Probing The Rheological Structure Across The Eastern And Southern Edges Of The Tibetan Plateau Using GPS Observed Postseismic Displacements

The spatial coverage of this thesis is focused on but not limited to the eastern and southern margins of Tibetan Plateau.The main contents of this thesis include long-term motion pre-earthquake,seasonal variation in the vertical component,and sepeceially focused on the studies of mechanisms of postseismic deformation following the 2008 Mw7.9 Wenchuan earthquake and 2015 Mw7.9 Gorkha,Nepal event and on their lithospheric rheological structure.The first two points of content,interseismic deformation and seasonal variation in the vertical component,are the important foun-dation for extracting the three-dimensional deformation field from the observed GPS data.A series of GPS raw RINEX data in China mainland and its margin regions from 1998 to 2015 were processed using precise GPS processing software GAMIT/GLOBK,from which present day velocity field across the Tibetan and Plateau were obtained.In the procedure of daily solutions,the thesis presents the methods of different cor-rection models and subnet division in detail.I also introduce the method of combing regional loosely constrained solutions with global solutions from SOPAC through com-mon IGS stations to get whole loosely constrained daily solutions,that contain position coordinates,orbit parameters and atmospheric delays and their covariances.Finally,loosely constrained daily solutions were relied into the ITRF2008 using seven param-eter translation through over 50 stations distributed globally.A standard time series analysis were employed to estimate velocities in three coordinate components,seasonal variations,offsets due to earthquakes or instrument changes,ignoring postseismic defor-mation at this stage.Average horizontal velocity field from 1998 to 2007 were extracted for the study of Wenchuan earthquake in the following section.Pre-earthquake velocity field from 1998 to 2015 were estimated for the Gorkha,Nepal earthquake as well.As the by-products of time series analysis,coseismic displacements for the 2001 Kokoxili earthquake,2010 Yushu earthquake,2013 Lushan earthquake and 2015 Gorkha,Nepal earthquake were briefly introduced.Considering the important role of vertical deformation for the study of mechanisms of postseismic deformation,and strong seasonal variations in Longmen Shan and Nepal areas,this thesis also investigates seasonal variations in the vertical component using data from continuous GPS stations and GRACE.Both the results of the vertical defor-mation in the GPS time series and that from GRACE-derived vertical time series show that the southwestern region of China and Himalayan Nepal region suffer from strong seasonal variations due to water loadings and other period signals.The peak-to-peak displacements in these two regions can reach 20?30 mm,which are much larger than that in northwestern,northeastern,southeastern and north China.The vertical time series derived from GPS data generally agree well with the GRACE-derived results,es-pecially for stations located in.Himalayan-Nepal and southwestern China.The average amplitude of GPS observed seasonal deformation is slightly bigger than that of GARCE,and there are small difference in phase.This thesis first focus the GPS observed phase shift phenomenon at stations on different side of Longmen Shan.Vertical time series of GPS stations located in the western side of Longmen Shan always reach peak position in March-April each year,however peak position appear in May-June at station in the Sichuan Basin.This interesting phase shift phenomenon is also occurred in Nepal re-gion.The phase shift phenomenon,observed over a small region is probably caused by-surface runoff of water loading moving from high elevation to low elevation region.It may be attributed to different terrain climate pattern differences and different elastic parameters in different regions.Based on above results of pre-earthquake velocity fields of seasonal variations in Longmen Shan and Nepal regions,this thesis investigates the mechanisms of postseismic deformation following the Wenchuan and Nepal earthquakes and probe the lithospheric rheological structures.The main results are presented below.Transient postseismic deformation is carefully isolated by subtracting long-term motions in the ITRF2008,seasonal deformation in the vertical component and un-known offsets from raw GPS position time series.Therefore three-dimensional displace-ments spanning 2009 to 2015 at 156 station were finally obtained.The postseismic deformation pattern following the Wenchuan earthquake is characterized by clear a asymmetric feature,large postseismic deformation in the