Crustal Deformation Characteristics Of The Xianshuihe Fault From InSAR And GNSS Observations

Since 60Ma~50Ma,the Indian plate has been colliding with the Eurasian plate,causing the uplift of the Qinghai-Tibet Plateau and the lateral escape of large amounts of crustal material eastward and southeastward.The Sichuan-Yunnan block,located on the southeastern margin of the Qinghai-Tibet Plateau,is one of the areas with strong south-east escape activities due to the continuous influence of plate boundary dynamics.As the north-eastern boundary of the Sichuan-Yunnan block,the Xianshuihe-Xiaojiang fault system inevitably undergoes high-rate left-lateral strike-slip movement,and its northwestern segment,the Xianshuihe fault has the most active structural activity and the highest frequency of earthquakes.Based on InSAR and GNSS,two kinds of satellite-borne geodetic techniques,we studied the crustal deformation characteristics of the Xianshuihe fault,and the conclusions are as follows:1.We used a data set of 94 continuous GNSS sites,77 campaign GNSS sites over10 years,28 and 30 interferograms of T33 and T135 decending Sentinel-1 to study the strain characteristics and three-dimensional deformation characteristics of the Xianshuihe fault.The analysis results are as follows:The cross-fault profiles of GNSS velocity field show that the strike-slip rates of the northwest,middle,and southeast segments of the Xianshuihe fault are 7.91 ± 2.07 mm/a,9.19 ± 2.37 mm/a,and 8.38 ± 2.38 mm/a,respectively.There is no obvious compression and shortening with left-lateral strike-slip movement;The maximum shear strain rates are significant in the area around Xianshuihe fault,accompanied by obvious dilatation.In other words,the strong tensile strain and shear motion enable the Xianshuihe fault zone to accumulate enough strain to generate strong earthquakes.The rotation rate field indicates that the Xianshuihe fault zone is left-lateral,and both sides of the fault move in a clockwise direction,which is consistent with the southward and southeastward escape of the Sichuan-Yunnan block.The interferograms corrected by GACOS reduces the phase standard deviation and the phase-topography correlation coefficient by 19.85% and 59.11%,respectively,eliminating some atmospheric errors.The InSAR velocity field profiles show that the creep rate increases gradually from Luhuo to Kangding,and the LOS creep rate in the Zhuwo-Luhuo,Luhuo-Daofu,Daofu-Bamei and Bami-Kangding sections are0.15±0.87 mm/a,1.57±0.37 mm/a,1.77±0.47 mm/a and 1.90±0.37 mm/a,respectively.The three-dimensional velocity decomposition results using InSAR and GNSS show that the eastward component and the southward component are significantly weakened when passing through the Xianshuihe fault.Meanwhile,the eastward velocity component gradually weakens from Zhuwo to Shimian,and the southward component of the southwest side of the fault is obviously higher than that of the northeast side.The vertical velocity in the area near the fault is about 0.89 mm/a.2.We used the observations of 69 GNSS stations for about 17 years and 30 Sentinel-1A images of T135 to study the characteristics of interseismic motion in the LuhuoDaofu segment.The methods and conclusions are as follows:In order to study the interseismic movement characteristics of the Luhuo-Daofu section,we process and integrate the InSAR and GNSS data covering the area.The InSAR velocity field and interpolated GNSS velocity field are transformed to faultparallel direction,and the GNSS data are transformed into the InSAR reference frame by the bootstrap method.In the data integration process,we prioritize the retention of far-field tectonic movement and near-field high-resolution deformation characteristics.Based on the integrated velocity field,we formulate a simple screw dislocation model which includes shallow creep,intermediate locked and deep slip layers,and use the Markov Chain Monte Carlo method to invert for the fault slip rate,shallow creep and locked depth of the Luhuo-Daofu section.The profile analyses of the InSAR velocity field show obvious surface sinistralcreep in the Luhuo-Daofu section,with the creep rate varying along the strike direction in the range of 1.0~5.2 mm/a.Specifically,the rate gradually increases initially,followed by a decrease,but then again increases.The creep rate of Luhuo section is significantly lower than that of Daofu section.The screw dislocation modelling indicates that the creep depth of the Luhuo-Daofu section is ~9.8 km,the shallow creep rate is 3.27~4.18 mm/a,and the fault slip rate is 8.12~9.30 mm/a.The earthquake recurrence interval of the Luhuo segment is 370~410 years,and the thickness of the locked layer is ~1 km,showing a low earthquake risk.The earthquake recurrence interval of the Daofu segment is 59~65 years,and the thickness of the locked layer is~7.5 km,suggesting that sufficient strain can be accumulated there.The seismic moment released by shallow creep is ~1.32×1016 Nm/a,accounting for 23%~38% of the accumulated moment due to tectonic loading.In the past 40 years,the seismic moment in the locked area has reached 3.62×1017 to 7.88×1017 Nm,equivalent to Mw5.6~Mw5.9.Therefore,its seismic risk is necessarily noteworthy.