Determination Of Solid Earth Tide Displacements In China Using GPS Technique

The tidal deformation consists of two main components: Solid Tide and Ocean Tide Loading(OTL).Solid Tide is the periodic deformation of the solid Earth under the influence of the periodic gravitational force of near-Earth celestial bodies such as the Sun and Moon,and the amplitude can reach tens of centimeters.The periodic response of nonrigid Earth to the redistribution of seawater mass is called Ocean Tide Load,which can be several centimeters in the vertical direction,which can reach up to several centimeters in the vertical direction.Although the effect of Solid Tide in precise point positioning is much larger than the effect of OTL,Solid Tide modeling is typically performed using the standard modeling procedures provided by the IERS2010 specification,with accuracy up to sub-millimeter levels in most areas.In coastal areas,OTL can be up to several centimeters in the vertical direction and can vary up to several millimeters between modeling methods.Therefore,in addition to theoretical modeling,direct acquisition of tidal deformation by modern geodetic observation has become a popular research direction,and GPS(Global Positioning System)has become the main research tool for acquiring tidal displacement because of its high temporal and spatial resolution and high accuracy.Due to the close relationship between tidal deformation and the internal structure of the Earth,the observation of tidal deformation by GPS is not only a supplement to the theoretical modeling,but also provides independent constraints for constraining the internal structure of the Earth.This thesis first introduces five common global tidal models(including FES2014 b,TPXO9-Atlas,DTU10,EOT20,and NAO99b)and three Earth models(PREM,STW105,and AK135);and describes the calculation method of OTL.Then,based on the static method,we obtain the ocean tidal load displacement parameters of the eight major tidal constituents using the observation data of 242 continuous GPS stations for more than 10 years.The comparison results with the theoretical modeling values show that the GPS observation accuracy of tidal displacements can reach sub-millimeter level.Further comparison and analysis between GPS observations and modeling values showed that the four tidal models,except DTU10,were in good agreement with GPS observations.Considering the observations of tide gauge stations,the statistical results show that the most suitable ocean model for ocean tide load displacement modeling is TPXO9-Atlas and the most suitable earth model is PREM.At the same time,we find that the residuals between observed values and theoretical IVmodeling values,for the M2 and O1 tidal,show obvious long-wavelength,coherent spatial patterns over the region.198 inland stations which are more than 200 km from the coastline are used to analyze the modeling errors of solid tide.After removing the modeling errors of solid tide,as shown in statistical results,the weighted root mean square(WRMS)misfits of inland stations in east,north,and vertical are respectively 0.04 mm,0.07 mm,and 0.11 mm by M2,while 0.05 mm,0.07 mm,and 0.11 mm by O1.The WRMS misfits of 44 coastal stations in the three directions are respectively 0.18 mm,0.27 mm,0.59 mm by M2,while0.10 mm,0.16 mm,and 0.30 mm by O1.After considering the solid tide modeling errors,some coastal stations still have large errors.The misfits mainly come from the modeling errors of OTL.In addition to the ocean tide model,it also includes the influence of factors such as seawater density and earth model.This paper first considers the effects of seawater density and compressibility based on the WOA18 ocean data set.The results show that in China the OTL displacement modeling is not sensitive to the variation of seawater density and compressibility.Next,in the asthenosphere,the anelastic dispersion results in a reduction of the shear modulus of about～8.5%(semidiurnal tide),～9.1(diurnal tide)respectively.Considering the anelastic dissipation effect,the residuals in the vertical direction of M2 and O1 tidal can be reduced by 0.50 mm and 0.25 mm,respectively.Finally,we evaluate the applicability of four 3D earth models based on GPS observation results,and find that except for the USTC earth model,which is affected by sedimentary layer,the other three earth models can effectively improve the consistency between the GPS observations and the theoretical modeling results.Among them,the residual misfits between the global crustal model CRUST1.0 and GPS observations is the smallest,indicating that the modeling accuracy of OTL in China is greatly limited by the accuracy of the crustal structure.Based on the above results,we believe that the observation accuracy of GPS is enough to inferring Earth’s shallow structures.