| Ongoing plate convergence between India and Eurasia during the past?55 million years has created the Tibetan Plateau,a region with average elevation of?4500 m,area of over 600×1000 km~2,and active faulting and crustal deformation extends more than2000 km into central Asia.Approximately one-half of India's 36–40?mm/a northward motion is partitioned in the Tibetan Plateau,resulting in crustal thickening,shortening,folds,and complex fault systems.The active crustal deformation causes diverse styles of strain accumulation and release on crustal faults,expressed as distinct faulting behavior or earthquake cycles.Investigating into crustal fault deformation and earthquake cycles in the Tibetan Plateau using space-based geodesy,i.e.,Global Positioning System(GPS)and Synthetic Aperture Radar Interferometry(In SAR),has started 30 years ago.Currently,high spatial-temporal resolution geodesy provides us with abundant data and sufficient resolution to study the ground deformation associated with earthquake cycle processes.In this dissertation,we focus on the interseismic deformation along three boundary large strike-slip fault systems of the Tibetan Plateau,the Altyn Tagh fault,the Haiyuan fault system and the Xianshuihe-Anninghe-Zemuhe-Xiaojiang fault system(XAZX).We use GPS(1999-2018)and In SAR(2003-2016)geodetic observations,along with2D dislocation and 3D block models,to invert for slip rates and interseismic fault coupling,assess seismic hazard and investigate earthquake cycles along these faults;moreover,study the kinematics of deformation across the Tibetan Plateau.Main research contents and results are as follows:1.Interseismic fault deformation along the Altyn Tagh fault using GPS(1999-2017)and In SAR(2003-2011).Results show that:(1)By integrating GPS and In SAR,we show two regions with apparent uplift and one with subsidence along the western Altyn Tagh fault.Horizontal GPS and In SAR data are consistent after correcting for contributions from the localized deformation,highlighting the importance of accounting for the vertical deformation during the interseismic In SAR data interpretations.(2)We correct for the block rotation contributions to the fault-parallel geodetic velocities,this has enabled us to robustly estimate the slip rate and locking depth of the western Altyn Tagh fault using a simple 2-D elastic dislocation model.A best fit is obtained with a left-lateral strike-slip rate of 8.1±0.4 mm/a and locking depth of15.1±3.4 km.Our result implies that previous studies(2-D model results)have overestimated the slip rate of the western Altyn Tagh fault up to 36%and a relatively slow slip deficit accumulation rate is found.(3)We show no apparent asymmetric interseismic velocities across the western Altyn Tagh fault,ruling out the requirement of lateral variations in elastic strength and/or dislocation shift with respect to the fault trace.Besides,our results imply the western Altyn Tagh is locked at shallow depth,negating the inference that there is shallow creep along the fault as proposed by Xu and Zhu(2019).(4)A quantitative analysis of the GPS and In SAR data suggests the crustal deformation in the northwestern Tibetan Plateau is a combination of strike-slip faulting,localized crustal shearing,shortening,subsidence and uplift.It further suggests that vertical motion is not directly controlled by crustal fault activity.(5)We introduce an elastic block model to estimate the fault slip rate,interseismic fault coupling(ISC),and seismic moment accumulation rate along the Altyn Tagh fault.The estimated left-lateral strike-slip rate of the ATF decreases eastward from 12.8±0.4mm/a to 0.1±0.2 mm/a.Results show a heterogeneous distribution of ISC along the fault,with the fault locking depth varying from 5 to 20 km.The seismic moment accumulation rate is high along three segments between Sulamu Tagh and Aksay bends,where the accumulated seismic moment over the last five centuries could be balanced by earthquakes with magnitudes of Mw 7.6-7.8.(6)We suggest that a hybrid kinematic model that includes both block-like and continuous deformation is needed to better delineate the crustal deformation of the Tibetan Plateau.2.Fault locking and surface creep on the Haiyuan fault system,northeastern Tibet,constrained from GPS(1999-2017)and In SAR(2003-2010).Results show that:(1)The comparison between GPS,Leveling and In SAR Line of Sight(LOS)rates show discrepancies at a few mm/a levels,reflecting long-wavelength systematic errors or vertical deformation in In SAR and levelling data.We solve this problem by combining GPS with high-pass filtered In SAR data to produce a continuous LOS rate map around the Haiyuan fault system.(2)Our geodetic data reveals three creeping segments separating four locked asperities.We confirm previous studies showing surface creep on the Laohushan fault(?103.6°E–103.9°E).We further identify 3–5 mm/a surface creep along a 10 km long segment for the western Haiyuan fault(HYW,?104.2°E–104.3°E)and 2–4 mm/a surface creep along a 43 km long segment for the eastern Haiyuan(HYE,?105.3°E–105.7°E)segments.(3)Both present-day creeping segments are located along the surface rupture trace from the M?8 1920 earthquake and they experienced 3 m and 3 to 10 m of co-seismic offsets respectively.This observation suggests that either the M?8 1920 earthquake was able to cross pre-existing creeping segments or that large earthquakes show heterogeneous relocking,with creep lasting decades along some parts of the rupture.(4)In calculating the seismic moment deficit along the Haiyuan fault system,we show that the Laohushan fault is capable of an Mw 6.8–6.9 earthquake even though its easternmost is creeping.The Haiyuan fault could generate an Mw 5.9–6.3 earthquake depending on whether asperities break as isolated seismic events or cooperate to produce a larger rupture.Present-day fault coupling along the Haiyuan fault system highlights segmentation or partial failure of the fault during earthquakes.3.Heterogeneous interseismic coupling and earthquake cycle along the Xianshuihe-Xiaojiang fault system,eastern Tibet,constrained by multi-temporal GPS(1999-2018)and In SAR(2014-2016).The main conclusions reached are as follows:(1)An elastic block model confirms predominantly left-lateral strike slip motion with slip rates of 7–11 mm/a along the XXFS.The Xianshuihe fault appears to be predominantly creeping,while the Anninghe fault,Zemuhe fault and Xiaojiang fault are found to be highly coupled.(2)Afterslip following the large(M 7.6 in 1973)and moderate-size(M 6.9 in 1981)earthquakes appears to be long-lasting(possibly up to 40 years).Several arguments also indicate that relatively weak coupling further south also contain a transient contribution arousing from past earthquakes in 1955 and even 1893.This view is further supported by surface creep rate decaying through time and possibly having stopped during the last years,therefore starting a new cycle of increasing slip deficit accumulation.(3)We identify that the Kangding-Bamei segment includes a?30 km-long creeping section(latitude 30.2°N to 30.4°N)and find that creep rate accelerated during the 2008–2014 period,possibly as a result of stress increments induced by surrounding large earthquakes and among them the 2008 Mw 7.9 Wenchuan earthquake.(4)Finally,quantitative calculation of moment budget highlights relatively high seismic potential on the Anninghe,Zemuhe and Xiaojiang faults,where the elastic strain deficit could be balanced by Mw 7.5,Mw 7.2,and Mw 7.4 earthquakes respectively.In summary,we used abundant GPS and In SAR data to study the interseismic fault deformation along the Altyn Tagh fault,Haiyuan fault system and Xianshuihe-Xiaojiang fault system.Our results not only delineate the slip rates of these strike-slip faults,but also reveal the high-resolution deformation characteristics of these fault system and crustal deformation around these faults.Our results also give quantitative assessment of the seismic risk along those faults.Furthermore,our results reveal for the first time the new aseismic(shallow)creeping segments along the Haiyuan fault and Xianshuihe fault,and determine the spatio-temporal distribution of the creeping.Our results provide a new understanding of the earthquake cycle along these faults. |
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