Deformation Characteristics And Displacement Distribution Of The Surface Rupture Zone Of The 1985 Wuia Mw6.9 Earthquake

The spatial distribution,deformation patterns,and co-seismic displacement distribution of co-seismic surface rupture zones not only record information about earthquake rupture and surface deformation but also reflect the characteristics of fault activity,regional stress state,and crustal movement.The co-seismic displacements along faults are important information about the surface rupture process during seismic activity.Conducting statistical surveys of displacement points and constructing a distribution of co-seismic displacements along faults are beneficial for understanding the activity characteristics of faults and assessing seismic hazards associated with them.Therefore,investigations of seismic surface rupture zones and measurements and statistical analyses of co-seismic displacements play a significant role in understanding the structural activity of the faults.This paper combines various methods such as traditional field investigation of surface rupture,trenching,image interpretation,differential GPS topographic profiles,and Structure-from-Motion(Sf M)photogrammetry to provide effective technical support for obtaining post-earthquake high-resolution and high-precision surface rupture data,thereby allowing for more extensive and comprehensive data acquisition.The Pamir Frontal Thrust,located in the convergence zone of the Pamir and South Tianshan Mountains,experienced a Mw 6.9 earthquake on August 23,1985,followed by a Mw 6.0 aftershock 20 days later.The surface rupture zone formed by this seismic sequence exhibits a distinct and well-defined linear pattern.It primarily consists of a variety of tectonic landforms associated with oblique thrusting activity,including numerous reverse fault scarps,thrust fault scarps,pressure ridge,and strikeslip fault scarps.These structural landforms have been well-preserved even after several decades since the earthquake event.After the earthquake,several experts and scholars conducted mapping and image interpretation of the seismic surface rupture zone.However,the existing fundamental data remains limited,and a detailed interpretation of the entire rupture zone is lacking.Due to limitations in displacement measurement techniques and difficulties in transportation,there is a sparse distribution of co-seismic displacement survey points and insufficient data on fault dip angles.Furthermore,there are still gaps in the statistical analysis of basic data such as slip measurements.Therefore,this study employs methods such as Unmanned Aerial Vehicle(UAV)measurements and field surveys to obtain high-precision and high-resolution topographic data of the surface rupture zone caused by the 1985 Wuqia earthquake.Building upon previous work,the study aims to further investigate and refine the morphology and displacement distribution of the Wuqia earthquake surface rupture zone.The causative fault of this earthquake,the Tuomluoan Fault,underwent reverse slip to the north,reaching above the Mingyaole Anticline,and formed a series of fault scarps on river terraces.The intense activity of the underlying Mingyaole Anticline significantly controls the cumulative surface deformation and co-seismic deformation of the Pamir Frontal Fault.In this earthquake,the latest surface rupture occurred along pre-existing fault scarps,providing us with a rare opportunity to discuss how deep-seated pre-existing faults influence the deformation of shallow fault structures through detailed analysis of the seismic surface rupture zone.This paper is based on field investigations and utilizes various data sources such as Google Earth satellite imagery,Worldview stereoscopic images,unmanned aerial vehicle(UAV)photogrammetry using Structure-from-Motion(Sf M)techniques to generate Orthophotos(DOM)and Digital Elevation Models(DEM),and differential GPS topographic profiles.These data were employed to conduct a detailed study of the surface rupture zone in Wuqia.The main conclusions obtained from this research are as follows:(1)Detailed spatial distribution and deformation patterns of surface ruptures.The eastern end of the surface rupture caused by the Wuqia earthquake is located east of the Kalanggoulvke River,while the western end is near the Kalabeili Hydropower Station dam site.The total length is approximately 22 km,and there are significant differences in the geometric morphology and deformation patterns of surface ruptures on both sides of the epicenter,which can be divided into eastern and western segments.The eastern surface rupture exhibits an arc-shaped distribution,with significant variations in strike,kinematics,and dip angles of the underlying faults.The western segment shows a more linear trace with more stable dip angles and kinematics.(2)More comprehensive distribution of co-seismic displacements.We have redefined the near-surface fault offset generated by the Wuqia earthquake and obtained the distribution of displacements along the strike of the surface rupture.The vertical fault offset ranges from 0.1 to 3.3 m,with the maximum value near the W11 km.The strike-slip fault offset ranges from 0.5 to 1.6 m,primarily occurring in the right-lateral oblique thrust segment at a distance of W4-6.5 km.By combining the fault dip angles revealed by natural outcrops and artificial trench profiles,the displacement vector magnitude ranges from 0.3 to 5.1 m,with directions ranging from340°-80°.The maximum total displacement value is located at 6 km along the rupture.(3)For reverse earthquakes,the cumulative-slip displacement is more suitable for calculating seismic moment,and the moment magnitude estimated using cumulative-slip displacement and maximum displacement provides a closer approximation to the instrument-recorded magnitude.(4)The eastern and western segments of the surface rupture zone caused by the 1985 Wuqia earthquake exhibit significant differences in spatial distribution and deformation patterns.The slip vector also rotates from a northward direction to the NE-NEE direction.The prominent differences are related to the variations in preexisting structural development on both sides of the epicenter.The migration activity near the epicenter has resulted in the formation of numerous curved faults with parallel slip,providing pre-existing weak planes for the seismic fault in the western segment.In contrast,the eastern segment has smaller dip angles and weaker folding effects.It does not develop significant curved faults or pronounced pre-existing weak planes.Under horizontal compression stress,it follows the Anderson’s fault mode and forms low-dip angle fault planes that cut across the stratigraphic layers.Consequently,the corresponding surface rupture traces are more curved in this segment.