  We show theoretically that according to the disentanglement behavior under
composite noise environment, the Hilbert space of a two-qubit system can be
divided into two separate parts: a 3-dimensional subspace in which all states
disentangle asymptotically, and the rest in which all states disentangle
abruptly. The violation of additivity for entanglement decay rates under weak
noises [see, PRL 97, 140403 (2006)] therefore can be explained in terms of such
division of the Hilbert space.
