[en] An HTGR core responds nonlinearly to an earthquake input. Nonlinear systems are different from linear systems. Academically, there is no characteristic natural frequency in the no linear vibratory system. Instead, there are resonant peaks that can develop. These are related to the amplitude and frequency of the forcing function as well as to the system characteristics and the transients that exist at the time. In particular, the frequency of peak response is strongly affected by the magnitude of the input amplitude. As in linear systems, resonance can be produced when the forcing frequency is equal to or less than the natural frequency. In addition, in a nonlinear system only, resonances can be produced when the forcing frequency is above the ''natural'' frequency. When this occurs, the resonance is referred to as a subharmonic resonance, that is, the frequency is 1/n times the forcing frequency. The occurrence of subharmonic resonance in a series of blocks with clearance between blocks and with springs on the outermost ends is the subject of the paper. The response of an idealized nonlinear one degree of freedom system to two sine waves is discussed. The influence of the elastic characteristics and damping is also examined