Detection of the oscillations of Earth's core by a collaboration observation with superconducting gravimeters and laser strain meters-Comparison between the Kamioka site and Matsushiro site-

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This research project was planed to detect weak normal mode signals from the Earth's core based on the observations with two superconducting gravimeters (SGs) at Kamioka and Matsushiro and by a comparison between the SG and laser strain meters set at Kamioka. H. Ikeda of the Tsukuba University and Severine Rosat of JSPS fellow did join to this project as the cooperative researchers though the term of the project.We have done the followings ;1.Developing a new non-linear analysis method for the normal mode analysis, which can treat both the frequency and Q-value of normal modes as unknown parameters,2.Analysis of the SG records mainly of the Sumatra-Andaman earthquake (Mw=9.0-9.3), which were obtained from the world wide SG network including Kamioka, Matsushiro and the Syowa Station in Antarctica,3.Improving the quality of the laser strain meter and the analysis of the records of the Sumatra-Andaman earthquake,4.Improving the estimation accuracy of gravity effect of the regional atmosph … More eric air mass changes,5.Investigating the accuracy of modern global ocean tide models, which are used in the correction for the ocean tides, and also studying the non-tidal sea surface height variations by comparing some model computation results with the actual data obtained from the ocean bottom pressure measurements in Off-Sanriku,6.Improving the calibration accuracy of the scale factor of SG by using an absolute gravimeter.In December 26th 2004, two months later when the SG was newly installed at Kamioka, the Sumatra-Andaman earthquake occurred. The fundamental radial mode _oS_o, which consists of a global deformation of the Earth with changes in its circumference, has been strongly excited by this event and its time decay has been observed till the occurrence of the second Sumatra event in March 28th 2005 at many of SG sites. A notable result obtained by this research project is; we have succeeded to reveal that the _oS_o mode shows a geographical amplitude distribution depending on not only the latitude but also on the longitude, which may reflect a large scale lateral inhomogeneity in the Earth's structure. The observed geographical pattern is consistent with the model prediction based on a 3-D seismic tomography model.Based on the two SG data sets for the Sumatra-Andaman event, which were obtained at Strasbourg (France) and Canberra (Australia) and using the new analysis method developed by this project, we have tried to find out such weak core signals as the Slichter triplets and the core undertones. Although we found some peaks to be closely related to these modes, the analysis results have not revealed such inner core signals yet, because some spectrum peaks fond are not observed commonly at these two sites.The effect of tsunami waves excited by the Sumatra event was also observed by several instruments set at the Syowa Station including the SG. We have compared the predicted tsunami effects with those observed from the SG and SST-1. Improvement of the accuracy of the bathymetry around the Syowa station is essential to improving the comparison accuracy between the observations and predictions. Less