Co-seismic And Post-seismic Deformation Mechanisms Of The M_w7.8 Nepal Earthquake In 2015

With the continuous development of modern geodetic technology,InSAR,GPS and gravity satellite can be used to monitor the co-seismic and post-seismic surface deformation caused by earthquakes,which provides data support for the study of seismogenic mechanism and internal structure of the earth.Firstly,this paper studies the practicability of spherical and viscoelastic spherical displacement theory based on the co-seismic displacement caused by the M_w7.8 earthquake in Nepal in 2015 monitored by 19 GPS stations in Nepal and its adjacent areas.In order to better compare and analyze the differences between the two spherical displacement theories in calculating co-seismic displacement,two spherical displacement theoretical models are used respectively.The co-seismic displacement field of the quasi-Nepalese earthquake is compared and analyzed.Secondly,based on the GPS data of Nepal one year after the earthquake monitored by 51 GPS continuous observations,this paper studies the afterslip and viscous relaxation effects after the M_w7.8 earthquake in Nepal.The main results obtained in this paper are as follows:1.Comparing the measured GPS co-seismic displacement with the co-seismic displacement calculated by the spherical displacement theory and the viscoelastic spherical displacement theory respectively,we can see that there is a good consistency in both spatial distribution and magnitude,which verifies the reliability and practicability of the two spherical displacement models.By comparing and analyzing the co-seismic displacement field of Nepal earthquake simulated by the two kinds of spherical displacement theory,we can see that the two kinds of spherical displacement theory have good consistency in calculating co-seismic displacement,and there is no obvious difference.2.The coseismic displacement fields simulated by the two spherical dislocation theories reveal that the earthquake was a thrusting earthquake.The coseismic displacement caused by the two theories mainly concentrated in the region near the coseismic and maximum aftershocks.The horizontal displacement in the north-south direction is more significant than that in the East-West direction.The simulation results of the two theories also reveal that the fault of the M_w7.8 earthquake in Nepal has the component of left strike-slip.3.This paper explores two different post-earthquake deformation mechanism models:?1?single post-earthquake afterslip effect model?model ??,?2?combined effect model of post-earthquake afterslip and viscous relaxation?model ??.The results of model ? show that the fault residual slip mainly occurs at the depth of 20-35 km in the downdip area of co-seismic rupture;the residual slip is mainly thrusting,accompanied by right-lateral strike-slip components,of which the maximum thrust and strike-slip components are 20 cm and 11 cm,respectively;the seismic moment released by the post-earthquake residual slip is1.23×10²⁰ Nm,equivalent to M_w7.33 earthquake.The distribution of residual fault slip obtained by model ? is consistent with model I,but the cumulative slip is slightly smaller,and the released seismic moment is 1.1×10²⁰ Nm,which is equivalent to M_w7.32 earthquake.The study of model ? shows that the optimum values of lithospheric elastic layer thickness and mantle viscous coefficient in Nepal seismic source area are 40 km and 2×10¹⁹ Pa·s,respectively.In summary,the post-earthquake fault afterslip effect played a dominant role and the viscous relaxation effect played a minor role in the 1-year period after the Nepal earthquake,which also revealed that the co-seismic fault slip of the Nepal earthquake induced the activity of deeper faults after the earthquake.4.The post-earthquake displacement evolution process of M_w7.8 earthquake in Nepal is simulated by using viscoelastic spherical body dislocation theory.The results show that the post-earthquake displacement caused by viscous relaxation effect mainly concentrates in the epicenter and the vicinity of the maximum aftershock.With the increase of time,the post-earthquake displacement changes gradually remarkably.The change of horizontal displacement in the north-south direction is larger than that in the East-West direction,and the change of vertical displacement is the largest.From the simulation results,it can be inferred that the effect of viscous relaxation will be more significant as time goes by after Nepal earthquake,and it will replace the residual slip effect of faults and play a leading role in post-earthquake deformation at a certain time.