Postseismic Processes Of Large Earthquakes,and Rheology Of The Lower Crust And Upper Mantle In Tibet

The Himalya-Tibet Orogen(HTO)is the largest deformation zone due to the continent-continent colliding that has been well studied.It is a natural laboratory for the study of the lithospheric deformation and rheological properties.Study on postseismic deformation processes of large earthquakes in Tibet has been hot research topic in geoscience and is the focus of this thesis.Through modeling the postseismic deformation processes of earthquakes in Tibet,I hope to quantitively constrain rheological properties of the lower crust and upper mantle in Tibet and distribution and evolution of the afterslip following the earthquake.We constructed viscoelastic three-dimensional finite-element models(FEM)of each large earthquake based the coseismic rupture,the layered structure of the lithosphere,and rock physical properties.I study two main postseismic processes,that is,afterslip of the fault,and viscoelastic relaxation in the lower crust and upper mantle,to understand the temporal and spatial evolution of the postseismic surface deformation.Model results indicate that the afterslip of the fault mainly controls the early-stage postseismic deformation in the near field,while the viscoelastic relaxation in the lower crust and upper mantle mainly controls the postseismic deformation in the middle-and far field.Other deformation processes may be secondary-order processes and be responsible for higher-order surface deformation.Test models have revealed that the aseismic afterslip takes place mostly over downdip and along-strike edges of the rupture area.The afterslip decreases with the distance to the rupture area.The modeled afterslip of the fault decays rapidly with time.The total accumulative afterslip in the same time window has a positive correlation with the moment magnitude of the earthquake,that is,the larger the earthquake,the more the afterslip.For example,the maximum accumulative afterslip in the first ten years after the earthquake is up to 1.0 m for 2008 Wenchuan earthquake(or the 2001 Kunlun earthquake),about 0.8 m for the 1997 Manyi earthquake is,and only 0.15-0.22 m for the 2010 Yushu and 2021 Maduo earthquakes.Afterslip following the Wenchuan and Kunlun earthquakes is equivalent to a modeled earthquake of Mw7.4-7.5.Afterslip following the Manyi,Maduo and Yushu earthquakes is equivalent to modeled earthquakes of Mw7.2,Mw6.9 and Mw6.2,respectively.Model results indicate the decay rate of afterslip may be different depending on the earthquake size.Afterslip following the Manyi and Yushu decays the quickest,and the Kunlun and Maduo cases are in the middle.The afterslip following the Wenchuan earthquake decays the slowest.On the basis of the optimal viscosities of modeling these earthquakes,I obtain the regional rheological structure of the lower crust and upper mantle in Tibet.Our results indicate that viscosity in the lower crust beneath both the Qaidam Basin and the western Qinlin is larger than～5× 1019 Pa s.The optimal steady-state viscosity in the lower crust is 1-2×1020 Pa s,which is similar with that in the upper mantle.The viscosity in the lower crust beneath the Kunlun Range and Longmen Shan is～5×1019 Pa s and 12×1019 Pa s,respectively.The viscosity in the upper mantle is larger than～2×1020 Pa s,which is at the same order of magnitude as that in the upper mantle beneath Qaidam Basin and Sichuan Basin.The optimal steady-state viscosity in the lower crust beneath the eastern and central Bayankala(or Qiangtang)is 1019 Pa s,and the range of the steady-state viscosity in the lower-crust is 0.3-2 × 1019 Pa s.However,a higher viscosity in the lower crust beneath the western Bayankala(or Qiangtang)is up to～5× 1019 Pa s.The steady-state viscosity may increase from the interior of plateau to the west and north,and the viscosity may decrease from the interior of plateau to the southeast.In Tibet,the range of the steady-state viscosity in the upper mantle is 1019-1021 Pa s,and the optimal viscosity in the upper mantle is 3-10×1019 Pa s.The viscosity in the upper mantle gradually increase from south to north.Based on the best-fit model of the Wenchuan earthquake,I calculate the distribution and evolution of the stress of the Wenchuan fault and neighboring active faults,evaluating Coulomb stress change over the active crustal faults and the change in the maximum shear stress in the lithosphere.I compare such stress change with the evolution of the aftershocks following the Wenchuan earthquake.Aftershocks took places mostly in the area with postseismic stress perturbation no less than 100 KPa within 10 years after the earthquake.The accumulative number of aftershocks positively correlates with the magnitude of the stress perturbation,that is,the larger the stress perturbation,the more aftershocks.All the aftershocks larger than Mw5.0 took place in the area with stress perturbation more than 100 KPa.Southern portion of the Longmenshan fault and middle portion of the Xianshuihe fault underwent continuous increase in the Coulomb stress,which may be responsible for the occurrence of the Mw6.6 Lushan earthquake and Mw6.0 Daofou earthquake.Other crustal fault underwent Coulomb stress change no more than 10 KPa since the 2008 Wenchuan earthquake.