In this paper, the seismic phase bulletin of 14381 earthquakes from January 1, 2009 to June 30, 2018 in the Weihe-Yuncheng Basin and its adjacent region were selected and analyzed. After removing the records with incomplete event information and insufficient station information, 11856 seismic events remained. A basic requirement for the double difference location method is that the distance between the pairs of seismic events is much smaller than the distance between the events and the stations and the linear scale of the velocity inhomogeneous body on the wave propagation path, so that the travel time difference between two earthquakes and the same station is only determined by the relative position between the two seismic events and the velocity of the seismic wave. In this case, the error caused by insufficient understanding of crustal structure can be effectively reduced and the result of relocation can be more accurate. Due to the large area, the whole study region was divided into three smaller parts for relocation of the events in order to reduce the influences of local structures. 8106 seismic events recorded by 52 stations were relocated using the double-difference location algorithm. It is found that the results constrained by the grid searching method are basically consistent with those obtained by other methods. The reliability of focal mechanism is affected by the number of initial motion and the azimuth distribution of the station. Therefore, when inversion of focal mechanism solution is carried out, earthquakes with more than 10 clear initial motion phases are selected, and the maximum azimuth gap between two stations with clear initial motion is required to be less than 90°. The azimuth coverage of the initial motion on the source sphere was measured according to azimuth and take-off angle distributions, and the focal mechanism solutions with poor coverage were eliminated. The contradiction ratio of focal mechanism solutions is less than 0.2. The average difference of b-axis of the best fitting solutions is less than 20°. Finally, the focal mechanism solutions of 346 seismic events with M_L≥2 were determined with initial motion of P and S waves. Normal type and strike-slip type earthquakes are widely distributed, accounting for more than 60% of all seismic events, and most of them are concentrated near fault zones. Before the formal inversion, the study area was divided into 1°×1° grids, and a series of damping coefficients were set to obtain the trade-off curve between the residual error of data fitting and the length of the stress field inversion model. The crustal stress field of 1°×1° grid in Weihe-Yuncheng Basin was obtained based on focal mechanism solution and stress tensor damping inversion method, and a certain number of depth profiles vertical to the faults were constructed for the analysis. The results show that compared with the original locations of seismic phase bulletin, the distribution of seismic events after relocation is more concentrated along the fault strike in plane. Vertically, they are densely distributed along the fault plane. There are more earthquakes in and around Shanxi graben, but the magnitude is generally small. The seismic activity in Weihe rift is relatively weak. Before the relocation, the focal depth distribution was concentrated in 5~10km, but after the relocation, the focal depth distribution changed significantly. The earthquakes were concentrated in the range of 10~25km, the overall focal depth was concentrated in the range of 20km, and a small number of earthquakes occurred in the range of 25~35km. The focal depth in the basin is relatively shallow with depth range of 5~15km. The focal depth at both ends of the basin tends to deepen, and the deepest depth can reach about 30km, which is consistent with the results of previous studies. The results of the depth profiles show that most of the fault planes in the study area have a large dip angle, similar to the occurrence of the surface, and some fault planes are even nearly vertical. The motion properties of fault structure and focal mechanism indicate that the faults in the study area are mainly normal and strike-slip ones. The results of stress field inversion indicate that the R values, which indicate the stress state, of the other regions are all less than 0.5 except for some areas in the southeastern margin of the research area. The stress state of Weihe-Yuncheng Basin tends to be tensile, and the maximum horizontal principal stress direction is nearly EW in Weihe rift and NNE and NEE in southern Shanxi rift, which is basically consistent with previous studies.

The Gansu Xiahe M5.7 earthquake on 28 October 2019 is located between the Lintan-Tanchang Fault and the northern Xiqinling Fault. The earthquake sequence offered significant information for interpreting the focal mechanism solution and analyzing the seismotectonics, which is helpful for the estimation of the level of future seismic risk in this region.
There are 180 aftershocks recorded by Gansu seismic network within 650 hours after the Xiahe M5.7 main shock up to November 24, 2019. The waveforms recorded by 20 seismic stations that have a distance less than 200km to the Xiahe M5.7 main shock are collected. HZT station, the nearest station to the main shock has an epicenter distance of 19km. Locations of 94 earthquakes including the main shock are obtained after locating the earthquake sequence using HYPO2000 method, each earthquake is with more than 3 recognizable Pg phases during location. The result of location shows that the aftershock epicenters are concentrated and mainly distributed in an area of 20km×20km near the epicenter of the main shock, the hypocentral locations are distributed in the depth of 1~16km.
After locating the Xiahe M5.7 main shock and the aftershock sequence, we calculated the focal mechanism of the main shock using gCAP and P waveform polarity method, and relocated the main shock and aftershocks by the method of double difference algorithm.
