  The short-term memory effects recently observed in vibration-induced
compaction of granular materials are studied. It is shown that they can be
explained by means of quite plausible hypothesis about the mesoscopic
description of the evolution of the system. The existence of a critical time
separating regimes of ``anomalous'' and ``normal'' responses is predicted. A
simple model fitting into the general framework is analyzed in the detail. The
relationship between this work and previous studies is discussed.
