Investigating Crustal Deformation And Fault Activity Using GNSS Measurements

Eathquake geodesy has opened a new chapter in accurately measuring and studying evolution history and seismic behavior of the present continental deformation dynamic system,which contributes to clarifying crustal deformation process,plate movement feature,fault coupling law,stress transmission mechanism,earthquake preparation behavior and earthquake rupture pattern.Space geodetic techniques?such as Global Navigation Satellite System?GNSS?,Interferometric Synthetic Aperture Radar?InSAR?and satellite gravity observation technology?provide a powerful tool to quantitatively describe crustal motion characteristics,to scientifically assess fault movement behavior and to optimally explain relevant geophysical phenomena with an unprecedented temporal-spatial resolution and high precision.With rapid development and global popularization of GNSS technology,we are provided with valuable insights into understanding the crustal deformation characteristics,plate movement patterns and fault activity laws,and with scientific guidances for evaluating seismic risk,realizing earthquake prediction and establishing earthquake warning.Based on the related theories and methods of seismic geodesy,this study uses GNSS time series to extract interseismic,coseismic and postseismic deformation;uses interseismic velocities to investigate crustal strain,block motion and fault coupling;uses coseismisc offsets to image fault slip distribution and analyze static stress changes;and uses postseismic measurements to quantify postseismic deformation mechanisms and illuminate lithospheric rheological structure.I extract and model the interseismic,coseismic and postseismic deformation of the 2008 Mw 7.9 Wenchuan earthquake,the 2015 Mw 7.8 Nepal earthquake and the 2016 Mw 7.9 New Zealand earthquake,and analyze the crustal deformation characteristics and fault movement states at different periods during the earthquake cycle.The main studies and achievements are as follows:1)This study designs a general GNSS coordinate time series analysis scheme to extract interseismic,coseismic and postseismic surface deformation,which includes gross error elimination,parameter estimation,spatial filtering and rate interpolation.For the Wenchuan earthquake,the interseismic rates?IGS08?point to east-southeast,with the horizontal and vertical rates in the intervals of[29.9,57.3]and[-7.1,8.4]mm/yr,respectively.The coseismic deformation of the Sichuan Basin is characterized by thrust motions,with the northwest-directed displacement and vertical subsidence.The postseismic deformation is consistent with coseismic motion patterns,which is obvious in the western Sichuan Plateau and insignificant in the Sichuan Basin?<5 mm?.For the Nepal earthquake,the interseismic horizontal velocity field?IGS08?is towards the northeast,and the horizontal and vertical velocities are from 39.4to 54.7 and from-113.5 to 7.0 mm/yr,respectively.The coseismic deformation is mainly located in central Nepal,with dominated thrust motions and slightly dextral strike-slip characteristics.The postseismic deformation flollows the coseismic futures,with maximum horizontal and uplift motions being located at stations of CHLM?33.4 mm?and MKLU?30.0mm?,respectively.For the New Zealand earthquake,the long-term rates move to northwest,with the magnitudes of[28.2,47.2]and[-8.8,2.7]mm/yr for the horizontal and vertical components,respectively.The coseismic deformation shows dextral oblique-slip movements in north Canterbury and right-lateral strike-slip motions in the Marlborough fault system.The postseismic deformation continues to move towards northeast and decays slowly in space,the maximum horizontal and uplift motions are at stations LOK1?42.4 mm?and CMBL?42.3mm?,respectively.2)This study introduces the theory,content,method and significance of interseismic deformation modeling of seismic geodesy,and summarize the method to invert interseismic velocity field for crustal strain distribution,block motion parameters and fault activity characteristics.Crustal deformation investigation shows a dominated principal compressive strain along the whole Himalayan tectonic belt,with the mean and maximal values of maximum principal strain rate of 10.0 and 35.5 nanostrain/yr and the mean and minimum values of minimum principal strain rate of-25.1 and-85.6 nanostrain/yr,respectively.The maximum shear strain rate has an average value of 35.1 nanostrain/yr and a maximum value of 87.8 nanostrain/yr,indicating that the Himalayan tectonic belt has accumulated a large amount of elastic energy.The