Using the Marginal Spectrum Method to Extract Spherical Free Oscillation of Strong Earthquakes

The research on earth free oscillation has become one of important methods to detect the internal structure of earth. This paper uses the marginal spectrum method based on Hilbert Huang transform for the first time, and the Tonghe station´s borehole strain data without removing solid tide, then we extracted spherical free oscillation mode that are motivated by Wenchuan M8.0 earthquake, Chile M8.8 earthquake and the east coast of Honshu of Japan M9.0 earthquake, the results are very close to the PREM mode. Among which, the Wenchuan earthquake is tested 24 spherical fundamental frequency modes, the Chile and Japan earthquakes are both 76, which consist of ₀S₀ and ₀S₂~₀S₇₆. The marginal spectrum method can effectively extract the earth free oscillation because of these strong earthquakes´ mode have spectrum divided phenomenon.