Characteristics Of Non-tectonic Crustal Deformation From Surface Loads Around Chinese Mainland And Correction Model

Since the early1990s the space geodetic technology GPS has been developed rapidlyand widely applied to various fields. As a revolutionary tool for detecting crustalmovement and tectonic deformation on various scales all over the world, the GPS is veryconvenient for surveying with high accuracy, large scope, all-weather and low cost. Totimely grasp the opportunity of application of GPS to monitoring the earth for earthquakeprevention and disaster reduction, the national major scientific project “ObservationalNetwork for Crustal Movement of China” and “Monitoring Network for ContinentalTectonic Environment of China” have been implemented in our country from1997to2000and2007to2012, respectively. The observation network, which is of high-accuracy andhigh-density, consisting of260continuous GPS stations and2056non-continuous GPSstations built around China mainland, has become an important infrastructure platform fordetecting the present crustal movement with high accuracy and analyzing the evolutionprocess of tectonic deformation quantitatively.In recent years, with further optimization and development of the GPS constellationsystem, observation devices, data processing software and IGS (International GNSSService), given the data with continuous24hours under well observation circumstance andstandard measurement mode, it is possible to achieve daily coordinates of the station inglobal reference framework with an accuracy of several millimeters by using the routineGPS data processing（the accuracy can be better than3mm and6mm in horizontal andvertical direction respectively）. By using years of continuous observation data ornon-continuous observation data of different GPS stations from CMONOC and otherprojects, one can get the GPS time series with high-accuracy. Based on these time series,one can not only estimate the velocity of each station, but can also analyze obviousnon-linear variations, such as co-seismic displacement, post-earthquake relaxationdeformation, and slow slip earthquakes of active faults. However, when the study focuseson the tectonic movement or crustal deformation related with the possibility of occurrenceof earthquakes, some systematic errors in GPS site coordinate time series cannot beignored, especially some non-tectonic deformation, because crustal deformation or sitedisplacement caused by non-tectonic load, such as time-varying atmospheric tide, regionalocean tide, and land water migration, always has the same magnitude as tectonicdeformation. Therefore, it is an important issue how to effectively separate and eliminatevarious non-tectonic deformation from GPS time series, so as to better serve detections andanalysis of earthquake risks. In this thesis, the effect of non-tectonic load deformation on GPS time series aroundChina is discussed firstly. Then the characteristics of deformation induced by time-varyingatmospheric tide, regional ocean tide, and land water load are analyzed quantitativelybased on high-accuracy continuous GPS observation data. At last correction tonon-tectonic load deformation from GPS continuous and non-continuous time series ofChina is made by using different geophysical models. The specific contents are concernedwith following several aspects:I. By deducing and summarizing the optimized method and advanced strategy inGPS data processing, the GPS data processing software GIPSY and post-processingbalancing software QOCA from JPL are adopted to process continuous GPS data ofdifferent area around China in order to acquired the GPS time series withhigh-accuracy, which provides a foundation for further separation and analysis ofcharacteristics of non-tectonic deformation in different areas.Not only the biases from random errors, model biases and parameter errors，but alsonon-tectonic deformation caused by time-varying surface loads, such as atmospheric tide,regional ocean tide, land water load, can affect GPS time series. To effectively getnon-tectonic deformation of different areas around China from high-accuracy GPS timeseries, this work selected some typical regional continuous GPS stations, and used precisepoint positioning (PPP) of GPSY software and Ambiguity resolution for the whole network(Ambizap) based on principal of fixed point in GPS data processing. Meanwhile, in orderto optimize the key model of GPS data processing, the international advanced prioritroposphere delay model ECMWF （European Centre for Medium-Range WeatherForecasts）and troposphere projection function VMF1(Vienna Mapping Function1) areemployed, and the higher-order ionosphere model is adopted to correct the ionosphereeffect in GPS data processing. Ultimately, tectonic deformation (like the rate of tectonicmovement, seismic influence and etc) and site abnormal effect of each site are eliminatedby using QOCA (Quasi-Observation Combination Analysis). Thus, non-tectonicdeformation of each area around China is highlighted.II. Effects of atmosphere tide load on non-tectonic deformation around Chinahave been analyzed and studied. Considering partial “tide” atmosphere component inatmosphere pressure data, which are commonly used all over the world, a low passfilter with20orders is designed to eliminate this effect and well adopted in GPS timeseries around China.Ocean water redistribution, which is originated from sun/lunar gravitation tides andlocal thermodynamic processes, will lead to non-tectonic load deformation. Based on high-accuracy GPS time series, one can observe and detect the atmosphere loaddeformation. The atmospheric load contains tidal load and non-tidal load. The maximumvalues of atmospheric load deformation on the earth can reach centimeters, which mainlyappears in high-latitude areas.The study shows that the vertical deformation induced by atmosphere tide is ofsubmilimeter level around China, while the north and east component are smaller,equivalent to10percent of vertical component. Among them, amplitude of loaddeformation induced by semidiurnal tidal (S1) is related with latitude, i.e., amplitudes ofload deformations of same latitude are identical, but the phases are different with longitude.This work mainly integrates atmosphere pressure data with Green Function to computenon-tectonic atmosphere load deformation. Nevertheless, current international atmospherepressure data are mainly from re-analysis products provided by different weather