The northeastern margin of the Qinghai-Tibet Plateau located near the middle and northern section of the north-south seismic belt in China is a place where historical seismicity is extremely high and many strong earthquakes of magnitude above 7 occurred, including the August 8^th 2017 M7.0 Jiuzhaigou earthquake and the May 22^th 2021 M7.4 Maduo earthquake. There are many active faults in this region, most of them are NWW trending, among which the Maqin-Maqu segment on the east verge of the East Kunlun Fault is especially dangerous because a seismic gap exists on it. The 2017 M7.0 Jiuzhaigou earthquake and the 2021 M7.4 Maduo earthquake which occurred near the seismic gap are still not able to remove the danger of a future strong earthquake along the Maqin-Maqu segment. The seismic activity of a certain region is closely related to the variation of regional stress field and the physical properties of underground medium. With the development of the broadband digital seismic networks, source parameters, attenuation characteristics of seismic waves and site responses can be deduced from three-component seismic signals of regional small and medium earthquakes. Study results on the current state of regional stress and underground medium properties can provide basic information for the establishment of strong ground motion attenuation model and the assessment of seismic hazard in the study area.
In this study, the geometric attenuation model, Q value, source parameters of 444 earthquakes and the site responses of 118 stations are obtained near the northeast margin of Qinghai-Tibet Plateau using the joint inversion method based on the three-component S-wave data of small and medium-sized earthquakes occurring during 2010 to 2019. The results shows that the geometric attenuation model in the study area conforms to a form of 3-segment piecewise function, while the inelastic attenuation model meets Q(f)=401.8×f^0.2963 based on this geometric attenuation model. Among the site responses of 118 stations, 88 site responses show characteristics of rock site, 20 site responses show amplification effect, particularly in the high frequency, and 10 site responses show the overall value below 1, which may be affected by the spatial anisotropy of velocity structure and Q value. Through the correlation study of local magnitude, moment magnitude, stress drop and apparent stress, it is found that the moment magnitude has a linear correlation with the local magnitude as a whole, and the local magnitude has a positive correlation with the earthquake stress drop and apparent stress under the same moment magnitude. There is a significant statistical correlation between stress drop and apparent stress and an obvious linear correlation in logarithmic coordinate system. Under the same moment magnitude, the ratio of the apparent stress to the stress drop is higher in the earthquake with lower local magnitude, which means the seismic rupture is more sufficient and the radiation energy is relatively small. While the ratio of apparent stress to stress drop is relatively low in the earthquake with higher local magnitude, indicating the energy loss during fault rupture is relatively small and the radiation energy is relatively high.

The Weixi-Qiaohou Fault is located in the west boundary of Sichuan-Yunnan rhombic block, and also the north extension segment of active Red River fault zone. Strengthening the research on the late Quaternary activity of Weixi-Qiaohou Fault is of great theoretical and practical significance for further understanding the seismogeological background in northwest Yunnan and the structural deformation mechanism of the boundary of Sichuan-Yunnan block. Based on the 1︰50 000 active fault mapping and the research results of the National Natural Science Fund project, this paper mainly elaborates the latest active times of the fault and paleoseismic events along it revealed by exploration trenches at Matoushui, Shiyan, and Yushichang. Matoushui trench revealed three faults developed in late Pleistocene and Holocene pluvial fan accumulation, and the latest ages of faulted strata are(638±40)a BP and(1 335±23)a BP, respectively. The Shiyan trench revealed six faults, three in the western section and three in the eastern section. The three faults in the western section dislocated the late Pleistocene and Holocene accumulation, and the ¹⁴C ages of the latest faulted strata are(4 383±60)a BP, (4 337±52)a BP