The results show that the parameters of the focal mechanism solution are as follows: For the nodal plane Ⅰ, strike is 185°, dip is 56°, and rake is 127°; for the nodal plane Ⅱ, strike is 312°, dip is 48°, and rake is 48°.The relocation results show the occurrence features of the seismogenic fault that dips northeast with dip angles about 47°~54°, which is near to nodal plane Ⅱ of the focal mechanism solution.
The epicenter of Xiahe M5.7 earthquake is closer to the western Lintan-Tanchang Fault, which has a complex geometry structure composed of several parallel and oblique secondary faults. Before Xiahe M5.7 earthquake, the activity of the western segment of the fault was weak. The Lintan-Tanchang main fault and its secondary faults are part of the transition zone of structural transformation between the Eastern Kunlun Fault and the north margin of West Qinling Fault, which caused several moderate earthquakes occurring near the Lintan-Tanchang Fault since 2003. The Minxian M_S6.6 earthquake 2013, which is the largest earthquake of these earthquakes, has a seismogenic fault which is a secondary fault of Lintan-Tanchang Fault, this fault merges into the Lintan-Tanchang main fault in the deep part.
The result of the relocation shows that the hypocentral location of Xiahe M5.7 is not on Lintan-Tanchang main fault but on a branch fault, since the epicenter of the earthquake has a certain distance from the western segment of Lintan-Tanchang main fault. Previous studies are lacking about seismogenic fault of Xiahe M5.7 earthquake. In this paper, based on the focal mechanism of Xiahe M5.7 earthquake, which is same to the mechanisms of the moderate strong earthquakes occurring near the Lintan-Tanchang Fault in the past twenty years and the feature of Lintan-Tanchang main fault with its secondary faults, we speculated that the seismogenic fault of Xiahe M5.7 earthquake is one of the secondary faults of Lintan-Tanchang Fault, and it has the same structure as other secondary faults that the fault merges into the Lintan-Tanchang main fault in the deep part. The seismogenic fault of Xiahe M5.7 earthquake has a strike angle of about 312° and a dip angle of about 48°, the strike angle is consistent with the intensity distribution from the post-earthquake investigations.

Due to the interaction between the Tibetan plateau, the Alxa block and the Ordos block, the western margin of Ordos(33.5°~39°N, 104°~108°E)has complex tectonic features and deformation patterns with strong tectonic activities and active faults. Active faults with different strikes and characteristics have been developed, including the Haiyuan Fault, the Xiangshan-Tianjingshan Fault, the Liupanshan Fault, the Yunwushan Fault, the Yantongshan Fault, the eastern Luoshan Fault, the Sanguankou-Niushoushan Fault, the Yellow River Fault, the west Qinling Fault, and the Xiaoguanshan Fault.
    In this study, 7 845 earthquakes(M≥1.0)from January 1st, 1990 to June 30th, 2018 were relocated using the double-difference location algorithm, and finally, we got valid locations for 4 417 earthquakes. Meanwhile, we determined focal mechanism solutions for 54 earthquakes(M≥3.5)from February 28th, 2009 to September 2nd, 2017 by the Cut and Paste(CAP)method and collected 15 focal mechanism solutions from previous studies. The spatial distribution law of the earthquake, the main active fault geometry and the regional tectonic stress field characteristics are studied comprehensively.
    We found that the earthquakes are more spatially concentrated after the relocation, and the epicenters of larger earthquakes(M≥3.5) are located at the edge of main active faults. The average hypocenter depth is about 8km and the seismogenic layer ranges from 0 to 20km. The spatial distributions and geometry structures of the faults and the regional deformation feature are clearly mapped with the relocated earthquakes and vertical profiles. The complex focal mechanism solutions indicate that the arc-shaped tectonic belt consisting of Haiyuan Fault, Xiangshan-Tianjingshan Fault and Yantongshan Fault is dominated by compression and torsion; the Yellow River Fault is mainly by stretching; the west Qinling Fault is characterized by shear and compression. The structural properties of the fault structure are dominated by strike-slip and thrust, with a larger strike-slip component. The near-north-south Yellow River Fault is characterized by high angle NW dipping and normal fault motion.
    Based on small earthquake relocation and focal mechanism solution results, and in combination with published active structures and geophysical data in the study area, it is confirmed that the western margin of Ordos is affected by the three blocks of the Tibetan plateau, the Alax and the Ordos, presenting different tectonic deformation modes, and there are also obvious differences in motion among the secondary blocks between the active faults. The area south of the Xiangshan-Tianjingshan Fault has moved southeastward since the early Quaternary; the Yinchuan Basin and the block in the eastern margin of the Yellow River Fault move toward the SE direction.