Theory of Earth’s free oscillation is revisited. Firstly, we summarized the underlying formulations, such as the equation of motion and its conversion into numerically integrable form and then explained computational procedures including the treatment of inner core-outer core boundary and core-mantle boundary, while the latter information has not been explicitly given in most publications. Secondly, we re-calculated the periods of Earth’s free oscillation modes (period >200 s) for PREM model. In doing so we acquired the values of modes missing in Dziewonski and Anderson (1981). As a case observation, one seismogram after 2011 Tohoku earthquake recorded at Daejeon, Korea (KIGAM seismic station) was briefly analyzed to identify free oscillation mode excitations on its spectra. The material in this article will be most clear guide for those on calculating the Earth’s free oscillation mode.

Abstract
 Introduction
 Equation of Motion
 Integration of ConvertedEquation of Motion
 Computational Result
 Observation of 2011 TohokuEarthquake at Daejeon, Korea
 Discussion and Conclusion
 References

• Tae-Woong Chung(Sejong University, Seoul 05006, Korea)
• Jin-Soo Shin(Korea Institute of Geoscience and Mineral Resource, Daejeon 34132, Korea)
• Sung-Ho Na(KASI SLR Observatory, Sejong 30060, Korea (on leave from Ajou Univ.)) 교신저자