Crustal Velocity Structure and Seismic Activity of the Central Qilian Block

LIANG Ci'an; SHEN Xuzhang; LI Minjuan; LIU Jinrui

doi:10.15953/j.ctta.2025.205

Abstract：The Qilian Orogenic Belt,located at the northeastern margin of the Tibetan Plateau with the Central Qilian Block at its core,plays a significant role in regional tectonic evolution.To reveal the crustal velocity structure of the Qilian Orogenic Belt and investigate the relationship between velocity structure and seismic activity,this study utilized P-wave and S-wave arrival time data recorded by seismic networks in Gansu,Qinghai,and other regions under the China Earthquake Networks Center.A double-difference tomography method was applied to relocate 6,617 seismic events in the study area,and 3D P-wave/S-wave velocity structures as well as velocity ratio distributions were obtained.The double-difference tomography produced several interesting results:(1)The Central-Northern Qilian Fault Zone exhibits distinct segmentation in activity along its eastern and western sectors.The eastern segment is dominated by shallow-seated faulting(0-10km),whereas the western segment features deeper-seated tectonic activity(15-25km).This indicates more pronounced upper crustal thickening in the western sector of the Qilian orogenic belt.Large earthquakes are concentrated in high-velocity anomaly regions of the upper-middle crust,suggesting that brittle crust is more prone to fault rupture.(2)Both velocity structures and velocity ratio distributions show a vertical transition from high to low values,indicating that the upper-middle crust of the Qilian Orogenic Belt is more rigid,whereas the middle-lower crust may include dehydration reactions of hydrous minerals.(3)There is a significant interface separation between the Central Qilian Block and the North Qilian Orogenic Belt in the upper-middle crust,which corresponds well with the south-deep and north-shallow distribution characteristics of sedimentary rock layer thickness in the Qilian Orogenic Belt.