Study On Postseismic Deformation Following The 2008 Mw7.9 Wenchuan Earthquake

A sequence of great earthquakes have occurred in China and its surrounding subduction zones since 2000,such as the 2001 Kunlun earthquake,the 2004 Sumatra earthquake,the 2008Wenchuan earthquake,the 2011 Tohoku-Oki earthquake,and the 2015 Nepal earthquake.There events produced static stress changes due to the coseismic ruptures and transient stress variations in their postseismic phase in Chinese mainland.The redistribution of stress fields caused by large earthquakes can induce or inhibit regional seismic activity.Therefore,the detailed studies on the coseismic and post-seismic deformation mechanisms of these large earthquakes with large range of influence and long duration are of great significance for the assessment of seismic risks.The2008 Mw7.9 Wenchuan earthquake struck the eastern edge of the Tibetan Plateau and ruptured along the Longmenshan fault system with reversing and right lateral strking mechanism.The stress changes caused by co-seismic and sustained postseisic transients the accumulation of Coulomb stress in the major active faults in and around Longmenshan and may increase the probability of earthquakes occurring on these faults.To determine the sustained impact of the postseismic relaxation following the Wenchuan earthquake,GPS observations from 2008 to 2015 are used to analysis the mechanisms of the postseismic deformation and to probe the first-order rheological structure of the eastern margin of Tibetan Plateau.The main tasks of this dissertation are as follows:?1?In order to isolate postseismic transients from raw GPS time series,an adaptive program named Pytsfit was designed and coded in Python.The program Pytsfit can be run in batch mode to estimate long-term velocity,offsets due to earthquakes or instrument changes,seasonal deformation and postseismic transients simulately.It can also extract postseismic deformation from GPS time series covering time interval after the earthquake by tightly constraining non-postseismic terms to priori parameters.Additionally,it can also plot time series figures for different combination of signals one required?such as original time series and post-earthquake deformation time series?.?2?Secular velocities at GPS stations of China Movement Obsevation Network of China?CMONOC?and Sichuan CORS are interplolated using the Greenspline method based on preseismic velocities between 1999-2007.Postseismic transients following the Wenchuan earthquake are isolated after removing long-term interseismic velocities from raw GPS time series using Pytsfit program.The postseismic time series after the Wenchuan earthquake shows that the Wenchuan earthquake continued to relax after 7 years of earthquake.The relaxation speed rapidly decayed in the first two years after the earthquake,and then became slowly.The postseismic deformation in the western Sichuan Plateau is significantly larger than that in the Sichuan Basin.The cumulated postseismic deformation in the near-field in the western Sichuan Plateau reachs approximately 100mm,whileas only 10 mm in the Sichuan Basin.Although there are significant contrast in the magnitude of postseismic deformation,the direction of postseismic deformation in both sides of the Longmenshan fault is generally similar to that of coseismic slip.?3?The PSGRN/PSCMP is employed to infer the rheological structure for the western Sichuan Plateau and Sichuan Basin by fitting the predictions and cumulated postseismic displacements in different intervals,seperately.Finally,the best-fitting steady-state viscosity of the low-velocity layer in the western Sichuan Plateau is 2.51×10¹⁸Pa s,and the steady-state viscosity in the middle-lower crust is 3.98×10¹⁸Pa s.In addition,the optimal Maxwell viscosity of 1.0×10²⁰Pa s and elastic thickness of 40 km are loosely constrained for the Sichuan Basin by fitting GPS dada from 2008 to 2015.The viscoelastic relaxation models can well fit the far-field measurements but fail to explain the near-field data,implying that other mechanisms,such as afterslip,should be considered.Several different coseismic slip models are applied to assess their affection to the viscoelastic relaxation simulations.I find that different coseismic slip models only differentiate predictions in the near-field.?4?The simulated values underestimate the GPS observations in the west of the longriba fault zone and along the xianshuihe fault zone,and overestimate the measurments in the northeastern direction of the western Sichuan plateau.This dissertation argues that the western Sichuan plateau itself has the different physical properties of the earth's media.If the western Sichuan plateau is divided into three zones:the southwest,the central and the northeast,the viscosity of the western crust may be lower than that in the central of western Sichuan plateau,and the viscosity in the northeast maybe greater than that in the central of western Sichuan plateau.?5?In order to investigate the afterslip distribution on the Yingxiu-Beichuan fault,the fault geometry from Wang et al.?2011?is adapted.A northwest-prone decollement zone to the northern section of the Beichuan fault is extened.Afterslip distribution on the fault plane was inverted based on the elastic dislocation theory from cumulated postseismic displacements from 2010 to 2015.Considering the sparsity of GPS sites and the spatial complementary relationship between coseismic slip and afterslip,afterslip is penalized in the coseismic slip regions.This dissertation adopts two constraint schemes:"tight constraint"?afterslip upper bound is constrained to 0.1 m in patches where coseismic slip is greater than 2m?and"loose constrained"?afterslip upper bound is constrained to 0.1 m in patches where coseismic slip is greater than 6 m?to invert the spatial distribution of afterslip,respectively.the"loose constraint"models yield a relatively smaller WRMS than the"tight constraint".The preferred afterslip model favors some what slip on patches where ruptured larger than 2 m during the earthquake,indicating the small size asperities inferred by Wang et al.?2011?maybe unreasonable or these small asperites can switch from velocity weakening to velocity strengthening in different period.?6?Predictions from the optimal afterslip model can fit the near-field GPS onservation along the Longmenshan fault zone,but underestimate the far-field data,further indicating that Wenchuan postseismic deformation afterslip the Wenchuan earthquake is the result of multiple postseismic mechanisms.Because the simulated values from the afterslip models are also systematically smaller than the GPS observations in the west of the Longliba fault zone and along the Xianshuihe fault zone,it suggests the possibility of differences in the characteristics of the earth's media properties on the southwestern zone of the western Sichuan plateau.