  Statistical properties of nonlinear systems revealed in an unexpected way
already in Fermi-Pasta-Ulam paper, are responsible for the appearance of
substructures. We represent a method within S-tree diagram formalism for
investigation of the substructures of N-body gravitating systems in a defined
time interval. This approach enables one to consider the link between
substructures of the system and its relative instability and is efficient for
the study of the substructures in clusters of galaxies.
