Rotational component of seismic ground motion
An algorithm for spectral density of the rotational (rocking) component of seismic ground motion has been derived. The nonstationary stochastic process representing total ground motion has been decomposed into the wave components coming from body & Rayleigh waves. Respective wave contributions were then differentiated with respect to spatial coordinate and combined back again. A shift of higher frequency components in the evolutionary spectral density of rotation has been observed.

For more information see: Probabilistic Engineering Mechanics, vol.8, 1993, pp.5-14,