Seismogenic structure of the 2020 Jiashi, Xinjiang Ms 5.4 and Ms 6.4 moderate earthquakes.

In January 2020, two moderate earthquakes with magnitudes of MS 5.4 and MS 6.4 occurred in the nappe structure of the boundary between the Southern Tianshan Mountains and Tarim Basin. To investigate the seismogenic structure of these two events, we systematically analyzed the earthquake sequence locations, focal mechanisms, and stress field in the source region. Using the P and S arrival-time data from January 1, 2009, to July 31, 2021, recorded by 18 seismic stations of the Xinjiang network, we obtained precise seismic event locations. Results show that the temporal and spatial distribution of the foreshock and aftershock sequences displays obvious differences in migration behaviors. The former is mainly distributed along the NNW direction, whereas the latter is distributed along the Ozgertau fault in the EW direction and characterized by a double-layer feature. Furthermore, we derived the focal mechanism solutions of the MS ≥ 4.0 events, including the MS 5.4 foreshock and the MS 6.4 main shock. The inversion results illustrate that the MS 5.4 foreshock is a strike-slip event, whereas the main shock and seven aftershocks are thrust events. Based on our results and the regional geological background, we conclude that the seismogenic structures of the MS 5.4 foreshock and MS 6.4 main shock are related to different faults. The MS 5.4 event is located in a blind "quasi-transformation" fault with the NNW direction, and the MS 6.4 earthquake occurs on a blind thrust nappe fault in the EW direction on the Kepingtage (Kalpintag) nappe. Our results indicate that the Tienshan orogenic belt has a complex tectonic environment. The difference in the strikes and dips of the MS 5.4 foreshock and MS 6.4 main shock reflects the stress compression in the near-NS direction in the source region. [ABSTRACT FROM AUTHOR]