Previous studies suggest that tidal friction gives rise to the secular deceleration of the Earth rotation by a quantity of about 2.25 ms/cy. Here we just consider additional contributions to the secular Earth rotation deceleration. Atmospheric solar semi-diurnal tide has a small amplitude and certain amount of phase lead. This periodic global air-mass excess distribution exerts a quasi-constant torque to accelerate the Earth＇s spin rotation. Using an updated atmospheric tide model, we re-estimate the amounts of this atmospheric acceleration torque and corresponding energy input, of which the associated change rate in LOD（length of day） is-0.1 ms/cy. In another aspect, evidences from space-geodesy and sea level rise observations suggest that Earth expands at a rate of 0.35 mm/yr in recent decades, which gives rise to the increase of LOD at rate of 1.0 ms/cy. Hence, if the previous estimate due to the tidal friction is correct, the secular Earth rotation deceleration due to tidal friction and Earth expansion should be 3.15 ms/cy.
The Earth is a tri-axial body, with unequal principal inertia moments, A, B and C. The corresponding principal axes a, b and c are determined by the mass distribution of the Earth, and their orientations vary with the mass redistribution. In this study, the hydrologically induced variations are estimated on the basis of satellite gravimetric data, including those from satellite laser ranging （SLR） and gravity recovery and climate experiment （GRACE）, and hydrological models from global land data assimilation system （GLDAS）. The longitude variations of a and b are mainly related to the variations of the spherical harmonic coefficients C 22 and S 22, which have been estimated to be consisting annual variations of about 1.6 arc seconds and 1.8 arc seconds, respectively, from gravity data. This result is confirmed by land surface water storage provided by the GLDAS model. If the atmospheric and oceanic signals are removed from the spherical harmonic coefficients C 21 and S 21, the agreement of the orientation series for c becomes poor, possibly due to the inaccurate background models used in pre-processing of the satellite gravimetric data. Determination of the orientation variations may provide a better understanding of various phenomena in the study of the rotation of a tri-axial Earth.
The total capacity of Three Gorges Reservoir（TGR） and Danjiangkou Reservoir（DJR） is large and has significant seasonal fluctuations, which give rise to crustal instability. In this research, we focus on studying the temporal and spatial variation of crustal deformation in Hubei Province caused by reservoir impoundment of TGR and DJR.The Digital Elevation Model, historical hydrological information, GPS monitoring data and load-induced deformation model are combined to monitor the crustal deformation. The modeled results indicate that in the trapezoidal area between the TGR and DJR, the average vertical deformations at different latitudes have different variation tendencies. The vertical deformation modulus and fluctuation amplitude are larger at the latitude of 33 N/32.5 N from 2003 to 2006 and at the latitude of 31 N/32.5 N from 2008 to 2014, while the latter are much larger than the former. Moreover, from2008 to 2014, the frequency and the intensity of seismic activities are all enhanced significantly in this region. The modeled results at the GPS sites are consistent with the vertical displacement of GPS monitoring results in trends and the waveform. It can be inferred that the seasonal deformation is elastic. The horizontal deformation components have the same variation trends with that at each GPS monitoring station,which demonstrates that the whole region is moving toward the southeast. The spatial variation of crustal deformation demonstrates that the impoundment of TGR in2003 causes significant vertical displacements, with the maximum modulus of 32 mm downward located in Xiangjiang River＇s estuary. When the water storage increases, the maximum value will become larger, and the location will move toward the upstream.Besides, the earthquakes occurred more frequently in the region with maximum deformation modulus.
At seasonal and intraseasonal time scales, polar motions are mainly excited by angular momentum fluctuations due to mass redistributions and relative motions in the atmosphere, oceans, and continental water, snow, and ice, which are usually provided by various global atmospheric, oceanic, and hydrological models（some with meteorological observations assimilated; e.g., NCEP, ECCO, ECMWF, OMCT and LSDM etc.）. Unfortunately, these model outputs are far from perfect and have notable discrepancies with respect to polar motion observations, due to non-uniform distributions of meteorological observatories,as well as theoretical approximations and non-global mass conservation in these models. In this study,the LDC（Least Difference Combination） method is adopted to obtain some improved atmospheric,oceanic, and hydrological/crospheric angular momentum（AAM, OAM and HAM/CAM, respectively）functions and excitation functions（termed as the LDCgsm solutions）. Various GRACE（Gravity Recovery and Climate Experiment） and SLR（Satellite Laser Ranging） geopotential data are adopted to correct the non-global mass conservation problem, while polar motion data are used as general constraints. The LDCgsm solutions can reveal not only periodic fluctuations but also secular trends in AAM, OAM and HAM/CAM, and are in better agreement with polar motion observations, reducing the unexplained excitation to the level of about 5.5 mas（standard derivation value; about 1/5-1/4 of those corresponding to the original model outputs）.