and(4 274±70)a BP, respectively; the other three faults revealed in the eastern part of the trench offset the Holocene fluvial facies accumulation, the ¹⁴C age of the latest faulted strata in the footwall of the main fault is(9 049±30)a BP, and the ¹⁴C ages of two sets of faulted sag pond deposits in the hanging wall are(1 473±41)a BP and(133±79)a BP, separately. Five active faults are revealed in Yushichang trench. Among them, the F₁ and F₂ dislocated the gray-white gravelly clay layer and the black peat soil layer. The ¹⁴C age of the gray-white gravelly clay layer is(1 490±30)a BP, and ¹⁴C ages of the upper and lower part of the black peat soil layer are(1 390±30)a BP and(1 190±30)a BP, respectively. The F₃ and F₄ faults offset the gray-white gravelly clay layer, the black peat soil layer and the brown yellow sand bearing clay, and the OSL age of brown yellow sand bearing clay is(0.6±0.2)ka. The F₅ fault dislocated the gray-white gravelly clay layer, its ¹⁴C age is(1 490±30)a BP. According to the relationship between strata and the analysis of dating data, the Yushichang trench revealed two seismic events, the first one occurred at(1 490±30)~(1 390±30)a BP, as typified by the faulting of F₅, the second paleoseismic event is represented by the faulting of F₁, F₂, F₃ and F₄.The F₁ and F₂ faulted the gray-white gravelly clay layer and the black peat soil. Fault F₃ and F₄ dislocated the gravelly clay, the peat soil and the sandy clay, and a seismic wedge is developed between fault F₃ and F₄, which is filled with the brownish yellow sandy clay. The OSL dating result of the brownish yellow sandy clay layer is(0.6±0.2)ka. Judging from the contact relationship between strata and faults, F₃ and F₄may also faulted the upper brownish yellow sandy clay layer, but the layer was eroded due to later denudation. Therefore, fault F₁, F₂, F₃ and F₄ represent the second event. Combined with the analysis of fault scarps with a height of 2~2.5m and clear valley landform in the slope near the fault, it is estimated that the time of the second paleoearthquake event is about 600 years ago, and the magnitude could reach 7. The trench at Gaichang reveals that the seismic wedge, soft sedimentary structure deformation and the medium fine sand uplift(sand vein)and other ancient seismic phenomena are well developed near the fault scarp. All these phenomena are just developed below the fault scarp. The vertical dislocation of the strata on both sides of the seismic wedge is 35cm, and ¹⁴C ages of the misinterpreted peat clay are(36 900±350)a BP and(28 330±160)a BP, respectively, so, the occurrence time of this earthquake event is estimated to be about 28 000a BP. If the fault scarp with a height of 2m was formed during this ancient earthquake, and considering the 0.35m vertical offset revealed by the trench, the magnitude of this ancient earthquake could reach 7.The Matoushui trench revealed three faults, which not only indicated the obvious activity of the faults in late Pleistocene to Holocene, but also revealed two paleoseismic events. Among them, the OSL age of the faulted sand layer by fault F₁ is(21.54±1.33)ka, which represents a paleoearthquake event of 20 000 years ago. The faulted strata by fault F₂ and F₃ are similar, which represent another earthquake event. The ¹⁴C dating results show that the age of the latest faulted strata is(638±40)Cal a BP, accordingly, it is estimated that the second earthquake time is about 600 years ago. A clear and straight fault trough with a width of several ten meters and a length of 4km is developed from Meiciping to Matoushui. Within the fault trough, there are fault scarps with different heights and good continuity, the height of which is generally 3~5m, the lowest is 2~3m, and the highest is 8~10m. Tracing south along this line, the eastern margin of Yueliangping Basin shows a fault scarp about 5m high. After that, it extends to Luoguoqing, and again appears as a straight and clear fault scarp several meters high. In addition, in the 2km long foothills between Hongxing and Luoguoping, there are huge rolling stones with diameters of 2~5m scattered everywhere, the maximum diameter of which is about 10m, implying a huge earthquake collapse occurred here. According to the length, height, width and dislocation of the rupture zone, and combined with the experience of Yiliang M≥7 earthquake and Myanmar Dongxu M7.3 earthquake, this earthquake magnitude is considered to be ≥7.