Observation of seismic core modes from a superconducting gravimeter record

Abstract

There have been few reliable reports of observation of seismic core modes, in spite of their geophysical importance. In this study, we analyze low-frequency seismic signals from a superconducting gravimeter (SCG) located at the Esashi Earth Tides Station, Japan, to make observations of core modes. Spectral analysis of an SCG record from the great Macquarie Ridge earthquake, which occurred during the period of our test runs of a SCG, by means of the sompi method has yielded well-resolved signals which are most likely to be the core modes 2S 2 and 6S 2. Close investigation of the analysis results, however, has revealed that there may be systematic bias in their Q estimates due to some methodological problems. Our tentative conclusion is that the Q values of the core modes observed here are relatively low (probably not exceeding 1000), and also that their amplitudes are larger than theoretically predicted from a spherically symmetric Earth model. At any rate, the present results demonstrate that the SCG also has excellent potential as a long-period seismometer.