In this paper, we first transferred the normal height of ASTER GDEM v2 to GPS ellipsoidal height based on the EGM96, and analyzed the precision of this digital elevation model in the northeastern margin of Tibetan Plateau （NETP） combining with 89 ground GPS measurements. The results demonstrate that the standard deviation of the difference between ASTER GDEM and GPS results is 9.3 m, and the precision of ASTER GDEM in this region is approximately 10 m. We also calculated the free-air gravity anomalies using the relative gravity data and DEM model in NETP. The results show that the gravity anomalies are generally negative with local positive values, ranging from -156 to 43 reGal （10-5 m/s2）. At last, we compared the EGM2008 free-air gravity anomalies （FGAs） with the ground gravity measurements, and their spatial patterns are similar. While the point-to-point difference between the modeling and measuring results shows great discrepancy. The free-air gravity anomalies of EGM2008 in this region range from -154 to 96 mGal, and the difference between EGM2008 and the ground measurements ranges from -102 to 50 regal. The mean difference is -17.34 mGal, and the standard deviation is 46.69 mGal, which demonstrates that the EGM2008 has poor precision in the northeastern margin of Tibet Plateau.
Guoqing ZhangWenbin ShenYiqing ZhuYing WangYawen She
The Hilbert-Huang transform （HHT） is used to analyze the time series from nine gravimeter （SG） stations and 22 broadband seismometers to investigate the anomalous signals superconducting prior to the great 2010 Maule earthquake. The results show that seven SG time series and 20 broadband seismometer time series have anomalous signals lasting about one to three days before the earthquake occurrence. The anomalous signals appear around the frequency bands 0.07Hz and 0.15Hz in SG records while around the frequency band 0. 13Hz -0.2Hz in seismic records, and the reason why they appear in different bands might be attributed to the intrinsic nature and different sensitivities of different kinds of instruments. Because more than 87% records have the anomalous signals prior to the earthquake, and no typhoon event is found in our chosen time window, we may conclude that the anomalous signals might be precursory signals of the great 2010 Maule event. However, we do not rule out other possible excitation sources.
The 2011 Tohoku-oki earthquake,occurred on 11 March,2011,is a great earthquake with a seismic magnitude Mw9. 1,before which an Mw7. 5 earthquake occurred. Focusing on this great earthquake event,we applied Hilbert-Huang transform（ HHT） analysis method to the one-second interval records at seven superconducting gravimeter（ SG） stations and seven broadband seismic（ BS） stations to carry out spectrum analysis and compute the energy-frequency-time distribution. Tidal effects are removed from SG data by T-soft software before the data series are transformed by HHT method. Based on HHT spectra and the marginal spectra from the records at selected seven SG stations and seven BS stations we found anomalous signals in terms of energy. The dominant frequencies of the anomalous signals are respectively about 0. 13 Hz in SG records and 0. 2 Hz in seismic data,and the anomalous signals occurred one week or two to three days prior to the event. Taking into account that in this period no typhoon event occurred,we may conclude that these anomalous signals might be related to the great earthquake event.
In this study,we selected 18 SG（superconducting gravimeter）records from 15 GGP stations with 99 vertical and 69 horizontal components of IRIS broad-band seismograms during 2004 Sumatra Earthquake to detect the splitting of higher-degree Earth’s free oscillations modes（0S4,0S7〈sub〉0S10,2S4,1S5,2S5,1S6）and 12 inner-core sensitive modes（25S2,27S2,6S3,9S3,13S3,15S3,11S4,18S4,8S5,11S5,23S5,16S6）by using OSE（optimal sequence estimation）method which only considers self-coupling.Results indicate that OSE can completely isolate singlets of high-degree modes in time-domain,effectively resolve the coupled multiplets independently,and reduce the possibility of mode mixing and end effect,showing that OSE could improve some signals’signal-to-noise ratio.Comparing the results of SG records with seismic data sets suggests that the number of SG records is inadequate to detect all singlets of higher modes.Hence we mainly selected plentiful seismograms of IRIS to observe the multiplets of higher modes.We estimate frequencies of the singlets using AR method and evaluate the measurement error using bootstrap method.Besides,we compared the observations with the predictions of PREM-tidal model.This study demonstrates that OSE is effective in isolating singlets of Earth’s free oscillations with higher modes.The experimental results may provide constraints to the construction of 3D Earth model.
