Seismogenic Fault Of The 2016 Hutubi Ms6.2 Earthquake

On December 8,2016,the Hutubi Ms6.2 earthquake occurred on the thrust nappe structure belt in front of the North Tianshan Mountain,which is a strong compression area between the North Tianshan Mountain and the Junggar Basin.The area develops a series of subparallel thrust faults,and the structural deformation is complex.Different research institutions release the Hutubi Ms6.2 earthquake focal mechanism,which demonstrate that the earthquake occurred on a large-scale low-angle reverse fault dipping to the south or a high angle back-thrust secondary fault dipping to the north.The seismogenic fault of the Hutubi earthquake is critical to understand the seismogenic structure and the structural deformation patterns of the North Tianshan area.However,as no apparent coseismic surface rupture and the insensitive far-field seismic observation record,the seismogenic fault of the earthquake remains controversial.Near-field borehole strain observation provides important constraints for studying the seismogenic fault of the Hutubi earthquake.In this study,we deploy 11 borehole strain observation stations in the North Tianshan area and select the 4 near-field observation stations to analyze the tensile and compressive characteristics of the coseismic strain steps.The results show that all stations record the coseismic tensile and compressive strain steps.According to the seismic source parameters given by seismic wave inversion,seismic reflection profile,and aftershock relocation,based on the elastic dislocation theory,we use the PSGRN/PSCMP program to calculate the coseismic maximum principal stress field of four observation stations under different source parameters,and analysis the characteristics of each component by integrating the spatial distribution of four components observation stations.Finally,we compare the simulated coseismic strain steps and observed coseismic strain steps,the results show that the seismogenic fault of the Ms 6.2 Hutubi earthquake is likely a high-angle blind back-thrust fault with a north-dipping fault plane near the southern margin fault of the Junggar Basin.Based on this,we construct a 2D finite element model including main thrust fault and secondary back-thrust fault to analyze the influence of secondary thrust fault on regional vertical deformation.The results show that the existence of thrust fault makes the near-fault area appear obvious local uplift,and the main thrust fault and secondary thrust fault jointly control the deformation of the North Tianshan area.These results not only provide a mechanical basis for understanding the seismogenic fault of the 2016 Hutubi earthquake but also provide a key for understanding the present-day crustal deformation model in the North Tianshan area.