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.