Consider the three-story building below that is experiencing horizontal ground motion.

Suppose that we model the building as a two-degree-of-freedom oscillator with ground motion *y*(t). The side-to-side ground motion can be represented by the harmonically time-varying function y(t) shown above. This system has two degrees of freedom, having two natural frequencies and two "natural modes" of vibration.

*Frequency of ground motion near the first natural frequency of the building*

*Frequency of ground motion near the scond natural frequency of the building*