This paper deals with the problem of damage localization and quantification using the Frequency Response Function
(FRF) subjected to earthquake ground excitation. The damage identification equation is derived from the motion
equation of the system before and after damage. The FRFs of the intact and damaged system as well as the system
matrices of the intact system are required to solve the damage identification equation. The system matrices can be
obtained from the state matrices identified by subspace identification technique. According to the numerical study of a
frame structure, only a few discrete frequencies from the FRFs which are close to the system natural frequencies are
sufficient to achieve a satisfactory precision of damage extent. The proposed technique was applied to an experimental
data of a 6-story steel frame structure and its efficiency was confirmed.