Scattering of seismic waves by lithospheric heterogeneities produces seismic coda which typically look random. Consequently stochastic modelling methods are usually applied to these coda [1--6>. Here, we introudce a technique, based on previous work [7>, for evaluating the predictability of time-series data as an indicator of underlying determinism. When analyzed using this method both earthquake and explosive source seismic coda display strong short term predictability which is not consistent with a random generation mechanism.