area strain rate is from-88.8 to 16.9 nanostrain/yr,with the average value of-15.1 nanostrain/yr,suggesting the dominant compressive deformation in this area as well.The interseismic fault coupling investigation shows that the strong locked width of the whole main Himalayan thrust fault exceeds 100 km and the deficit rate of interseismic moment tensor is 1.51×10²⁰ Nm/yr.The locking depth under Nepal increases rapidly from west to east and the depth of strong locking reaches up to 30 km in the central region.I also find that the 2015 Mw 7.8 Gorkha mainshock and the largest Mw 7.3 aftershock are directly located in the highly locked areas.3)This study introduces the coseismic deformation modeling in seismic geodesy,summarize the related concepts,research contents and significance of coseismic deformation modeling,and emphatically introduces the research ideas and methods of coseismic slip inversion and Coulomb failure criterion.I study the coseismic deformation of the Wenchuan earthquake by constructing two different fault models?FM1 and FM2?and find that the two modeling results are highly consistent in slip area,peak motion,slip direction and data-model correlation.Therefore,I conclude that the coseismic model of the Wenchuan earthquake is insensitive to deep fault geometry and need to constrain it with post-earthquake deformation that has a larger spatial wavelength.I have collected the most comprehensive GNSS data for the coseismic modeling of the Gorkha earthquake,and find that the coseismic slip is characterized by a dominated thrust motion and slightly dextral strike-slip movement.The coseismic slip is distributed between the epicenters of the mainshock and largest aftershock,ranging from 11-20 km depth and with the maximum slip of⁶.26 m.The released moment by this earthquake is⁷.13×10²⁰ Nm,corresponding to Mw⁷.8.The first-order rupture characteristics of the New Zealand earthquake are investigated by using abundant coseismic data and a simplified fault model.The results show that the model can well explain the surface deformation,with the coseismic slip mainly located at shallow crustal faults and showing a maximum slip of²5 m.The cumulative moment release of the earthquake can reach up to⁸.28×10²00 Nm?Mw⁷.9?.The increased Coulomb stress changes in the mainshock regions reach up to MPa level and exhibit a deformation diversity in different depths,undoubtedly revealing the complexity of the multi-fault rupture.4)This study introduces the postseismic deformation modeling in seismic geodesy,summarize the related concepts,main contents and significance related to postseismic deformation,and emphatically introduce the physical meaning,constitutive relation,space-time characteristic and postseismic interpretation of poroelastic rebound,postseismic afterslip and viscoelastic relaxation.I find that the inversion results based on FM1 and FM2 are quite different,and FM1 can well explain the post-earthquake deformation of the Wenchuan earthquake,I therefore conjecture that the optimal fault model of the Beichuan fault?BCF?is a listric fault connecting a shallowing sub-horizontal detachment.The postseismic reverse deformation is mainly caused by the reverse afterslip on the shallow BCF,which maybe result from the local dynamic overshoot of shallow slip during the coseismic stage.I model and analyze the post-earthquake afterslip and viscoelastic relaxation associated with the Gorkha earthquake.The results show that the afterslip mainly appears in the northern part of the coseismic rupture zone and controls most of the post-earthquake deformation,whereas the viscoelastic deformation process contributes much small and the middle-lower crustal viscosity undelying the southern margin of the Tibetan Plateau is conservatively estimated to be 1.6×10¹⁹ Pas.I also suggest that the shallow portion and western segment of the seismogenic zone are still unruptured during both the coseismic and postseismic phases and multifaceted evidence consistently confirms that the Nepal region is still under a high seismic risk.I analyze the temporal-spatial variations of the afterslip and slow slip triggered by the New Zealand earthquake,and find that the afterslip occurred not only on shallow crustal faults,but also on deep subduction plate boundary.The seismic-triggered shallow east coast slow slip event has a small magnitude?¹5 cm?and shows extremely active in short term?2-3 weeks?,which is likely triggered by the dynamic stress changes.The deep slow slip motion has a large amplitude?>30 cm?and a long duration?>1 year?,which is possibly triggered by the static stress changes.