Dear @Kazuya.Nishiyama,
There are many forum posts that explain the use of FAST v7 + MBC3 + Eigenanalysis for determination of full-system natural frequencies and mode shapes. However, this process is much more convenient in OpenFAST through the Automated Campbell Diagram Code, which automates the process and visualizes the modes: GitHub - OpenFAST/acdc: ACDC: Automated Campbell Diagram Code.
We’ve also discussed how to include Seismic effects in OpenFAST: [Question]How to perform seismic load analysis.
Best regards,