Plateau and limited motions in the Sichuan Basin,implying different rheological properties.Significant postseismic displacements are observed at the Plateau,especially in the zone confined between the Longmen Shan thrust fault and the Longriba right-lateral strike fault.The azimuths of the displacements in the south of 32° are nearly normal to the Longmenshan fault,and the directions of the vectors in the north of 32° are closely parallel to the Long-menshan fault.Fax-field postseismic displacements are also captured in the west of the Longriba fault and in the middle-to-south segment of the Xianshuihe fault.Up-lift deformation occurs in the Plateau.This thesis investigated the contributions from poroelastic rebound model by differencing displacements from dislocation models using different Poisson ratios.The result shows the poroelastic deformation concentrate along the near-field of the Longmen Shan fault and decays rapidly away from the fault.In general,the displacements from the poroelastic relaxation are smaller than the GPS observed displacements.Postseismic deformation following the Wenchaun earthquake can be explained well using multiple mechanisms of viscoelastic relation and afterslip models.Far-field postseimsic deformation can only be contributed to viscoelasitc re-laxation in the lower crustal and upper mantle and the near-field deformation can be recovered by using both viscoelastic relaxation and afterslip on the fault plane.The transient postseismic deformation constrained lithospheric rheological structure show strong contrast in lithospheric rheologies.The best-fitting viscoelastic relaxation model requires a high-viscosity(10ivPa s)upper mantle underlying the elastic layer of the Sichuan Basin.However,the viscosities of the lower crust and upper mantle beneath the Plateau are one to two orders smaller than that of the Sichuan Basin.The con-strained viscosities in Tibet's lower crust and upper mantle are coincide with results from previous studies on earthquakes within the Tibet Plateau.We also notice that the strain rate variations before and after the Wenchuan earthquake along the middle-to-south segment of the Xianshuihe fault are in fact due to far-field viscoelastic relaxation following the Wenchuan earthquake rather than suspected precursor of earthquake.As the last part of this thesis three-dimensional GPS coordinate time series both in Nepal and in south Tibet were analyzed and calculated the initial one-year postseis-mic displacements.We investigate the poroelastic rebound,afterslip and viscoelastic relaxation individually and attempt to separate the contributions from different mecha-nisms by evaluating the misfit between observed displacements and simulations,and by independent verification using physically reasonable stress-driven afterslip models.The modelling results show that no single mechanism satisfactorily explains observations of postseismic deformation following the Gorkha earthquake.A kinematic afterslip model can explain the GPS observed three-dimensional displacements with small misfit,how-ever the inversion favors a very broad distribution and afterslip at far down-dip of the rupture,which is found to be physically implausible based on stress-driven afterslip models.A stress-driven afterslip model shows that afterslip occur immediately below the coseismic rupture and gradually decays from 200 mm to nearly zero at the base of the model.A viscous relaxation,model can fit the far-field GPS data in south Tibet,however it fails to predict the near-field data in north Nepal.The poroelastic rebound model alone is also incapable of explaining the observations.By reconciling all these mechanisms,we find that the combination of afterslip and viscoelastic relaxation can not only improve the misfit but also make the residual affterslip model more physically meaningful.The afterslip releases 7%of coseismic moment,corresponding to a Mw7.0 earthquake.This low afterlip-to-coseismic moment ratio is relate to the lack of after-slip across the seismogenic depth range,with down-dip afterslip in the brittle-ductile transition regime releasing a relatively small amount of slip.The lacking of afterslip within the shallow unbroken fault section implies that it is still locking and may rupture in future.Our preferred model also illuminates the laterally heterogeneous rheological structure between India and south Tibet,with a transient viscosity of 6 × 1018 Pa s,steady state viscosity of 6 × 1019Pa s in Tibet's lower crust aud higher viscosity of 1020 Pa s in the Indian upper mantle.The transient ald steady state viscosities of the upper mantle beneath Tibet are colstrailed to be 1×1018,Pa s and 1 × 1019 Pa s respectively,based on a very far field GPS station.