Abstract
In relation to the observations that are carried out in horizontal tunnels, several topics are described as applications of the observations to geodynamics studies . The topics discussed are; the free core nutation (FCN), the fluid core resonance (FCR) and the Earth's free oscillations. Long-term observation data obtained from the horizontal tunnels are useful to studya time variation in the FCN and/or the FCR parameters (i.e. its eigen-period, Q-value and strength of the resonance) and to study also the coupling and splitting of the Earth's normal modes. Collaborative observations by strainmeters and gravimeters shall give us useful data to discuss the topics mentioned above.
