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.
First, we would like to thank Parker for his discussion and comments on our work. Parker’s main concern [1] is that the sea level is oscillating with important multi-decadal periodicities but absolutely not positively accelerating. He also argued that the global mean sea level (GMSL) acceleration computed by Jin et al. [2] seemed too large. His
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.
As one of the ocean sudden natural disasters,the tsunami is not easily to differentiate from the ocean variation in the open ocean due to the tsunami wave amplitude is less than one meter with hundreds of kilometers wavelength. But the wave height will increases up to tens of meters with enormous energy when the tsunami arrives at the coast. It would not only devastate entire cities near coast,but also kill millions of people. It is necessary to forecast and make warning before the tsunami arriving for many countries and regions around the Pacific rim. Two kinds of data were used in this study to extract the signals of 2011 Tohoku tsunami and 2014Iquique tsunami. Wave undulations from DART（ Deep-ocean Assessment and Reporting of Tsunamis） buoys and SLA from altimetry could extract the tsunami signals generated by this two earthquake. The signals of Tohoku tsunami were stronger than that of Iquique tsunami probably due to the 2011 Tohoku tsunami was generated by a magnitude 9. 0 earthquake and the 2014 Iquique tsunami was triggered by a magnitude 8. 2 earthquake.
The search for the elusive Slichter triplet requires elaborate analysis of the elastic-gravi- tational mode characters and the non-stationary behavior of noisy time-series. A typical question is that it is difficult to characterize the excitations with attenuation by diffusion when their intensity is low compared to noise. Thus the theory for deriving the modes＇ frequencies is still controversial, and various scholars tried to search for the Slichter triplet in superconducting gravimeter （SG） records, but failed. One of the main causes might be due to the inappropriate use of datasets. We present in this paper synthetic experiments on the selection of record length, sampling rate and number of SG records under the Global Geodynamics Project （GGP） to detect the damped harmonic signals hidden in noises based on the optimal sequence estimation （OSE） method. Moreover, our results show that the existing observation conditions arouse restrictions and it might be impossible to detect the Slichter triplet excited by single excitation source based on Fourier spectrum analysis. Thus we suggest a stacking way of combining several seismic events in the case that the excitation mechanism has so far been unclear.
According to the general theory of relativity, two clocks placed at two different positions with different geopotentials run at different rates. Thus one can determine the geopotential difference between these two points by comparing the running rates of the two clocks. Using the most precise optic-atomic clocks whose stability achieves 10 is level and the time transfer technique with comparison accuracy higher than 10ps level by 100 m coaxial cable, the relativistic geodesy method for determining the geopotential may be realizable in the near future. In this paper, we propose, design and describe in detail an approach for determining the geopotential difference between two points based upon a simulation experiment. We select two stations A and B whose ground distance is within 100 m, height difference being about 30 m. Each station is equipped with an atomic clock whose instability is 3.2 × 10-16/√τ （where τis time in second）. And the two stations are connected with a coaxial cable for time transfer. Our simulation experiment results show that the accuracy could reach 0.16 m2/s2 （equivalent to 1.6 cm in height） level after a fourhour period of observation.
