Quantitative Study On Geometry And Kinematics And Three Dimensional Structural Model Of The Longquan Shan Fault Zone

The Longquan Shan fault zone（LQFZ）is located in the forebulge of the Longmen Shan foreland thrust belt and is the eastern boundary of the eastern margin of the western Sichuan foreland basin.Due to the weak Cenozoic shortening rate and seismicity,as well as lack of Cenozoic sedimentary strata in the LQFZ region,the fault activity and seismic risk is poorly known.Meanwhile,the segmentation deformation characteristics of the LQFZ and the three-dimensional spatial distribution of faults are not clear,and there is a lack of evidence for the earthquake disaster assessment.Based on the Area-Depth-Strain（ADS）method and fault-related-fold theory,the depth of the detachment layer and the geometry of the fault are quantitatively constrained,and the shortening amount and strain of each segment of the LQFZ are quantitatively analyzed.We use 62 high-resolution seismic reflection profiles,14drilling wells,1:200,000 regional geological maps,and 30m resolution DEM data,to interpret the structure of the LQFZ in detail.Combined with the shortening amount/strain data of each section and the three-dimensional structural characteristics of the fault zone,we further segment the LQFZ.In this study,a new three-dimensional structural model of the LQFZ is constructed.Combined with the shortening amount/strain data of the LQFZ constrained by the seismic reflection profiles,the boundaries are at the Renhe County and Yucheng County,and the LQFZ is divided into three structural segments,i.e.,the northern segment,the central segment and the southern segment.ADS method analysis displays that the shortening amount of the northern segment is about 1.1-1.4km,and the average shortening strain is 5.99%;the shortening amount of the central segment is 1.1-3.0km,with an average shortening strain of 9.55%;the shortening of the southern segment is1.1-1.6km,with an average shortening strain of 4.89%.Based on the three-dimensional modeling process of the SKUA-GOCAD platform,this study establishes the three-dimensional geometry of the east and west branch faults of the LQFZ,the middle-lower Triassic detachment layer and the bottom interfaces of the Cretaceous,Jurassic,upper Triassic,middle-lower Triassic and Permian strata.Both the detachment layer and the strata tend to become shallower from north to south and from west to east,and the thickness of each stratum gradually decreases from north to south.As for the fault plane,the northern segment of the LQFZ is composed of two thrust faults dipping to NW and SE,respectively;the central segment is a structural wedge,which is mainly composed of a thrust fault dipping to the SE and a detachment,as well as a NW trending thrust fault in the southern part of the central segment.The southern segment consists of two NW trending thrust faults.The dip angles of the Zhongjiang-Lingxianmiao fault（f₁）and the Zhenyangchang faule（f₄）is relatively uniform in the strike,40°～60°and 20°～30°respectively.The dip angle of the Jintang-Shigaopu fault（f₂）obviously increases in the south.The Renshou-Zhoujiapo fault（f₃）can be divided into two parts,and the dip angle in the south is significantly smaller than that in the north.The three-dimensional model of the LQFZ provide the basis for the numerical modeling and stress-strain analysis of the region,as well as the earthquake risk assessment and earthquake hazard research in the southern Longmen Shan.Aiming at the Ms 5.1 earthquake in Qingbaijiang District,Chengdu City,Sichuan Province on February 3,2020,a quantitative and comprehensive study on the seismogenic structure is carried out.The epicenter of the earthquake is located in the northern segment of the LQFZ in the Western Sichuan Basin.There are east and west branch faults in the northern segment of LQFZ,and the distance between the two faults and the epicenter is close.We try to use multiple constraints,through geophysics,seismology,and geodesy,to conduct a quantitative constraint on the Qingbaijiang Ms5.1 earthquake.In this study,CAP waveform inversion method is used to obtain the focal moment centroid depth and the focal mechanism solution of the earthquake.The focal moment centroid depth is 5 km,which indicates that the earthquake is related to shallow fault activity;the focal mechanism solution is 18°/32°/100°for nodal plane I,186°/59°/84°for nodal plane II.The Hyposat location method and the Double-difference location method are used to relocate the Qingbaijiang earthquake sequence.The epicenter of the main earthquake is at 30.73°N,104.48°E,the 60 aftershocks are mainly distributed near the east branch of the northern segment of the LQFZ,which has good correlation.The occurrence of the east branch of the fault is consistent with nodal plane II of the focal mechanism solution.It is a low angle reverse fault with NW dip.The coseismic deformation field of InSAR near the epicenter is extracted,and the maximum coseismic deformation is 4 cm.The deformation caused by the earthquake is uplift in the northwest of the epicenter and depression in the southeast.The largest depression is between the epicenter and the east branch fault of the LQFZ,which is consistent with the characteristics of thrust activity of the east branch fault.Based on multidisciplinary and multiple constraints,the Qingbaijiang Ms5.1 earthquake structure in 2020 is analyzed in detail,the seismogenic fault is determined,which provides a certain scientific basis for fault activity analysis and seismic risk assessment in Longquanshan area.Using the geophysical Vs data and earthquake catalog in the eastern margin of Tibetan Plateau,the seismic risk of the southern segment of the LQFZ was revalued through the S-wave velocity tomography,seismology and fault system,this work estimate that the magnitude of future earthquakes is around 5-6 in the southern segment of the LQFZ.