centers(ECMWF and NCEP). Due to complicated signals related with model defect, noise andsampling rate, these atmosphere pressure data contain partial "tide" atmosphere in it.Theoretical calculation shows that this kind of partial "tide" atmosphere will lead to biasesin GPS time series with the magnitude of0.5-1mm and0.1-0.2mm in vertical andhorizontal direction, respectively, thus generating spurious signal in GPS time series withperiod of5to6days and seminal and annual, which can not be ignored in the GPS dataprocessing. To this end, a low pass filter Butterworth with the order of20is designed;which has a cutoff frequency of1week/day and a stop-band attenuation of35dB. Byadopting this filter, the partial "tide" atmosphere can be effectively eliminated, thusatmosphere non-tide component can be acquired with good reliability.III. By comparing the differences of several global ocean tide models aroundChina’s seas, a regional ocean tide model with high spatial resolution is adopted tocorrect tide load deformation, which is an effective way to eliminate spuriousnon-tectonic deformation signal of long-period in GPS time series. Meanwhile, byanalyzing the current satellite altimetry data, this work concludes that non-tide oceanloading deformation is not reliable enough now and accuracy of the non-tide oceanmodel needs to be enhanced further.With a long coastline, China faces the Pacific Ocean in the east and Indian Ocean inthe south. Both the tidal and non-tidal variations of oceans will lead to non-tectonicdeformation around China’s seas, especially in coastal areas. Because several global oceantide models have different spatial resolutions in Chinese seas, it is found that a regionalocean tide model with high spatial resolution is incorporated, the coastal GPS verticaldeformation induced by M2tide wave will be improved by a magnitude of1.1mm on average, and the maximal correction value can reach5mm. If this regional ocean tidemodel with high spatial resolution is not taken into consideration, then spuriouslong-period signal will be generated in GPS time series. And as for ocean non-tidal loaddeformation, only few costal GPS time series can be improved if ocean non-tidal loaddeformation from GPS time series is corrected based on currently high-accuracy satellitealtimetry. It is the reason that currently satellite altimetry data is still not accurate enough,thus elimination of ocean non-tidal load deformation in GPS time series is still on the way.In view of maximum amplitude of ocean non-tidal deformation is lower than1mm; thisthesis suggests ignoring ocean non-tidal load deformation currently when performing GPSdata processing and time series analysis.IV. Combining respective advantages of the GRACE time-varying gravity fieldand global land water data NCEP, this work attempts to design a kind of dataassimilation method and get the land water data around China. This method has theadvantage in both time and space resolution. Based on the above, the land watermigration load deformation of different regions is computed. Then, analysis andevaluation of this method in GPS time series are performed.The magnitude of crustal deformation induced by land water is very considerableamong all values of non-tectonic deformation. The magnitude of deformation differenceinduced by land water load can reach centimeters around China. The GRACE time-varyinggravity field and global land water model （NCEP）are currently most important data forstudying land water load deformation. Both of them can reflect short-term variation of landwater. Among them, the GRACE time-varying gravity field contains not only snow/soilhumidity but also surface water and ground water variation. Thus it is of higher accuracy.But GRACE has disadvantage of low time resolution. The global land water model（NCEP）can provide soil humidity and snow load in a depth range from0to2meter andis of higher time resolution, but not includes surface water and ground water data.According to the advantage and disadvantage of these two kinds of data, this work tries todesign a data assimilation method by taking NCEP as effective complementation forGRACE in short term, in this way the land water load data with advantages ofhigh-accuracy of GRACE on a long-time scale and high resolution of NCEP on short timescale can be jointly employed. Based on the assimilation result, this thesis analyzes thecharacteristics of land water deformation over China. And by adopting the wave analysismethod, it discusses the correlation between land water load deformation and GPS timeseries, and then analyzes the improvement of GPS time series corrected by the simulationresult in low frequency component. The results show that when the land water load deformation is larger than GPS noise, the correction coefficient is larger. But for the regionwith lower SNR (signal noise ratio), the correction coefficient is smaller, even negative.V. Based on continuous GPS observation data and GRACE assimilation data,this work adopts the Support Vector Regression (SVR) to jointly invert the regionalnon-tectonic load deformation model, and this method is validated by analyzing theimprovement of continuous GPS and non-continuous GPS time series correctednon-tectonic load deformation based on the assimilation results in two typical regionswith larger significant differences of surface water variations over China.High-accuracy GPS can directly detect the surface non-tectonic load deformation, butit usually also contains deformation from local effects and environment model errors andother factors. Based on the geophysical model，"theoretical deformation" induced byvarious kinds of surface loads can also be acquired, but the poor accuracy of thegeophysical model and time-space resolution of input data may lead to deformation biases.To solve this problem, this work tries to take the continuous GPS observation data as basicobservations and GRACE assimilation data as a prior constraint, and adopts SVR (SupportVector Regressing) to jointly invert the regional non-tectonic load deformation model. Theapplication and validation in western Yunnan and the Longzhong plateau area areperformed. The result shows that the joint inversion method can utilize advantages of bothGPS and GRACE data, with a prior constraint from the known physical model, which doesnot solely originate from continuous GPS. The joint inversion results can obviouslyimprove the time series of continuous and non-continuous GPS stations.