age data and geomorphologic feature.Interior stratum of the Datong Basin is mainly featured with fold with the major axis in northwest direction.According to the relation of fault and fold deformation,Datong Fault is a transversal tear,which is due to uneven compression of the folds in different parts and the NNE-oriented regional compressional stress.It is common among the NE-trending faults in northeastern Qinghai-Tibet plateau.These NE-trending faults aren't large,and most are located in the active plate.They are all nearly vertical to the axis of the folds and compressive basins.