The Global Positioning System (GPS) derived bedrock displacements respond to multiple geophysical effects, ranging from surface elastic loads to tectonic sources or viscoelastic uplifts stemming from Earth’s viscous mantle. In this study, the GPS‐inferred vertical crustal velocities are rigorously estimated in mainland China. We integrate the GPS vertical velocity field with Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow‐On (GFO) data, adopting an empirical Spatial Structure Function (SSF), to image tectonic deformation in mainland China with respect to the International Terrestrial Reference Frame (ITRF) 2014. We present four profiles across China, which indicate that our new robust results are superior to kriging. Furthermore, we use the GRACE/GFO products to account for elastic deformation due to surface mass changes to isolate tectonic deformation signals at GPS sites within mainland China from 2002 to 2019. By integrating GPS and GRACE/GFO measurements, our results reveal the long‐term spatial patterns of vertical tectonic motion in different blocks in mainland China. We conclude that significant steep velocity gradients occur at tectonic block boundaries that are attributable to locking and elastic strain accumulation on active block boundary faults.

To advance geodynamics and geophysical research, high precision Global Navigation Satellite System (GNSS) observations of velocity and position are needed. In this study, we construct a 3‐D model of a ~5.9‐year periodic signal for the Earth surface displacements. This model is mainly based on the determined time sequences and the Y₂₂ spatial pattern of this ~5.9‐year periodic signal from the global Global Positioning System (GPS) observations. Based on the constructed model, we calculate synthetic temporal 3‐D displacements on the Earth surface and the velocity changes, and we compare them to observed GPS displacements. Our results reveal that the ~5.9‐year signal contributes to surface displacements and velocity fields. Maximum displacements for the N, E, and U components can reach up to 0.28, 0.55, and 1.69 mm, respectively; maximum velocity effects can up to 1.69 mm/year for the U component (depending on record length). For those studies which need millimeter or even submillimeter accuracy (e.g., tectonic deformations, glacial isostatic adjustment, and deformations caused by core motions), these effects cannot be simply ignored. Our results also confirm that the model we present here can heavily reduce the ~5.9‐year signal in the original GPS time series. We suggest that our model should be used to pretreatment the GPS data at the observation level, which could improve signals used for geophysical or geodynamic interpretations.

Temporal gravity retrieval simulation results of a future Bender-type double pair mission concept, performed by five processing centers of a Sino-European study team, have been inter-compared and assessed. They were computed in a synthetic closed-loop simulation world by five independent software systems applying different gravity retrieval methods, but were based on jointly defined mission scenarios. The inter-comparison showed that the results achieved a quite similar performance. Exemplarily, the root mean square (RMS) deviations of global equivalent water height fields from their true reference, resolved up to degree and order 30 of a 9-day solution, vary in the order of 10% of the target signal. Also, co-estimated independent daily gravity fields up to degree and order 15, which have been co-estimated by all processing centers, do not show large differences among each other. This positive result is an important pre-requisite and basis for future joint activities towards the realization of next-generation gravity missions.

Surface and deep potential geophysical signals respond to the spatial redistribution of global mass variations, which may be monitored by geodetic observations. In this study, we analyze dense Global Positioning System (GPS) time series in the Eastern Tibetan Plateau using principal component analysis (PCA) and wavelet time-frequency spectra. The oscillations of interannual and residual signals are clearly identified in the common mode component (CMC) decomposed from the dense GPS time series from 2000 to 2018. The newly developed spherical harmonic coefficients of the Gravity Recovery and Climate Experiment Release-06 (GRACE RL06) are adopted to estimate the seasonal and interannual patterns in this region, revealing hydrologic and atmospheric/nontidal ocean loads. We stack the averaged elastic GRACE-derived loading displacements to identify the potential physical significance of the CMC in the GPS time series. Interannual nonlinear signals with a period of ~3 to ~4 years in the CMC (the scaled principal components from PC1 to PC3) are found to be predominantly related to hydrologic loading displacements, which respond to signals (El Niño/La Niña) of global climate change. We find an obvious signal with a period of ~6 yr on the vertical component that could be caused by mantle-inner core gravity coupling. Moreover, we evaluate the CMC’s effect on the GPS-derived velocities and confirm that removing the CMC can improve the recognition of nontectonic crustal deformation, especially on the vertical component. Furthermore, the effects of the CMC on the three-dimensional velocity and uncertainty are presented to reveal the significant crustal deformation and dynamic processes of the Eastern Tibetan Plateau.

The density jump at the inner core boundary of the Earth is poorly constrained by seismic data. One sensitive measure of the jump is the eigenperiod of the inner core wobble (ICW). Although theoretical studies on the ICW have been ongoing for decades, no observed ICW signal has been reported. To increase the detection level, here we use a recently proposed AR‐z method to analyze the 1960–2017 polar motion time series. We finally identify a +8.7 ± 0.2 year prograde motion with an ~2.67 ± 0.04 milliarcsecond amplitude. After confirming that this signal is almost a stationary oscillation and that atmospheric/oceanic/hydrological effects cannot seem to excite this signal, we carefully suggest that this ~8.7 year signal is possibly the ICW. According to this suggestion, we reconstruct a new 1‐D density model with a 507 ± 15 kg/m³ inner core boundary density jump. This new model can also provide a better fit than Preliminary Reference Earth model to the seismic overtone 2S1.

An airborne gravity survey was carried out to fill gaps in the gravity data for the mountainous areas of Taiwan. However, the downward continuation error of airborne gravity data is a major issue, especially in regions with complex terrain, such as Taiwan. The root mean square (RMS) of the difference between the downward continuation values and land gravity was approximately 20 mGal. To improve the results of downward continuation we investigated the inverse Poisson’s integral, the semi-parametric method combined with regularization (SPR) and the least-squares collocation (LSC) in this paper. The numerically simulated experiments are conducted in the Tibetan Plateau, which is also a mountainous area. The results show that as a valuable supplement to the inverse Poisson’s integral, the SPR is a useful approach to estimate systematic errors and to suppress random errors. While the LSC approach generates the best results in the Tibetan Plateau in terms of the RMS of the downward continuation errors. Thus, the LSC approach with a terrain correction (TC) is applied to the downward continuation of real airborne gravity data in Taiwan. The statistical results show that the RMS of the differences between the downward continuation values and land gravity data reduced to 11.7 mGal, which shows that an improvement of 40% is obtained.