Shen Wenbin

Effects of the Earth＇s triaxiality on the polar motion excitations

Abstract: his study aims to evaluate the significance of the Earth’s triaxiality to the polar motion theory. First of all, we compare the polar motion theories for both the triaxial and rotationally-symmetric Earth models, which is established on the basis of the EGM2008 global gravity model and the MHB2000 Earth model. Then, we use the atmospheric and oceanic data (the NCEP/NCAR reanalyses and the ECCO assimulation products) to quantify the triaxiality effect on polar motion excitations. Numerical results imply that triaxiality only cause a small correction (about 0. 1–0.2 mas) to the geophysical excitations for the rotationally-symmetric case. The triaxiality correction is much smaller than the errors in the atmospheric and oceanic data, and thus can be neglected for recent studies on polar motion excitations.

Anomalous signals before 2011 Tohoku-oki Mw9.1 earthquake, detected by superconducting gravimeters and broadband seisimometers

Abstract: 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 Hilber-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.

Hydrologically induced orientation variations of a tri-axial Earth's principal axes based on satellite-gravimetric and hydrological models

Abstract: 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.