Data obtained by GRACE (Gravity Recovery and Climate Experiment) have been used to invert for the seismic source parameters of megathrust earthquakes under the assumption of either uniform slip over an entire fault or a point-like seismic source. Herein, we further extend the inversion of GRACE long-wavelength gravity changes to heterogeneous slip distributions during the 2011 Tohoku earthquake using three fault models: (I) a constant-strike and constant-dip fault, (II) a variable dip fault, and (III) a realistically varying strike fault. By removing the post-seismic signal from the time series, and taking the effect of ocean water redistribution into account, we invert for slip models I, II, and III using co-seismic gravity changes measured by GRACE, de-striped by DDK3 decorrelation filter. The total seismic moments of our slip models, with respective values of 4.9×1022 Nm, 5.1×1022 Nm, and 5.0×1022 Nm, are smaller than those obtained by other studies relying on GRACE data. The resulting centroids are also located at greater depths (20 km, 19.8 km, and 17.4 km, respectively). By combining onshore GPS, GPS-Acoustic, and GRACE data, we obtain a jointly inverted slip model with a seismic moment of 4.8×1022 Nm, which is larger than the seismic moment obtained using only the GPS displacements. We show that the slip inverted from low degree space-borne gravimetric data, which contains information at the ocean region, is affected by the strike of the arcuate trench. The space-borne gravimetric data help us constrain the source parameters of a megathrust earthquake within the frame of heterogeneous slip models.
In this paper, we study how coseismic deformations calculated in 1066 Earth models are affected by how the models treat Earth discontinuities. From the results of applying models 1066A (continuous) and 1066B (discontinuous), we find that the difference in Love numbers of strike-slip and horizontal tensile sources are bigger than dip-slip and vertical tensile sources. Taken collectively, discontinuities have major effects on Green’s functions of four independent sources. For the near-field coseismic deformations of the 2013 Okhotsk earthquake (Mw 8.3), the overall differences between theoretical calculations in vertical displacement, geoid, and gravity changes caused by discontinuities are 10.52 percent, 9.07 percent and 6.19 percent, with RMS errors of 0.624 mm, 0.029 mm, and 0.063 μGal, respectively. The difference in far-field displacements is small, compared with GPS data, and we can neglect this effect. For the shallow earthquake, 2011 Tohoku-Oki earthquake (Mw 9.0), the differences in near-field displacements are 0.030 m (N-S), 0.093 m (E-W), and 0.025 m (up-down) in our study area with the ARIA slip model, which gives results closer to GPS data than those from the USGS model. The difference in vertical displacements and gravity changes on the Earth’s surface caused by discontinuities are larger than 10 percent. The difference in the theoretical gravity changes at spatially fixed points truncated to degrees 60, as required by GRACE data, is 0.0016 μGal and the discrepancy is 11 percent, with the theoretical spatial gravity changes from 1066B closer to observations than from 1066A. The results show that an Earth model with discontinuities in the medium has a large effect on the calculated coseismic deformations.
The purpose of this study is to explore nonhydrological mass transfer in China continent. For this purpose, gravity recovery and climate experiment (GRACE) data were obtained to study the spatial distribution of time variant gravity signals in China continent. Then, from auxiliary hydrological data processed according to the current hydrological model, a new more comprehensive hydrological model of China continent was constructed. Finally, the time variant signals of this new hydrological model were removed from the time variant gravity field computed from GRACE data, thus obtaining a description of the nonhydrological mass transfer of China continent. The physical sources and mechanisms of the resulting mass transfer are then discussed. The improved, more realistic, hydrological model used here was created by selecting the hydrological components with the best correlations in existing hydrological models, by use of correlation calculation, analysis, and comparison. This improved model includes water in soils and deeper strata, in the vegetation canopy, in lakes, snow, and glaciers, and in other water components (mainly reservoir storage, swamps, and rivers). The spatial distribution of the transfer signals due to nonhydrological mass in China continent was obtained by subtracting the combined hydrological model from the GRACE time-variable gravity field. The results show that the nonhydrological signals in China continent collected in GRACE data were mainly positive signals, and were distributed in the Bohai Rim and the northern and eastern parts of the Tibetan Plateau. The above nonhydrological mass transfer signals have been studied further and are discussed. The results show that the nonhydrological mass migration signals in the Bohai Rim region originate primarily from sea level change and marine sediment accumulation. The mass accumulation from Indian plate collision in the Tibetan Plateau appears to be the main reason for the increase in the residual gravity field in that region.
Water budget closure is a method used to study the balance of basin water storage and the dynamics of relevant hydrological components (e.g., precipitation, evapotranspiration, and runoff). When water budget closure is connected with terrestrial water storage change (TWSC) estimated from Gravity Recovery and Climate Experiment (GRACE) data, variations in basin runoff can be understood comprehensively. In this study, total runoff variations in the Yangtze River Basin (YRB) and its sub-basins are examined in detail based on the water budget closure equation. We compare and combine mainstream precipitation and evapotranspiration models to determine the best estimate of precipitation minus evapotranspiration. In addition, we consider human water consumption, which has been neglected in earlier studies, and discuss its impact. To evaluate the effectiveness and accuracy of the combined hydrological models in estimating subsurface runoff, we collect discharge variations derived from in situ observations in the YRB and its sub-basins and compare these data with the models’ final estimated runoff variations. The estimated runoff variations suggest that runoff over the YRB has been increasing, especially in the lower sub-basins and in the post-monsoon season, and is accompanied by apparent terrestrial water loss.