Tectonic deformation and seismic mechanism of the 2021 Aksai MS5.5 earthquake
[Objective]On August 26,2021,an Ms5.5 earthquake occurred in Aksai,Gansu Province.The epicenter is located along the southern piedmont of the Danghe Nan Shan.This event garnered significant attention because of its deformation characteristics and seismogenic mechanisms.Existing studies have mainly focused on emergency response and seismic activity analyses;however,there is a lack of research on tectonic deformation and seismic mechanisms.This study aimed to fill this gap by analyzing the deformation characteristics of the earthquake zone and revealing its seismogenic mechanism.[Methods]This study employed seismological methods combined with interferometric synthetic aperture radar(InSAR)technology to investigate the tectonic deformation and seismic mechanism of the 2021 Aksai Ms5.5 earthquake.Combining focal mechanism solutions,precise earthquake locations,and InSAR results,the seismogenic fault and its geometric and kinematic parameters were determined and validated through geological field surveys.[Results]This study applied joint inversion with both local and teleseismic waveforms(the generalized cut-and-paste joint,gCAPjoint)to source parameters.The fault solutions strike 315°,dip 41°,rake 81°,depth 6.9 km.We relocated the Aksai earthquake and its aftershocks using the hypoinverse and double-difference method(HypoDD),and accurate locations of 88 earthquakes were obtained.The 2021 Ms5.5 earthquake sequence in Aksai is distributed near the southern Danghe Nan Shan Fault,with a fault dip toward the NE.The co-seismic deformation field indicated by InSAR matched the macro-epicenter with the precise location results,confirming the reliability of the precise location.Both the ascending and descending orbit surface deformation fields showed uplift near the epicenter with similar magnitudes and signs in the line-of-sight direction,indicating that the earthquake rupture was mainly thrusting.Fault scarps near the epicenter along the southern piedmont of the Danghe Nan Shan were recognized in the field and satellite images.Combined data from focal mechanism solutions,precise earthquake locations,and InSAR coseismic deformation fields,along with field geological survey results,indicate that the seismogenic fault of this event was the southern Danghe Nan Shan Fault,with a strike of 315°,dip of 41°,and rake of 81°.[Conclusion]This study indicated that the seismogenic fault of this event was the southern Danghe Nan Shan Fault,which is a thrust fault.The fault solutions strike 315°,dip 41°,rake 81°,depth 6.9 km.Because of the northward extrusion thrust of the Qinghai-Xizang Block,the seismic activity in the northern part of the Qaidam Block has significantly increased.The future seismic risk of the eastern section of the Altyn Tagh Fault and western Qilian Shan should be emphasized.[Significance]This study provides new insights and methods for researching active tectonics.It holds significant scientific importance and innovation in understanding seismogenic mechanisms and structural transformation,as it helps to understand the mode and magnitude of slip transfer between the strike-slipping of the Altyn Tagh Fault and the shortening of the Qilian Shan and also contributes to a better evaluation of the seismic risk in this region.
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