In this paper, the influence of the rotation speed on the earthquake response of rotors is investigated by means of a simple model with only two degrees of freedom. Some worth noticing properties of the eigenmodes and the transfer functions in the presence of solid gyroscopic effect are discussed. In particular, the importance of the relative contribution of the forward and backward modes is evaluated using basic properties of the transfer functions. In the case of a white noise excitation the backward mode contribution dominates the response, for high values of spin velocity. Depending on the frequency content of the excitation and the rotation speed, an important amplification of the response may occur due to gyroscopic effects, as a result of the amplification of the backward mode response. Nonlinear boundary conditions reduce, to some degree, the differences between the responses of rotating and non-rotating systems, but, even in this case, the importance of the contribution of the backward mode in the response appears clearly. Rotors immersed in a dense fluid with moderate confinement exhibit different behaviour and amplification is possible in the case of resonant response of the second forward mode.
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